Primitive Archer
Main Discussion Area => Bows => Topic started by: Selfbowman on January 10, 2021, 09:38:22 am
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If early wood is not as dense and would crush first does it not domino the set thru out the limb.
I’ve heard that what is in the middle of the limb does no work it just holds the tension on the back and
Compression on the belly apart so to speak.
So What is the fix to compensate for the weaker link?
Possibly by trapping. I’m looking forward to hearing the take on this from everyone. Especially Marc, Mark , Bownara, DC, and Badger to mention a few.
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Before we make too many assumptions regarding the tendencies of clearly designated early and late wood in some species, let's be sure to keep diffuse porous woods and tropical woods in mind. They take set too.
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Thanks dances with squirrels good point. I can’t agree about crooked wood makes a better bowyer though. There is art in straightening the grain also. I’ve built a couple snake bows following the grain and built a bow that you can drop a half dollar thru mid limb . Bows full of shock and so on none of which made me a better bowyer. Arvin
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Yes, even non open grained wood takes set.
We talking Osage the is but little doubt that early chaulky growth is the weak part the wood. Specially if that early growth is a rather thick line. It really reduces the density of the stave as a whole.
If noticed when doing statics if you have really nice rings but the ratio of early to late leans toward large early chaulky and they are thick the wood will try to separate in that line. Yes , that is the weakest part of Osage.
HH~
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Shawn I think it applies to lots of woods . Oak, walnut, maple, pine, lots of them. Maybe not blood wood , Purple Heart etc.
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Send yer snakey staves to me then, Arvin. The snakier the better! :OK
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Don’t have any .I like better bows.😁😁😁😁just a dig in fun. Please don’t take that personal. Arvin
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Yeah, I always see these snaky bows on the Podium. Aaaah, not really. Well, not ever.
But that early growth is however the weak part of the Hedge for sure.
Shawn~
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The bull crap is getting old fast enough. Can I hear from some smart guys that can put things in language that the country bow can understand?
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It's all good Arvin.
Podium. Lol. Took me a minute to figure out what the heck you were even talking about.
Sounds like a good name for the snakey osage bow I'm working on. "Nopodium".
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Adapt and overcome. If there is a lot of early wood, leave it a little wider. Jawge
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I’ve heard that what is in the middle of the limb does no work it just holds the tension on the back and
Compression on the belly apart so to speak.
This is mostly correct, but the inner core of the limb isn't doing nothing at all, it is just contributing much less than the wood near the surfaces.
For a rectangular cross section limb the stresses (and strain) are zero at mid thickness and increase linearly to a maximum at the surface, both back and belly. This arrangement means that the majority of the work/energy storage is done in the wood near the back and belly surfaces. For the rectangular cross section it works out that half the work is done in the outer 15% of the limb.
Say you have a limb that is 0.5" thick, 50% of all the work is being done by a strip that is 0.075" thick on the belly and back. The other 50% is done by the 0.35" of wood in the core, which is why I said it doesn't do nothing, but it contributes at a much lower level per ounce of wood.
So What is the fix to compensate for the weaker link?
Possibly by trapping.
Your fix (if there is one) depends on if the early wood is weaker in tension, compression or both. If it is weaker in both then there won't be much you can do aside from lowering the stresses everywhere by making the bow longer and/or wider in the limbs. If it is only weaker in compression then trapping will help by reducing the stresses on the belly. It won't solve the problem but it will improve the situation for any tension strong wood.
Mark
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I think the weak link in the bow building process is the builder that doesn't understand the properties of the wood being used or practice appropriate tillering methods and/or the design considerations for the wood that is used. Wood bows will take set, it is inevitable because of the properties of wood and what it is being asked to do. Set is not necessarily a problem in a wood bow. Some of our wood bow icons from history indeed liked some set in their bows. We are not dealing with modern materials and we should not expect a wood bow to act the same as a FG bow; oranges to apples.
Of course early wood is weaker than latewood and especially in ring porous wood like osage, mulberry the oaks and others. But I haven't noticed that the early wood adversely affects the strength or performance of a bow. Some of my best, hardest hitting bows have been osage with paper thin rings. With a good solid, clean back ring, proper tillering and design methods and patience anyone can make a well performing wood bow.
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Thanks Pat I appreciate what you said . I am not trying to build a hard hitting shooter . I’m going for the hardest hitting bow for flight. Arvin
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I think you're sort of wondering if the crunchy layers are sort of like a bad glueline in a laminated bow and if they actually allow some creep in addition to the more common types of set caused by compression.
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Yes Pat. It does not just crush but kinda fold in one direction or the other. I don’t think much would show in width even under microscope. But probably where its happening . Just a guess. In length it would seem that it would delaminate because of thickness.
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I would guess any weaknesses in the early wood are overcome by the late wood. Even if the early wood does take set earlier, the late wood would just push it back to its original shape. I think the issues with higher ratios of early wood have more to do with overall density. If early wood does fail in compression earlier I think we would see frets or hinges at the point where growth rings feather out on the belly but I rarely see compression failures in such a specific spot on the limb.
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Arvin, I dont know what my opinion is worth to you but....
From an aviation stand point, the crunchy wood is just like the honeycomb core in composite skin structures. We lay a piece of thin aluminum the bond to it a layer of honeycomb and another layer of aluminum. The resulting structure is very light weight and amazingly stiff. It has all of the properties we want in a bow.
If I can find one, I'd like to make a bow that has one ring in the back, crunchy wood in the middle, and one ring on the belly. I dont think crunchy wood is a weak link. However, a badly proportioned stave of that early crunchy wood could cause a problem easily.
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I can answer better than what I just did.
To add a more relatable example, structurally speaking: corrugated cardboard uses the same principle.
As for how to proportion the core (crunchy wood) it will vary depending on the stiffness of the belly and back. The stiffer they are, the thicker the core can be to support them. If the core is too thick, the tension and compression properties will be exceeded of the back or belly, or even lead to a torsional/sheer failure in the core.
This being an organic material we really have to learn from experience what a ring count looks like in proportion to the early/late growth, then know what stresses it can handle. A good way to Guage your progress will be the mass formula by Badger. Another is just measure the specific gravity of the wood before building and document your results. Eventually you could just look at a staves ring count, and cut one inch square of wood, and know exactly what that stave can do.
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I can answer better than what I just did.
To add a more relatable example, structurally speaking: corrugated cardboard uses the same principle.
As for how to proportion the core (crunchy wood) it will vary depending on the stiffness of the belly and back. The stiffer they are, the thicker the core can be to support them. If the core is too thick, the tension and compression properties will be exceeded of the back or belly, or even lead to a torsional/sheer failure in the core.
This being an organic material we really have to learn from experience what a ring count looks like in proportion to the early/late growth, then know what stresses it can handle. A good way to Guage your progress will be the mass formula by Badger. Another is just measure the specific gravity of the wood before building and document your results. Eventually you could just look at a staves ring count, and cut one inch square of wood, and know exactly what that stave can do.
Interesting.
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Yes the winter growth is weak and I want the ratio as thin as possible, but speaking of Osage i have had some that the whole limb was summer growth, 1 ring and it took some set also. :)
Pappy
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Yes the winter growth is weak and I want the ratio as thin as possible, but speaking of Osage i have had some that the whole limb was summer growth, 1 ring and it took some set also. :)
Pappy
Mark Was the growth rings real big? I’ve noticed the thick late rings have more lunar ring size as well. The ones I’ve seen do not seem real dense. You can’t have everything. Arvin
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O ya it was a large one and to be honest I don't like large thick rings, I would much rather have fairly thin rings with a very fine winter growth , and well seasoned, I find they hold more of what you do to them and don't change much if any after shooting them a bunch, don't know about set, don't really measure it, but will say my best bows come from finer ring well seasoned Osage. HHB would be my next favorite for white wood but again well seasoned. :)
Pappy
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All this info is interesting and the points are valid. But to strictly answer the question about the early wood being the weak link causing set...I don't think so. Set is caused when the belly wood is compressed to the point of cell colapse and once it is colapsed, it stays colapsed...thus causing the set. That crunchy early wood, it seems to me, would colapse under compression extrememly easily and shear under tension very easily. The only way...again, this is just what I am assuming/educated-guessing...the only way that early wood crushing could actually cause the set we see is if it, in a colapsed-cell state, was strong enough to overpower the late wood directly around it on the belly side in the compression plane that is resisting compression forces...and that ain't happening. Can the early wood in a stave with a large amount of early wood to late wood ratio affect the late wood's overall ability to resist compression forces? I'm sure it can have some negative impacts, but to what degree, I havn't a clue. That said, I seriously doubt it is anywhere near the top many reasons that a bow takes set. Like has been mentioned before...set is far more likely due to poor tillering and mistakes by the boyer in the tillering process than anything to do with the early wood ratio of the stave.
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Arvin, I dont know what my opinion is worth to you but....
From an aviation stand point, the crunchy wood is just like the honeycomb core in composite skin structures. We lay a piece of thin aluminum the bond to it a layer of honeycomb and another layer of aluminum. The resulting structure is very light weight and amazingly stiff. It has all of the properties we want in a bow.
If I can find one, I'd like to make a bow that has one ring in the back, crunchy wood in the middle, and one ring on the belly. I dont think crunchy wood is a weak link. However, a badly proportioned stave of that early crunchy wood could cause a problem easily.
I wouldn't say we want that in a bow. We need a bow to flex, not be stiff.
You could likely test the crunchy layer theory by gluing lams of bamboo stacked like growth rings with a thick layer of crunchy glue holding them each together.
Hard to say if there is a glue that mimics early wood.
One thing is pretty sure, early wood is often not the greatest bond between rings when a piggyback split can often be achieved right down one.
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Arvin, I dont know what my opinion is worth to you but....
From an aviation stand point, the crunchy wood is just like the honeycomb core in composite skin structures. We lay a piece of thin aluminum the bond to it a layer of honeycomb and another layer of aluminum. The resulting structure is very light weight and amazingly stiff. It has all of the properties we want in a bow.
If I can find one, I'd like to make a bow that has one ring in the back, crunchy wood in the middle, and one ring on the belly. I dont think crunchy wood is a weak link. However, a badly proportioned stave of that early crunchy wood could cause a problem easily.
I wouldn't say we want that in a bow. We need a bow to flex, not be stiff.
You could likely test the crunchy layer theory by gluing lams of bamboo stacked like growth rings with a thick layer of crunchy glue holding them each together.
Hard to say if there is a glue that mimics early wood.
One thing is pretty sure, early wood is often not the greatest bond between rings when a piggyback split can often be achieved right down one.
I think gorilla glue is pretty bubbly?
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Arvin, I dont know what my opinion is worth to you but....
From an aviation stand point, the crunchy wood is just like the honeycomb core in composite skin structures. We lay a piece of thin aluminum the bond to it a layer of honeycomb and another layer of aluminum. The resulting structure is very light weight and amazingly stiff. It has all of the properties we want in a bow.
If I can find one, I'd like to make a bow that has one ring in the back, crunchy wood in the middle, and one ring on the belly. I dont think crunchy wood is a weak link. However, a badly proportioned stave of that early crunchy wood could cause a problem easily.
I wouldn't say we want that in a bow. We need a bow to flex, not be stiff.
You could likely test the crunchy layer theory by gluing lams of bamboo stacked like growth rings with a thick layer of crunchy glue holding them each together.
Hard to say if there is a glue that mimics early wood.
One thing is pretty sure, early wood is often not the greatest bond between rings when a piggyback split can often be achieved right down one.
Stiffness is just resistance to bending. We don't want a bow that bends, we want a bow that resists bending. Like in a hydraulic system, pressure is a measure of resistance to flow, not a flow rate. So, when I look at it from that angle, I want a bow that has the most resistance to bending as it can without failure.
thays all theory anyway. I haven't had much opportunity to test it in depth. However, much like Pappy, my favorite bows have thin rings, been that way for years.
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It definitely is but I'm not sure how crunchy it ends up in comparison to earlywood. It still seems a bit like spongy foam rather than being brittle.
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Maybe some foamy epoxy? Well, now I wonder, do we even know why early wood is so crunchy? What are its actual properties?
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Arvin, I dont know what my opinion is worth to you but....
From an aviation stand point, the crunchy wood is just like the honeycomb core in composite skin structures. We lay a piece of thin aluminum the bond to it a layer of honeycomb and another layer of aluminum. The resulting structure is very light weight and amazingly stiff. It has all of the properties we want in a bow.
If I can find one, I'd like to make a bow that has one ring in the back, crunchy wood in the middle, and one ring on the belly. I dont think crunchy wood is a weak link. However, a badly proportioned stave of that early crunchy wood could cause a problem easily.
I wouldn't say we want that in a bow. We need a bow to flex, not be stiff.
You could likely test the crunchy layer theory by gluing lams of bamboo stacked like growth rings with a thick layer of crunchy glue holding them each together.
Hard to say if there is a glue that mimics early wood.
One thing is pretty sure, early wood is often not the greatest bond between rings when a piggyback split can often be achieved right down one.
Stiffness is just resistance to bending. We don't want a bow that bends, we want a bow that resists bending. Like in a hydraulic system, pressure is a measure of resistance to flow, not a flow rate. So, when I look at it from that angle, I want a bow that has the most resistance to bending as it can without failure.
thays all theory anyway. I haven't had much opportunity to test it in depth. However, much like Pappy, my favorite bows have thin rings, been that way for years.
Still needs to bend though. Bow woods come in very low resistance to bending and very high. Both are highly regarded as long as they bounce back.
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Maybe some foamy epoxy? Well, now I wonder, do we even know why early wood is so crunchy? What are its actual properties?
That might work but you'd have to do some tests to determine if you were actually mimicking it.
We do know why, there's lots of info on the actual differences between early and latewood and why they are different. Or not really in some cases.
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People have been making their living, obtaining food and protecting themselves for over 10,000 year with wood bows. Apparently the early wood was never a problem for them even though many of their bows had set. Blaming the materials we use is the easy way out. Learn proper tillering, learn the properties of the wood you are working with, learn the affects of the design you use with the wood used and work with patience if you want a wood bow with minimal set and reasonable cast. :OK
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People have been making their living, obtaining food and protecting themselves for over 10,000 year with wood bows. Apparently the early wood was never a problem for them even though many of their bows had set. Blaming the materials we use is the easy way out. Learn proper tillering, learn the properties of the wood you are working with, learn the affects of the design you use with the wood used and work with patience if you want a wood bow with minimal set and reasonable cast. :OK
I agree totally but I would be willing to bet they have been testing one another to see who shot their arrow the farthest for the same time. Had to impress the ladies somehow even 10000 years ago. 😁😁😁
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Tim Baker was attempting something similar to Sleek’s idea with some type of hide glue foam and a sinew back a few years ago. I don’t think it went anywhere though.
