Primitive Archer
Main Discussion Area => Bows => Topic started by: Woody roberts on August 29, 2020, 08:35:09 am
-
Hello, first post I’m fairly new to archery and bow making so there is a lot that I don’t know. Ive built 5 bows so far. The first and the last are still shooting. The other 3 have broke after a few hundred shots. Well the Elm bow broke the first time I pulled it back.
The first bow is a red oak board backed with rawhide. 64” long ,8” handle section. I would call it a flat bow. Finished it drew 30 lb at my 26” draw and pushed around 115 fps. Heavy or light arrows made little difference.
I piked it to 60” which raised draw weight to 40 lb. Arrow speed went to 118/120. About the same.
The last bow is bamboo backed Ash. The Ash blank had a 1/4” resawed off and turned end for end before glueing up. This bow was more English long bow style 69” long. Finished at 35 lb @ 26” and pushed an arrow about 120’ fps. Very smooth to shoot and as accurate as I can shoot it.
I piked it to 64” which raised draw weight to 46 lb. just what I was looking for. Maybe gained 5’ fps.
I’m confused by the added draw weight not making any difference in arrow speed.
I realize all wood bows are individual but are there any given designs that have a higher ratio of arrow speed vs draw weight without going to reflex/deflex/ recurve design?
Thanks, Woody
-
I've only been at this a few years, but here are a few thoughts.
First of all, you didn't mention arrow weight, but those speeds are pretty good for a target bow shooting 10 grains or more per pound of draw weight. Considering how smooth and accurate the ash bow is, I can only wish my first few where like it.
As to the piking dilemma, piking a bow puts more stress on it, which causes more set, which in turn reduces cast, so there comes a point when any advantage to piking is lost. Also, all else equal, shorter bows are slower per draw weight than longer bows because they store less energy per draw weight. Finally, are you sure you're drawing the heavier weight version exactly as far as you drew the lighter weight? Sometimes heavier bows are accidentally under drawn. No offence :)
I hope this helps explains some of your issues.
Holmegarrds, D-bows, and American Flatbows are very efficient designs when well made. Heat tempering and Perry-reflexing are tricks that increase cast on almost any design.
-
Yes that does explain some things. I believe I’m coming to full draw. Both bows shoot best with 550 gr arrows. That’s the heaviest I have right now. A 31” carbon arrow with no field point doesn’t go much faster.
Can I heat treat without damaging my glue? My next bow is already glued up. It’s 3 piece bamboo, ash with a red oak belly.
Ps. Posting here is really difficult.
-
Posting gets easier after a few posts. Speed is a strange beast. Everything affects it. Really. First, decide on the grams per pound(gpp) you want to use and stick to it. The standard is 10 gpp. It means you need a different arrow for each weight bow which is a pain but it gives some consistency to your results. Using one arrow for different DW just confuses the issue. DW and DL must be measured reasonably accurately. Consistency is important and your release is important. Unless you are a speed freak like me don't worry about speed too much. As long as the bow doesn't feel doggy you're good. For target just use the lightest arrow that flies true.
-
Double the draw weight doesn't double the speed... it's the law of diminishing returns.
A 70# isn't twice as fast as a 35#.
There's a sweet spot at say between 35# and 45# where it's relatively easy to make a good smooth fast bow.
As you go heavier, yes they'll get a bit faster, they'll also throw a heavier arrow.
The skill of the bowyer makes a huge difference too, once you've mastered the basics, you can try all the tricks to build faster bows, but be warned... you don't get 'owt for now't so what you gain in speed you may loose in longevity, stability or smoothness.
Del
-
The main thing here is wood combination choice.
A bow feels tension and compression forces.
Wood is normally able to stretch further than it can compress.
This means that you need to balance out the properties of the materials you are using.
Bamboo backings are very resistant to stretching.
Red oak and ash are not great in compression.
A more optimal belly wood to use with a bamboo back would be yew/osage/ipe. All woods that excel in compression.
If you have access to red oak it can and will make a great self board bow. Just get very straight grain (important!). If you can't find any straight grained boards then back them with rawhide or woven linen cloth.
Red oak is more resistant to stretch than it is able to resist compression. To aid it in compression you can heat treat it and trap the back - this is making the back narrower than the belly. On a flatbow the cross section of a 'trapped' limb would be a trapezoid. Both heat treating and trapping will make a big difference to how much set red oak takes.
Look up what a pyramid bow is and stick to that design. The bend should then be an arc of a circle tiller and easier to judge for a beginner.
Your main goal to improve speed should be to tiller slowly and never pull the bow past a problem.
Make a tillering gizmo for help with the board bows.
Make your bows 68 inch ntn for a 28 inch draw. Do not bother piking bows if you come in under weight....pretty much a waste of time.
No you can't safely heat treat a bow with a glueline.
Good luck and post pictures as you tiller to get some expert eyes on your bend :)
-
there are times ,,,bows can be piked to increase draw weight and cast,,
-
Wood is normally able to stretch further than it can compress.
