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May 22, 2023

Carbon Fiber

The secret life of carbon fiber—cycling’s most ubiquitous material. Since carbon

The secret life of carbon fiber—cycling's most ubiquitous material.

Since carbon fiber essentially took over the performance cycling world nearly two decades ago, the bike industry has continued to crank out ever-lighter, stiffer, and more impressive designs that were impossible to make in metal.

But carbon fiber products can also be more obscure and mysterious than their metal counterparts. Here are 11 facts you might not know about the material.

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It just seems that way. It's true, most of the cycling industry's carbon-fiber products do come from China, Taiwan, and, increasingly, Vietnam and Indonesia. But some carbon fiber frames and parts are also made in U.S. bike factories (Allied, Guerrilla Gravity, Zipp, Enve, and Argonaut, for example), Portugal, Tunisia (Look), and Italy (Sarto, Basso, Colnago's C68).

Despite its reputation as an aerospace material, carbon fiber is actually as easy to work with on a small scale as metal tubes. Smaller American shops and even individual builders can work with carbon, and not just the finished tubes available from companies like Italy's Dedacciai or Rock West in Utah. Builders like Craig Calfee, Nick Crumpton, and Matt Appleman, all of whom exhibited at the North American Handmade Bike Show, create their own frames from carbon sheet in much the same way that big builders like Giant do.

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Carbon fiber itself is typically made of a material called polyacrylonitrile (PAN) fiber. A less common precursor is called pitch. The PAN fiber is cooked to extremely high temperatures, which burns off all the non-carbon material and leaves a series of very thin, long fibers. The more processing, the more the stiffness increases.

It's a sophisticated industrial process, and only a few companies worldwide have the capability to make raw carbon fiber. As of 2010, per a U.S. Department of Energy report, more than 90 percent of the world's carbon-fiber supply comes from just six companies: Toray, Teijin (Toho Tenax), Mitsubishi Chemical Corporation, Zoltek, Hexcel, and Cytec. Toray, Teijin, and Mitsubishi account for almost two-thirds of global carbon fiber production. Zoltek, Hexcel, and Cytec are headquartered in the U.S. and make a little less than a third of the world's carbon fiber.

Carbon fiber comes in various forms: raw thread or sheet fiber (both continuous fiber), and short-tow fiber that can come as either sheets, or clumps of chopped, very short-length fiber. Chopped fiber is used in products like pedal bodies. Sheet style is most common in bike frames, and is what it sounds like: continuous sheets of carbon fiber (called plies), which look like bolts of fabric and are typically about as wide, coming wrapped on giant spools. Raw thread is far less common in the bike industry; only a handful of bike makers like Giant and Time have the technical capability to work with raw thread.

We say "carbon fiber," but what we really mean is "carbon-fiber composite"—it's not the only material in your frame. That's because carbon fiber's incredible stiffness properties come at a price: On its own, it is quite brittle and prone to splitting and cracking. To preserve its abilities, it's suspended in a glue-like material called epoxy resin before it's molded, forming a composite material. Most carbon fiber used in the bike industry comes in this form, which is called "pre-preg."

The resin accomplishes two tasks. First, it holds the carbon together—both the individual fibers within a ply and, once cured, the orientation of two plies to one another. Second, resin adds crucial toughness and durability. It's slightly plastic, deforming under impact to help absorb sharp blows like a crash or a rock strike, without creating a crucial split between the fibers.

Every bike company liberally salts its literature on frame design with buzzwords like high-modulus or even "ultra high-modulus." But those terms aren't the bike industry's to toss around. Carbon fiber is graded by its stiffness, rated in terms of tensile modulus, or how much a material will deform under load. For carbon fiber, those ratings and the stiffness range each covers are set by the Japan Carbon Fiber Manufacturers Association (JCMA).

Most of the carbon fiber used in the cycling industry is standard modulus or intermediate modulus; on more expensive frames, higher grades do come into play. But that doesn't sound very sexy, so there's often a bit of grading on the curve as companies slip high-mod and UHM into their copy. Some companies are moving away from massaging the grading system altogether, simply creating their own carbon grading system with company-specific marketing terms like FACT or Advanced Grade.

