snsmith-

I'm sure other people can give you better info than me but,

Carbon fiber is the stiffest, and therefore great for strengthening something to make it more resistant to flex.

Fiberglass is not as stiff as carbon fiber, but still stiffer than most things such a balsa, so it does compliment balsa for stiffening/ strengthening. Its more "springy" than carbon and will result in more weight to acheive the same "stiffness"

Kevlar tends to be more for reinforcing for ultimate loads, as in it just wont give up completely, unlike carbon which will snap when overloaded. So kevlar tends to be good for reinforcing something to prevent if from breaking apart, but lousy for stiffness.

If i were reinforcing something to make it stronger and stiffer and didnt want to pay for carbon than my next choice would be fiberglass. I would reserve kevlar for something that was impacted where ultimate strength was key not stiffness.

Now these are only my opinions and someone more knowledgable than myself may very well feel otherwise. I think MHester was a good person to ask.