But it changes the relative stiffness, no?
I think I see what you are asking about. A wood backing doesn't 'act' stiffer as such, it really is stiffer when only loaded in tension. When working with a composite glue up (that is, multiple lams of different materials) you have to do any calculations taking into account the properties of each lam as it will be loaded. The back lam will only see tension, so you would need to use its tension modulus and the belly lam will only see compression so you would use its compression modulus. The core will see both tension and compression, so you would want to use its flexural modulus (the modulus you get when you measure with a bend test, like Alan mentioned a few posts back).
Since I threw another term in there (engineers, eh?
), I think some explanation is in order. There are 3 ways to measure the stiffness (modulus) of the material. You can load it in tension, compression or bending. Tension and compression are 'pure' loading conditions where all the material sees the same load, but bending (or flexural) puts the test piece in both tension and compression at the same time. The end result is that bending gives you an 'average' modulus of sorts, whereas tension and compression tell you purely about that one load direction.
For a selfbow the flexural modulus is the most accurate one to use because that is the exact loading that the limb sees. For lams you would want the actual tension and compression numbers, but those are hard to get without sophisticated lab equipment. Flexural modulus can be measured reasonably well in a home shop with a bit of care.
Did that get it?
Mark