Probably an old question

[bjrogg]

I hear you DC. Add sinew and it's like adding another bow or something.
Bjrogg

[sleek]

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.

[Selfbowman]

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

[SLIMBOB]

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.

[sleek]

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.

[sleek]

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 

A good simple answer as Arvin put it, is tiller for no set, taking all the needed steps to avoid it.

[PatM]

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.

[sleek]

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

[SLIMBOB]

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.

[Nasr]

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.

[NicAzana]

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.

[Marc St Louis]

Which contributes more to cast, the back or the belly? Opinions?

I've always believed that it is a balance between the two

[gutpile]

both... if the belly overpowers the back.. boom...gut

[DC]

 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.

[Deerhunter21]

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!!