Source?
-
I’ve never had a belly of a bow blow. Does it happen? Does it fail and cause dead mass ? I think so. Interesting topic . Arvin
-
Excessive set, frets, and chrysals are all considered belly failures. If any are too concentrated in an area the back can break there.
Piking works best when the bow is to long to begin with. I typically leave extra length on my staves for just this reason.
-
Ok. I have an Ash board with pretty decent grain. The growth rings run side to side pretty square with the board. 6’ long by 2-1/2” wide. I googled pyramid bows and am going to give this a go.
Questions
Should the original outside of the tree be the back?
Should I back this bow with rawhide, bamboo or something I can come up with. I have nothing but plastic Sheetrock tape here at the moment?
It will be 67-1/2 n-n and I’m shooting for 40+ -50 lbs @ 27” draw
All thoughts, concerns, advice, criticism welcomed.
Thanks Woody
-
If the grain is straight, the surface of the board will be fine for a back as is. A little wiggly grain is also fine on a flat ringed board.
If the grain is marginal, rawhide is a good idea. From what I have heard, bamboo is much more likely to overpower ash, and it's really more protection than you need. Rawhide is more proportional, and less expensive.
You may want to start the bow @ 70" so you can pike it if necessary. At around this length, 1" off each limb increases draw weight by 5#. Generally, piking from extra long to ideal length is better than from ideal to too short.
Anyway, that's how it works for me. :)
-
Wood is normally able to stretch further than it can compress.
Source?
Common sense matey :)
-
Wood is normally able to stretch further than it can compress.
Source?
Common sense matey :)
Unfortunately it's false. ;)
-
Wood is normally able to stretch further than it can compress.
Source?
Common sense matey :)
Unfortunately it's false. ;)
Pat I'm confused I always thought that too... They say something in the TBB along the lines Of, most wood has better tension strength then compression, excluding yew, ERC, juniper, and a few others... hence why you trap the back on wood such as hickery to take stress of the belly and put it on the back where hickery excels in tension strength and and the rounded/ trapped belly of a elb takes tension of the back and puts in on the belly? ..... also woody Roberts... you should check out a bendy handle bow... There my favorite design...
-Fox
-
Wood is normally able to stretch further than it can compress.
Source?
Common sense matey :)
Unfortunately it's false. ;)
Before I start making popcorn are you guys talking strength or distance?
-
Strong in tension simply means it is harder to bend and resist breaking. Elasticity is another thing all together. Wood does not stretch very much at all, nearly all the bend is because of compression being elastic. That is why when you heat bend something you strap the back so it cannot stretch and break
-
Wood is normally able to stretch further than it can compress.
Source?
Common sense matey :)
Unfortunately it's false. ;)
Before I start making popcorn are you guys talking strength or distance?
It can only mean distance when worded like that.
-
It can only mean distance when worded like that.
@Patm, bownarra
there's lots of tests results out there from folks who like to break boards, but of course as bowyers we are more concerned with set taking.
do you have a source for "distance" or elongation before set in tension of any species?
-
It is known that wood breaks when stretched a minimal amount. The elongation distance is very short before that happens. Tensile "set" is likely not an issue at all with wood bows in most cases.
That's likely why tensile members in wood buildings are replaced by steel.
-
That's likely why tensile members in wood buildings are replaced by steel.
My understanding of why wood is not often used as a tensile member in wood buildings is because of a limitation in the practicality of making adequate mechanical connections in the field. Tension shears the wood along the grain at a connector, an example being a bolt in the end of a board pulling out the end. Actually wood is used quite a bit in tension in wood truss designs, but requires large carefully glued gussets or properly sized nailplates.?
Please consider this quote from the flight bow section.
Q. Are record quality bows pretty much all Osage and Yew? What would be the top five?
A. Hickory for self bows, Ipe backed with hickory or bamboo is at the top for backed and multi-lam bows.
Do you think the hickory backing on an ipe backed with hickory is stretching further than the back on a
hickory selfbow? what about the bamboo? could the advantage of a multilam be that the bamboo is stretching more and storing more energy than the ipe belly?
Tensile "set" is likely not an issue at all with wood bows in most cases.
If that is true, we should be looking into constructing better wood backings
-
Got the Ash pyramid bow built today. 67-1/2” N to N. 40 lb at 26”. Thinking about heat treating the belly. What do you all think.
Ive only shot 4 arrows so far. I thought it might be better to heat treat before it picked up anymore string follow.
Any advice appreciated.
-
[quote author=willie link=topic=68728.msg964764#msg964764
Do you think the hickory backing on an ipe backed with hickory is stretching further than the back on a
hickory selfbow? what about the bamboo? could the advantage of a multilam be that the bamboo is stretching more and storing more energy than the ipe belly?
Tensile "set" is likely not an issue at all with wood bows in most cases.
