Primitive Archer
Main Discussion Area => Bows => Topic started by: Kidder on June 02, 2022, 01:21:05 am
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Is there any correlation between design stress and performance? In other words, all else being equal, will a higher stressed design bow result in greater performance? One example of what I’m thinking is a 50# 66 inch bow versus the same 50# bow but only 60 inches overall. Assuming that is the case, will the performance degrade over time where the performance of the low stress design overcomes that of the high stress design after extended usage? Thanks for entertaining my wandering curiosity.
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Every piece of wood has its own set of properties. How elastic is it? How able to resist compression? How close are its resistances to stretching and compressing? Plus others!
Your job as a bowyer is to 'listen' to the wood and hear what it is telling you.
The more skilled/experienced the bowyer the closer he can get to optimal strain on the wood.
One reason I always suggest people should trace the outline of the back before ever bending the stave - during tillering you can placed the bow back against the original outline and see where/how the set is appearing.
Also get your head around 'tiller logic' as soon as possible....once you understand how width taper is married to thickness taper and that thickness taper determines 'tiller' you can make any bow in any design and KNOW how it should bend to optimally strain the wood.
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If you give wood enough width to handle the strain in the areas it needs it.It holds up over time.There is no certain script that fits all bows and designs.All things are not equal then.All woods are not the same either in density/shapes of backs/elasticity and the list goes on.Evaluating the wood and knowing it before design and tillering is started.Width's and thickness's are different then too.If the design is going to be more stressed you need more wood to handle it.
The cleaner the wood the more stressed design it can handle.Even then it's a fine slow line to walk and not always accomplished.
To me that's the beauty of self bows or sinewed bows for that matter.
It's one of the reasons I think why these type bows will out shoot FG bows as most all those are scripted.There are FG bow makers out there though that do not conform to the script,but they have their limits too.
Even the highly stressed bows will shoot quiet as a mouse with or without silencers [I like the looks of them] if they are tillered correctly/settled in and take years and years of shooting/hunting seasons and still shoot with the same performance.
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Well said mike, Ed. Sometimes our own thoughts and ideas or ambitions get in the way of listening to the wood.
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I think there’s likely a sweet spot for performance. Too stressed will obviously result in set and worse performance. Understressed May mean more mass or wind resistance in the limbs. I think there’s still a question of mass given Badger’s experience with wide limbed bows that we’re not necessarily heavier than narrow limbed counterparts.
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Is there any correlation between design stress and performance?
Yes. You will get more performance as stresses go up until you go too far and the wood starts taking set or otherwise failing due to the high stresses. As the others have noted, the fundamental problem with wood is its inconsistency in material properties. You never know quite what the limits are for the piece of wood you are working on and you can't tell the peak until you have gone past it.
Mark
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Firstly I would like to thank Alan Case for publishing his valuable work for other bowmakers and then recommend to read his comments /see the graphs/ here ( and several previous pages too ) :
http://www.primitivearcher.com/smf/index.php/topic,69529.345.html .
I ( as not very experienced bowmaker ) do not know other way to do asked task, than to test ( similarly to avcase's method, but preferably using adviced 4 point bending setup ) on appropriate samples of wood from prepared stave, determine the proportional limit and decide what set is still acceptable along different parts of limbs for your bow design. Then use bendmetering at the final part ( at least ) of tillering to not overcome the desired set ( in every stations along the limbs ).
To resolve the problem mentioned by Mark - "you can't tell the peak until you have gone past it" - I suppose one could make the bow intentionally thicker/stronger/ from the beginning and after overcoming the peak ( evenly ), you will remove damaged belly wood without changing the tillered shape of the bow and by such way achieving desired draw weight and performance.
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Ok I have some questions?
Is it possible to reduce mass to prevent set in a computer program?