A good carbon-fiber frame has a blend of carbon fibers of different types, each used in different places in a frame for very specific purposes. High-modulus fiber is expensive (thanks to all that processing), so bike companies judiciously use relatively small amounts in key areas like the downtube, bottom bracket, and chainstays to resist pedaling forces and make the bike stiffer. But they’re placed in the mold along with standard- and intermediate-modulus and high-strength fiber to create the kind of durability, performance, and ride quality a good carbon frame has.

Carbon fiber is a great bike material for two reasons. First, it's stiffer at lower weight than almost any other material we know of. Second, unlike metal, carbon fiber's stiffness can be finely manipulated; its stiffness properties apply only unidirectionally, or along the long axis of the fibers themselves, so stiffness can be tuned based on how the carbon-fiber composite is oriented, or placed in the mold. That's called anisotropism. Metals, by contrast, are isotropic, exhibiting the same strength and stiffness properties along any axis of the material.

Engineers use sophisticated software programs that take into account the grade of carbon, the resin, the shape, size and orientation of the carbon-fiber ply, and even where it's located in the mold. That's how frames are optimized for extreme lightness or rigidity, or both, but those programs and the graduate-degree expertise to run them are expensive. That—and the cost of the High Modulus carbon needed to realize the most heavily tweaked designs—is what makes the nicest carbon-fiber bikes so much more expensive than even their mid-line peers.

That classic, cross-woven, carbon-fiber look is actually not much of a performance attribute. A woven layer adds some stiffness, but one of its major purposes is to add a top sheet that's okay to ding, scratch, and dent, since most of the structural properties of the frame or part come from the unidirectional fiber underneath.

It also provides a cosmetic layer that showcases a builder's craftsmanship, since lining up the edges of a woven sheet in the mold to create a perfect seam requires significant expertise. Increasingly, though, bike and part makers are skipping a woven top layer altogether and just letting the unidirectional fiber serve as the cosmetic skin.

Custom framebuilding is often associated with steel and titanium, but every carbon frame, wheel, handlebar, or other part made with carbon-fiber sheet requires intensive hands-on work, whether the frame is a custom model from a small builder like Crumpton, or a production model from Giant or Trek.

The individual plies of carbon are often cut from a sheet by a computer-controlled machine, but that's about the only automated process throughout production: A carbon-fiber composite frame or part must be assembled in a mold precisely according to the layup schedule, a process that has yet to be automated. In smaller facilities, one person may lay up an entire frame. In larger ones, workers will create a layup for a specific portion of a frame, like a bottom bracket area, then pass the part along to other workers to lay up around tubes, and place in the final mold for curing.

Even the post-cure process—where a frame is broken out of the mold, cleaned up, and painted—requires close attention. Expert workers examine the frame or part for problems like wrinkles in the carbon, or spots where the layers aren't smoothly compacted, which could cause a failure.

It's a common misconception that once a carbon-fiber frame or part is damaged, it's toast. But like a steel or titanium frame, a carbon-fiber frame can sometimes be repaired. The process isn't much different: The damaged section is cut out and then bridged with new material that's cured, sanded, and painted to match. With the most expert carbon repairs, you may not even be able to see the affected portion.

But this is a job for experts. There are DIY carbon-fiber repair kits, but they’re primarily intended for products like kayak paddles. Bicycle frames are complex, load-bearing structures that have to take the stress of forces like sprinting and high-speed cornering. If your carbon-fiber frame is damaged, the manufacturer may have an in-house repair process, or you can go with a reputable third-party like Calfee Design. But under no circumstances should you attempt a repair yourself.

The rise of carbon fiber has allowed an explosion of distinctive frame styles. It's possible to identify a Pinarello or a Specialized just from a frame's silhouette. But that distinctive industrial design also makes frames particularly susceptible to copying by counterfeiters.

It's relatively simple to create a mold from a real version of a finished frame (or even detailed photos of the product). But the materials, layup, and construction methods may differ completely. Many counterfeiters lack the sophisticated manufacturing capability and will use cheap filler materials to mimic the outward appearance of the real thing in counterfeit bikes. As Specialized's former test lab director Santiago Morales said of counterfeits: "It's not an engineered product in any way; it's like a canvas that's painted to look the same."

Joe Lindsey is a longtime freelance journalist who writes about sports and outdoors, health and fitness, and science and tech, especially where the three elements in that Venn diagram overlap.

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