I don't think when we build a bow we are accurate to the point where it is neccessary to really know exactly what the properties of the wood are. I believe it is safe to say that most woods are considerably stronger in tension than compression. This tells me that I can use a wood for the backing that is a lower sg than the belly wood. I have used white oak to back osage and ipe with good results. I have used maple to back hickory and several other slightly heavier woods with good results. I have also seen many guys on here use very strong backs to back weaker bellies and have good results where I would have been afraid of the back overpowering the belly. I have gotten to where I don't worry too much about it anymore if things are pretty close.
