The woven fiber you see on many parts and frames is not really just a cosmetic layer in most cases. That material is in fact real carbon and it is generally more expensive than unidirectional fiber as it has to be woven together. The woven fiber is not as strong or as stiff as unidirectional, but has one key property: toughness. With plain unidirectional material, the fibers are stronger and stiffer because they are not bent at the fiber intersections of the weave, but if you crack this material, there is nothing to stop that crack from propagating within a ply as the crack can run along the resin lines between fibers. 90% of the time the unidirectional fiber is all you need, but if you are drilling any holes or have any areas of likely crack formation or impact, woven materials are really ideal. If you look at one of our rims you see that there is woven material at the spoke bed and tire bed to eliminate crack formation and propagation from those holes, and also on top of the rim at the impact zone of the rim as woven materials generally have higher elongation before crack properties as well as keeping cracks from growing. If we were to use unidirectional materials in these zones, the rims could be made just as strong and impact resistant, but at a weight penalty. Also in parts like handlebars the woven can serve to hold the unidirectional onto the part, as a part that is subject to high deformations like a handlebar, can have individual strands of carbon peel away from the part if the part is deflected substantially. A few years ago I was in china visiting a factory and a company was working on a light carbon downhill mtn bike bar and they were trying to do it entirely in unidirectional, and the bar was sitting on a test machine with a massive nest of carbon strands peeled up and wrapped around the stem, where the free ends of the unidirectional fiber peeled away during the impact testing and subsequent loading allowed the fibers to continue peeling away. It looked like a really nasty bird's nest! By the time that product got to market they had put a thin ply of woven fiber over top of the unidirectional so that the part looked didn't gain much weight, but wouldn't have this problem.
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