Primitive Archer
Main Discussion Area => Bows => Topic started by: DC on August 19, 2019, 01:47:18 pm
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Which contributes more to cast, the back or the belly? Opinions?
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Good question...
IMO the belly, I believe wood will elongate less than it compresses and thus the back is there to stop it breaking, whereas the belly does the work.
Of course you can't have one without the other and they need to be well matched.
I think most of us would accept that bamboo is particularly good as a back and, not only doesn't it break (often ;) ) but it adds to the cast, but is that because it allows you to get the maximum out of the belly wood.
I reserve the right to be wrong of course!
Del
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The belly. Unless you also sinew the bow.
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If your question was, " which is more susceptible to rob cast the most, back or belly" I'd say belly. Wood typically compresses more than it can stretch, making the belly take damage often before the back breaks, and a broken back has no cast. However a properly designed back can complement an otherwise bad belly. So, a back can force a belly outside its envelope and cause it to perform poorly. Behind every belly is a back.
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I like Dels answer but I think it depends some on the wood type and design.
DBar
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Interesting question. Which came first the chicken or the egg?
I'm thinking it has to be teamwork. I think a well tillered bow will eventually give up cast from wear on the belly so it would seem that we need a good belly. But if we overpower the back we end up with a failure of tension and a broken bow. Not much cast there. It seems to me they need to work together in perfect harmony to achieve the optimum cast.
Bjrogg
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Think about it......is the wood tension or compression strong?
DBar
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Think about it......is the wood tension or compression strong?
DBar
I'm not a rocket scientist but I'm thinking it better have at least as much tension strength as it's compression or it's going to be in more than one piece.
Bjrogg
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The material properties of wood will explain this. Wood is stronger in tension than compression but its weakness in either property isn't expressed the same.
You also end up with the apparent paradox in building construction where wood is not even used in tension.
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Mabey theres a push pull. its easier to push than to pull. so the back is getting pulled apart pushing the belly together. so the bellys getting pushed together more than the backis getting pulled so i would think that there is more energy in the belly being ready to push itself back apart more than the back is ready to pull together.
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now thinking about it the back seems like it would want to snap back creating speed while the belly pushes hard creating power.
As with del i reserve the right to be wrong.
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Mabey theres a push pull. its easier to push than to pull. so the back is getting pulled apart pushing the belly together. so the bellys getting pushed together more than the backis getting pulled so i would think that there is more energy in the belly being ready to push itself back apart more than the back is ready to pull together.
Every action has an equal and opposite reaction. The forces WILL and DO balance out, even when the belly or back begin, and are well into, failure. As failure happens, the forces decrease but do remain balanced. So it's not a question of what pushes are pulls harder between the belly and back , it's a question of what can handle the most PSI before it fails and design the bow so that the belly and back can both handle the same psi before failure.
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Cool, lots of responses. I remember reading somewhere that steel wouldn't make a good backing because it has no(or very little) stretch. That could just have been someones opinion but I've remembered that. I'm not sure I believe it because I tend to think that it's the belly that does the work.
About the "have to be matched" idea. A little thought experiment. If we have a completed bow and somehow through magic we could double the strength of the back without changing the mass or thickness would the belly suddenly fail? Do you think the performance would change at all?
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When drawn and released, does the back pull the belly along, or does the belly push on the back. It would seem to me that ideally they work together with neither out running the other. But that’s just my guess.
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When drawn and released, does the back pull the belly along, or does the belly push on the back. It would seem to me that ideally they work together with neither out running the other. But that’s just my guess.
If you were to make a bow out of two steel bars and a hinge(the back) and a spring(the belly) you could say that the belly is doing all the work and the back just holds it together. You could turn the whole contraption over and bend it the other way and say the back is doing all the work. The problem with wood is that there is a little give on both sides. I guess it boils down to which has the most springyness. Sinew just makes it worse to think about ;D
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I hear you DC. Add sinew and it's like adding another bow or something.
Bjrogg
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Sinew doesnt change the equation, it's just so stretchy that the bow has to bend more in order to load up the back enough to get the full use of the sinew. While the back loads up slowly, it also loads up the belly proportionate to the back. If the back is under 100psi of tension, the belly is under 100psi of compression, assuming a bow that has an equal width back and belly. If the back is twice as strong as the belly, you should narrow the back by half its width so the resistance to forces equal.
