[Peter Matteson]

Originally Posted by Andy A.

A great deal of attention is paid to composite materials great strength and low weight, but I've always been more interested in it's incredible stiffness. Aluminum has always struck me as being a fine material where strength (most measures of strength anyways) and low weight is a concern, but it's softness can worry me. If the bicycle and R/C car industry are any indicators, Carbon Fiber's stiffness sure is winning people over.

Not entirely true I can get a sub kilo Titanium bike frame (litespeed) as well as a CF one (Look, Giant, Trek, etc.). The true advantage with carbon fiber, in that specific application, is the ability to tune the frame through the layup. By putting the material where it is needed for stiffness and making structural shapes not necessarilly possible in metal a frame builder can make a lateraly stiff structure that is vertically compliant to dampen road shock (vibrations transmitted from uneven pavement) and minimize rider fatigue. Anyone with a CF bike who's had an AL one will tell you this.

This is harder to do with aluminum because the tubes need to be stiff to minimize deflection in order to have a reasonable fatigue life. They do this by using large OD Al tubing. The result is a frame that transmits a lot of bumps.

CF alone isn't stiff, think of a CF fishing pole. Uni-directional fibers laid cured together in a matrix to have flexibility. It is the structural shapes and fiber orientations that you can use with it that allows is to be stiff.

Once again it all comes down to knowing the material you are working with and how to optimize it. Don't just use something because you can. Learn the whys and hows so you can use it correctly.