Carbon fiber rims are made of two components; carbon fiber and epoxy resin. When molding a carbon fiber wheel, pieces of pre-preg carbon fiber are laid into molds. Pre-preg is raw carbon fiber that is impregnated with an epoxy resin. Prior to the invention of pre-preg, raw carbon fiber was laid in place and epoxy resin was applied over the carbon fiber, which was a messy and difficult process. Pre-preg makes for a much cleaner and workable manufacturing process.
Once the pre-preg carbon is laid in the mold, it is placed in an oven where it goes through a prescribed heating cycle know as a ramp cycle. The oven with the mold starts off at a low temperature, ramps up to a set point temperature and then lowers to an exit temperature. Each stage has a predetermined time. If you were to graph the temperature over time, a ramp shape is created, hence the name ramp cycle.
When the carbon fiber exits the mold, it is a hard finished part. However, all epoxy resins have what is called a “glass transition temperature” where the hardened epoxy turns from a glass back to a liquid. When this happens, the carbon fiber object becomes soft and will fail.
Epoxy resin design and selection is critical for carbon fiber rims. Most of the myths mentioned in the opening of this article date back from when carbon rims were first being manufactured in the 90s and early 00s. At that time, epoxy resins could only handle the high temperatures needed for brake heat if they produced a brittle part. You could not find a resin that produced a durable rim that also handled handled high heat.
Today, epoxy resins have made major advancements and we can produce rims that handle high heat and are more durable than aluminum.
Now that we understand more about carbon rim manufacturing, let’s discuss the myths we discussed in the opening paragraph above.