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Tim Baker was attempting something similar to Sleek’s idea with some type of hide glue foam and a sinew back a few years ago. I don’t think it went anywhere though.
His idea was going a different route and he quit for whatever reason without reaching a conclusion. I reached out to him several times over it, but his interest in it had passed.
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From an aviation stand point, the crunchy wood is just like the honeycomb core in composite skin structures. We lay a piece of thin aluminum the bond to it a layer of honeycomb and another layer of aluminum. The resulting structure is very light weight and amazingly stiff. It has all of the properties we want in a bow.
I think the cell structure of early wood is not quite this well ordered. Also, your honeycomb is not oriented in the direction of bend like it is in wood.
I think that most ring counts are fine enough and even enough to treat the wood as a homogenous material so long as there is not excessive early wood near the back or belly.
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Often, always, unless its a pyramid ( im hinting at Arvin success ) the early wood is found on the belly as the rings terminate through tillering.
your point about the orientation of the honeycomb is a well thought out one but not applicable. The grain structure does not matter in a core as long as it is a function of 90° to the stresses.
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Early wood on the belly surely just sits there passively. About like roughing up a belly surface with a rasp. It can't have any functional integrity.
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Early wood on the belly surely just sits there passively. About like roughing up a belly surface with a rasp. It can't have any functional integrity.
It doesn't, and that's why I think ( key word ) it going into the belly is detrimental to a bow.
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Maybe to the degree its "mass" slows the bow but that has to be minuscule on a typical bow. A streak or two of graininess amounts to nothing.
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I think you're sort of wondering if the crunchy layers are sort of like a bad glueline in a laminated bow and if they actually allow some creep in addition to the more common types of set caused by compression.
I believe energy is stored when when adjacent wood cells are subject to shear stress as well as tension or compression. Otherwise we would be able to build a thin skinned honeycomb cored bow from wood.
higher percentages of lighter early wood in the core can't help much. energy storage is likely higher closer to the back and belly surfaces, but even though the contribution is less towards the center of the limb, total energy storage is a sum of the whole
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Otherwise we would be able to build a thin skinned honeycomb cored bow from wood.
We can't?
Mark
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Arvin, I dont know what my opinion is worth to you but....
From an aviation stand point, the crunchy wood is just like the honeycomb core in composite skin structures. We lay a piece of thin aluminum the bond to it a layer of honeycomb and another layer of aluminum. The resulting structure is very light weight and amazingly stiff. It has all of the properties we want in a bow.
If I can find one, I'd like to make a bow that has one ring in the back, crunchy wood in the middle, and one ring on the belly. I dont think crunchy wood is a weak link. However, a badly proportioned stave of that early crunchy wood could cause a problem easily.
I wouldn't say we want that in a bow. We need a bow to flex, not be stiff.
You could likely test the crunchy layer theory by gluing lams of bamboo stacked like growth rings with a thick layer of crunchy glue holding them each together.
Hard to say if there is a glue that mimics early wood.
One thing is pretty sure, early wood is often not the greatest bond between rings when a piggyback split can often be achieved right down one.
I think gorilla glue is pretty bubbly?
Few types of branded gorilla glue Sleek what you are talking about is polyurethane glue. Expanding foam is basically the same stuff.
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For folks wanting to understand set a bit better. I strongly recommend building a quicky d-bow. Pull it too far to induce set......then take it to the bandsaw and cut it in half....so you seperate back and belly and end up with two lams :)
One thing I will say is remeber the balanced seesaw (teeter-totter?)....IF we manage to balance the force back and belly we get significantly less set. I wish somebody else would do this experiement.....you might find it interesting. Trapping may well be the way to better distances for you.
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Otherwise we would be able to build a thin skinned honeycomb cored bow from wood.
We can't?
Mark
it could be built, but I do not believe you will get the energy storage. for example
a hickory bow vs. an otherwise similar dimensioned bow constructed with a front and back hickory lam on a balsa core
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I always feel more comfortable with the belly ring at least 1/8" thick. I don't think I have ever finished a bow that had an early wood belly. I have had plenty of osage staves with paper thin rings that worked out great. I have actually gotten to where I like the thin ringed osage. It doesn't seem to make much difference as long as the belly ring is late wood. Just an observation on my part I don't claim any factual expertise on.
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it could be built, but I do not believe you will get the energy storage. for example
a hickory bow vs. an otherwise similar dimensioned bow constructed with a front and back hickory lam on a balsa core
Ah, I understand. If you keep the external dimensions the same then you wouldn't get the same energy storage, because the core is normally doing some work in storing energy. But building a bow as you describe with the dimensions adjusted to suit the construction method should be the same or better (due to higher efficiency in how it uses the wood) than a one piece bow. It would essentially be a wood version of a foam core fibreglass lam bow and those work just fine.
Mark
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I always feel more comfortable with the belly ring at least 1/8" thick. I don't think I have ever finished a bow that had an early wood belly. I have had plenty of osage staves with paper thin rings that worked out great. I have actually gotten to where I like the thin ringed osage. It doesn't seem to make much difference as long as the belly ring is late wood. Just an observation on my part I don't claim any factual expertise on.
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I’ve been attempting this Steve but to hit our weight classes makes this a bit difficult. For me at least. Arvin
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Stiffness is just resistance to bending. We don't want a bow that bends, we want a bow that resists bending. Like in a hydraulic system, pressure is a measure of resistance to flow, not a flow rate. So, when I look at it from that angle, I want a bow that has the most resistance to bending as it can without failure.
Still needs to bend though. Bow woods come in very low resistance to bending and very high. Both are highly regarded as long as they bounce back.
"In terms of looking at the raw mechanical data of woods, the best bow woods tend to be those that have a low MOE and a high MOR. (Stated another way, the best bow woods tend to be those that will bend easily, and not break.)"
From https://www.wood-database.com/wood-articles/bow-woods/
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One thing I will say is remeber the balanced seesaw (teeter-totter?)....IF we manage to balance the force back and belly we get significantly less set. .... Trapping may well be the way to better distances for you.
Yes, IMHO what matters most is the well balanced relative ratio of the tensile strength of the back half and the compressive strength of the belly half; as well as the relative ratio of MOR to MOE of the whole stave.
Cf. https://www.wood-database.com/wood-articles/bow-woods/
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Early wood on the belly surely just sits there passively. About like roughing up a belly surface with a rasp. It can't have any functional integrity.
It doesn't, and that's why I think ( key word ) it going into the belly is detrimental to a bow.
Detrimental how? Unless its a pyramid bow or a bow with extremely thick late wood growth rings, there has to be some early wood coming into the belly surface. I'm just thinking about the osage bows I've built...how could you not have early wood coming into the belly of the working portions of the limbs without it being a pyramid design? I agree...its not just sitting there passively. It is getting compressed and sheared and doing whatever weak, pithy wood does when it gets compressed and released, but for it to contribute any way to set, it's physical properties would have to overwhelm and overcome the physical resistance-to-compression properties of the latewood layers it's sandwhiched between. If the latewood in the compression plane is able to withstand full draw without it's cells crushing an colapsing, it will bounce back, and I can't see how any amount of that pithy early wood could stop it.
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Early wood is barely even continuous in structure. I think we're thinking that layer of early wood exposed on a belly has about the structural strength of a coat of glue with some sand strewn on it. It would be like dead weight. Might as well just sand it off .although the weight has to minimal.
I think people wondering if it contributes to set are thinking it might creep or break down and allow some shifting of the growth rings perhaps.
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Yes Pat I’m in that group. Making it the weak link.
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Think of an Oreo cookie . . . . .?
Snap one
Then scrap the whyte guts out put the black outer shells together now snap one.
Physics by Hostess and Keebler
Not hard for a Nug to figger.
Shawn~
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Think of an Oreo cookie . . . . .?
Snap one
Then scrap the whyte guts out put the black outer shells together now snap one.
Physics by Hostess and Keebler
Not hard for a Nug to figger.
Shawn~
I like that. And I just gained two pounds experimenting.
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Early wood is barely even continuous in structure. I think we're thinking that layer of early wood exposed on a belly has about the structural strength of a coat of glue with some sand strewn on it. It would be like dead weight. Might as well just sand it off .although the weight has to minimal.
I think people wondering if it contributes to set are thinking it might creep or break down and allow some shifting of the growth rings perhaps.
If this is the case, how would board bows or bows where the early wood is not exposed on the belly be effected?
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Board bows of typical orientation would actually have more early wood exposed as linear lines but they wouldn't have that "between lam" shear potential.
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Board bows of typical orientation would actually have more early wood exposed as linear lines but they wouldn't have that "between lam" shear potential.
That’s my thought. If the shear potential is why early wood is a weak link, then it seems that board bows would take less set on average than stave bows.
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Board bows of typical orientation would actually have more early wood exposed as linear lines but they wouldn't have that "between lam" shear potential.
That would theoretically make a big difference, and its my understanding that this may be the best orientation for a bows rings if you can get good clean wood.
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Board bows of typical orientation would actually have more early wood exposed as linear lines but they wouldn't have that "between lam" shear potential.
That would theoretically make a big difference, and its my understanding that this may be the best orientation for a bows rings if you can get good clean wood.
Would be a good experiment to find a wide flatsawn board with enough width for a quarter sawn/rift cut piece and a flat sawn piece to build a couple identical bows from to test. Still not totally homogenous but as close as it one could probably get.
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So if I cut a bow out of a straight grain stave and back it . Growth rings turned 90 degrees.
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The most scientific option would be to make a few slats of different grain orientation from that same stave (or any single board) and test them individually with bending tests. Building one bow won’t tell us much.
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So if I cut a bow out of a straight grain stave and back it . Growth rings turned 90 degrees.
Yeah, thats the idea, but you shouldn't need to back it. And you would need to make it wider than normal as well due to the reduced surface area of the late growth taking compression and tension loads on the belly and back. So basically measure how wilde all the early growth rings are and subtract their total width from the bows actual width. If you make a 2 inch wide bow and those early rings add up to .25 inches, make the bow an extra .25" wide to accommodate it.
With your pyramid bows always having a solid ring on the back and belly, id be interested to see any difference made in cast and set with a bow made this way from you. Any losses due to the sheer forces on the early wood would be eliminated. Be an interesting experiment considering how consistent you are with your bows performance.
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Sleek the tapers on the pyramid where the growth changes won’t separate? That’s why I’m thinking a backing. I certainly don’t know what is being designed here. I think I can build it though. Don’t know if it will stay together though. Arvin
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Sleek the tapers on the pyramid where the growth changes won’t separate? That’s why I’m thinking a backing. I certainly don’t know what is being designed here. I think I can build it though. Don’t know if it will stay together though. Arvin
That’s the question in the other thread I started. For white woods, they almost never break along the early wood lines if the grain is straight, and often not even if it isn’t very straight.
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Ok selfbow it is . I like to make them more anyway. It will be a few days it’s wet here for the next few days . Probably next week
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There are 3 types of board cuts. First there's plane or flat sawn where the butt grain looks like this =. Second there's rift sawn where the butt grain looks like this // and finally, there is 1/4 sawn where the butt grain looks like this ||.
So the first 2 allow for some grain run out ...maybe 2 per limb for a 50# bend in the handle bow. 1/4 sawn must have none at all. Something to keep in mind for this thread. If you cut a a 1/4 sawn slice out of stave seems like you'll get run outs.
My board bows over the years seem to take less set but I only like rift sawn boards. I don't keep records. This is anecdotal.
Jawge
Jawge
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Jawge that was my concern. I don’t know anything about this I think I made one maybe two board bows in my life. Are you saying two ring run offs in one limb? I will try to find some scraps big enough to do a test. Arvin
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Jawge that was my concern. I don’t know anything about this I think I made one maybe two board bows in my life. Are you saying two ring run offs in one limb? I will try to find some scraps big enough to do a test. Arvin
I think Jawge is referring to runout from back to belly. If you planed down the wiggles in that stave, you would cut through fibers that could split regardless of early or late wood growth. I would try and carefully create a clean back following the forward and backward wiggles of the stave.
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Ryan have you done this before? Just asking. And if so what was your results?
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Ryan have you done this before? Just asking. And if so what was your results?
I haven’t done this with a stave. Just a handful of board bows.
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With good success? What kind of wood? A board comes from a tree how could it be different? Pics please if possible?
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Mostly red oak, hickory, and maple. I did a couple backed bows with maple backs and cherry and walnut bellies. Here's a full selfbow's. Red oak paddle recurve, poplar longbow, red oak bendy handle bow, and hickory longbow.
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No doubt you can tiller! I was more interested in pics of the selfbow backs and especially at the transitions.
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Here's a few pics corresponding to the FD pics. The last one is blurry but the best one I have of the grain on the back.
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I took the back down to a growth ring so I could lay the bows back out from there . If we was going to follow the grain. I peeled the sides of the stave some as to follow the grain. Found some checks in the sides of the stave. Would not worry me at all in a normal selfbow. Probably not to deep but could be a failure in the making. May need to try another stave.
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More pics
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That stave is impressively straight. You could possibly get away with planing down one side and going from there, though I can't say I've seen many if any osage board bows. I would also worry about the checks if I couldn't get past them or glue them together because side checks (shakes) appear to happen along the growth rings specifically.
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Yes equally as impressive as a crooked one! 😁😁😂
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Straight osage is so rare I almost hate to see that experimented with :-\ I'm sure it will turn out great :)
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Arvin, I have never found edge grain osage to be all that reliable. Osage splits easily and can split when you start to taper the bow. I would back it if you go edge grain.
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Arvin, I have made a dozen or so ash backed bows, all more or less pyramid. In the beginning I was concerned also about grain runoff at the edges, on the working limb and handle section. In the end it never was a problem. Dunno about osage 8)
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Yes smik but it had a joint under it if I understand correctly.
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Straight osage is so rare I almost hate to see that experimented with :-\ I'm sure it will turn out great :)
Yes I agree. I know there could be a record bow in that stave.
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Another stave . No checks in this one. Down to one growth ring and block planed close on on side.
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Yes smik but it had a joint under it if I understand correctly.
I can't imagine this matters anyhow - the joint is in the neutral zone. I haven't done a edge grained selfbow, but I know it's been done successfully. Show us how its been done :)
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Shavings
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Arvin, I have never found edge grain osage to be all that reliable. Osage splits easily and can split when you start to taper the bow. I would back it if you go edge grain.