My thinking is if you know the property’s of the wood . Which we can get close on doing bend test and float test. Let’s say the test reveal the wood is 30 percent stronger in tension. Would it make sense to reduce the back of the bow in width 30 percent to let the belly’s compression catch up in strength as not to fail to compression. Trapping the bows back. This would also use the mass to its full potential would it not. Just asking.I would like to see some of the guys playing with computer designs to try this. This may be a good time to figure out perfect diminishing mass using 8 oz on a 28 inch limb. Maybe not. You smart guys please ponder this ! Arvin
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+1 to what everyone else said. The TBB performance chapters have detailed tests on power stroke as well. The same bow of 45-48 pounds (if I recall correctly) was tested at low 20’s draw and shot 135 fps. It was retillered over and over to 45-48# out to 28” in one inch increments, and it gained 20+/-feet per second in the process. Power stroke is also important.
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Ok I have some questions?
Is it possible to reduce mass to prevent set in a computer program?
My thinking is if you know the property’s of the wood . Which we can get close on doing bend test and float test. Let’s say the test reveal the wood is 30 percent stronger in tension. Would it make sense to reduce the back of the bow in width 30 percent to let the belly’s compression catch up in strength as not to fail to compression. Trapping the bows back. This would also use the mass to its full potential would it not. Just asking.I would like to see some of the guys playing with computer designs to try this. This may be a good time to figure out perfect diminishing mass using 8 oz on a 28 inch limb. Maybe not. You smart guys please ponder this ! Arvin
Hey Arvin,
To answer your questions in order:
1) A bend test doesn't tell you how much stronger your wood is in tension or compression, it simply gives you the stiffness of the wood in bending (the modulus of elasticity) and the stress level where set starts to occur (assuming you test like Alan Case did in your other thread linked to above). You could figure the tension/compression balance out by trapping bend test samples different amounts and examining how they fail until you find the one that fails nearly simultaneously in tension and compression. You might run out of wood before you find this point, though.
2) The effects of trapping are not linear like that at all. Removing 30% of the back width will not raise the tension stresses by 30%. The change in stresses is based on how much you shift the neutral axis of the limb cross section and that is dependent on the back width along with how much of the side is removed and the final cross sectional shape.
3) I would say the perfect mass distribution is what you get with a quasi-pyramid back profile and constant limb thickness. The constant thickness keeps the strain/stress the same throughout the working limb length and the width changes to optimize the amount of wood used to the minimum required to keep the strain/stress the same everywhere.
Mark
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Mark I’m thinking removing the edges of the back of the bow will reduce the tension possibility’s. Not add to. Reducing the mass should also make increase speed in the limb returning to brace. Redneck thinking.🤠🤠
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as you make the bow shorter,, there will be a point of diminishing return,, where the perfromance will go down,, even the draw weight is the same,,
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Mark I’m thinking removing the edges of the back of the bow will reduce the tension possibility’s. Not add to. Reducing the mass should also make increase speed in the limb returning to brace. Redneck thinking.🤠🤠
Trapping will help balance the tension and compression capacity of the wood and let you get closer to the maximum performance the wood can achieve but it isn't magic.
Mark
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Ok Mark thanks for the answers.
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For the same reason we round the belly on yew bows I think...because with yew the weaker side is the back, imho.
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Is there any correlation between design stress and performance? In other words, all else being equal, will a higher stressed design bow result in greater performance? One example of what I’m thinking is a 50# 66 inch bow versus the same 50# bow but only 60 inches overall. Assuming that is the case, will the performance degrade over time where the performance of the low stress design overcomes that of the high stress design after extended usage? Thanks for entertaining my wandering curiosity.
When I've previously said things along the lines of what I say below, I have attracted criticism for my expression of this craft as being too divorced from the romance of bowmaking by feel. However, given your question I feel an answer such as the below merits this sort of info. Using methodologies from materials science and mechanical engineering allows us to answer questions such as yours, and has helped me to personally teach maybe 180 students how to make dependable, high performance wooden bows.
Yes, there is a correclation between stress and performance. If the stress is very (very) low, it will be because the stress is being distributed across a whole lot of wood. Either through very long or very wide limbs (or both). If your bow is very high stress, it will be because the bow is very short or very thick (or maybe both).
In the former case, all that wood across which the stress is distributed presents a whole lot of excess mass. More of the energy stored in bending the bow has to be expended on accelerating the heavy limbs.
