Carbon Fiber Vs. Aluminum Rim Braking

Have you heard any of these statements when talking about carbon wheels?

1. Carbon rims are only for racing.
2. Carbon rims are not safe when braking on long descents because they melt.
3. Carbon rims are not as tough as aluminum rims.
4. Carbon rims have poor braking performance when compared to aluminum rims.

We’ve heard them, too. The truth is, these statements are false, if you are buying wheels from a reputable wheel manufacturer. We will go into more depth on the aforementioned statements, but first, let’s start with a little history and carbon fiber tech talk.

Rims were originally made from aluminum, not carbon fiber.

At the time, the majority of bikes used rim brakes as their braking system. To slow down the bike, rim brakes use brake pads that squeeze the rim surface which creates friction.

For aluminum wheels, a good, durable rubber brake pad works well and heat is not an issue. However, for carbon fiber rims, heat must be properly managed during braking. Let’s discuss how carbon fiber rims are made and what created the heat issues.

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.

Well, if it were 1997, it wouldn’t be a bad idea to only race on carbon wheels, based on what we just learned about early epoxy resins being brittle.

However, today, carbon fiber rims are tougher than aluminum rims as they are able to take more impact force. For this reason, we recommend you ride your carbon wheels all the time. There are two major benefits to this.

1. If you’ve paid for a nice set of wheels, then riding them and enjoying them is a good idea.
2. If you plan to race, then we want you as familiar with your gear on race day as possible. If you’ve been training on a set of wheels all year and break out race wheels for race day, you will not be as comfortable as you could be with these wheels.

I am guessing if you’ve been following along, you already know where this statement came from. Original carbon rim design did have issues with heat. Braking on long descents with rim brakes creates heat on the rim which, with original carbon fiber construction, took the rims to the glass transition temperature resulting in rim failures.

Today, because of pre-preg and updated epoxy resin, you don’t have this braking/heat issue when buying wheels from a reputable manufacturer.  A word of caution: you can still find cheap rims being made that cannot stand the heat produced by long braking.

All FLO wheels pass the ISO 4210-7 test standard for braking and heat. Make sure any wheels you buy meet this standard.

This myth also goes back to the days when resins were brittle. Today, resins allow for the production of durable parts. In fact, the number of dented or damaged aluminum rims I’ve seen over the years far outweighs the number of issues seen with carbon rims.

One thing worth noting is that when aluminum and carbon fail, they fail in very different ways. Aluminum typically bends and rarely cracks. For the most part, it does not look very catastrophic but is most likely unsafe to ride. Carbon fiber on the other hand looks really messy when it breaks. Typically you see carbon fiber fragments and it looks rather catastrophic. I don’t have proof of this, but I do feel that the visual aspect of failed carbon help perpetuate the myth that aluminum is still tougher than carbon.

Yes, this also dates back to the original rims. When designing a rim brake system for carbon wheels you have to think of both the epoxy resin system and the rubber compound of the brake pads. When these are designed together they work very well. We consider the braking performance of our carbon rims to match that of our aluminum rims. For the best results, we recommend our carbon brake pads that ship with all rim brake FLO carbon wheels or Swiss Stop Black Prince pads. For our aluminum rims, any good alloy pad will work.

Again, disc brakes are changing things. With a disc brake wheel, the rim material doesn’t matter when it comes to braking performance.

If you are worried about buying carbon-fiber rims for any of the reasons above, we understand why. The stories have been spread for nearly three decades. Today, when buying from a reputable manufacturer, you do not have to worry about toughness or braking performance. Carbon fiber wheels can be your everyday wheels and race wheels. So if considering between carbon fiber and aluminum, we will point you toward a set of carbon wheels. The benefits are vast, but to just name a few: aerodynamics, compliance, and weight. Still want to know more? Feel free to email us or give us a call and we’d be happy to answer any questions you have.