Originally Posted by
PeteHski
I don't believe that is true, but I'm not by any means a composites expert. There are properties of both unidirectional and woven bi-directional carbon that are beneficial to the combined structure. There may well be more emphasis on unidirectional layers in a high-end modern bike frame, but woven carbon is not merely for cosmetic use.
A 'woven' piece of carbon is nearly always cosmetic. It doesn't provide any difference in stiffness in any direction because 50% of the material is always positioned perpendicular to the other 50%. A layer of unidirectional on the other hand can be positioned at an angle to the next layer if needed and completely change the flex/stiffness characteristics of the the part. During a visit to the Cervelo CA facility we made samples that were the size/shape of a popsicle stick. We did 2, the first had all the fibers running the length of the sample, the second had each layer at an opposing 45* angle to the length of the sample. The first you pretty much couldn't bend the length of the part, but was torsionally very flexible. The second you could easily bend lengthwise but was torsionally very rigid. Both were 4 sheets thick. Damon Rinard (head engineer for Cervelo at the time) said that weave was pretty much only used for cosmetic finish layers. It's strong against impacts but doesn't really change the flex you've designed into the layers under it.