If that is true, we should be looking into constructing better wood backings
[/quote]
Even if it does, it still can only stretch a tiny amount or it will break. The amount of "working" stretch has to be tiny.
I'm not sure what you mean by "constructing" a better wood backing. We don't do that. We just harvest what nature made.
-
It seems this topic has shifted and developed quickly:) Any engineers here with input on the mechanical properties of wood, or even just the terminology?
A moderate heat tempering never hurt anything except glue lines -- Just be careful not to let the wood get black.
Post photos when your done!
-
Got the Ash pyramid bow built today. 67-1/2” N to N. 40 lb at 26”. Thinking about heat treating the belly. What do you all think.
Ive only shot 4 arrows so far. I thought it might be better to heat treat before it picked up anymore string follow.
Any advice appreciated.
Woody, do you know the cause of the string follow? hope the site will let you post some pics soon, and please let us know if going off track in your thread is offensive. Sometimes discussions just kinda evolve in strange places.
-
Any engineers here with input on the mechanical properties of wood, or even just the terminology?
yes, and sometimes the terminology get difficult, as folks here come from many different backgrounds, but you should ask technical questions if woody is ok with it.
-
I don’t mind the thread straying at all. I’m taking it all in and trying to learn.
Ive decided against heat treating the bow for now. Ive put close to 100 arrows through it and is very smooth and accurate. Only getting 125 fps with 550 gr arrows. I would like to have more speed but I’m confident I can kill a deer with either of my 2 bows I have.
Ive built six in the last 2 weeks and see no reason to stop. Only 3 have broke so far. I have a bear reflex and a bear recurve but no interest in hunting with a bow I didn’t build myself.
Ive knaped points before and I know where a good patch of river cane is so that will be next. Ive always been an avid hunter but since the kids are all out of the house I only need a couple deer a year. Most of my deer the last few years have been taken with a 45 colt revolver. I like to hunt close.
Thanks guys. Woody
-
As I've mentioned a few times before.....
Take a bow and draw it far enough to take a few inches of set.
Cut that bow along the nuetral plane.
Take a look at the back and belly pieces and the shape they now are when 'released' from each other.
Doing this will greatly help you when it comes to making better bows :)
Pat there are many reasons why steel beams are used in construction over wood. Before we start going down a pointless rabbit hole - are they using pristine split 'staves' in construction.....no.....can you compare apples and oranges.....
I did study mechanical engineering for a few years if you want to dicuss buildings etc but they have very little to do with bow making in the real world.
Theory and numbers on sheets are useful to a point but you have to have a greater knowledge to see how said numbers fit into the bigger picture or else you will end up with a view point that is limited at best.
-
Your description proves the error. The belly shortens.
-
Your description proves the error.
Tensile "set" is likely not an issue at all with wood bows in most cases.
PatM. I think you are being too dismissive of the back being a working part of the limb.
granted, the back does not stretch as much as the belly compresses when a bow is drawn, but if (roughly speaking), wood is twice as strong in tension than in compression, the back is still stretching 1/2 as much as the belly is being compressed. Considered as a whole, the back contributes 1/3 of the deformation that stores the draws energy. Being that the deformation in the back is in a materiel that is twice as strong, it is worth considering.
The fact that set is not observed in the back, could be a good thing worth noting.
-
Your description proves the error. The belly shortens.
granted, the back does not stretch as much as the belly compresses when a bow is drawn, but if (roughly speaking), wood is twice as strong in tension than in compression, the back is still stretching 1/2 as much as the belly is being compressed.
In a one piece (not a lam bow with different woods) rectangular cross section limb the back stretches exactly as much as the belly compresses, until set occurs. While many woods are much stronger in tension than compression, they have the same MOE (ie - stiffness) in both compression and tension until the compression side begins to fail. Different cross sections such as a trapped shape or high crown back do not have equal amounts of strain at the back and belly surfaces, but that is a property of the cross section shape and not the wood.
Mark
-
In a one piece (not a lam bow with different woods) rectangular cross section limb the back stretches exactly as much as the belly compresses, until set occurs. While many woods are much stronger in tension than compression, they have the same MOE (ie - stiffness) in both compression and tension until the compression side begins to fail. Different cross sections such as a trapped shape or high crown back do not have equal amounts of strain at the back and belly surfaces, but that is a property of the cross section shape and not the wood.
Mark
I'm having a tough time wrapping my head around this. Maybe I'm confusing strength with something. If the back is stronger than the belly doesn't that shift the neutral plane? And if the neutral plane is shifted doesn't that change the "strain ratio". It would put more or less pressure on the belly depending on which way the NP is shifted. Now I'm starting to doubt my understanding of which way the NP moves.
-
D/C I agree with you that the strength of the back increases the neutral plane. Here is where I am not 100% clear, my understanding is that if a back resists bending and is stronger that the belly can be a bit thinner. But I admit to not having a real clear understanding of how tension actually affects the bow beyond personal experience and observations.