But Steve then I would have to go after your backed bow record.😁😂😂😂
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Ok Smik. You got your huckleberry. Next question. Which side of the limb do you heat to straighten and put my reflex in??? If I’m the huckleberry you have to be Tom. Don’t tell me straight bow cause then you lose you huckleberry. Arvin
PS if it’s failure could you chip in on the next $100 stave. 😁😁😁
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Golly mention money and it gets quite. 😁😁😁seriously I’m interested in this. I’ll build a test limb today and try it. If I drilled a hole in the tip to hold a 50 caliber muzzle loader ball put this limb in a vice would that be enough to resistance to simulate a limb load of one limb? Arvin
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Straighten from side to side So run off will be even.
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Pic
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hey Arvin, glad to see you take the challenge. very interesting. huckleberry is a bit late to answer: it's your risk but also your profit in the end ;) nevertheless, my offer would have been that - in case you screw up this 100buck stave - I'd send you a pair of yew billets, if you pay for shipping :)
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Haha
I'll be yer Huck Arvin. That Super pyramid is shooting up a storm. Next time out I will run it against any selfbow for for a 30 shot round and put up a $100 and see who wants a run at the Super.
I know I tillered that bow starting at 20'' very very slowly and sanded it down alittle at time. then would leave it strung for an hour at brace height then give a few warm ups and took note of tiller changes.
When you get tips width to 1/4" its got to be dead nuts on string line up & down middle and limbs have to be timed up for how your gonna shoot it. 3 under on this Target Super. I compared it to your 40 you gifted me and had shoot off with yours. The profile is much slimmer from fade to tip. Which generally makes a much harder bow to shoot because of grip pressure and grip location. I fixed that by a way over sized grip with a thumb rest. your really locked it when you grip and present bow to target and under draw tension. Line up that rear sight for target range and its like shooting my custom 5" 45ACP.
I will say it took a long time to get bow exactly where I wanted string line and tiller for this type of rig. Its a tad shorter than what I really wanted but this really sped the bow up. Very forgiving with arrow spines. I shot of some my friends heavily tapered 11/32nds shaft and 70grn points on its first event last year. Bow was lacking the positive tiller on bottom limb so I heated and flipped bottom a tad more and boy that fix made the difference. It ran 2nd Place in a big shoot in IN. was new so I was down on day one. But day two she shot the field in dirt! One arrow in shootoff cost me the Money. My fault now the bows.
She will debut 2021 at Pre Spring Fling in the reset tiller it has now. She will run with your best I am confident.
Yer friend
Shawn~
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Ok Shawn but you don’t have any worry’s about beating me in tournaments. I concede cause all I can do well is hit the earth a long ways out there. Built a test limb. The run off grain held fine in the test. But it took set. Being one limb in a vise makes for a hard tiller. Going to go for it but probably only once. Arvin
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Pic
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Looks like fairly even distribution of set, so the tiller is probably fine. Did you go a little wider on it than normal?
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Sleek 1-3/4 at fades. Should be able to get 2-1/8 on the stave. Continue ext week . Need to take the fishing shack back to the pole in Baffin Bay. We had to rebuild after hurricane.
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Ok fishing shack home.Back on bow. The back is flat and bow roughed out. To the heat bench.
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First to straighten the limb.
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Now to the caul.
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It will be interesting to see what happens when the clamps come off... :OK
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Pic of edge straight grain osage.
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First time off the caul. That edge grain will pop once finish is on it. Not bad coming from a stave.
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I will say this you can get the back flat this way! Don’t know about set and such yet but we are going to find out. Arvin
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Radius the back edges well before floor tiller. Normally its not critical but in this case I'd keep the odds in my favor.
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Second visit to the caul.
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Pic
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I will Sleek I did on the test limb along with a good hard rub from my shaft tamer
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Well the back is done except for tip overlays. This is as straight as I could get it.
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Closer
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Other end
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That really looks good. How wide is it, how wide compared to your normal design, and are you going for your typical 50 pound draw?
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2-1/8 at fades. I’ve built them narrower and wider. It should work if not the test has been done.
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Arvin, that is impressive. I REALLY hope that it does what we hope it will do and you break yet another record. I'm totally impressed with how you took an idea and turned it into a totally working prototype...congrats. :OK
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Thanks Bob . We’ll see. Tip overlays. Do you think I need to glue something across the back at the handle to keep side plate from splitting out if strung wrong? Just a hair to much side pressure.
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I'm not sure I understand your question, but tip overlays are good for preventing the sting being able to slit growth rings. I don't know about the handle area, but someone else might. I always have a piece of leather on my side plates anyway, but I just use barge cement on them. If I wanted them for protection, I guess I would make them fit the entire site window area and I would use epoxy to attach them...
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How about this? It don’t want to reflex any more.
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Did you cut a dish into the back of the handle there?
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Yes going for 2” reflex. Well I’ll let that glue dry good for a few days and go quail hunting.
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Im watching. Done this with elm. No issue. Shot no better no worse than on flat grain. You leave fades long?
Shawn
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I’ve never seen anyone cut into the back of the handle like that. Seems like a recipe for disaster. Something you’ve done before Arvin?
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Elms interlocking grain took it like a champ.
Cut in how Ryan?
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It will be interesting to see the results. If it does take a little more set than you want, maybe you could do another experiment and rough the back with some 36/40 grit, apply a thin bamboo backing, and glue back in the lost reflex.
~I'm not saying that it will lose reflex, only that if it does... :) :OK
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Elms interlocking grain took it like a champ.
Cut in how Ryan?
Looks like he took a semicircular cut out of the back and glued in a new piece to fill it in.
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Ryan I glued a thin piece of Osage on the back area to revive a shelf also moving the back of the handle back and the limbs forward. I also glued a thicker piece on the handle belly. It’s not going to bend in that handle. No I have not done it before. We are doing experiments here.well see.😁😁😁
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There is no rings to violate either. 8x stronger in thickness.
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Bob that would only happen on the back of this bow. I’m not sure I like this kind of build. I’m giving it my all though. Arvin
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I reenforce handle all the time. The issue is where grain peels off in fade. Where i would just make it longer where that vertical grain must be cut off for a sight window. Dont wanna have to shoot it off the hand!
HH~
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Gotcha. Even if the handle is rigid it’s still under a good amount of stress. That close to the fades would be uncomfortable for me. Hope it works out.
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Why you extend to fade to keep stress out where sight window cut in occurs. Works fine. I just never did it with osage. Look if it does pull a grain its going up or down limb and not in half like a ring failure. Just peel a grain vertically.
Like it Arvin. . . Post an update when ya can.
Shawn~
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Well back from quail hunt . Great time but few quail. Arvin
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Well I cleaned the handle up a bit . Now to finish up the tip overlays and start the tiller.
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Tip overlays done . On long string to 20” 50#. Back to heat bench for hopefully last straightening and will heat temper the limbs. Now to string it.
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From the bench of America's most esteemed selfbowman. Freakin nice.
Shawn~
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I see what you did there with the handle now. I like the idea.
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Looking sharp. Put a big center cut shelf in it. Keep speed up wirhout arrow brushing sight widow on way down range!
Shawn~
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Lost most of this page. Here is the bow with horn on the belly.
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I thought I had lost it...myself... that looks great...side profile now? I really can't wait to see how this one does... :OK
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I finally got the chance to read this whole thread Sunday and reply to it. Then the page went away. Interesting thread Arvin. Thanks for conducting this experiment with us.
Bjrogg
PS I never used horn, but isn’t it really good in compression? I’m guessing it would help eliminate set. That’s just my uneducated guess though.
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Did the server cause the page to go away?
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That looks really good Arvin. I need you to teach me how to glue horn.
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Did the server cause the page to go away?
I’m thinking replies that happened during switch might not have come along.
Bjrogg
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Yes Brian that’s what I think. Pat kinda warned us to stay clear for a day or . Oh well you know how old men are.😀😀😀
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Yes Brian that’s what I think. Pat kinda warned us to stay clear for a day or . Oh well you know how old men are.😀😀😀
Was any switching of servers done? Still says the site is not secure.
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I thought the site would be down during the switch, but I looked a few times during the time mentioned and it was still up.
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Ok it’s 50@25. Hysteresis is creeping in . 1-1/2 back to 2”
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The bow is coming in at 35oz right now. Probably 2-3 oz to be taken out of handle. Ok Steve where are you? About 6# to remove . How much mass is that. 67” ntn 2” at fades. I think with the horn it should hit 28oz or so. I’ve taken all I want to off the horn. So off the back or narrow smart guys. I will guess off the back. Arvin
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Don't narrow it at all, just tiller the back like you were saying.
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Wait. Did you radius the belly yet like I was telling you about to reduce back strain by 30%?
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Maybe I missed something, but what is the point of building a bow to test the compression strength of edge ring osage if you put horn on the belly? It looks like a great bow, but what about the test?
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I'm pretty sure that it began to take set earlier than Arvin's "normal" selfbows...the horn belly was done to save the stave and to also test the horn belly.
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The horn is going to test the backs ability to hold. The belly already showed it can't take the strain like normal ring orientation.
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No sleek but that would make taking 6# off a lot easier. Hungry now. Be back soon
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I'm pretty sure that it began to take set earlier than Arvin's "normal" selfbows...the horn belly was done to save the stave and to also test the horn belly.
Yes I’m not going to test the stave to more failure than normal. No need to. Answer edge grain is not as good by its self. Going to try the horn on belly. If that doesn’t work there will be one last try. Reduce the back and put boo on it. This will make the edge grain act as a series of I beams separated by the early wood. Then I give up. The y’all have to go back to the computer.
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The horn is going to test the backs ability to hold. The belly already showed it can't take the strain like normal ring orientation.
Horn is not as stiff as wood and bends easier.
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The horn is going to test the backs ability to hold. The belly already showed it can't take the strain like normal ring orientation.
Horn is not as stiff as wood and bends easier.
I think the point is to get the compression in line. Even the tension and compression. Without set.
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The horn is going to test the backs ability to hold. The belly already showed it can't take the strain like normal ring orientation.
Horn is not as stiff as wood and bends easier.
Then its perfect for this!
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Yes. There was a big argument a few years back where even the horn bow guys were of the opinion that horn would easily break a wood back. Obviously a wood back can still break with a horn belly but they were saying it was practically going to blow apart the back early on as if it forced the back into greater tension stress right away.
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Yes. There was a big argument a few years back where even the horn bow guys were of the opinion that horn would easily break a wood back. Obviously a wood back can still break with a horn belly but they were saying it was practically going to blow apart the back early on as if it forced the back into greater tension stress right away.
I honestly thought horn was stiffer, thats why I reccomend reducing its width on the belly.. I hope he hasn't done it yet, or I'm going to be the reason his bow is messed up.
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I'm pretty sure that it began to take set earlier than Arvin's "normal" selfbows...the horn belly was done to save the stave and to also test the horn belly.
Yes I’m not going to test the stave to more failure than normal. No need to. Answer edge grain is not as good by its self. Going to try the horn on belly. If that doesn’t work there will be one last try. Reduce the back and put boo on it. This will make the edge grain act as a series of I beams separated by the early wood. Then I give up. The y’all have to go back to the computer.
The one thing I wonder is kinda what Pat was saying. We don’t necessarily want stiff. We want a bend designed to spread out stresses and out evenly. And a material that wants to spring back to its original shape preferably very quickly.
Cardboard is stiff, but it doesn’t seem to bend well to me. It’s seems like it would just kink over.
Bjrogg
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Not saying that will happen here.
Bjrogg
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Here is the bow we are working on and a Osage with Horn on the back. The one with horn on the back took no set. That one is same length but 1-1/2 at fades. 48@28
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Horn is not as stiff as wood and bends easier.
+1
I honestly thought horn was stiffer, thats why I reccomend reducing its width on the belly..
Bovine horn is ~35-40% as stiff as typical bow hardwoods. Its big advantage is that it will survive strains that are several times more than the wood will.
Mark
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Oh it weighs in at 26oz
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I take the no set back. It probably took 3/4” set
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Hey, for fun, try shoot an arrow at 26 inches if you haven't tillered it out yet...
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I understand that the horn likely won’t take set in compression but will that be off set by the extra mass on the belly? This design doesn’t seem to suit horn without any reflex. Seems like too many variables to know what’s going on. I have nothing to base this on but I had the same sense as Pat that the horn won’t necessarily over-stress the back. Board bows seem to break often enough without horn... (lol)
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Here is the bow we are working on and a Osage with Horn on the back. The one with horn on the back took no set. That one is same length but 1-1/2 at fades. 48@28
Arvin, I seem to recall you posted about the bow with the horn back, but could not find much. Can you link to a previous thread?
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No Willie but I will open another can of worms. Some say the back does not stretch. I put some painters tape on on he back once and it did not brake after taking the bow to full draw so the tape either did not stretch and that would make the back the neutral plane or the tape stretched. I will do another test today by clamping a steel tape or strap to the back and mark both ends and take it to full draw and see if back stretch’s. The tape should move some guessing 1/8 to 1/4 inch if the back is not the neutral plane. If not the back does not stretch. Leaving the belly to take it all. Then I would say horn would be good for the belly. Arvin
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Ok smart guys I need math on a given length. Let’s go with 60” of tape or strap 1/2 thick limb. I’ll try to find the straightest limb bow I have to find out about the neutral plane being at the back or not. In 20” of bend if the back is not the neutral plane how much should the tape move? Arvin
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You cant generalize backings and say they stretch or don't stretch. Tape and sinew will stretch, bamboo and steel much less so.
When people say wood backs don't stretch they mean that most of the movement in a bending bow is on the belly side. Wood simply can't elongate much before breaking.
So a bow is more about the belly grudgingly compressing like a hard rubber ball than the back being like an elastic
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No Willie but I will open another can of worms. Some say the back does not stretch. I put some painters tape on on he back once and it did not brake after taking the bow to full draw so the tape either did not stretch and that would make the back the neutral plane or the tape stretched. I will do another test today by clamping a steel tape or strap to the back and mark both ends and take it to full draw and see if back stretch’s. The tape should move some guessing 1/8 to 1/4 inch if the back is not the neutral plane. If not the back does not stretch. Leaving the belly to take it all. Then I would say horn would be good for the belly. Arvin
I would think tape would be just like a brown paper bag backing. i did a measurement of how much my bow back stretched when drawn a while back.... i cant remember how much it stretched, it wasnt a lot though, but it did stretch. i think i tested it by measuring the whole bow nock to nock undrawn by putting a string up against the bow and marking the string where the nocks were (and then measuring it ofc). then i had my dad draw it and i took the same string, put one mark on one nock and put it up against the back and marked where the other nock was now. if i were to try to guess how much space was between the markings for the same nock, drawn vs undrawn... i would say around a quarter inch.... but dont quote me on that. it was about a year ago. all i can remember for sure was that the nock lined up on the string (when drawn) further than when undrawn.