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I to will guess. I think the proper tiller makes this all even. Then you have no set! Maintaining the no set with use is the key. Wish I had that mastered! Then there is the Osage , white woods, and then there is sinew backed, bamboo backed, heck I even tried gems bock horn on the belly of a selfbow . Yes it took set. I was told that Osage and horn had about he same compression. Nice bow though. Arvin
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I don’t often regret never having studied engineering but this is one of those times I do. I would think, that if the back outruns the belly upon release, set has occurred and if the belly outruns the back, a tension failure is around the corner. Arvin’s take seems right to me. Tiller brings the 2 into harmony with one another.
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I to will guess. I think the proper tiller makes this all even. Then you have no set! Maintaining the no set with use is the key. Wish I had that mastered! Then there is the Osage , white woods, and then there is sinew backed, bamboo backed, heck I even tried gems bock horn on the belly of a selfbow . Yes it took set. I was told that Osage and horn had about he same compression. Nice bow though. Arvin
That's a balzy project and faaascinating outcome Arvin.
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I don’t often regret never having studied engineering but this is one of those times I do. I would think, that if the back outruns the belly upon release, set has occurred and if the belly outruns the back, a tension failure is around the corner. Arvin’s take seems right to me. Tiller brings the 2 into harmony with one another.
Tiller is a broad term. Tiller must match top and side profile, and the bend radius must match the thickness of the limb, with the weight of the bow being equal to the width of the limb. Tiller is a BIG term, with very in depth analysis. Yes, tiller it correct and you have no problems, but what do you need to know, understand, and do to get it all right? EVERYTHING >:D
A good simple answer as Arvin put it, is tiller for no set, taking all the needed steps to avoid it.
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Sinew doesnt change the equation, it's just so stretchy that the bow has to bend more in order to load up the back enough to get the full use of the sinew. While the back loads up slowly, it also loads up the belly proportionate to the back. If the back is under 100psi of tension, the belly is under 100psi of compression, assuming a bow that has an equal width back and belly. If the back is twice as strong as the belly, you should narrow the back by half its width so the resistance to forces equal.
The point is that it does stretch. Wood really doesn't.
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Sinew doesnt change the equation, it's just so stretchy that the bow has to bend more in order to load up the back enough to get the full use of the sinew. While the back loads up slowly, it also loads up the belly proportionate to the back. If the back is under 100psi of tension, the belly is under 100psi of compression, assuming a bow that has an equal width back and belly. If the back is twice as strong as the belly, you should narrow the back by half its width so the resistance to forces equal.
The point is that it does stretch. Wood really doesn't.
Yeah, I agree to that
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With sinew the back is ALLOWED to stretch, letting the belly compress less. Using DC’s steel bar example, if you glued a steel strap to the back of the bow, it would not stretch much at all making the belly do all the work. Massive set. If you glued a thinnish steel bar to the belly of that bow, the belly would not compress and the back would be forced to carry the entire load...until it exploded.
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100% percent its the back. My back is what holds the bow at full draw right before I release it. My big belly on the other hand is only good for getting in the way.
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Cool, lots of responses. I remember reading somewhere that steel wouldn't make a good backing because it has no(or very little) stretch. That could just have been someones opinion but I've remembered that. I'm not sure I believe it because I tend to think that it's the belly that does the work.
About the "have to be matched" idea. A little thought experiment. If we have a completed bow and somehow through magic we could double the strength of the back without changing the mass or thickness would the belly suddenly fail? Do you think the performance would change at all?
I think, with no elasticity in the back, the belly must do all the work, because when the bow bends, the only part that can give is then the wood. That would mean that the 'neutral plane' of the bow would be essentially at the back of the bow. I guess if you just made the wood part thin enough, to where the thickness of wood on the bally side of the neutral plane was the same as on a normal bow, it would be able to work. However, depending on how much work a wood back actually does in a bow (and depending on the thickness and spring quality of the steel), I think the poundage wood be lower. Metal is also a lot heavier than wood.
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Which contributes more to cast, the back or the belly? Opinions?
I've always believed that it is a balance between the two
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both... if the belly overpowers the back.. boom...gut
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A little bit of arithmetic says that stretching a 66" bow 1% is .66". Should be easy enough to measure. Has anyone actually put a tape measure along the back of a bow and then bent it? I'm wondering if it does stretch.
