Lifting the Veil On Carbon Fiber

It seems that for the last ten to fifteen years carbon fiber has been the sexy bicycle building material. It is light, stiff and expensive. It looks exotic. It is not a metal, and how cool is that, right? Well, sometimes not so much. It can fail in frightening ways. How much is really known about this material is a question recently asked by the folks at mountainbike.com. Last week they published a fascinating article on how carbon fiber bike frames and components are made and how and why they can fail. This is must read stuff for the attorney handling or considering a product liability action and for the consumer thinking about throwing down big bucks for a bike made of what the magazine calls "Black Magic." Here is a link to the full article. Here is a sampling:

Why carbon in the first place? For one, there's what engineers call the "black aluminum" analogy: A sheet of carbon composite would be just as stiff and strong as an identical sheet of aluminum, but a third lighter. Then there's "tunability." While metal frame shapes are by nature isotropic (they exhibit the same properties of strength and stiff ness on any axis), carbon fiber composites are anisotropic (those properties exist only along the axis of the fiber direction). So a skilled composites engineer can "tune" the way a tube responds to diff erent loads simply by orienting the carbon fibers in various directions. There are, of course, drawbacks. First, you need said skilled composites engineer. You cannot randomly orient fibers and expect a good result. The art lies not in merely alternating the ply orientation, but also the direction and number of plies—the layup schedule. Second, because manufacturing is incredibly labor-intensive, with almost everything done by hand, it must be obsessively controlled for quality.

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At C-Tech, a programmed machine cuts giant rolls of three-foot-wide prepreg sheet into an array of smaller pieces needed to assemble a frame, from swaths of intermediate or high-modulus fiber large enough to cover an entire down tube to small, twoinch- square swatches of high-strength standard-modulus fiber that will be valuable reinforcement structures. It's an exacting process—each ply of carbon must be cut to size, and with the proper fiber orientation.

Once the plies are ready, they are shipped to the layup room. There, workers assemble a layup kit and follow the layup schedule to precisely assemble whatever part they're working on. A single frame built at C-Tech may have as many as 500 individual plies of carbon, of varying modulus, sizes, locations and fiber orientations. Because of this complexity and the level of human involvement, the layup room is both the nexus for quality control and where most of the headaches lie.

"You can do everything right and have a great engineer and design and materials, and if the layup guy had a bender the night before and forgets a part of the frame, it's toast," says Tomac general manager Joel Smith, who moves his entire family to Taichung, Taiwan, for three months during the prime manufacturing timeframe to ensure quality at Tomac's partner factory, A-Pro.