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Ok I went Lowe’s and got some plastic lumber strap I found center on the 60” strap marking each end and clamped it on the unstrung bow. I then braced it . This was the result. Better explanation please Pat.
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Results
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Way more than I would have expected, I was thinking you might see about 1/10 of an inch. Are you pretty confident that was a good test?? If that is the case it implies that wood stretches about the same as it compresses.
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How about if the banding is permanently attached.
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intresting... now im curious to see how much a bow compresses compared to how much it stretches... that would mean we would have to measure only one limb and only the amount that bends... i would but my bow is kinda out of commission... i gotta fix my bow now so i can test some!! (lol)
also i should point out, that this is two bending limbs plus the handle which is not bending, so the amount were seeing is twice the amount of one bending limb.
i also feel like including the handle in the measurements might change the end result... it might not but thats just my thought... another thing i wanna test...
Also, is the band as taught as when it measured the unstrung bow? i
Also, what wood is it? (it looks like osage but just making sure!) different woods differ in tension and compression strength so it would affect how much the belly compresses and the back stretches
m sorry for the questions? this is just all super interesting to me!!
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Ok smart guys I need math on a given length. Let’s go with 60” of tape or strap 1/2 thick limb. I’ll try to find the straightest limb bow I have to find out about the neutral plane being at the back or not. In 20” of bend if the back is not the neutral plane how much should the tape move? Arvin
Strain is measured in inches of deflection per inch of length. So 1% strain would equal 1" of stretch over a 100" length. I guarantee that the neutral plane is not on the back, it is not physically possible for that to be the case on an all wood bow.
Results
It is a pretty crude measurement but you have about 1/4" of change over 60". That equates to ~0.4% strain getting to brace. That seems about right, as the limbs typically see close to half their maximum stresses getting to brace on a typical flat bow. On reflexed bows they see more than half getting to brace.
Way more than I would have expected, I was thinking you might see about 1/10 of an inch. Are you pretty confident that was a good test?? If that is the case it implies that wood stretches about the same as it compresses.
That is a reasonable result based on the numbers I calculated above. Yes, the back will stretch as much as the belly compresses with a rectangular limb section on a self bow. Why is there so much skepticism about this? It is basic mechanics of materials and has been well proven for well over a century now, maybe much longer than that.
How about if the banding is permanently attached.
One end has to remain free indicate the length change. If you solidly anchored both ends of the band it would just change length along with the bow.
Mark
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Thanks Mark . Now can you give me the approximate length change at brace so I can decide weather to trap belly or back to work prevent set?
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Ok smart guys I need math on a given length. Let’s go with 60” of tape or strap 1/2 thick limb. I’ll try to find the straightest limb bow I have to find out about the neutral plane being at the back or not. In 20” of bend if the back is not the neutral plane how much should the tape move? Arvin
Strain is measured in inches of deflection per inch of length. So 1% strain would equal 1" of stretch over a 100" length. I guarantee that the neutral plane is not on the back, it is not physically possible for that to be the case on an all wood bow.
Results
It is a pretty crude measurement but you have about 1/4" of change over 60". That equates to ~0.4% strain getting to brace. That seems about right, as the limbs typically see close to half their maximum stresses getting to brace on a typical flat bow. On reflexed bows they see more than half getting to brace.
Way more than I would have expected, I was thinking you might see about 1/10 of an inch. Are you pretty confident that was a good test?? If that is the case it implies that wood stretches about the same as it compresses.
That is a reasonable result based on the numbers I calculated above. Yes, the back will stretch as much as the belly compresses with a rectangular limb section on a self bow. Why is there so much skepticism about this? It is basic mechanics of materials and has been well proven for well over a century now, maybe much longer than that.
How about if the banding is permanently attached.
One end has to remain free indicate the length change. If you solidly anchored both ends of the band it would just change length along with the bow.
Mark
Mark, I agree with you ALMOST all the way. I disagree that a belly and back stretch and compress the same on a rectangle cross section bow. I disagree because if that were the case, then a bows wood would be equal in tension and compression strength and we know that not to be the case because all woods we are familiar with are either stronger in compression or tension. Which leads to Arvins question of trapping. We trap to compensate for the difference in strength.
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Plastic strips do not mimic the wood exactly, so even if you locked it in place as you should have it only tells you how much that material might stretch.
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No Pat the strips are a constant length in the radius . If you don’t trust the test do your on if you have a bow without 3 inches of set.
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Mark, I agree with you ALMOST all the way. I disagree that a belly and back stretch and compress the same on a rectangle cross section bow. I disagree because if that were the case, then a bows wood would be equal in tension and compression strength and we know that not to be the case because all woods we are familiar with are either stronger in compression or tension. Which leads to Arvins question of trapping. We trap to compensate for the difference in strength.
You are confusing strength and stiffness. Assuming the wood is the same stiffness in both compression and tension then it will deflect the same amount on both belly and back. If it is not the same stiffness both ways (there was a thread on this a while ago with an indeterminate conclusion on this) then it will not deflect the same amount on both sides. The ultimate strength has nothing to do with the amount of deflection until you reach the limit and a failure occurs.
Thanks Mark . Now can you give me the approximate length change at brace so I can decide weather to trap belly or back to work prevent set?
All flatbows are very similar but not the same. Assuming you have a typical bow hardwood the compression limit on set will be around 0.55-0.6% strain. The tension limit will often be around 0.9% strain. If your bow has a flat unbraced profile and you use a 6.5" or so brace height that will get you around 0.4% strain like Arvins bow above. Max strain at a 28" draw will be around 0.7-0.75% strain. You would need to trap enough to shift the neutral axis more than 10% to completely eliminate set, which is a lot.
The amount of trapping required can be calculated more accurately if you know the maximum strain your design will see and the strain level your wood can take before set occurs. You have to balance the reduction in belly strain with the increase in back strain. For maximum performance the goal is to have both belly and back reach their limit at the same moment, but that is not always possible depending on the two limits. You can always get closer to balanced than you get with a basic rectangular cross section, though.
Mark
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Plastic strips do not mimic the wood exactly, so even if you locked it in place as you should have it only tells you how much that material might stretch.
the strip isnt and really shouldnt be locked on. the strip isnt bending with the bow, its just a consistent length thats used to measure before vs after its strung.
im pretty sure hes just using the clamp to make sure the strip doesnt fall when he takes the picture.
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Same results with my world record bow.
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I just did the same test on a finished bow and got exactly the same result Arvin got.
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Ok so i have a question, do you get a different result with bend in the handle bows?
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Try it deerhunter. It’s going to be close to the same in any bow. Yes a stiff handle should make slightly different variables .
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i would if i could, i havent made a bend in the handle bow... im working on a stave that i can turn into a bend in the handle bow though :BB.
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I don't understand why you even need a strip just draw two pencil marks on the back of a bow and measure them. and if you want to know whether the back moves more or the belly just draw perpendicular lines on the edge of the limbs and see whether the lines on the back open more or the lines on the belly side closed more.It will probably vary depending on which part of the limb you're measuring to because some limb bends more than the rest
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The shortest distance between two points is a straight line.
Idk, just wanted to sound smart.
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to me set is caused by over stressing the bow during tillering.. even a heavy early wood bow can be made with no virtually no set provided it is tilled properly and slowly.. using a cable system and never pulling within 5 lbs of projected target weight range is a good way to reduce and or eliminate set provided bow wood is properly dried also and the moisture content correct..... gut
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Gut do your bows never take set? If so I’ll send you a bow blank and give you the needs I have for a finished bow expecting to get a bow back with out set. And what would that charge be.
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Arvin,
I am just catching up on your thread. This would be an excellent example for an engineering strength of materials course to help explain the relationship between stress, stiffness (elastic modulus), neutral plan, and strain. Without detailed measurements to go by, the amount of stretch (strain) you are measuring on the bow back seems reasonable. If you can measure it, you will probably find a similar amount of compression (shortening) on the belly side, then the neutral plane is right between the back and belly.
You might find some difference depending on how much of the belly-side thickness is horn. More horn = less strain on bow back and more strain on the bow belly.
I don’t have any experience with gemsbok horn. I have a small sample, but it is not in good enough condition for a 3-point bend test. It seems a little stiffer than my water buffalo horn, but half to a third as stiff as a decent piece of hickory of same thickness.
Alan
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Allen is horn less stiff because it stretches more or compresses more? Would it have less stretch if glued to a piece of hardwood for example?
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I just did this same test on a hickory back osage and the line barely moved less than 1/8". Thinking I will go back and recheck it.
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There is the stiffness versus strength difference so you'd have to show that a selfbow material does act as stiff in tension as it does in compression.
Does a wood back 'act" stiffer because wood tends to be twice as strong in tension as it is in compression? There has to be some truth in that for trapping to work as well as it does.
For backed bows you could likely do all sorts of extreme combinations in a test scenario.
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Allen is horn less stiff because it stretches more or compresses more? Would it have less stretch if glued to a piece of hardwood for example?
This is a good question. The only data I have comes from 3-point bending tests, which is a product of the combined tensile and compression properties. I don’t know why the tensile modulus would be much different than the compression Modulus. Usually the two are pretty similar to each other.
Similar to wood, the strength of horn in tension may be different than in compression. I don’t know much about using it in tension.
I think this is what you are suggesting. To verify if there is a difference in stiffness and compression Modulus, you could laminate a thick-ish piece of horn to a thin piece of something very stiff, like unidirectional glass to make a composite beam. With it supported at two ends, place a weight on the center and accurately measure the deflection. Then flip it over and repeat the test. If the deflection is the same, then the tensile and compression Modulus of the horn is the same.
Alan
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Allen is horn less stiff because it stretches more or compresses more? Would it have less stretch if glued to a piece of hardwood for example?
This is a good question. The only data I have comes from 3-point bending tests, which is a product of the combined tensile and compression properties. I don’t know why the tensile modulus would be much different than the compression Modulus. Usually the two are pretty similar to each other.
Similar to wood, the strength of horn in tension may be different than in compression. I don’t know much about using it in tension.
I think this is what you are suggesting. To verify if there is a difference in stiffness and compression Modulus, you could laminate a thick-ish piece of horn to a thin piece of something very stiff, like unidirectional glass to make a composite beam. With it supported at two ends, place a weight on the center and accurately measure the deflection. Then flip it over and repeat the test. If the deflection is the same, then the tensile and compression Modulus of the horn is the same.
Alan
Brilliant experiment
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There are guys who make glass bows with a horn belly. They do behave a bit oddly.
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Does a wood back 'act" stiffer because wood tends to be twice as strong in tension as it is in compression? There has to be some truth in that for trapping to work as well as it does.
The strength and stiffness are essentially unrelated properties. Strength is the maximum load the material can carry before it fails, stiffness is how much it deflects under a load. Trapping works by balancing the strains so that the weaker side and the stronger side get closer to using the same amount of their maximum strength. It does not change the inherent stiffness of the material.
Mark
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I don’t know why the tensile modulus would be much different than the compression Modulus. Usually the two are pretty similar to each other.
Alan - there is a small difference. See for example "Conners, T. & Medvecz, P. (1992). Wood As a Bimodular Material. Wood and Fiber Science 24, 413-423".
In wood tensile modulus is about 8 % greater than compression modulus. But, of course there is a lot of variation. For example, in sugar maple the difference can be almost 28 %.
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Does a wood back 'act" stiffer because wood tends to be twice as strong in tension as it is in compression? There has to be some truth in that for trapping to work as well as it does.
The strength and stiffness are essentially unrelated properties. Strength is the maximum load the material can carry before it fails, stiffness is how much it deflects under a load. Trapping works by balancing the strains so that the weaker side and the stronger side get closer to using the same amount of their maximum strength. It does not change the inherent stiffness of the material.
Mark
But it changes the relative stiffness, no?
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But it changes the relative stiffness, no?
I think I see what you are asking about. A wood backing doesn't 'act' stiffer as such, it really is stiffer when only loaded in tension. When working with a composite glue up (that is, multiple lams of different materials) you have to do any calculations taking into account the properties of each lam as it will be loaded. The back lam will only see tension, so you would need to use its tension modulus and the belly lam will only see compression so you would use its compression modulus. The core will see both tension and compression, so you would want to use its flexural modulus (the modulus you get when you measure with a bend test, like Alan mentioned a few posts back).
Since I threw another term in there (engineers, eh? (A) ;D), I think some explanation is in order. There are 3 ways to measure the stiffness (modulus) of the material. You can load it in tension, compression or bending. Tension and compression are 'pure' loading conditions where all the material sees the same load, but bending (or flexural) puts the test piece in both tension and compression at the same time. The end result is that bending gives you an 'average' modulus of sorts, whereas tension and compression tell you purely about that one load direction.
For a selfbow the flexural modulus is the most accurate one to use because that is the exact loading that the limb sees. For lams you would want the actual tension and compression numbers, but those are hard to get without sophisticated lab equipment. Flexural modulus can be measured reasonably well in a home shop with a bit of care.
Did that get it?
Mark
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I got some metal strap. 60” long marked the unstrung bow. Clamp one end at mark after brace camp the other end . Less friction clamp at other end. Check at brace , 14” 21” 28”. Pretty wild just how much wood can stretch! Arvin
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‘This is at 21”
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Alan - there is a small difference. See for example "Conners, T. & Medvecz, P. (1992). Wood As a Bimodular Material. Wood and Fiber Science 24, 413-423".
In wood tensile modulus is about 8 % greater than compression modulus. But, of course there is a lot of variation. For example, in sugar maple the difference can be almost 28 %.
Thanks for that paper, it is always nice to have some references to actual research. That much difference in sugar maple indicates that I should be heavily trapping every maple bow I make.
Mark
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This pic is of metal strap laying on the bow unstrung. That represents the blue tape. The next dark pencil mark is the bow at brace. The green mark is at 21”
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I’ll be playing with this on a variety of bows! Arvin
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So if I’m following you correctly Arvin. The back side of this bow stretches just over 3/8” when drawn only to 21”. Seems like it would get considerably longer at 28”
Bjrogg
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Yes Brian. That is the bow I’m working on and it’s not been to 28 yet. I’m blown away. If no set that would mean it is compressing that much also. I’ve got a all new respect for wood.
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Alan - there is a small difference. See for example "Conners, T. & Medvecz, P. (1992). Wood As a Bimodular Material. Wood and Fiber Science 24, 413-423".