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i can do it when i get home and tell you what i find. im going to do this to the belly to! this is interesting!!
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Most wood can only stretch about half a percent before failing. The old test of cutting apart a bow along the middle after some use tells the story. The back goes back to straight and the belly may even show more set than it did as a whole bow.
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Most wood can only stretch about half a percent before failing. The old test of cutting apart a bow along the middle after some use tells the story. The back goes back to straight and the belly may even show more set than it did as a whole bow.
I thought about that and that tells you how much the belly has collapsed but it doesn't tell you what the back is doing at full draw.
i can do it when i get home and tell you what i find. im going to do this to the belly to! this is interesting!!
I thought I would try it too but it may be a two person job to avoid leaving the bow at FD.
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It tells you that if the bow doesn't break it still can't be stretching much at all. It HAS to be compressing more
I guess the argument would be does wood store more energy by stretching .5% or compressing however much it does at full draw.
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i did the experiment with my dad. my bow is 65" ntn has about 1.5 inches of set. around 45# at 26". the backs lenght before it streached was 65". after it was streached it was 66"ntn. if the bow was perfectly tillered each inch along the bow would be streching 0.01538461538 per inch along the length of the bow. please tell me if i missed to say anyting. i know I said this before but seeing how much a bow bends i would say it definentially compresses more than it streches. i think there is a line where the back will stretch and the belly compresses and it can be uneven with the back stretching less than the bow compresses. but then again what sleek said is that they both take the same amount of psi but what if it takes more compression to take the same amount of psi as the back?
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Thanks. So that's a 66" bow pulled to 26". Not overly stressed I would think but it still stretched 1.5%. Interesting. :D
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now how to find how much it will compress
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Thanks. So that's a 66" bow pulled to 26". Not overly stressed I would think but it still stretched 1.5%. Interesting. :D
Does that make us professional board stretchers?
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Thanks. So that's a 66" bow pulled to 26". Not overly stressed I would think but it still stretched 1.5%. Interesting. :D
Does that make us professional board stretchers?
I guess that does! (-S
also DC i dont want to be a stickler but when we do stuff like this the tinyest thing can change the whole thing. its 67 tip to tip and 65 nock to nock
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Laws of math apply here, just dont ask me to do it lol...
If you know the circumference of the back of a drawn bow, and its thickness, you can calculate the circumference of the belly of the bow. That will tell you how much the belly is compressing vs its straight length. Of course, being as the back and the belly is separated by the distance set by the thickness of the limb, there is a ratio between the back and belly that can be applied.
Now if I could only remember how to math....
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dont worry about it sleek, ive just been taught like 3 years ago what we need and still using it 8). only problem, my bow i think is eliptical tiller and i need to tweak one side a couple of scrapes. i should really post it. well ill post it right now.
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That’s spot on sleek to the common sense guy. The thickness will be a portion of the equation. Correct me if I’m wrong. .050 of glass on the belly and .050 on the back of a glass bow the part in the middle just keeps them apart. Yes or no? So Marc’s comment on them doing the same amount of work will give you a perfect tiller for that piece of wood. I think. Arvin
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dont worry about it sleek, ive just been taught like 3 years ago what we need and still using it 8). only problem, my bow i think is eliptical tiller and i need to tweak one side a couple of scrapes. i should really post it. well ill post it right now.
Well, as I am quite used to playing with a short deck, I figured another way. Put the image on the computer and use an arc rendering tool to trace the back and belly, then measure the length of each arc drawn. A CAD program would be great.
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Further musings....Red and Green style
Full draw the bow and while at full draw, lay a strip of duct tape on the back. No need to trim it, just lay it flat on the back. Let down on the bow and trim the tape excess that hangs over the tips. Pull the tape and measure it. Its length is the length of the strung bow.
Do the same on the belly, law tape at full draw, and mark the tips with sharpie. Then yank it off before letting the bow down or it will stretch the tape out as when you let down.
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Sleek duck tape will stretch. Arvin
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Sleek duck tape will stretch. Arvin
yeah, That's why I was saying to lay it on the bow AT full draw, but I suppose pulling it off the bow will stretch the tape also....
I'm certain there is a computer program that can do this with an FD image.
Could just trace the bow at full draw on a chalkboard... back and belly, then measure those lines.
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Spot glue paper on one and if it tears the paper the back stretched. Might be a test. Arvin