Everybody knows that carbon wheels look good, but do they actually have a significant impact on your performance? Well, yes as it happens. Not only do they improve the way your bike handles, but they also give you an aerodynamic advantage.
Written By Christopher Kautz
Race wheels are the best possible upgrade you can make to your current bike. High performance wheels can greatly improve how your bike handles, its aerodynamics and overall ride quality. While there are many different styles of race wheels for different types of racing, triathlon race wheels should be designed to provide an aerodynamic advantage because wind drag creates the majority of the resistance faced by a cyclist in a non-drafting triathlon.
One hallmark of aerodynamic wheels is a deeper rim than found on standard wheels. Manufacturers use carbon fibre to build deep, aerodynamic rims because it is strong, stiff and light. Constructing an ultra-deep rim from aluminium is unfeasible because the rim would weigh a tonne. There are two types of carbon fibre rims: those entirely constructed of carbon fibre and those that use an aluminium rim bonded to a carbon fibre structure. Understanding some basic differences between aluminium and carbon fibre can help you determine which of these two types of wheels will best suit your needs.
Unlike aluminium, the carbon fibre used to build wheels is made of two different materials, carbon and resin, which is why it is called a composite. The long strings of carbon fibre resolutely resist bending and give the material its stiffness, and the resin acts as the glue that holds them together. Carbon fibre is different from metals in a few key ways.
Shaping: A carbon fibre rim is formed by laying the material in a mould to give it its shape and is cured at high temperatures. Shaping the material with a mould, rather than welding tubes together, allows a manufacturer to develop components with clean aerodynamic shapes that are unattainable with metals.
Directional stiffness: Metals are equally stiff in all directions but carbon composites are not. The carbon fibres themselves are very stiff and resist bending, but the resin that holds them together is relatively soft and does not contribute significantly to the material’s stiffness. Since only the carbon fibres give the material its stiffness, it is only stiff in the direction the fibres are oriented. This has a massive impact on carbon frame design and also makes it very difficult to construct a carbon clincher.
Heat dissipation: Aluminium and other metals transfer heat quite well, but carbon fibre does not. As a result, wheels with carbon fibre rims do not brake as effectively as wheels with metal brake tracks. These characteristics of composites give full carbon wheels two primary advantages over wheels with an aluminium brake track bonded to a deep carbon fibre rim. The first, and more important of the two, is the aerodynamic shape. The wheel manufacturer can actually make a rim that has a superior aerodynamic profile when using only carbon fibre rather than combining a metal brake track with a deep carbon fibre rim. The full carbon Zipp 404 Carbon Clincher, built with Zipp’s proprietary Firecrest rim shape, shows the potential of wheels built only with carbon composites. According to Josh Poertner, the lead engineer at Zipp, this wheel has similar aerodynamic drag numbers to the Zipp 808 Clincher, which has a much deeper rim but is built from a combination of aluminium and carbon.
Weight is the second advantage of a full carbon wheel over an aluminium and carbon fibre combination. Even though aerodynamics is more important to performance than weight, all other things equal, a lighter wheel is a faster wheel because it accelerates more quickly. The biggest drawback to full carbon fibre rims is the braking characteristics. Since a carbon fibre brake track doesn’t dissipate heat as effectively as an aluminium brake track, the heat generated by braking builds up in the wheel and reduces stopping power. As a result, most athletes racing on carbon fibre rims use carbon-specific brake blocks to prevent heat from building up in the brake track. These blocks are also less abrasive and protect the rim from being damaged by a rough pad.
The question to ask yourself when selecting wheels is whether the full carbon wheelset provides enough added benefit for you, or if its limitations outweigh the benefits. If you are not comfortable changing brake blocks when you change your wheels, you should most likely look to a wheelset with an aluminium braking surface. However, if maximum aerodynamic advantage with minimum weight is your goal, then a full carbon fibre wheel will likely be the better choice for you, just as it is for most professional cyclists and triathletes.
Christopher Kautz is the president of the Master Bike Fitters Association, owner and founder of PK Cycling and one of the originators of the fit studio concept. His clients include numerous Ironman world champions, Tour de France veterans and Olympians, as well as thousands of age-group athletes. Visit Pkcycling.com.