In wood tensile modulus is about 8 % greater than compression modulus. But, of course there is a lot of variation. For example, in sugar maple the difference can be almost 28 %.
Thanks for that paper, it is always nice to have some references to actual research. That much difference in sugar maple indicates that I should be heavily trapping every maple bow I make.
Mark
I was going to quote similar. True tensile tests are not that common. I have seen numbers showing about 7500 for compressive in Maple and about 15000 for tensile. Very likely Hickory is similar.
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Alan - there is a small difference. See for example "Conners, T. & Medvecz, P. (1992). Wood As a Bimodular Material. Wood and Fiber Science 24, 413-423".
In wood tensile modulus is about 8 % greater than compression modulus. But, of course there is a lot of variation. For example, in sugar maple the difference can be almost 28 %.
This is good info. 28% for Sugar Maple is quite a bit of difference, but I see that was from a single separate study, so methods and the way samples were prepared might be a little different than the other studies. I am thinking of trying two experiments, one with horn, and one with sugar maple to see how it compares.
Thanks!
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Mark,
did you have a chance to review the test data from the Canadian Forest Lab, where sugar maple excelled in certain tests?
I am thinking those tests might serve as a proxy for tensile testing.
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Does anyone following Arvins testing method have access to a yew longbow with a sapwood back? I am curious how much of the elasticity in yew is attributable to the sapwood. A close to "circle of arc' tiller could be useful to easily make a calculation comparing the back stretch measurement to the whole.
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Conners and Medcevz are referencing Schnieder et al
https://www.cabdirect.org/cabdirect/abstract/19920656871 (https://www.cabdirect.org/cabdirect/abstract/19920656871)
for the sugar maple data.
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Does anyone following Arvins testing method have access to a yew longbow with a sapwood back? I am curious how much of the elasticity in yew is attributable to the sapwood. A close to "circle of arc' tiller could be useful to easily make a calculation comparing the back stretch measurement to the whole.
Yew bows with no sapwood are still very elastic. It likely owes a lot of its attributes from being a very balanced wood naturally.
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I was going to quote similar. True tensile tests are not that common. I have seen numbers showing about 7500 for compressive in Maple and about 15000 for tensile. Very likely Hickory is similar.
Those sound like yield/rupture numbers and not modulus. I would expect sugar maple to be closer to 20,000psi for MOR but wood varies so much that you never know unless you measure the piece you are actually using.
Mark,
did you have a chance to review the test data from the Canadian Forest Lab, where sugar maple excelled in certain tests?
I am thinking those tests might serve as a proxy for tensile testing.
I have looked at it, but I don't recall the particulars. I will have to review that.
Conners and Medcevz are referencing Schnieder et al
https://www.cabdirect.org/cabdirect/abstract/19920656871 (https://www.cabdirect.org/cabdirect/abstract/19920656871)
for the sugar maple data.
That link requires a login to access.
Mark
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I’m also curious about Arvin’s stretch test. I wonder if it would be different if it had a single growth ring instead of a edge grain.
Arvin is that the bow with edge grain and horn on belly?
Bjrogg
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I’m also curious if it has horn on belly and no set. Maybe the belly isn’t compressing as much as the back is stretching? Maybe elastic back is better for no set. Maybe why sinew and horn such a good combination? The capability of the back to stretch would help to keep the belly from having to crush.
Bjrogg
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I was going to quote similar. True tensile tests are not that common. I have seen numbers showing about 7500 for compressive in Maple and about 15000 for tensile. Very likely Hickory is similar.
Those sound like yield/rupture numbers and not modulus. I would expect sugar maple to be closer to 20,000psi for MOR but wood varies so much that you never know unless you measure the piece you are actually using.
Yes, I was just showing that they are different
I think they do say that tensile strength and modulus of rupture are very similar in numbers.
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Yes, I was just showing that they are different
I think they do say that tensile strength and modulus of rupture are very similar in numbers.
Ah, yes. The properties certainly do vary significantly between the two directions. Modulus of rupture is roughly equivalent to the yield strength of a ductile material. Because wood has no ductility it just blows when you pull too hard on it.
Mark
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That link requires a login to access.
Mark
I was going to see if I could find the the referenced article when I had more time, but it is notable that Conners and Medcevz are only quoting numbers from the Schnieder article, Mechanical properties of polymer-impregnated sugar maple. which may or may not be relevant. from related quotes and abstracts....
The treated sugar maple wood showed increases in MOE and MOR in bending by 20.0 and 28.0 %, respectively, as it was treated with a vinyl-type monomer to 51 % WPG. ...
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I’m also curious if it has horn on belly and no set. Maybe the belly isn’t compressing as much as the back is stretching? Maybe elastic back is better for no set. Maybe why sinew and horn such a good combination? The capability of the back to stretch would help to keep the belly from having to crush.
Bjrogg
Horn had a lower elastic modulus than wood, so it is less stiff and will be compressing more than the back is stretching. Horn is also able to handle much more compression strain than wood, so this isn’t a problem.
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Thinking out loud here.
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That's a very interesting thought Arvin
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Will it shoot anymore accurately? My most accurate selfbows have lower string tension at brace height and tend to get heavy at FD.
My hunt rigs have high string tension at brace but are not quite as deadly accurate as the the above type.
And. . . the above type are even more accurate if they have taken a tad of set.
HH~
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Come on Shawn set robes cast in all of them. It’s about building all of them better. That’s all. Arvin
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Sure is. Elk wont know there's bone or horn on it.
Accurate tools are the coolest tools for me. Lets face it, you know well, the best shootin selfbows send an arrow thru riser not around it. You proved that in 3D and my guess is you'll do it in unlimited class in flight.
Shawn~
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Set is wood cells that are shortened without actually collapsing completely.
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Set is wood cells that are shortened without actually collapsing completely.
😊ok Pat I agree. But my example is a thought of the direction in which cells fell. If the back stretches out then the early wood cells would be pulled toward the tips. Yes or no? While we think about that which direction are the belly cells going ? 😀😀😀
Arvin
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All of this to see if what we all probably know about early wood must be the weakest link. I’m still interested and trying to give myself a good visual of what’s happening in wood under stress and leverage. The stretch test opened my eyes. I’m getting a more clear mental vision. Arvin
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Seems like it has been ignored but the effect of weak early wood would have to be strong enough to pull against the return of the late wood rings. Compression failures across limbs appear to happen across late woods often enough that it seems to be a weakness of wood to fail in compression in general. In bows with a larger proportion of early wood they need to be wider to take less set and will take less set if designed well which means it is a matter of overall density.
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Arvin,
in the circled area of your pic reposted below, does the angle of the short lines intend to show the top latewood slipping sideways from the bottom latewood? If so, this would be shear.
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Edit: this pic is left up to provide context to the following posts by others.
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Yes your pic is more correct . I just build them I can’t draw well. And yes we would be talking sheer in this Scenario. This may cause the bow to return to its shape before being strung in a time release creeping. This happens till full breaking in. But because of the limb stretching it causes sheer.
Arvin
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to illustrate better what I hoped to explain in my previous post.
the shear stress at the areas with the reds dots is more than at the green dots
I was of the impression sheer force is strongest in the neutral zone.
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to illustrate better what I hoped to explain in my previous post.
the shear stress at the areas with the reds dots is more than at the green dots
I was of the impression sheer force is strongest in the neutral zone.
If the bow was a constructed of a uniform or homonegous materiel, you would be correct, sleek.
I will try to edit my posts above to be more accurate. shear stress between layers of early wood and late wood is a different question. a more complex solution is in order, for sure
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to illustrate better what I hoped to explain in my previous post.
the shear stress at the areas with the reds dots is more than at the green dots
I was of the impression sheer force is strongest in the neutral zone.
If the bow was a constructed of a uniform or homonegous materiel, you would be correct, sleek.
I will try to edit my posts above to be more accurate. shear stress between layers of early wood and late wood is a different question. a more complex solution is in order, for sure
What conditions must be met for my statement to not be correct?
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sleek, shear in beams can be across the beam or along the beam. laminar shear is what Arvin is thinking here, I believe.
PM sent.your inbox is full
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Yep I guess thats what it’s called. 😀 Arvin
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Cleared a spot for your pm.
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Ok bow and test pieces on way to laboratory. We are going to get to the bottom of this.
😀😀😀😀
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Meanwhile I’m going to do stretch test and ring location in the limb for comparison. Pretty obvious of set. But not bad in all them. They are all 67” long. Handles vary.
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Is that a burl they are sitting on?
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So Arvin, what is your favorite style of bow? ;D
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;D ;D Golly Kevin all those beautiful bows and you see the cypress table they are setting on😀. Here’s a burl.
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The one with two world records .
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The one with two world records .
You sure have a style of your own! Nice to see the subtle differences in profile. Sounds like you’ve nearly perfected it based on those medals.
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Here is what the rope is for. There’s a buzzard coming in the background. Man this could get like Waldo
Just passing time till Super Bowl.
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;D ;D Golly Kevin all those beautiful bows and you see the cypress table they are setting on😀. Here’s a burl.
I like odd stuff like that. Can't help it. :)
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So Arvin, what is your favorite style of bow? ;D
The two recurves on the left also come close if not braking the other ones records. We’ll see in September.
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beautiful bows Arvin! The two on the bottom look like they will do the job...that recurve with straight limbs should be good for plenty of yards. :OK
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I just hated seeing this huge thread get to page 3... I'm sure I'm not the only one looking forward to seeing how far they shoot at this year's competition. I hope everyone is staying warm through this crazy cold spell..."below zero" coming in Texas...? wow... (S)
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I will be getting some results soon I hope. I’ve learned engineers don’t get in as big a hurry as the builders . Learned that in my construction carrier a long time ago😀😀😀
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I’ve learned engineers don’t get in as big a hurry as the builders .
In the engineers defense it is a lot cheaper to wait a bit and have it right the first time than to try and correct problems on the fly. We are both stuck waiting on Canada Post to ship things faster than pony express used to. It hasn't really mattered much the last 10 days or so, been so cold that I couldn't work in the shop anyway. Temps down in the -30C range with wind chills close to -50C some days. Just brutal.
Mark
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Typical Engineer answer. 😀😀😀😀I was referring to my other engineer. 😀😀😀😁oh well. Hey I can get away with it now! 😀😀😁😁 Arvin
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I will be getting some results soon I hope. I’ve learned engineers don’t get in as big a hurry as the builders . Learned that in my construction carrier a long time ago😀😀😀
I'm just a "hack~shooter~hunter", so I can express opinions AND go slow... :) It's good to have friends that are engineers and builders... :OK
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Oh don’t be confused with Arvin. They can do the math!!! That makes them the smart guys! Arvin
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Typical Engineer answer.
We're trained to keep our backsides well covered. (-S
Mark
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I asked if Arvin was willing to send a couple scraps of osage and gemsbok horn to test out since I had zero prior experience with either one. A few days later, a crate the size of a small coffin arrives with way more than a few scraps!
I have never found good test data on osage, so I decided to start with the standard basics to measure density, and bending stiffness/elastic modulus. I figured if this went well, I would move on to more exotic testing to see if I can measure any difference between the elastic modulus in tension versus compression. Next, I’d like to see if I can measure the stress level where set begins to occur in tension and compression. Finally, I would like to see if any data I do gather can be applied in a useful way for building an optimal bow using these materials.
Alan
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The first thing I need to do is make up some decent test samples. For the osage, I decided to turn one of Arvin’s stave off cuts into several tiny 10” long rectangular boards. I split these into two groups. The first group is flat grained, or the best I could do to make flat grained samples on a bandsaw. It wasn’t easy to perfectly follow the rings like Arvin would when making a bow from a stave, but I think I got a good mix. Each sample had about one and a half growth rings that ranged from late-early-late growth, to early-late-early growth. I figured this would give a good indication of how much the properties of the test sample is affected by where each type of growth ring is located. A couple even had some runout between the early and late growth, and I figured this would be interesting as well.
I was really blown away by how easy it was to cut and shape this osage compared to about any other type of wood I worked. It flew through my bandsaw like a hot knife through butter! I have never worked anything like this. It sanded easy, it was easy to work with rasps and other hand tools. I could see why Arvin likes this stuff so much!
(https://live.staticflickr.com/65535/50945175837_3c9da0cea9_c_d.jpg)
Alan
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I ran each sample through several passes in the drum sander to get a uniform thickness and width to the nearest thousandths of an inch.
(https://live.staticflickr.com/65535/50944376778_3320e78404_c_d.jpg)
Here is some of the end product. Each sample is labeled and I measured the length, thickness, width, and mass of each piece. I have a piece of white water buffalo horn, but still have to add the gemsbok horn pieces to the collection.
(https://live.staticflickr.com/65535/50945072461_bc784ddf3e_c_d.jpg)
Alan
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Next, I’d like to see if I can measure the stress level where set begins to occur in tension and compression. Finally, I would like to see if any data I do gather can be applied in a useful way for building an optimal bow using these materials.
Alan
very cool objectives. Watching with interest (-P
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I upgraded my little test fixture. This is for a 3-point bend test, which is pretty standard method. The sample is supported on the ends and a load is applied at the center. The deflection is read using the dial indicator. This one is pretty stout and is producing nice repeatable results. So it’s time to take some measurements and see what we get.
(https://live.staticflickr.com/65535/50944376773_2ba5c84866_c_d.jpg)
Alan
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@Avcase
Nice work setting up an experiment :)
Looking forward to the results.
Del
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Before I even get started measuring the wood samples, I measured a couple of steel samples to compare against known properties. Steel is unique because the elastic modulus of all steel is the same. It doesn’t matter if I use mild steel, or tool steel. All steel has a Modulus of elasticity of 29-30 msi. So this is a good check of my setup.
This is what I got:
Sample 1, E = 30.8 msi, (.014” thick by .682” wide)
Sample 2, E = 27.0 msi, (.085” thick by .395” wide)
This is close, but I’d like to find why I am getting a lower value for the thicker steel sample. It could be due to some flex in the fixture. I’ll check things over and try again.
Alan
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Thanks Alan for sharing the test on this thread. I now will know how to Do some test myself.
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Aha! I figured it out. The thicker steel sample I measured has a thin zinc plating, which skewed the results. I measured some very clean steel samples and averaged 29.9 msi, which gives me confidence that the equipment and method that I am using is accurate.
Next step, I’ll measure the elastic modulus and density of the horn and wood samples that I prepared.
Alan
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Thanks Alan for sharing the test on this thread. I now will know how to Do some test myself.
It is like a simple spine test. There is some math involved, but I put the formulas in a spreadsheet to take care of that. I just need to provide the thickness, width, applied load, and deflection, and I let the computer do the rest.