In the latter case, the bow gets dangerously close to having stress that either causes high amounts of set (which diminshes performance) or breaking (which summarily ends all performance). A bow which is tillered to a high level stress and repeatedly exposed to that stress is likely to deteriorate in performance over time. I understand that it's not uncommon for wooden flightbows last a few shots and then break down (or break up).
So the aim of the game is to find that amount of working stress that walks the fine line between sufficient wood to withstand the stresses over time, and of sufficiently high stress to minimise the mass. In your example of the two 50# bows, ideally they'd both have the same amount of stress. There may be some who don't believe that, but mechanically it's true. A piece of timber with an ideal amount of working stress should be made into bows that experience that amount of stress regardless of the length or drawn shape.
While it is true that the variability of wood's mechanical properties can be quite broad, it is also the case that this phenomenon doesn't matter much so long as the stave is of sufficient size to extract a sample for bend testing first. There are probably many bowyers who would argue that this isn't 'listening to the bow/wood', but I'd argue that it is in fact listening very carefully after having asked the wood some very specific questions.
It is entirely possible to measure the mechanical properties of a piece of timber and from those calculations make a dependable, high performance bow. I've done it for years. What's more, I've outsourced the calculations to spreadsheets. I've written a reasonable amount about it here: https://ozbow.net/phpBB3/viewtopic.php?f=34&t=5450 (https://ozbow.net/phpBB3/viewtopic.php?f=34&t=5450) and here: https://ozbow.net/phpBB3/viewtopic.php?f=34&t=13765 (https://ozbow.net/phpBB3/viewtopic.php?f=34&t=13765). You can do the same with free software and engineer your own bows.
If you've the patience to go through that first link, you'll see its possible to make a bow with perfectly even stress and strain throughout the entire length of the bow, and that you can achieve this regardless of the particular drawn profile of the bow, be it English Warbow or short Mollegabet. If you go through the second, you'll see how I record bow properties in shorthand. Of note, by combining my bend test library with Tim Baker's, I found that while a wood like Rock Maple might vary in its mechanical proprties between samples, it is very often the case that the variation is sufficiently small not to matter a great deal.
I suspect the practice of rounding the bellies of yew bows is more a matter of convention than engineering best-practice.
AY
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Paul Comstock spent quite a bit of effort analyzing the subject of highly stressed bow designs in “The Bent Stick.” I’d venture to say you’ll find lot’s of useful info in that short highly informative paperback.
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ok here is a good example,,
take a 40 inch self bow,, and draw it 20 inches to 50#,, shoot it through a chrono with 500 grain arrow,, I would say 140 fps would be good,,
now take a 66 inch self bow 50# draw and draw it to 28 inches ,, not so strained ,, shoot the same arrow,, 170 fps is achievable,,165 being more achievable,, ;D
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Bradsmith, the change in speed and disgance of the arrow in your example is due to the power stroke, which changes how much energy is stored and then released into the arrow.
Ideally, both bows would be designed and built to have equal strain/stress.
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yes but one bow is stressed more than the other,,
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No, it shouldn't be. That's what I'm trying to say. No matter what the design, profile, drawn shape, draw length or draw force... a piece of wood should be used so that it has the same maximum stress.
It probably is the case that people doing it by feel often make some bows with higher or lower stress depending on the dimensions and whatnot. But in an ideal world, and with the help of some scientific and engineering principles, a crossbow prod pulling 200 lb at 12 inches should be subject to exactly the same working stress as a 72" longbow pulling 45# at 28".
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ok
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No, it shouldn't be. That's what I'm trying to say. No matter what the design, profile, drawn shape, draw length or draw force... a piece of wood should be used so that it has the same maximum stress.
It probably is the case that people doing it by feel often make some bows with higher or lower stress depending on the dimensions and whatnot. But in an ideal world, and with the help of some scientific and engineering principles, a crossbow prod pulling 200 lb at 12 inches should be subject to exactly the same working stress as a 72" longbow pulling 45# at 28".
