Carbon fibre
Carbon fibre
There are a few reasons why carbon fibre has become by far the most popular material for a performance road bike, but the central one is strength-to-weight ratio.
Carbon is incredibly light, which is how brands like Cervélo and Trek have managed to create frames that weigh under 700g, but strong enough that these bikes can actually be ridden to their limits in the toughest races in the world, not slowly down to the shops taking care to avoid potholes.
But while carbon is strong enough to withstand the wattage put through them in a sprint finish, it’s comparatively brittle, as you’d find out if you hit your toptube with a hammer (we can’t emphasise enough how much you shouldn’t ever consider doing this). Where metal will dent, carbon will crack and render your pride and joy totally unusable. The way to avoid this (other than not hitting your bike with a hammer) is to not crash and, specifically, avoid immovable objects like lampposts, parked cars or walls as these combined with the speed you’re travelling may prove terminal. But we suspect most of you actively try to avoid that no matter what bike you’re riding…
The reason carbon works for bikes is that the majority of different forces on a bike all work in the same direction, so the frame only needs to withstand forces in those certain directions. Metals are isotropic, that means that their properties are the same in all directions, so a metal bike frame will be far better at withstanding forces acting upon it in directions other than which is was designed for. In other words, if you design a metal bike tube to be very strong if you sit on it, you’ll get lateral strength thrown into the bargain too. Carbon, on the other hand, is bundles of carbon fibres held together by resin. It can be extremely strong in an intended direction, but you won’t necessarily get that strength in other directions. Take the toptube on Trek’s flagship 690g Émonda frame as an example. You can physically squeeze it so the carbon flexes vertically between your fingers, but it still has the necessary strength and lateral rigidity to be raced at the highest level.
Carbon has also allowed designers to be much more creative with their designs. Steel tubes are typically round and the classic thin-tubed design of years gone by was adhered to for weight reasons as much as anything, because framebuilders couldn’t afford to use too much material. But with carbon, you can make large, aerodynamically shaped tubes and not worry about weight, because it’s so light. Look at Canyon’s Aeroad as an example, a frame with a huge downtube like that made out of steel would quite simply weigh a ton, but the carbon frame weighs less than 1kg, and Canyon’s engineers have also been able to tweak the carbon layup for comfort, without impacting on its rigidity or aero prowess.
The ride properties of carbon are well documented. The best carbon frames are extremely stiff which makes for great power transfer at the bottom bracket and little to no flex anywhere on the frame (unless it’s been designed that way). The flip side of this is that they can be rather uncomfortable, but bike brands have become increasingly adept at balancing stiffness and comfort. In fact, a whole bike genre has been born out of it – the ‘endurance’ bike.
So if carbon fibre is light, stiff, versatile and (sometimes) comfortable, why would you choose anything else? Firstly, it’s not cheap – although it has become significantly more affordable in recent years. Or, rather, a top-level carbon frame isn’t cheap. As is the case with all frame materials, there’s good carbon and, well, less good carbon, and at the lower end of the market the argument for carbon again other frame materials, particularly aluminium, is less persuasive.
Still, it’s what the pros use and, for many riders that counts for a lot. All World Tour teams ride carbon bikes, but that hasn’t always been the case…