So far, I am just looking at pure bending of the samples and I am keeping the applied load low enough that it isn’t inducing set. Bending is a combination of compression and tensile stress. After I finish measuring this, I will see if I can measure any differences in compression versus tension properties, and I will up the applied load to measure the stress point where set begins.
Alan
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Here are density values that I measured so far. Arvin’s osage is about 50 lb/ft^2, compared to 78 for my water buffalo horn, 45 for my sugar maple, and 490 for steel.
(https://live.staticflickr.com/65535/50950297408_0bffcb0eee_c_d.jpg)
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So far, I am just looking at pure bending of the samples and I am keeping the applied load low enough that it isn’t inducing set.
Alan,
What stress levels are you testing at for the linear portion?
Mark
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Anyone else notice edge was denser
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Alan,
What stress levels are you testing at for the linear portion?
Mark
Hello Mark,
I am starting off with some fixed weights. The thickness and width of the samples vary, but I am keeping it to pretty low strains of up to 0.2% for now. It is low enough that the sample returns to its original position after the load is removed. I was even keeping the load on for a couple minutes and watching the gauge to see if was slowly relaxing, but this hasn’t occurred so far. At this point, I wanted to avoid any permanent changes, or set.
Later, I will start applying gradual increases in the load until I start to see the sample start to take a temporary set.
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Anyone else notice edge was denser
Good observation. I noticed that the edge grain was much more consistent from sample to sample. The variation in density on the flat grain depends much more on the proportion of early to late wood mix. These samples are pretty thin, so the flat grain samples only have about 1-1/2 growth rings. A sample with early-late-early mix was measuring lower than the ones that had a late-early-late wood mix.
Alan
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Anyone else notice edge was denser
[/quote
Yes I did but I don’t know what that represents. Other than the limbs should be able to be maybe more narrow. Arvin
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Anyone else notice edge was denser
[/quote
Yes I did but I don’t know what that represents. Other than the limbs should be able to be maybe more narrow. Arvin
I'm surprised because you just made an edge ring bow and you said it started taking set earlier than your normal bows. If it is denser edge ringed I would think it would take less set normally, unless the crown of the back is just enough to even things out. Maybe that was the deciding factor on your edge ring bow vs your normal one ring bows. If you had trapped the back it might have made a difference?
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Allyn I sent the the bow to Alan also. It’s still no t been pulled past 25” where it started to take set. I wanted Alan to be able to see and measure the bow in progress if need be.
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Arvin...thank you for the dedicated search! :OK It looks like we may know more before this is over...
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Arvin I'm very excited about these test. I love seeing the results of science over speculation. I know it won't help me become a better bowyer yet but down the line it'll make a difference
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I’m making samples for the gemsbok horn so I will have that added in soon also.
I need to make a correction on the description of flat grain #4 sample to “Late Growth-Early Growth-Late Growth.” This is why the density is a little higher than flat grain #3, which is mostly early growth with a late growth ring sandwiched in the middle. It is still surprising how uniform the density is from one sample to the next. *Edit* Disregard. I’ll just update the descriptions in the table and make sure it correctly describes the growth ring mix for each sample.
I measured the Modulus of Elasticity/stiffness properties and will get these uploaded in a bit.
Alan
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Here is some Modulus of elasticity numbers:
Osage flat sawn average 1.86 msi. Osage Edge Grain average is 2.18 msi. I don’t have a good explanation why the edge grain measures consistently stiffer. The flat sawn samples seem to be much more sensitive to grain runout, which makes sense. It is interesting that the stiffest edge grain sample had a small pin-knot that ran right through the middle of the piece for it’s entire width. The wood around this pin knot seems more dense, which stiffened the sample.
Note the column for the bending stress and strain. I am taking these readings at around 25% of the stress where the sample will start yielding, or taking set, so this is best case numbers for fresh wood.
I included a column that calculates the ratio of Stiffness to density. Check out how low the horn compares to the wood samples.
I am curious how the yield stress numbers will compare. First, I need to figure out the best way to measure these numbers! I will try a few ways of adding an increasing load to the point where it starts to either take some set, or the load-deflection graph becomes non-linear.
(https://live.staticflickr.com/65535/50954070338_6fcfd6c7c5_o_d.jpg)
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Does wood in a bow even follow Hookes Law? or is it bent to far for that? (I think I know what I am talking about)
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So rock maple looks like it's kicking osages butt
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Edge grain would be stiffer because it acts like vertical lams rather than flat lams. The same way a baseball bat is struck on edge rather than the flat face. Parallel bars for gymnastics used to be Ash with the rings oriented vertically rather than flat like a typical bow.
This seems counter to what happened with Arvin's bow though.
I think Maple appears better on average because it is very homogenous wood.
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Pat you have any theories on why Arvin's bow took more set?
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Questions: 1 does the amount of early wood have a direct coalition in density when it comes to a float test? 60% late wood 40% early wood will float higher than say 70% late wood and 30% early wood?
2 : if we have more early wood in the bottom half of the limb will it lose strength in compression and if we have more early in the back half of the limb will we fail in tension?
3 would the best scenario be late ring on the back, then early ring , then late ring in the middle of the limb, then early ring , and last late ring on the belly? This will make the sheer the same in tension and compression? Yes or no? The sheer leaning toward the tips in tension and leaning toward the handle in compression.
4 all this happening in harmony and set being limited to very little
Thanks Alan and the rest of you guys input on what is causing this nagging problem called set.
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So rock maple looks like it's kicking osages butt
Not really, it is just stiffer. That is not necessarily a desirable characteristic in a bow wood. What is wanted is a very high ratio of strength to stiffness. This lets you bend the wood farther before it will fail (or take set). The reason osage is a better bow wood than maple is it will have a higher strength than maple while being lower stiffness. Yew also has this characteristic.
Mark
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Pat I tend to agree on you edge grain theory being more stiff but it could fail sooner due to not having a continuous late ring in tension.
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Pat you have any theories on why Arvin's bow took more set?
I would think decreased total latewood on the belly is the cause. If it was a left a correspondingly increased width it may have offset that.
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Alan, could you describe the relationship between these measures? I see different terms used a lot but haven’t seen it in the context of having specific numbers.
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Does wood in a bow even follow Hookes Law? or is it bent to far for that? (I think I know what I am talking about)
I think it does until the last few inches of draw. Below is a stress strain diagram for wood. the droop at the top of the curves is where set occurs and the wood is said to be undergoing plastic deformation. the lack of a well defined yield point is typical of wood. Wood testing reports often use the term "proportional limit" to describe the offset from the linear or elastic part of the graph described by Hookes Law. Wood is also said to be viscoelastic.
https://en.wikipedia.org/wiki/Viscoelasticity
We see this behavior when initial unstrung set is reduced by resting the bow for a while.
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Alan, could you describe the relationship between these measures? I see different terms used a lot but haven’t seen it in the context of having specific numbers.
Right now, I am measuring the Elastic Modulus of these different materials. This indicates how the material resists deformation under a load. For example, I came up with a number averaging 2.18 msi for edge grain osage. This is 2.18 million pounds per square inch. What does this mean? Picture a cube of this edge grain osage that is 1” wide by 1” tall, by 1” deep. Now imagine pulling opposite sides of this cube to stretch it. Let’s assume this osage cube is magic and will not break no matter how much force is used to stretch it. It will take 2.18 million pounds to stretch this cube enough to double its length to 2” long. Of course the wood will break before this happens, but it might be able to stretch almost 1% before breaking. 1% stretch = .01” with this cube. The amount of force required to stretch this cube 0.01” is 2.18 X .01 = 21,800 pounds!
A material with a higher elastic modulus number is stiffer than a material with a lower number. Here is another way to look at what this number means. Let’s say we make two identical bows in every way except the material that the bow is made with. Bow #1 is made with this edge grain osage with a 2.18 msi elastic modulus. Bow #2 is made from the water buffalo horn with a 0.85 msi elastic modulus. This means the osage used in bow #1 is just over 2-1/2 times as stiff as the horn used in bow #2. So if bow #1 happened to have a 50 pound draw weight at 28” draw. The draw weight of an identical bow #2 made from horn will be (0.85/2.18)*50# = 19.5#.
I hope this helps.
Alan
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Thanks Alan. That was very helpful.
As far as it applies to bows, if a wood is stiffer/less stiff than other woods then to make a bow of the same draw weight as another wood you would need to adjust the "width" proportionally to the other woods elastic modulus? Using your example with the two bows, would making the horn 2.5x wider help it equal the draw weight of the osage bow?
I'm guessing a different measurement is needed to assess if woods will bend to the different degrees before set.
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RyanY,
Yes, you got it.
There is a second material property that you hint at in your last paragraph which drives the design of the bows. This is the maximum bending stress that the material can handle before it takes on significant set or permanent damage, and I am working on that next. It may require trying a few things, but the basic process is adding increasing bending loads until the sample doesn’t quite snap back to its original shape once the load is removed. This will give a yield stress value for the wood. If this is known, than we can really start to have some fun.
There are a couple of things that can complicate this. One possible issue is that pesky viscoelastic behavior that Willie posted about earlier. This is where a material takes a temporary set or relaxes under load, but it gradually recovers after a short time with no apparent permanent damage. I think the horn will be most affected by this. The other issue is finding a best method to measure and model this viscoelastic effect. The wood samples will show some of this also. It may turn out that it is best to stay below the stress levels before there is any significant viscoelastic behavior because this contributes to hysteresis losses.
Alan
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I would think it would make more sense to narrow and thicken the theoretical horn bow.
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Questions: 1 does the amount of early wood have a direct coalition in density when it comes to a float test? 60% late wood 40% early wood will float higher than say 70% late wood and 30% early wood?
2 : if we have more early wood in the bottom half of the limb will it lose strength in compression and if we have more early in the back half of the limb will we fail in tension?
3 would the best scenario be late ring on the back, then early ring , then late ring in the middle of the limb, then early ring , and last late ring on the belly? This will make the sheer the same in tension and compression? Yes or no? The sheer leaning toward the tips in tension and leaning toward the handle in compression.
4 all this happening in harmony and set being limited to very little
Thanks Alan and the rest of you guys input on what is causing this nagging problem called set.
Arvin,
Question 1:
Yes, more early wood means lower density, so you might notice a difference in a float test. However, I was surprised at how little the mix of early and late growth rings affected the density of these samples.
Question 2:
I hope we can learn more about early and late wood with this little research project. I see early wood reducing the elastic modulus or stiffness in these tests, but I haven’t measured the impact on strength yet. This will show if it just means the bow with more early wood needs to be just a little wider in order to achieve equivalent performance of a bow of higher late wood content. I think a higher late wood content will always be better because the material will have more uniform properties, but it is just a guess.
Question 3:
It seems like it might be best to end with a solid late wood ring on the belly, but it depends on how elastic the early wood is. It also may depend on tensile versus compression properties. If the early wood is very elastic, then it might not matter much.
Question 4:
Harmony is good. :)
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Thanks for your hard work and explanations with this Alan. It is greatly appreciated.
If a wood is able to bend further than other woods before taking set, would it be fair to say that it is storing more energy for the same amount of mass? My thinking is that these bows could be narrower and thicker to achieve the same draw weight.
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Thanks for your hard work and explanations with this Alan. It is greatly appreciated.
If a wood is able to bend further than other woods before taking set, would it be fair to say that it is storing more energy for the same amount of mass? My thinking is that these bows could be narrower and thicker to achieve the same draw weight.
Yes, you are on the right track. The amount of energy that a material can store in bending is a product of the elastic modulus and amount of strain or deformation it can handle before taking a set. It can be taken a step further by throwing in the density in order to get a property that rates materials as ft-lb of energy per ounce of material.
Alan
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Updated table with gemsbok horn properties added. I was surprised how the density and elastic modulus is almost identical to the white water buffalo horn.
(https://live.staticflickr.com/65535/50957860928_aecbd0f320_c_d.jpg)
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I would think it would make more sense to narrow and thicken the theoretical horn bow.
My thinking is that these bows could be narrower and thicker to achieve the same draw weight.
Yes and yes. This is what make yew, ERC and the other junipers good bow wood. They can tolerate a fairly high amount of strain and let you make a narrower, thicker bow that performs very well.
Since we are on this, has anyone ever determined if narrower/thicker is more efficient than wider/thinner when it comes to bow limbs?
EDIT - I forgot to mention that I think a thicker/narrower limb should be more efficient but have not seen anything about that.
Mark
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"Thicker is quicker" is the saying . If the wood can take it.
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"Thicker is quicker" is the saying . If the wood can take it.
This makes sense to me. When designing for maximum performance (in most anything) you almost always want to stress your materials to the maximum they can take so you are using as little material as possible to do the job. In a bow that means designing so the bend is the maximum the wood can take, then setting the width to get your desired draw weight at that bend.
Mark
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Thank you for the tests, avcase! It´s much apprechiated!
Let´s assume there would be a wood that doesn´t take set, no matter how much it is bent. A bow made out of this material could have a very deep cross section (it must still be a bit wider than deep so it doesn´t twist). The deepest possible cross section would make the lightest bow for sure, which might be very advantageous especially for accelerating light arrows.
But in reality, the amount of strain a wood can take is limited. I think there is an optimal cross section for every combination of bow design and the wood being used. The optimum will be the highest possible cross section where the set is still low.
lonbow
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I ran a load-deflection curve on the Osage Edge Grain sample #1, and thought that this was starting to hint at some pretty cool data that is more applicable to us who build bows.
First, a quick description of the procedure that I used. I placed the sample in the bending fixture and lightly flexed it a few times before setting the zero point on the dial indicator. Next, I applied a weight for three seconds and recorded how much the sample deflected. I then removed the weight and watched to see if the needle on the dial returned to zero. I then repeated this procedure with increasing weight. Part way through this process, the needle would immediately return a few thousandths of an inch above zero, and it would then slowly return to zero over a few seconds. As the load increased even more, this offset increased. On the last two measurements with the highest weights applied, the deflection didn’t recover completely. Several minutes later, this still hasn’t recovered, indicating some more permanent set has already taken place. It isn’t much, just a few thousandth.
You can see this by comparing the curves on this graph. The Orange line shows the maximum total deflection versus applied weight. The blue line subtracts the short term recovery immediately after the load was removed. What this is starting to show is increasing hysteresis in the sample as the stress is increased. This is starting to get a little more interesting!
(https://live.staticflickr.com/65535/50960735757_5dfb15cfa1_c_d.jpg)
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Interesting, what is also important is the recovery that takes place on the first fraction of a second, anything that happens after the arrow leaves the bow is what we are concerned with. Hysteresis is time sensitive, the heavier the arrow the slower the limbs and less hysteresis. I like the test you are doing, it clearly demonstrates what we are looking for hear.