-
If "stiffness" is a resistance to bending then what causes stiffness? Bending stretches the outside and compresses the inside. Does it do anything else? If not then the whole thing revolves around stretching and compressing the wood. This is then complicated by how fast the wood recovers which is hysteresis, I believe. All this would mean that these three things pretty well cover it. Where am I wrong?
-
If "stiffness" is a resistance to bending then what causes stiffness? Bending stretches the outside and compresses the inside. Does it do anything else? If not then the whole thing revolves around stretching and compressing the wood. This is then complicated by how fast the wood recovers which is hysteresis, I believe. All this would mean that these three things pretty well cover it. Where am I wrong?
You see it exactly as I do and that is why I am still unclear on it. If the back does not stretch at all then the neutral plane would be right under the back. If you took very exact measurement and bent a piece of wood into a complete circle then compared the inside and outside diameters to what it was straight I guess we would know exactly what was going on. My guess is that there is a very small amount of stretch, say a strong wood might stretch .1% and a weak wood might stretch .2%. I just pulled those numbers out of my head.
-
One thing I do know for sure is that wood has very little hysteresis before it has taken set. Not much different than fiberglass.
-
One thing I do know for sure is that wood has very little hysteresis before it has taken set. Not much different than fiberglass.
I wonder what is happening at a microscopic level to cause this. I would think that a wood would have less after it was damaged. Like a new piece of rope is stiff and then softens up as you use it.
-
One thing I do know for sure is that wood has very little hysteresis before it has taken set. Not much different than fiberglass.
I wonder what is happening at a microscopic level to cause this. I would think that a wood would have less after it was damaged. Like a new piece of rope is stiff and then softens up as you use it.
In a perfect world your bow would have no memory of ever being bent. It would be like cutting a bow out to size and then shooting it with no change from how you cut it out.
-
In a one piece (not a lam bow with different woods) rectangular cross section limb the back stretches exactly as much as the belly compresses, until set occurs. While many woods are much stronger in tension than compression, they have the same MOE (ie - stiffness) in both compression and tension until the compression side begins to fail.
thanks for correcting me on that, I guess the back stores as much energy as the belly, if not more.
Maybe I'm confusing strength with something
strength and stiffness are often confused with each other. The commonly cited "wood is twice as strong in tension" phrase most likely comes from a table from The Mechanical Properties of Wood, by S J Record, 1914 reproduced below. If one cares to read the paragraph above the table, in this link, https://www.gutenberg.org/files/12299/12299-h/12299-h.htm#I_01 (https://www.gutenberg.org/files/12299/12299-h/12299-h.htm#I_01). some of the terminology used in discussions about wood strength and stiffness is discussed.
-
I'm having a tough time wrapping my head around this. Maybe I'm confusing strength with something. If the back is stronger than the belly doesn't that shift the neutral plane?
You are confusing strength and stiffness. Stiffness is how much the material deflects in response to a load. Strength is the maximum load the material can take before failing. In a same wood bow the back and the belly have the same stiffness, so they deflect equal amounts in response to the load of bending as the bow is drawn. But the wood is weaker in compression, meaning it will fail at a lower load, so the belly will fail before the back does. This shows up as set.
The confusion with strength and stiffness comes because bowyers will say a stiffer limb is 'stronger'. On an engineering basis that is incorrect terminology but is used all over the archery world and it clouds the distinction between the two.
You are correct that a materials stiffness mismatch will shift the neutral axis. This is why the above only applies to a bow made from one homogeneous piece of wood. Lam bows made with different woods in the various layers will not follow this rule because the different woods will have different MOE's and that will shift the neutral axis towards the stiffer wood.
Mark
-
But if the wood does not provide enough resistance for the back to be stretched by, why would it stretch?
The wood nay be the same material but each side is given a different task.
When wood is cranked in two by tension, what is the linear increase before breaking?
-
But if the wood does not provide enough resistance for the back to be stretched by, why would it stretch?
It does provide enough resistance, up to the point the belly starts to take set (fail). A belly that has reached this point becomes weaker in terms of stiffness, and then (as as you already know), a hinge happens fast.
When wood is cranked in two by tension, what is the linear increase before breaking? the actual distance?
assuming a 1/2 inch thick limb bent into a 24" radius (at the neutral plane), and the bow is about to break, then the back is at a radius of 24.25", and the belly is at a radius of 23.75".
3.142 x 2 x 23.75 = 149.3
3.142 x 2 x 24.25 = 152.4
if your limb is a 1/4 circle then
152.4/4 = 38.1
149.3 /4= 37.3
or about .80 difference or .4" more than the length of the resting limb.
-
I mean cranked in two linearly as it is in the machine.
-
I'm having a tough time wrapping my head around this. Maybe I'm confusing strength with something. If the back is stronger than the belly doesn't that shift the neutral plane?
You are confusing strength and stiffness. Stiffness is how much the material deflects in response to a load. Strength is the maximum load the material can take before failing. In a same wood bow the back and the belly have the same stiffness, so they deflect equal amounts in response to the load of bending as the bow is drawn. But the wood is weaker in compression, meaning it will fail at a lower load, so the belly will fail before the back does. This shows up as set.