So, if you have the formula, and if it's not a lot of work, could you explain using Brad's 2 bow example from the above post? I'm just guessing that your formula involves coming up with the ideal dimensions for a given bow/bow wood...? thanks.
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Mass distribution is more important than mass alone. That gets into surface area and pounds of stress per square inch. For a few years I have been on and off working on getting a formula for it. I have one that works well, but isn't exactly where I want it to be. I'm still trying to improve it.
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I think because all species and even individual samples of wood vary to such a large degree it is almost impossible to predetermine stress levels based on design beyond the standard rules of thumb we have come up with. You can fine-tune that a bit by monitoring set which is the ultimate practical limits of the bow anyway. But Ideally, I would agree that stress limits should be the same for any bow of any weight and design. Without engineering details on every piece of wood any practical useful chart to establish widths would be nothing more than the same estimates we use now.
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So, if you have the formula, and if it's not a lot of work, could you explain using Brad's 2 bow example from the above post? I'm just guessing that your formula involves coming up with the ideal dimensions for a given bow/bow wood...? thanks.
Yep, absolutely can do. Stand by for a day or two and I'll pul something together.
I think because all species and even individual samples of wood vary to such a large degree it is almost impossible to predetermine stress levels based on design beyond the standard rules of thumb we have come up with. You can fine-tune that a bit by monitoring set which is the ultimate practical limits of the bow anyway. But Ideally, I would agree that stress limits should be the same for any bow of any weight and design. Without engineering details on every piece of wood any practical useful chart to establish widths would be nothing more than the same estimates we use now.
Totally agree Badger. Fortunately, doing a bend test needn't take long. And the time it saves is immense too. I can make a longbow from a board that pretty much comes off the bandsaw tillered to within a couple of pounds of design.
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Aussie, 1st of all I wanted to publicly thank you for converting the mass formula to an excel spreadsheet for me many years ago.
As for a bend test, I was never able to convert a bend test to usable information that I could apply to making a bow. I think if you could lay out the method for doing this, if it is not too complicated it would be a huge step in making higher performing bows. I came out with several tests that were useful in comparing wood but I was never able to directly apply it to a design. Hopefully you can send us in the right direction here.
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the answer to your question is yes,, no graph or formula needed, just make the two bows and see for yourself,, its that easy
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the answer to your question is yes,, no graph or formula needed, just make the two bows and see for yourself,, its that easy
Brad, I agree somewhat. but if someone can tell me that the shorter bow needs to be (maybe) 3" wide and 3/8" thick to be strained the same as a longer bow that is "1 1/4" wide and 3/8" thick... it would be interesting and a good place to make 2 bows just to see how they compare in reality. I guess I'm a visual learner....
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me too thats a good point,, I would be thinking is the mass of the 3 inch bow gonna effect the performance,,
I would just have to make it,, shoot it through a chrono and see,, I dont know the answer,
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Aussie, 1st of all I wanted to publicly thank you for converting the mass formula to an excel spreadsheet for me many years ago.
As for a bend test, I was never able to convert a bend test to usable information that I could apply to making a bow. I think if you could lay out the method for doing this, if it is not too complicated it would be a huge step in making higher performing bows. I came out with several tests that were useful in comparing wood but I was never able to directly apply it to a design. Hopefully you can send us in the right direction here.
Most welcome Badger, a pleasure and an honour to help out.
Laying out the method can be easy to understand, but it isn't necessarily something I can cover thoroughly in a short space. Fortunately though a few years ago I did lay out the method, from plank of wood to engineered bow here: https://ozbow.net/phpBB3/viewtopic.php?f=34&t=5450 it might take a while to read through but I tried hard to make it accessible.
the answer to your question is yes,, no graph or formula needed, just make the two bows and see for yourself,, its that easy
BradSmith, if by this you mean that it's easy to show that a short bow will be inherently more stressed than a longer one, then no that isn't a fair demonstration of the hypothesis. All you will demonstrate is that a more stressed bow takes more set than a less stressed bow. The reason is without common metrics and methodology, there's no way to control for the amount of experienced stress in the bending bow. By this I don't mean that the stress is uncontrollable, but that you can't be sure that the stress in the two bows is the same.