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What if you attached something like a needle to the end of the test piece and photographed it say 1/4 of a second after release? Have the needle lined up like a dial.
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Steve,
I can take video of the test fixture so you can watch the dial as increasing loads are applied and quickly removed. It pretty clearly shows what happens and is precise.
Most bows are under about the same stress as I got to in this graph just sitting braced. I stopped at a bending stress of around 7500 psi (0.35% strain). I only held it at this level for about three seconds before letting it down, although I did this multiple times at each load. If I held a tape measure on the bow Arvin was measuring a few pages back, 0.35% strain would show the bow back stretching nearly 1/4”, and belly compressing by about the same amount. If I go back and continue this graph to about double this stress level, then it would show very clearly the short and long term effects for a typical bow. This will give three lines. One line showing the total deflection under load, a second line subtracting the short term hysteresis, and a third line subtracting out only the long term set. What I love to see is a clear point on this graph where there the set and hysteresis losses balances against the design compromises necessary to keep the stresses in check.
The long term set reduces the capacity of the wood to store energy in bending.
The short term hysteresis just eats up energy that would otherwise go to the arrow.
Alan
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You are smart!!!
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Alan, you mention that the lengthening and shortening in tension and compression would be the same in Arvin’s bow but I think we normally assume that with wood being stronger in tension that the back does not stretch as much. Or perhaps my thinking is wrong and the back does stretch as much but is not under as much strain as the belly (not sure if that’s the correct use of strain/how close it is to breaking?). It would make sense to me that the back and belly experience set at different rates due to the differences in tension and compression. If trapping can equalize these forces and increase the degree of bend before taking set then my thought is that it would force the narrow back to stretch further and could result in a slightly thicker bow for the same draw weight. Bit off topic from the early wood discussion but just a thought.
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Ryan The way I understand it is not that the back stretches less but that it handles tension stress better than the belly handles compression stress so trapping the back doesn't make the back stretch further It instead makes the same amount of stress take place on less wood
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If you picture two men one stronger and one weaker picking up a beam if they both hold one end the weaker man will give out first. Trapping the back is like having the stronger man move his grip up further so that he is supporting more of the weight of the beam
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So if we had a sample with more early wood on one side could we start with early wood side up test and then flip the sample and retest to find out the difference. We could trap accordingly.
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Ryan The way I understand it is not that the back stretches less but that it handles tension stress better than the belly handles compression stress so trapping the back doesn't make the back stretch further It instead makes the same amount of stress take place on less wood
That’s where I’m wondering if I am misunderstanding the properties of wood. My understanding is that wood can’t stretch as much as it can compress just purely from a length standpoint before failure. I guess failure can be measured differently also as a tension failure is likely a full on breakage of the fibers versus a slower crushing in compression.
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I may have missed it, but can someone explain to the less initiated what MSI AND GPA means?
Ok, I quit being lazy and looked it up. Just different units of measure for MOE and while the units are foreign to me, I at least can appreciate the trend in the charts.
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we normally assume that with wood being stronger in tension that the back does not stretch as much. Or perhaps my thinking is wrong and the back does stretch as much but is not under as much strain as the belly (not sure if that’s the correct use of strain/how close it is to breaking?). It would make sense to me that the back and belly experience set at different rates due to the differences in tension and compression.
Strain, in the engineering world, is simply a numerical percentage a materiel deforms (stretches or compresses) under a load. The deformation can be temporary (elastic) or permanent (plastic). Other sciences define the word strain differently, so it's not always easy to visualize the exact meaning with the commonly used word. For instance, one often refers to a muscle or tendon strain to imply ligament damage. Permanent only if the body cannot rebuild the ligament. An "elastic" example would be the normal working of a muscle.
As for how much strain wood can sustain before breaking in tension, it may well be twice the amount wood can sustain in compression, when one considers the strain limit on compression to be the point where wood takes set. wood, can of course take quite a bit more compression before it actually breaks (collapses on the belly, moisture content being the big factor)
whether wood stretches and compresses the same under a given load is an interesting question I am hoping Alan can determine. Normal wood tests to not usually show these these differences , as typical bend tests do both at the same time.
Whether wood experiences set in pure tension is also an interesting question, to us bowyers at least. And whether some woods are better than others should be of interest to lam bow builders
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Thanks Willie muscles mean more to me than numbers.😁😁😁
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I just tested out if the wood stretches or compresses more, when a bow is braced. I took one of my elbs and marked a distance of 30 cm on the back and belly the mid limb area. Then I braced the bow and measured the two distances bethween the pencil lines again. I measured 30.15 cm on the back and 29.85 cm on the belly. The back elongiated about as much as the belly shortened, which means that the neutral plane should be in the middle bethween the back and belly.
This being said I must confess that the differences of the measured distances were so tiny that there might be measuring errors. It´s just a rough estimation. I also don´t know if the length ratios change when the bow is fully drawn.
lonbow
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It was probably too small to measure accurately, but the concept is a really good one.
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I just tested out if the wood stretches or compresses more, when a bow is braced. I took one of my elbs and marked a distance of 30 cm on the back and belly the mid limb area. Then I braced the bow and measured the two distances bethween the pencil lines again. I measured 30.15 cm on the back and 29.85 cm on the belly. The back elongiated about as much as the belly shortened, which means that the neutral plane should be in the middle bethween the back and belly.
This being said I must confess that the differences of the measured distances were so tiny that there might be measuring errors. It´s just a rough estimation. I also don´t know if the length ratios change when the bow is fully drawn.
lonbow
Unless you were able to measure around the verve vs the strait line between the two points, the test wasn't an accurate one.
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I used a tape measure and I followed the curves on the back and belly of the bow.
lonbow
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I am ready to do some more testing. I had to make some adjustments to how the test samples were supported so that they didn’t slide around as the load was applied and released. This was becoming a problem with applying and releasing higher loads and larger deflections, so it was worth taking a little time to fix this.
I was also thinking about the method I am using to test these samples. The amount of time I apply the load to a test sample is a variable that can make a big difference in short term and long term set or yielding. The method I have been using is to apply a load for 5 seconds, take a reading, release the load and note where the indicator needle immediately returns to. I then give it a minute or two to allow the sample to settle down and recover and note how close to zero the indicator needle is. Then I increase the load and repeat the process.
As the loads get higher, the deflection will continue to increase if I didn’t release it after this 5 second time. This amount of time the load is applied can make a big difference when trying to determine when the material significantly yields. I notice that the Forest Service uses some very slow strain rates in their tests. I figured 5 seconds is reasonable because a bow is only held at full draw for a short time before release.
Another thing to think about. The bow may also be kept braced for many hours. I am sure this can induce significant stress relaxation. I see this with all my wood bows. If kept braced for any length of time, it can take a little while after unstringing to return to its original profile. I am not sure how to factor this in. I think for now, I will just stick to this consistent method of applying the load and holding for 5 seconds and then releasing the load. I can follow this up with a test that applies a load for much longer duration to simulate the effect of keeping a bow braced. Does this seem reasonable?
Okay, I will now load an osage flat grain and edge grain sample up to significant yielding and see how they compare.
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This amount of time the load is applied can make a big difference when trying to determine when the material significantly yields. I notice that the Forest Service uses some very slow strain rates in their tests.
I have some test data from the Canadian Forest Service which shows MOE in impact testing to be half again higher than static bend testing. The static test loads a 2 x 2 steadily over the course of 2 minutes (pine at elastic limit) compared to bouncing a 50 lb weight dropped from a foot or so.
Of course, snapshooters know this already :)
sugar maple and yellow birch out perform hickories in the the drop test, and doug fir is right up there with the hickories.
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Today I picked an average flat grain osage and edge grain osage sample and ran them through a much higher bending stress level. It seemed both were right about at the edge of taking on some set at this point. I probably should have taken it up one more notch but I am sure it would have shown both yielding.
This flat grain sample had a pretty continuous late growth ring on the compression side, with a slight runout between a late and early growth ring on the tension side (toward one end).
This chart is a little different than the last one. This chart plots bending stress versus bending strain. This allows direct comparison regardless of the thickness, width, or length of the test sample. Based on this first test, it appears 14,000-15,000 psi is about the maximum bending stress for a bow made with this osage before getting into some significant set. Strain at this maximum stress level is between .75% and .80%. The surprise for me is that the edge grain sample did a little better, but this is only one data point so far. I will do a few more.
0.8% on a 66” bow would be equal to the back stretching by .53” and belly compressing by .53” at full draw.
What do you think? According to these results so far, it seems as though Arvin’s edge grain bow should have ended up a little lower mass than a comparable flat grain bow!
(https://live.staticflickr.com/65535/50975510292_85c0f99a12_o_d.png)
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Thats really fascinating. If I'm not mistaken, Arvin had only .375 back stretch at full draw. That leaves .685 for the belly to compress. So the belly compresses 1.8 times further than the back stretches. I was running some numbers and got a rough calculation of slightly more. Do you believe a thicker sample would influence those numbers, or better put, would stretch and compression increase as sample thickness goes up? I was expecting higher compression numbers than this is why I ask.
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Thats really fascinating. If I'm not mistaken, Arvin had only .375 back stretch at full draw. That leaves .685 for the belly to compress. So the belly compresses 1.8 times further than the back stretches. I was running some numbers and got a rough calculation of slightly more. Do you believe a thicker sample would influence those numbers, or better put, would stretch and compression increase as sample thickness goes up? I was expecting higher compression numbers than this is why I ask.
I should have wrote that the maximum strain possible strain for a bow 66” long is .53”. Most bows will show less than this because there are many areas that are not pushed to the maximum strain. For example, the handle area, and bow tips, etc.
Alan
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I was gonna say Arvin measured across the non bending handle and he didn't measure the belly side. Also I wonder how it affects the compression that we only tiller on the belly side while the back remains untouched.
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Thats really fascinating. If I'm not mistaken, Arvin had only .375 back stretch at full draw. That leaves .685 for the belly to compress. So the belly compresses 1.8 times further than the back stretches. I was running some numbers and got a rough calculation of slightly more. Do you believe a thicker sample would influence those numbers, or better put, would stretch and compression increase as sample thickness goes up? I was expecting higher compression numbers than this is why I ask.
I should have wrote that the maximum strain possible strain for a bow 66” long is .53”. Most bows will show less than this because there are many areas that are not pushed to the maximum strain. For example, the handle area, and bow tips, etc.
Alan
So a better stretch test would be to measure only the working portion of a limb.
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So far, I measured a bending stress where the Osage wood starts to yield. This was around 14,000 psi. I wanted to get some idea of how this compared to the bending stress in real osage self bows. Is this too low, or is it almost inevitable that the typical osage bow is stressed considerably beyond this point?
As a quick sanity check, I measured Arvin’s edge grain bow and built a computer model using the Osage and gemsbok horn material properties. I braced and drew back the bow to 24” and monitored the bending stresses. The model calculated over 10,000 psi stress just to brace the bow and over 18,000 psi maximum bending stress at full draw. This means that the actual bow is certainly taking on some set already. The bow model gave a draw weight of 54.4#, and the last time the real bow was measured, it had a lower draw weight at the same draw length. I will need to measure this. A lower draw weight with the real bow hints at material changes.
(https://live.staticflickr.com/65535/50980816501_18e024894c_c_d.jpg)
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A lower draw weight with the real bow hints at material changes.
Watching a bow loose weight (before set can be seen), compared to what the model predicts, is always annoying.
Alan, are you still using supertiller-6_6 ?
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Watching a bow loose weight (before set can be seen), compared to what the model predicts, is always annoying.
Alan, are you still using supertiller-6_6 ?
Yes, but it is a really mutated version of it that is much more sophisticated and capable, but also less user friendly.
I am honing in on something with this wood testing. Not quite sure what yet. I have enough information that I could come up with a true no-set osage self bow. Then it gets into trade offs. Is it better to draw an inch or two past the yield point knowing that doing so eroded the material properties by a few percent? I suppose that is the big question.
I do find it interesting that edge grain osage is consistently stiffer than flat grain osage. My theory on that is that there may be some shear deformation in the early wood that allows the late wood rings to slide past each other a little. This would be kind of like a stack of paper. It is easy to bend the stack when the pages slide past one another, but impossible to bend them when they are stacked together on edge.
Today I did a little research on strain gauges. A strain gauge will give a very accurate measure of what happens on the tension and compression side of a bow limb. The investment required is pretty low, but it does require some time and brainpower to put the pieces together to make it work.
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Today I did a little research on strain gauges. A strain gauge will give a very accurate measure of what happens on the tension and compression side of a bow limb. The investment required is pretty low, but it does require some time and brainpower to put the pieces together to make it work.
this could get interesting (-P
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Alan if you used my weight of 50 @25 my scale could be off. I was told by Jim M . that it is off. Just saying .
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Today I did a little research on strain gauges.
Can you get glue on gauges that will live with the relatively high strains that wood can take? That would certainly be one way to calculate tension and compression MOE with a simple bend test.
Mark
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Alan if you used my weight of 50 @25 my scale could be off. I was told by Jim M . that it is off. Just saying .
Thanks Arvin. I’ll string it up and put it on the weigh machine.
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Can you get glue on gauges that will live with the relatively high strains that wood can take? That would certainly be one way to calculate tension and compression MOE with a simple bend test.
Mark
Mark,
The strain gauges I am ordering are good for measuring up to 2% strain. This is about double what I need for wood tests, but I won’t be able to take the horn samples to the limit. The gauges cost about $1-$2 apiece. The gauges require a little amplifier that directly interfaces with an Arduino microcontroller. For a few more dollars, I can even integrate a Bluetooth module to wirelessly beam the measurements directly to my phone! So I put in an order and expect to have some new toys to play with next week. These microcontrollers are useful for all kinds of mischief, so I hope to have a lot of fun with it.
Another future project will be to integrate a linear string pot encoder to the draw-weight machine that we use at our flight shoots. This can be used to generate real-time force-draw curves on a bow. With a little coding, I can have it calculate total stored energy and other useful stats in just a few seconds. I have wanted this for a long time. I am a happy nerd. Haha!
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Alan,
it would be nice if you could post some links to the parts and pieces you are putting together. I am sure someone will be asking later, once you show us your set up. Some of whats needed is getting down right affordable!
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Can you get glue on gauges that will live with the relatively high strains that wood can take? That would certainly be one way to calculate tension and compression MOE with a simple bend test.