The confusion with strength and stiffness comes because bowyers will say a stiffer limb is 'stronger'. On an engineering basis that is incorrect terminology but is used all over the archery world and it clouds the distinction between the two.
You are correct that a materials stiffness mismatch will shift the neutral axis. This is why the above only applies to a bow made from one homogeneous piece of wood. Lam bows made with different woods in the various layers will not follow this rule because the different woods will have different MOE's and that will shift the neutral axis towards the stiffer wood.
Mark
Is stiffness in any direction? In the Wood Database they show MOE as a bending force. Is strength then just stiffness(or any test) carried to destruction?
-
I mean cranked in two linearly as it is in the machine.
it probably does not matter how the wood is stressed. bending or pure tension. the machine samples are double tapered, and the tested crossection is not very long, but if it were 36" long, then it could stretch 7/16" before breaking (approximating from the above example)
-
I'm having a tough time wrapping my head around this. Maybe I'm confusing strength with something. If the back is stronger than the belly doesn't that shift the neutral plane?
You are confusing strength and stiffness. Stiffness is how much the material deflects in response to a load. Strength is the maximum load the material can take before failing. In a same wood bow the back and the belly have the same stiffness, so they deflect equal amounts in response to the load of bending as the bow is drawn. But the wood is weaker in compression, meaning it will fail at a lower load, so the belly will fail before the back does. This shows up as set.
The confusion with strength and stiffness comes because bowyers will say a stiffer limb is 'stronger'. On an engineering basis that is incorrect terminology but is used all over the archery world and it clouds the distinction between the two.
You are correct that a materials stiffness mismatch will shift the neutral axis. This is why the above only applies to a bow made from one homogeneous piece of wood. Lam bows made with different woods in the various layers will not follow this rule because the different woods will have different MOE's and that will shift the neutral axis towards the stiffer wood.
Mark
There do seem to be differences in stiffness in the compressive vs tensile directions. The first figure below shows tensile and compressive MOE data for a few species. Therefore under a given load I would expect nearly double the strain on the belly side vs the back. Please provide any additional info you have on the topic, this is interesting stuff.
http://support.sbcindustry.com/Archive/2010/june/Paper_096.pdf
-
Is stiffness in any direction? In the Wood Database they show MOE as a bending force. Is strength then just stiffness(or any test) carried to destruction?
It depends on the material. Something that is isotropic (same properties in every direction) like steel will have the same stiffness in all directions. A composite like wood is not isotropic and will show different properties depending on how it is oriented in testing. In wood the two major directions are with the grain and cross grain. For bows all the loads are with the grain and the properties need to be measured in that direction.
The MOE and modulus of rupture are in psi, which is force/unit area. It is the same units that stress and pressure are measured in. The units are not related to any particular type of loading, just the force/area.
A tensile test will give both MOE and ultimate strength. Below is a typical stress/strain chart from a tensile test of a ductile material like steel.
(https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Graphics/Mechanical/StressStrainCurve.gif)
A test graph for wood will look similar, except that the yield strength point for wood is the modulus of rupture and the curve will drop right off when that point is reached as the sample fails. Wood does not have the long, gradual arc at the top that a ductile material does. The ultimate strength will be given by the stress reached at MOR and the MOE is the slope of the curve in the linear-elastic region.
Mark
-
There do seem to be differences in stiffness in the compressive vs tensile directions. The first figure below shows tensile and compressive MOE data for a few species. Therefore under a given load I would expect nearly double the strain on the belly side vs the back. Please provide any additional info you have on the topic, this is interesting stuff.
That is interesting, it is the first test data I have seen that says the MOE is different for tension and compression in wood. I have downloaded the paper and will give it a read. That changes the behaviour of the limb substantially.
Mark
-
Hi Brandon,
welcome to the discussion. Wood is interesting stuff, and reading thru the paper, I would like to point out a few things that caught my eye.
1. the moisture content of the tested lumber is typical for the construction industry but not for bows. Higher MC wood does not perform well in compression. Below is a generalized graph from the FPL that shows how tension and compression vary with MC.
2, those knots!! It would be interesting to compare values obtained in the Korean study with tests performed on clear straight grained specimens of the same species.
Admittedly, the graph reflects "strength", rather than stiffness, but it is commonly observed wood is easier to bend when its green, so there may be some correleations.
-
It was the first data of the kind I had seen as well. Prior to this discussion I had taken it as an assumption that the belly side experienced greater strain, but after that was questioned I realized I didn't really have good reason for my assumption.
