The finished parts that we casually call “carbon fiber” are more than that. They are composites made of super-strong crystalline carbon fibers, held together by an epoxy resin. The proper name is Carbon Fiber Reinforced Plastic, or CFRP. Luckily, no one insists on it.
The idea of embedding fine, high-strength fibers in a plastic matrix goes back to 1920, when A.A. Griffith, employed in England’s Royal Aircraft Factory, showed that the low strengths of practical materials resulted from surface or internal defects. Without these defects, considerations of atomic bonding would predict tremendously greater strengths.
A good example is glass, which in handling accumulates microscratches on its surfaces. Application of even moderate stress causes one or more of these to open and propagate as a crack. Also revealed by Griffith’s analysis was that below some critical size (the Griffith crack length), such microcracks did not propagate. Herein lay a path to practical super-strong materials—to produce fibers too small to contain defects of that size.
Fiberglass-reinforced plastic (FRP) found application as radomes during World War II, and fiberglass-bodied cars such as the Kaiser Darrin and Chevrolet Corvette debuted soon after. The beautiful, hand-beaten aluminum fairings of racing Guzzis and NSUs of the 1950s disappeared, replaced by easily molded fiberglass. You coated the inside of a mold with mold release, then gelcoat, then resin, after which you could lay in and “wet out” one or more layers of woven fiberglass cloth. Thousands of boats were built in similar fashion, but the method of fiberglass application was different: the dreaded “chopper gun,” which spewed forth a mixture of prickly chopped-glass fibers and resin.=
A fixture of the 1960s and ’70s California bike-racing scene was the one-man fiberglass shop. Its proprietor wore jeans so covered in half-cured resin and fiber that they stood up by themselves, and in a distressing number of cases, his mind had been altered by the fog of MEK peroxide (catalyst for the polyester resin used in fiberglass work) in which he spent his days.
Even stronger fibers were on the way. When in doubt, look at atomic bond strengths and pick a likely element. Carbon got the nod. Let’s extrude tiny fibers of polyacrylonitrile (PAN) and then heat the daylights out of it in a nonreactive atmosphere. Everything but the carbon is driven off, and the carbon assumes a strong crystalline form. Or, do the same with pitch—a heavy hydrocarbon.
As with so many things, the idea is just the beginning. When in 1976 an “information officer” at AVCO handed me a demo piece of unidirectional carbon prepreg (fibers already embedded in a layer of uncured resin), I was naturally thinking about carbon reed valves for two-strokes. Twenty years later, Erv Kanemoto was trying such valves in his Honda NSR500. And 41 years would pass before Boeing rolled out its 787 Dreamliner, most of whose structure is CFRP.
Carbon fiber is available in many forms. So-called tow is bundled fibers wound onto a spool. John Britten used tow to make the “bones” of his “skin-and-bones” chassis structure of the early ’90s—wetting it out with resin after winding it in place.
Fibers can also be woven into fabric, and it is this black fabric, seen through a perfectly smooth resin surface, that tells us if a nonstructural part such as a fender or rider’s heel guard is made of CFRP. Unfortunately, the stiffness of carbon prevents it from lying limply, conforming to a mold as fiberglass does. To force it into place, it is “bagged”—a vacuum pump pulls the air out of the bag, and atmospheric pressure holds bag and cloth in place against the mold surface. Best cures take place in a heated autoclave.
For highest strength, CFRP laminates are made (like tires) from unidirectional fiber prepreg sheets whose fiber direction and number are planned to deliver the desired strengths in chosen directions. Prepreg has the advantage that the correct volume ratio of fiber-to-resin is provided in advance—no “wet-out” required. The sheets are coated with tackifier to enable easy assembly of the laminate. Prepreg is held under refrigeration until use to prevent cure from occurring in storage.
The tremendous strength of CFRP has delivered a large improvement in racecar-driver safety. In the days of aluminum structural “tubs,” crashes burst or crushed fuel bladders, leading to fire and loss of life. Cured high-strength CFRP can have the tensile strength of alloy steel with one-fifth the weight.
Why doesn’t every MotoGP bike have CFRP structure? There are two reasons, one of which was known in advance. First, the need for elaborate molding and curing arrangements meant that with a CFRP bike, Kenny Roberts and Kel Carruthers could not have sawn off their YZR Yamaha’s steering-head and welded it back on at a better angle. With CFRP, the teams would not be able to quickly fabricate improved chassis midseason—that would require changing all the tooling. Second, although full and informed use of directional materials has enabled wonderful aerostructures, the CFRP bike chassis built so far have been so stiff that they give no warning of grip loss.
Speaking of Ducati’s 2009 pyramidal airbox/steering-head beam chassis, rider Casey Stoner says, “On that thing, you’re in the corner doing exactly the same thing you did last lap, but now you’re on the ground and don’t know how you got there.”
The chassis problem is that three different stiffnesses are needed:
- High longitudinal bending resistance to handle braking force
- High twist resistance to keep the two wheels in plane
- Sufficient lateral flexibility to act as a primitive suspension when leaned-over in corners.
The words of long-serving 500 GP engineer Mike Sinclair deserve our attention: “Something must isolate tire grip from the inertia of the engine.” Rigidly attaching fork and swingarm to the engine has not worked. It might be that easy-to-use predictive software will appear that can guide the design of CFRP chassis to provide the required performance. Until then, teams will keep sawing and welding the heavier materials they’ve known for years.n