Probably an old question

[DC]

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

[Del the cat]

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

[PatM]

The belly. Unless you also sinew the bow.

[sleek]

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.

[Danzn Bar]

I like Dels answer but I think it depends some on the wood type and design. 
DBar

[bjrogg]

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

[Danzn Bar]

Think about it......is the wood tension or compression strong?
DBar

[bjrogg]

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

[PatM]

 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.

[Deerhunter21]

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.

[Deerhunter21]

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.

[sleek]

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.

[DC]

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?

[SLIMBOB]

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.

[DC]

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