Mark
Mark,
The strain gauges I am ordering are good for measuring up to 2% strain. This is about double what I need for wood tests, but I won’t be able to take the horn samples to the limit. The gauges cost about $1-$2 apiece. The gauges require a little amplifier that directly interfaces with an Arduino microcontroller. For a few more dollars, I can even integrate a Bluetooth module to wirelessly beam the measurements directly to my phone! So I put in an order and expect to have some new toys to play with next week. These microcontrollers are useful for all kinds of mischief, so I hope to have a lot of fun with it.
Another future project will be to integrate a linear string pot encoder to the draw-weight machine that we use at our flight shoots. This can be used to generate real-time force-draw curves on a bow. With a little coding, I can have it calculate total stored energy and other useful stats in just a few seconds. I have wanted this for a long time. I am a happy nerd. Haha!
I want this set up when you got it debugged and working right.
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The strain gauges I am ordering are good for measuring up to 2% strain.
That is lots for working on a conventional bow limb. Maybe not quite enough for the wheelie bows, but that isn't our concern here.
The gauges cost about $1-$2 apiece. The gauges require a little amplifier that directly interfaces with an Arduino microcontroller. For a few more dollars, I can even integrate a Bluetooth module to wirelessly beam the measurements directly to my phone!
Technology is awesome. If I told you how much gear I packed into the middle of a large portal crane to strain gauge that machine you would be horrified. Now all that is on one small amplifier and a phone app! Despite complaints to the contrary, we live in amazing times.
Mark
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Alan if you used my weight of 50 @25 my scale could be off. I was told by Jim M . that it is off. Just saying .
Arvin,
I just string it up and measured 49.8# at 24”. I adjusted the model so the brace height is identical, and it predicted 55.2# @ 24”. That may not be too far off knowing that the wood will have started to yield at only 18” of draw. Assuming the computer model is a perfect representation of the real bow, then the elastic modulus of the wood in the real bow has decreased by about 10%. The only way to find out how much the wood has changed is to retest the samples to much higher loads.
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How does this tie in with Steve's thoughts of avoiding set and hysteresis?
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Alan - very interesting!!
One thing about measuring draw force and draw force curve. You should take into account a relaxation of wood. Here is an example of one bow and two draw force curves. The bow in this example is a lot used (thousands of shots) bamboo-maple-ipe-laminate, about 40#@28". Before draw force measurements, I took about twenty full draw. My scale has two measurement option - a peak value and stabilized value. Depending on draw length, it took about 1–5 seconds to stabilize. Please, see the result, in the table and graph.
The blue curve is measured with the peak values. The red curve is measured with relaxed values - it takes a few seconds the draw weight value will stabilize.
There is one oddity - wood bow energy storage (potential energy) is too good. If compared to straight draw force "curve" (which value can be set to 100 %), then using maximum values, this bow stores 109,6 % of energy compared to straight line. But, if I use stabilized value, it stores 111,5 %!. As you can see from the graph, there is small hump in the beginning of draw force curve but with stabilized value, the maximum value is lower than peak value. So, it seems that wood bow stores a lot of energy but it is not so true because this phenomenon is caused by hysteresis, or relaxation, to be exact.
blue line - max. values
red line - stab. values
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That’s good to know if I understand. A fresh wood bow stores more energy. Yes or no. Would that be because of the more it’s pulled to full draw the longer it takes relax?
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yes, it seems quicker is stiffer. so the longer it is at full draw, the weaker it becomes (temporarily).
It would be interesting if Toumo could repeat his peak value test using the quickest snapshot like pull on a well rested bow
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Tuomo,
You show a good example of one of the complications of testing with natural materials. With these test samples, I am applying a load for about 5 seconds and then letting it go all the way to zero. I noted the immediate set or offset after letting off the load, and then gave it about 30 seconds to a minute to settle and stabilize and then I recorded this second offset. Then I increase the load and do the measurement cycle over again. I would get a little different stress-strain curve if I let down either quicker or slower.
I find it confusing to say energy storage exceeds 100%. An ideal maximum force-draw curve should look like a rectangle instead of a triangle. 112% compared to a triangle is really 56% compared to a rectangle.
Alan
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How does this tie in with Steve's thoughts of avoiding set and hysteresis?
This is a good question. I haven’t been stressing these wood samples far enough to see a major permanent change. A majority of the “set” I have measured so far is mostly temporary and recovers over time after the load is removed. What I should do is load the sample higher to where a more distinct yield point is exceeded.
Alan
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The strain gauges I am ordering are good for measuring up to 2% strain.
That is lots for working on a conventional bow limb. Maybe not quite enough for the wheelie bows, but that isn't our concern here.
The gauges cost about $1-$2 apiece. The gauges require a little amplifier that directly interfaces with an Arduino microcontroller. For a few more dollars, I can even integrate a Bluetooth module to wirelessly beam the measurements directly to my phone!
Technology is awesome. If I told you how much gear I packed into the middle of a large portal crane to strain gauge that machine you would be horrified. Now all that is on one small amplifier and a phone app! Despite complaints to the contrary, we live in amazing times.
Mark
Mark,
Some of the components have come into the “lab” which I am tinkering with. I’m still waiting on the actual strain gauges. I should be able to get a resolution of around .002%, which is good enough for this kind of work.
1.85% strain is about the maximum strain that unidirectional glass can handle on the compression side of the limb, so these strain gauges will work pretty well for a glass bow too.
Alan
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Some of the components have come into the “lab” which I am tinkering with. I’m still waiting on the actual strain gauges. I should be able to get a resolution of around .002%, which is good enough for this kind of work.
Please post some pics and maybe links for the bits you are using when you have it all in hand.
Mark
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Please post some pics and maybe links for the bits you are using when you have it all in hand.
Mark
Yes, I will do this as soon as I have something working.
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While waiting for the strain gauges, I decided to take these test samples all the way to breaking. I wanted to see if there was a clear point where pushing the wood further was not worth the gain. This first flat grain sample results was pretty inconclusive. It held together pretty well and was pretty close to linear almost to the breaking point. I am very impressed with a bending strain of nearly 1%. The lines on the graph show the first test that I did nearly two weeks ago compared to the one I did tonight (in blue).
It broke in an interesting way. It almost looked like it was neatly cut on the tension side, but that was where it transitioned from a late to early growth ring. Time to break some more!
(https://live.staticflickr.com/65535/51004361433_421ee1964b_o_d.png)
(https://live.staticflickr.com/65535/51005063016_754dbedcc0_c_d.jpg)
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Did it seem to loose some stiffness at .8%? I seem to see a slope change in the graph. No permanent set even if you pinch the break back together?
I am kinda disappointed it broke so soon, but not surprised as chasing rings on osage seems necessary with bows. My previous testing with some birch stock of square cross section needed a decent radius on the back corners to make it take very much strain past the proportional limit, and with your thin/flat aspect ratio, any cupping would create a higher local stress.
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Did it seem to loose some stiffness at .8%? I seem to see a slope change in the graph. No permanent set even if you pinch the break back together?
I am kinda disappointed it broke so soon, but not surprised as chasing rings on osage seems necessary with bows. My previous testing with some birch stock of square cross section needed a decent radius on the back corners to make it take very much strain past the proportional limit, and with your thin/flat aspect ratio, any cupping would create a higher local stress.
There wasn’t a significant change in stiffness at all, right up to when it broke. I was quite surprised how linear this wood behaved.
I am not having an issue with any obvious cupping with these test samples as they are loaded. The corners are not rounded off, but I figured they would start taking a set before this became an issue. I have many more that I will be breaking before I am done.
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This graph is interesting. At higher stresses, I am starting to see some trends comparing flat grain to edge grain test samples. Edge grain has higher elastic modulus and better consistency than flat grain, but the edge grain samples do start taking a greater amount of short term and long term set at lower stresses than the Flat grain. Any ideas on why this might happen?
(https://live.staticflickr.com/65535/51006767702_f710abf309_c_d.jpg)
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By the way, you are measuring the strain at the surface of the sample, right? Are you measuring on the 'back' or 'belly', or is this the same?
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Edge grain has higher elastic modulus and better consistency than flat grain, but the edge grain samples do start taking a greater amount of short term and long term set at lower stresses than the Flat grain. Any ideas on why this might happen?
looked like it was neatly cut on the tension side, but that was where it transitioned from a late to early growth ring.
the early ring ran out pretty close to where the load was applied ie @ maximum bend?
I do notice that once the samples are pushed well into the elastic range, values seem to average out as indicated by the convergence of the two curves.
If you consider the possibility of one datapoint being out of range, the graphs could be seen as having the proportional limit somewhat close to each other @ .63%
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This graph is interesting. At higher stresses, I am starting to see some trends comparing flat grain to edge grain test samples. Edge grain has higher elastic modulus and better consistency than flat grain, but the edge grain samples do start taking a greater amount of short term and long term set at lower stresses than the Flat grain. Any ideas on why this might happen?
(https://live.staticflickr.com/65535/51006767702_f710abf309_c_d.jpg)
The early wood comes all the way to the surface of the back with edge grain. Instead of one continuous ring on the back. While the edge grain is stiffer like a I beam. I bet this as a complex composite would be a awesome bow.
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Edge grain has higher elastic modulus and better consistency than flat grain, but the edge grain samples do start taking a greater amount of short term and long term set at lower stresses than the Flat grain. Any ideas on why this might happen?
My thoughts:
The stronger fibres (early wood/late wood?) in the edge grain samples are not as well distributed as with the flat sawn samples. When you chase a ring you get a continuous layer of strong fibres to carry the tension loads on the back. This is similar to your flat sawn samples. When you look at edge grain (ie - quarter sawn) the strong fibres and weak fibres are approximately equally distributed through the thickness. This means both of those fibre types share the loads relatively equally. If you look at the portion of the material that carries the bulk of the load through the thickness, the outer 30% of the thickness carries 50% of the load. In the flat sawn samples you might get a large proportion of the stronger fibres in that outer 30%layer and that reduces the set the sample takes. The edge grain versions don't get that same advantage and they show set sooner because of it.
By the way, you are measuring the strain at the surface of the sample, right? Are you measuring on the 'back' or 'belly', or is this the same?
Alan is not measuring anything, he is calculating the strain based on the deflection and sample thickness. Assuming the neutral axis is mid-thickness on the limb (that assumes the stiffness is the same in both compression and tension), the strain will be the same on both the back and belly. The maximum strain will always be located on the surface of the sample, regardless of where the neutral axis is located.
Mark
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Ok, I see. I feel like I should know that, but I'm still working through basic statics.
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Ok, I see. I feel like I should know that, but I'm still working through basic statics.
No worries. He is getting set up to measure strain directly but is not there yet. I look forward to seeing how that all goes.
Mark
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This is beyond interesting. Thank you all who put effort into this!
Indeed one has got to wonder about what would happen with one continues ring on the back. Would it make significant difference?
I always figured a tree that bends in the wind takes most tension and compression on the, let's say- 'flat ringed' side. The 'edge grain' is more in the center so would have to take less punishment. A flat sawn board therefore be stronger in both compression and tension. Does that make any sense at all?
Jaap
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So I'm wondering if this is where tillering comes into play. If the samples are uniform thickness and width that would be like a badly tillered bow, most of the bend taking place in the middle. If they had a straight width taper wouldn't they take less set? So maybe this is a good test of just the materials but doesn't show how soon set would start to show on a bow
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It will still manifest the same way. Whether you are isolating a smaller section or spreading it over a larger section you're still pushing the material to the same degree.
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Each point along the limb has a different amount of force acting on it, so it needs to be shaped to bend to it's maximum stress at that point. I believe these tests are telling us what the maximum strain is.
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Please post some pics and maybe links for the bits you are using when you have it all in hand.
Mark
Yes, I will do this as soon as I have something working.
I’m still waiting on the arrival of the strain gauges in the mail. Latest date is this Wednesday, so I probably won’t have any meaningful test data until this coming weekend.
Do a search on YouTube for “Curious Scientist Strain Gauge”, and he has a multi-part series with the same basic hardware that I am using. I spent some time over the weekend playing with the Arduino microcontroller and it is pretty amazing what you can do with these. I bought one of the larger kits for about $60 because I often volunteer for extra-curricular activities at the schools that focus on introducing engineering to middle school and high school students. The strain gauges are about $10 for a pack of 10.
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Each point along the limb has a different amount of force acting on it, so it needs to be shaped to bend to it's maximum stress at that point. I believe these tests are telling us what the maximum strain is.
Sure but that's when you don't want it to break. This test is supposed to break it.
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So I'm wondering if this is where tillering comes into play. If the samples are uniform thickness and width that would be like a badly tillered bow, most of the bend taking place in the middle.
you are correct Allyn.
If they had a straight width taper wouldn't they take less set?
yes
So maybe this is a good test of just the materials but doesn't show how soon set would start to show on a bow
It should work for an entire bow if the area directly under the load is typical of the whole sample. There is a similar 4 point bend test, (the test Alan is doing is called a 3 point, I believe), where the sample has the load applied at two evenly spaced points. It tests the entire sample and averages the value better. Each test has it's specific formula to calculate MOE . The 3 point test is generally considered adequate for clear wood, and is simpler.
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Thank you for clearing that up willie, you answered my question perfectly
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I may switch to a 4-point bend test. I don’t expect a significant difference, but it does have the advantage of providing an unobstructed space in the middle of the test sample for the strain gauge.
3 point bending is like bending the test sample over your knee. 4 point bending is similar to gripping the ends of the test sample in each hand and bending it in half. The formulas are a little different, but the methods for calculating the material properties are basically the same.
What i like about this is that we are getting some good measurements of the behavior and limits of bow materials without the additional uncertainties that come with specific bow designs or tillering. In the future, it would be interesting to build on this and look at heat treating, laminating effects, and use of pre-stressing in laminates.
Alan
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Anything new here...?
Great topic, so hopefully the story continues!
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That’s what the was wondering. I think someone got real busy . I did. 😁😁😁
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Another future project will be to integrate a linear string pot encoder to the draw-weight machine that we use at our flight shoots. This can be used to generate real-time force-draw curves on a bow. With a little coding, I can have it calculate total stored energy and other useful stats in just a few seconds. I have wanted this for a long time. I am a happy nerd. Haha!
I think I heard him say it uses the Arduino https://www.youtube.com/watch?v=teMO9oCl4YE (https://www.youtube.com/watch?v=teMO9oCl4YE)
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I would think just making a bow out of say good mulberry vs Good Hedge would get you a much faster bow by dropping lots of limb mass.
I like a heavy Hedge bow (mass weight). I do know that 13-14" risered 66" Super Pyramid is a zinger for a flat 40@28" bow. Flipped and with maybe 1/4" tips. Has short working limb but it'll flat punch a 600grn arrow out of it.
HH~