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The Evolution of Carbon Rims

by John Neugent

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John Neugent probably knows more about bicycle wheels than anyone else alive. Maybe more about bikes as well. He's spent his life in the bike business, at every level. He now owns Neugent Cycling, a firm devoted to delivering world-class equipment at the lowest possible price. If you are in the market for a set of wheels, please, check out John's site. He really knows his stuff. —Chairman Bill

At the turn of the century, carbon rims were really Formula One products. That is, they were almost exclusively for racing and more specifically triathlon racing. They were extremely fragile, and difficult to build. One well-known brand had only one builder in their factory who was approved to build wheels for their sponsored professionals.

As carbon rims developed they became much better, not so much with the carbon, but with the glues that hold the carbon in place. There is even a story about one name brand who gave a set of carbon wheels to everyone on a pro tour team and they were all broken after one training ride. But they continued to evolve.

When I first started building with carbon rims – around 2005 – I bought rims from a well-known USA brand. Even at that point, the rims were extremely difficult to build with and were incredibly fragile. A spoke tension slightly too high would cave in the side walls. To make the cavity in the middle of the rim you need a bladder and that bladder stays in the rim. Back then it was common to need to poke a hole through that bladder to insert a nipple. If you lost a nipple in the rim cavity it could take a hour to shake it out. Also, since the spoke bed was very uneven it took a long time to adjust the spokes because the uneven spoke bed effectively make each spoke a different length.

When you build a carbon rim wheel it’s unlike building an alloy wheel in that the carbon doesn’t bend. On an alloy wheel the spoke tension can, and will, bend the rim. On a carbon wheel you center the hub in the rim. Therefore the quality of the rim is very important.

But as this evolution continued the rims kept getting better and better.

Carbon Clicher rims

Carbon clincher rims have come a long way in a short time,

Almost all low spoke count wheels (a rear wheel with 24 or less spokes) wear out in the same spot. The drive side rear spokes crack the rim. In the last 10 years, I haven’t seen one of my carbon rims crack a spoke hole. I find this pretty amazing.

Therefore, I believe, carbon rims are stronger than alloy with one major exception. They can melt.

The carbon in the rim is held together with glue. If the glue heats up too much it will melt the rim – or in the case of a tubular rim, melt the glue holding the tires on. This happens at a very high heat (typically about 220 degrees Celsius or about 430 degrees Fahrenheit). The reason for this is that carbon doesn’t dissipate heat like aluminum alloy. All of the energy in stopping a bike is converted to heat by the brake and rim interface. So all of the energy going up a hill is turned into heat slowing down on the descent.

There are ways to mitigate this. Fanning the brakes (even a little) makes a big difference. Grabbing onto the brakes at the last moment before a downhill curve puts a lot more heat into the rims. I find that people who are aware of the issue and brake accordingly have no issues but people unaware or unaccustomed can occasionally melt a rim.

The good news is that if you are careful and know what you are doing, carbon wheels are as durable, and I would say more durable, than alloy wheels.

John Neugent was was one of the first to establish quality hand building in Taiwan around the turn of the century. He now owns Neugent Cycling, a firm devoted to delivering world-class equipment at the lowest possible price.