Yes, there are many questions I have about the paper as well. I know nothing about those species for one. The MC of 8-12% didn't catch my eye however. But I found that MOE data specific to directions parallel to the grain were hard to come by. I had found one additional source, but didn't really like the 'dogbone' shaped samples used. I'll include below if anyone is interested. It shows tension/compression ratios greater than 1 (but nowhere near double) for all species where they used the same specimen for both tests.
https://www.dora.lib4ri.ch/empa/islandora/object/empa%3A15782/datastream/PDF/Bachtiar-2018-Mechanical_behavior_of_walnut_%28Juglans-%28published_version%29.pdf
-
Good find. I was intrigued by the first two graphs in that study. the top graph is tension, and the long high line is for the longitudinal direction. In comparing it to the compression graph below, we see that the stress/strain line is relatively straight, right up until it breaks. A straight line is indicative of little or no set taking. The bend or flattening in the lower compression graph is set taking, ie where it is bending further, but it's not taking much more force to make it bend further.
-
As expected. Wood is spongy but not ductile.
-
Therefore under a given load I would expect nearly double the strain on the belly side vs the back.
The neutral axis doesn't shift that far. In the paper they calculated that 57.7% of the cross section was in compression (using composite beam theory). That means the belly will see 15.4% higher strains than if the MOE was the same in both tension and compression.
Mark
-
Absolutely. If the strain is double on belly side that does not mean the neutral axis will shift 1:1 proportionally toward the back.
The real question is how we use this info to build a better bow. Matching laminations? Sizing back lam? Dimensions for trapping?
It's a shame there is not more data specific to this topic. My vote is that someone here buys a tension testing machine and starts a database for us :)
-
The real question is how we use this info to build a better bow. Matching laminations? Sizing back lam? Dimensions for trapping?
pairing laminations of correct MOE and sizing thicknesses are both important. The benefits from trapping might have more to do with reducing limb mass. Trapping also has a down side of concentrating stress, putting more demands on material quality.
I believe testing for tension could be done in the shop without an expensive machine. Finding backings of various moe and also capable of exceptional strain may be worth the research for those trying to break records at the flight shoot, but when you consider the difference between a self bow vs. a backed bow in the broadhead 50# class, which might be representative of bows close to what the average guy shoots, the difference in cast is less than 4%.
-
I believe testing for tension could be done in the shop without an expensive machine. Finding backings of various moe and also capable of exceptional strain may be worth the research for those trying to break records at the flight shoot, but when you consider the difference between a self bow vs. a backed bow in the broadhead 50# class, which might be representative of bows close to what the average guy shoots, the difference in cast is less than 4%.
if I get enough flights in broadhead that might be less than 4%. I’m going after some other records . Then Steve and others are showing up with some real good bows. Arvin
-
Absolutely. If the strain is double on belly side that does not mean the neutral axis will shift 1:1 proportionally toward the back.
The paper shows the strain on the belly is not double, it is 15.4% more for the wood they were investigating.
The real question is how we use this info to build a better bow. Matching laminations? Sizing back lam? Dimensions for trapping?
That is the big question. If our woods are indeed twice as stiff in tension as compression that does seem to indicate that trapping is a very good idea for most same wood bows.
I have been looking at Perry reflex and getting sorted to try some experiments. I had calculated that I could lower compressive strains by ~4% if done properly, but that was assuming MOE was constant. I now have to redo the calculations with a 2:1 ratio of MOE and see how that affects the results. Either way I am still going to try building a few Perry reflex lam bows to see how it works out.
It's a shame there is not more data specific to this topic. My vote is that someone here buys a tension testing machine and starts a database for us :)
It is unfortunate we have so little data, it is hard to optimize a design when you are working without accurate material properties. You go first on the tensile tester and let me know how it works out. ;D
Mark
-
It seems like bowyers have already figured all this out but people are hopeful there's some magical other combination lurking out there.
Pairing the most extreme examples seems to still average out to the same end result when a bow is made properly.
-
It seems like bowyers have already figured all this out but people are hopeful there's some magical other combination lurking out there.
Pat,
My goal is not to find a magical combination (I don't think such a thing exists) but to be able to understand the mechanics of the bow limbs and materials in order to be able to design a bow on paper and build it with high assurance of getting what I want without building 100's of bows and building an empirical database. As an engineer it is normal to me to use math to define the world around me and allow me to design structures and machines that work the first time out of the box. This is fundamentally no different.
Bow making has room for everyone in it, from those that want to use basic hand tools and craftsmanship to reveal the bow inside a stave to those who want to make bows in a more precise fashion, through understanding of the physics involved. All of these approaches are legitimate in my opinion, it is really just different strokes for different folks.
Mark
-
you still have to build quite a few bows to develope the skill to execute the design,, I think,, maybe 100's
there is so much variation in the wood,, what ever approach you like,, and makes it fun for you,,or enjoyable,,is great,,
you can look at one of DC's or Marcs bows,,Badgers and many more,,,,,there is the design,, right in front of your eyes,, all the specs are there,, but to execute it, is a different story,, and so on,, getting the design on paper is like notating a violin part from Mozart,, there it is,, but to play it,,
-
It seems like bowyers have already figured all this out but people are hopeful there's some magical other combination lurking out there.