An aluminium rod and a dowel feel cold and tepid, respectively, when you pick them up in your workshop. But the reality is that if you measured them scientifically, they would actually be the same temperature. Observational studies are not always reliable reflections of physical phenomena.
The amount of plastic deformation (permanent set) a piece of wood (a bow) takes is dependent on the maximum amount of stress the wood is subject to during bending. More working stress = more set. Less working stress = less set. This is completely independent of draw length, draw weight or bow length.
We've all made short bows that took little set and long bows that took more. It has less to do with the length than it does with the stress as a proportion of the elastic limit of that piece of wood. The shorter bow with less set experienced a smaller proportion of its potential stress, and the longer one with more set was caused to endure a larger proportion of its potential stress.
On the weekend I'll bring together a few different theoretical bows with dimensions and if people wish they can make their own according to the dimensions. I'll do a short Mollegabet, a medium pyramid bow, and a longer D bow.
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Aussie thank you so much for the details and explanation,, I just ment if you build the two bows,, alot of your answers will be revealed,,, :)
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Oh, I see. Orrighty then.
Okay, so I pulled some stats and calculated some dimensions. These dimensions are for a sample of timber tested by Tim Baker many years ago. The design stress should result in a set of about 1.5". For what it's worth, the modulus of elasticity of this piece of timber was 16,415 MPa and the working strain is 0.085%.
64" pyramid bow (8" rigid centre not included - "Dist" = "dist from fade")
50# @ 28"
Dist_____Thck___Wdth
0.00_____0.47____1.58
4.67_____0.47____1.40
9.34_____0.47____1.18
14.02____0.47____0.92
18.69____0.47____0.63
23.36____0.47____0.55
28.03____0.47____0.47
71" bendy handle longbow
60# @ 28"
Dist_____Thck_____Wdth
0.00_____0.80_____0.87
3.94_____0.80_____0.86
7.87_____0.77_____0.84
11.81____0.73_____0.82
15.75____0.69_____0.80
19.69____0.62_____0.79
23.62____0.54_____0.77
27.56____0.48_____0.69
31.50____0.41_____0.54
35.43____0.36_____0.39
63" Modern Mollegabet
50# @ 28"
Dist_____Thck_____Wdth
0.00_____0.89____1.02
1.57_____0.78____1.00
3.15_____0.67____1.04
6.30_____0.56____1.05
9.45_____0.54____1.05
12.60____0.52____1.00
15.75____0.54____0.77
18.90____0.61____0.49
22.05____0.64____0.34
25.20____0.64____0.23
28.35____0.59____0.19
31.50____0.55____0.14
I've tried to upload a picture that overlays the drawn shapes of the longbow and the molly. My relationship with forum interfaces is often a fraught and tenuous one, though.
*EDIT*
Aha! It worked! For funzies, I also uploaded a snip of my bow dimensions calculating spreadsheet.
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How much wider would those designs have to be for 0.1” set?
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The short answer is "quite a bit, but I'm not sure".
It would take some re-figuring. I might have some time tomorrow.
The thing is though that reducing strain means making it wider and thinner. Going in this direction increases the mass per unit of stiffness. So while you might have less set (no set), the performance will suffer by virtue of more of the stored energy being required to accelerate the limbs.
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thats a very good point Aussie, I have made bows with little set ,, but shot slow,, to much mass,,
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The short answer is "quite a bit, but I'm not sure".
It would take some re-figuring. I might have some time tomorrow.
The thing is though that reducing strain means making it wider and thinner. Going in this direction increases the mass per unit of stiffness. So while you might have less set (no set), the performance will suffer by virtue of more of the stored energy being required to accelerate the limbs.
the 63 inch Molly needs to be almost twice it's width.
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I suppose you are quite right, on average. I tried to pick a timber that would be common in the US (almost none here), and I guess I picked a truly exceptional sample.
I'll run the numbers on some other samples and see if we get something more conventional. Closer to average.