How Durable is Carbon-Fiber? | Answers

By Karel Kramer, Photography by Adam Campbell

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Carbon Dating
Is there some info about the durability of the carbon-fiber some manufacturers use on their bikes? I’m not talking about “cosmetic” items, but major components like swingarms from Bimota, forks from Confederate or fuel tanks from Ecosse. I mean, rocks from the road can break them, or “light” falls could have consequences that wouldn’t happen with steel or aluminum. I’ve seen carbon-fiber getting old and fragile on bicycles. So, how durable are these super-light components?
Cesar M.
Austin, TX

Willie Amaradio is the owner of Lightspeed Performance Products ( Lightspeed makes protective carbon-fiber accessories, not major components, but he has done research into manufacturing frames and swingarms.

“With carbon-fiber the durability and strength has to do with the design of the part and the manufacturing process,” he says. “The nature of carbon-fiber is that it doesn’t fatigue like a steel part that flexes repeatedly, and then breaks. If you stay under the load threshold for a part it is fine and if you reach that threshold the part fails. It is unlikely that rock chips are going to bother a part, but crash damage might.

“The lifespan of a carbon-fiber part doesn’t have much to do with why we don’t see more swingarms or frames. Carbon-fiber is exceptionally rigid, and as we have seen with the switch from steel frames to aluminum, more rigidity is not always a good thing. To switch to carbon-fiber would require a whole new learning curve that would involve the whole bike, all the way to the tires. Considering the costs involved in manufacturing carbon-fiber parts, it doesn’t make sense to switch when current metal designs work so well.”

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So, unanswered is the important question of How the carbon component fails.

If a mission-critical part fails via stress beyond it's yield strength, is it a non-ductile, brittle, catastrophic  failure? Or does the part hold together after impact, at least long enough to keep the bike under rider control to make a safe stop? Think of a forged aluminum front wheel hitting a baseball-sized rock or deep pothole at speed coming round a blind turn. The wheel deflects ductile-ly, and does not fail catastrophically. Rider maintains control, or at least doesn't crash due to wheel failure.
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