Pat,
My goal is not to find a magical combination (I don't think such a thing exists) but to be able to understand the mechanics of the bow limbs and materials in order to be able to design a bow on paper and build it with high assurance of getting what I want without building 100's of bows and building an empirical database. As an engineer it is normal to me to use math to define the world around me and allow me to design structures and machines that work the first time out of the box. This is no fundamentally no different.
Bow making has room for everyone in it, from those that want to use basic hand tools and craftsmanship to reveal the bow inside a stave to those who want to make bows in a more precise fashion, through understanding of the physics involved. All of these approaches are legitimate in my opinion, it is really just different strokes for different folks.
Mark
For sure, but Archery The Technical side did all this for you decades ago.
-
Are moe values obtained in a bend test the average of the moe in compression and the moe in tension?
If moe tension varied much from moe compression, then it could be noticed when comparing the averaged moe value obtained from a bend test to those values obtained from a compression test.
-
What a journey it has been for me.
I try to let the stave dictate the design but that's after 2-300 bows.
Then there's changing the design on the fly because I thought it was the right one for the stave...
But what do I know I'm just an old farm boy at heart.
Jawge
-
Yes Jawge I’m lost too. But I’m trying to understand. Arvin
-
Arvin, just a different approach outlined here. I taught chemistry for so many years. I used bow making to relax so I kept the math/science/ engineering out of it as much as possible.
My wife always noted that I was in my happy place while building bows.
It is good that some enjoy the math of it all. :)
Jawge
-
I've always wanted to know how things work, even down to the molecular level. Just curious I guess. I'm not even sure how much of the knowledge gets transferred to my bow making because I really don't do a lot of conscious planning. I get an idea and go with it and take note if it works or not. I just wish there was an Issac Assimov of bow physics so I could get a clear understanding without the math ;D
-
For sure, but Archery The Technical side did all this for you decades ago.
Not really. It started down that path, but is merely a first step.
Mark
-
For sure, but Archery The Technical side did all this for you decades ago.
Not really. It started down that path, but is merely a first step.
Mark
But where are you in your ability to execute the science with a piece of wood?
-
Oh come on Pat. These guys are doing what they do best. And I bet there are some that can make a really good bow. Arvin
-
Oh come on Pat. These guys are doing what they do best. And I bet there are some that can make a really good bow. Arvin
He was just starting with board bows this year though . Sometimes it's best to learn more of the art before the science. Case in point, DC.
-
But where are you in your ability to execute the science with a piece of wood?
My last bow I got the limb thickness to within ~0.002" of what the design called for without too much trouble. When it was done I needed to do one light scraping on one limb to get the tiller where I wanted. Without the science I never would have got as close on the tiller as I did because I don't have years of experience to work by eye.
Lam bows are easier to be accurate with because I will use a thickness sander to get the components made accurately before I even start. I am trying to remove as much of the art as possible in order to increase the predictability. I don't have to be able to tiller as well as DC or Badger if I can design the bow on paper and then make it to those dimensions with good accuracy. Which is a good thing, because I am never going to be able to tiller as well as many of the veterans here.
Mark
-
i like to make self bows and stave bows,, and its difficult to measure some things,, but is rewarding when they turn out,,
-
But where are you in your ability to execute the science with a piece of wood?
I don't have to be able to tiller as well as DC or Badger if I can design the bow on paper and then make it to those dimensions with good accuracy. Which is a good thing, because I am never going to be able to tiller as well as many of the veterans here.
Mark
Those two things are the same thing.
-
another thread off the rails, and I am not surprised the OP has not been back since he posted
I don’t mind the thread straying at all. I’m taking it all in and trying to learn.
-
what were we talking about, (-S
-
what were we talking about, (-S
if you compare the stiffnesses (MOE) of your favorite hardwood in the charts posted above, it appears the back stretches more than the belly compresses. It's the stretching that stores energy. btw. For shagbark hickory, the back is 36% less stiff.
-
Ive been back guys. I’m trying to learn but danged if I can follow what you all are saying, much less add something.
Woody
-
Ive been back guys. I’m trying to learn but danged if I can follow what you all are saying, much less add something.
Woody
Just keep following on, it's amazing what your brain can pick up without you even being aware of it. Ask questions.
-
is it ok if I ask what about sinew on on the back, or is that too far off topic
-
I also would like to know. Saxton Pope liked it. I use artificial sinew for my leather work but I don’t think it will work because of the wax that’s on it.
-
We'll leave that one to the scientists. ;)
-
Those two things are the same thing.
Not in my world. IMO, tillering is reading the bend of the limb and adjusting the dimensions to get the bend you want. Making the bow to calculated dimensions is just woodworking. I am OK at the woodworking, very inexperienced at reading the bend.
if you compare the stiffnesses (MOE) of your favorite hardwood in the charts posted above, it appears the back stretches more than the belly compresses. It's the stretching that stores energy. btw. For shagbark hickory, the back is 36% less stiff.
