When it comes to bicycles, there is nothing more cutting-edge than carbon fiber. Carbon fiber has promises of lighter weight, excellent stiffness, shock dampening, and so much more. People tout carbon fiber as the be-all and end-all of bicycle design — the answer to every quest. But is it really so great?
Carbon fiber is used in almost every part on some bicycles today. Frames, forks, cranks, handlebars and stems, pedals, seatposts, saddle rails, and hubs and rims. Some wheels are now even being made with CF spokes. All together, these things can combine to build a bike that weighs as little as 15 lbs.
When it comes to building bicycles, three things one needs to consider are strength, stiffness, and weight. Carbon-pushers love to extol the virtues of carbon fiber as being both stronger and stiffer than steel. I've seen sources saying CF is 5x stronger and 2x stiffer than steel. It is also a fraction of the weight of steel for a given volume. Ideal for bicycles, right? Wrong.
What the carbon-pushers tend to leave out is that these claims of superior strength and stiffness to weight ratios are based on lab test of raw materials. For example, a strand of carbon fiber can withstand more pounds of tension force per square inch without breaking than a piece of steel of equal weight. However, all these lab test results don’t necessarily translate into superiority when built into a bicycle. Nobody really rides “carbon fiber” as it is tested in the lab. Carbon fiber is, as the name implies, a fiber — think “string” or “thread” — it is really only strong when in tension (pulling) but has no resistance to compression (pushing) or torsion (twisting). Nor does it have the ability by itself to hold a shape (unless you count "string-shaped" as a shape). Carbon fiber, as used in tubing and for bicycles, must first be woven into some kind of fabric, mesh, or perhaps a felt-like material — then soaked in some type of resin (plastic) and molded to hold it all together and give it shape. The orientation or grain of the fibers is important so the frame or the component can withstand the types of forces put on it. Unfortunately, a bicycle as a structure undergoes a number of different forces from many different directions as the bicycle is ridden — from twisting in one direction to the other, bending, compression and tension. These forces cannot always be predicted accurately (even in sophisticated computer models — although they keep getting better). And if the bicycle is subjected to forces that cannot be predicted (such as an accident or crash) the result can be catastrophic. A small nick or scratch in the plastic that holds all that carbon fiber together (a scratch perhaps barely visible to the eye) can spread quickly and without warning, suddenly leaving the rider riding a bicycle that has no structural integrity. Imagine riding a bike made of, well, string.
Stronger than steel? Hmmmm. A steel bike can be ridden with a dent, and that dent will not spread or break apart without warning. A sharp dent — one that creases the steel — can turn into a crack over time if repeatedly ridden. But that crack will develop gradually and won’t suddenly appear and then break apart in the middle of one ride. Yes, if one continued to ride the bike, it could fail eventually. But who is going to keep riding the bike when they can see this crack developing?
Occasionally, riders of carbon fiber bikes will claim that they were “just riding along” when suddenly the bike broke apart. First, it is probably more likely that the bike was in a minor incident — and after being inspected by the rider, it appeared that nothing happened to the bike. But the damage could be basically undetected, hidden deep inside the layup of carbon. Then on another ride, maybe a day, maybe a couple weeks, or maybe even a couple of months later (after the incident is long forgotten) the tiny imperceptible damage suddenly spreads and the bike is broken. Still, it isn’t all that uncommon that the CF bike (or component) fails and there was no accident damage as the cause. It could be poor design, or just a minor flaw in the manufacturing. Often, the flaw can be hidden and completely undetectable.
Crash damage involving CF is spectacular. Whereas steel bikes, and to a similar extent aluminum, may get bent, twisted or dented in a serious crash, a CF bike can break apart completely. It’s possible that the steel bike may still be un-rideable after a serious crash, but it is almost certain that the CF bike will not be ridden again.
And then there is repairability. A damaged steel frame (particularly if built with lugs) can be repaired at the fraction of the cost of replacement. And the repair will not compromise the frame’s integrity in any way — with a new paintjob, it can literally be as good as new. I've heard of people who say they can repair carbon frames, but I cannot imagine there would be any way to ensure that the repaired section would be truly as good as new.
A decently built steel frame should last a lifetime. But a CF frame really needs to be seen as a disposable purchase.
What about weight? Certainly, a bike built with a CF frame and with some CF components can easily shed 4 - 5 pounds from a complete bike. That weight is instantly noticeable when one picks up the bike. But I've covered in other posts how that weight loss doesn't necessarily make as much of a difference as the perception. If one actually races, it might be important -- but if one doesn't race, it's pretty insignificant. Speaking for myself, I have other priorities that make shaving a few seconds off the ride time pretty meaningless. Factor in the total weight of the bicycle and the rider, and those 4 - 5 pounds mean very little.
I go round and round with fellow cyclists on the point of carbon fiber frames, forks, and components. Some probably get sick of hearing it, but CF is simply not for me.