I'm not understanding how you are determining that? It looks to me like the tensile fiber stress at the elastic limit is 9000psi and the compressive stress at the elastic limit is 4620psi. This is for the air dried shagbark hickory. Am I misreading that?
Ive been back guys. I’m trying to learn but danged if I can follow what you all are saying, much less add something.
Woody
Ask questions and we will do our best to answer them in an understandable fashion.
is it ok if I ask what about sinew on on the back, or is that too far off topic
What about it?
Mark
-
Not in my world. IMO, tillering is reading the bend of the limb and adjusting the dimensions to get the bend you want. Making the bow to calculated dimensions is just woodworking. I am OK at the woodworking, very inexperienced at reading the bend.
Do your calculated dimensions not indicate a bend?
-
I'm not understanding how you are determining that? It looks to me like the tensile fiber stress at the elastic limit is 9000psi and the compressive stress at the elastic limit is 4620psi. This is for the air dried shagbark hickory. Am I misreading that?
No, you are not misreading the data supplied in the publication. The first scan shows the formula and raw data used for calculating the fiber stress at the elastic limit in the static bend test. As far as I can tell, the formula presumes the specimen to be homogeneous and having the neutral axis at the center of the cross section. I believe this is a common practice, as there are few uses for wood in tension near the elastic limit and the difficulty of conducting tension tests along with corresponding compression tests on the same sample, in order to actually determine the location of the NA in each specimen tested in bending
I cannot think of any reason why MOE data obtained from a dedicated compression test should differ from data obtained from a bend test unless there were actual differences between tension MOE and compression MOE.
For the air dried shagbark hickory tested above, the MOE at the elastic limit in bending is 1980 (x1000) vs. 2280 from the dedicated compression test., or 300 lower.
Since the bend test acts equally on the back and the belly of the specimen, and the MOE at the elastic limit represents both tension and compression stiffnesses, I have assumed it must be an average of the two. In order for the the bend test average to be 300 lower, the stiffness in tension must be 600 lower, or 1680. This makes tension MOE 76% of compression MOE. or compression MOE 36% higher than tension, depending on your point of reference. My statement of " the back is 36% less stiff.", in the post above is incorrect math. (it implies a difference of 820).
The last two scans are the data and formulas for the compression test
-
I just want to know what wood or combo will get me the fastest arrow speed at the lowest draw weight. Talking straight end longbows and hunting weight arrows. 50 lb is probably about my max.
-
(-P
-
I just want to know what wood or combo will get me the fastest arrow speed at the lowest draw weight. Talking straight end longbows and hunting weight arrows. 50 lb is probably about my max.
woody, in the other thread you said,
I still need practice on my tillering. I’m not ready to build my dream bow yet. ( whatever that is )
I am not sure why you need to be building your dream bow to use osage if it is locally available. Part of tillering practice is judging bend, but a good part of it is knowing your material and learning how anticipate set or not over stressing the material. For a fast straight stave bow, osage will allow you to keep the tips stiffer while doing more of the bending midlimb, and still have a reasonably shortish bow for hunting. The guy who can help with that design in osage (possibly topped with with a little sinew) is busy eating popcorn right now. :)
-
You guys are a hoot.
-
I just want to know what wood or combo will get me the fastest arrow speed at the lowest draw weight. Talking straight end longbows and hunting weight arrows. 50 lb is probably about my max.
Bamboo backed yew :D
-
Willie you are e spot on . I don’t understand the physics and math but if you get a smart guy willing to help the not so smart guy . Well the learning of mass removal and where to remove it is Hugh. I know a few smart guy’s that play along with me and do the math . Then we discuss the why an wherefores. While I understand the needs but not the math. Most helpful to everyone. Arvin
-
Do your calculated dimensions not indicate a bend?
They do, but that bend occurs as long as I get the dimensions right in the first place. My last bow was the first I tried this method on and it only required one slight scraping to get the tiller I wanted after the initial wood removal work. I didn't need to tiller it from nothing, I got 99% of the way there without ever pulling it on the tree. It would have been much harder for me to achieve the same result doing it by eye from a rough floor tiller.
No, you are not misreading the data supplied in the publication.
Ok, good. I missed the MOE on the compression page, what you are talking about now makes sense. I will go through the pages you just posted and come back to this.
I just want to know what wood or combo will get me the fastest arrow speed at the lowest draw weight. Talking straight end longbows and hunting weight arrows. 50 lb is probably about my max.
For the fastest bow you want the least amount of set with the lightest limb weight. That is, you want a bow that is not overbuilt beyond what is needed to have very minimal set and no extra material left to slow the limbs down. If you are willing to heat or glue in reflex that helps early draw weight and increases energy storage for higher performance.
If you have osage easily available it is probably the best choice for you and is a superb bow wood. Follow Badger's no set tillering method and you will be very close to the best you can get with your restrictions on the design.
Mark
-
Badgers no set tillering method? What is it and where do I find it?