Putting rim brake-equipped carbon clinchers to the test

Test from Alto Cycling compares carbon wheels' ability to dissipate heat

Video Wheels
Putting rim brake-equipped carbon clinchers to the test

This is what can happen when a carbon wheel does a poor job of dissipating heat.

While you can make a strong argument that rim brakes should be done away with and replaced with disc brakes in all cases, there are still many cyclists who haven’t (and likely won’t) make that switch anytime soon. Be it for reasons of weight, ease of use, or simply tradition, the road bike rim brake remains ever popular. The same can be said of carbon wheels, which offer the benefits of lower weight and better aerodynamics.

Unfortunately carbon rims and rim brakes don’t always go well together. This is largely due to the material’s lackluster ability to dissipate heat. And when carbon rims get too hot, two ugly possibilities arise: tire failure and total wheel failure. The results can be terrifying and may cause great injury to the rider. Indeed, it’s a safe bet that most people reading this post know someone who’s had a catastrophic experience with a carbon wheel.

With all that in mind, the team at Alto Cycling decided to do some testing, which you can view and read about below. It must be said that RoadBikeReview has not independently verified any of the information below. But outside appearances indicate that it was a fair test. First up here’s a short teaser video of what you can expect to see:

Now here’s the full test (video running time ~18 minutes). As you’ll see, testing took place in 20-minute phases — or until failure. Phase 1 utilizes a 7-pound weight applied to the brake lever, mimicking braking forces in the real world. For each additional phase the brake pads are replaced and the weight on the lever is increased by 2 pounds. All tests are run at the same 1200-watt output supplied by a motor, allowing wheel speed to vary based on brake force. Press play to see more.

Finally, here’s a synopsis for the rationale for the test and testing protocol from Alto Cycling CEO Bobby Sweeting. Again these claims have not been verified by RoadBikeReview. To learn more about Alto Cycling, please visit altocycling.com.

We build and program all of our own testing equipment at Alto, and we’ve been doing various brake track testing on our rims for the past two years. We test in 1 minute intervals with 30 second rest periods, and our 2017 rim model would run for about 5 cycles before delaminating. We never saw a failure in the field, but were curious if it would be possible to create a carbon clincher that would be completely unaffected by heat, even in the lab.

We began processing different resins with different additives (which are now proprietary and will not be disclosed), and researching ways in which we could increase the onset temperature of the resin. Essentially, that is the point at which the resin begins to soften. We then decided to filament wind the brake track while infusing these additives, which gives the fibers much tighter compaction and allowed the resin to flow more evenly throughout the structure. In our first round of prototype testing we were failing tubes and rim strips (more on that later), but were never able to fail one of the new rims.

Our retailers and customers would constantly ask how our rims compared to our competitors in brake track performance, but we honestly didn’t know. There was zero information online regarding this sort of thing, and it seemed like everyone was just buying into the marketing from each brand. We’d hear anything from 150 C up to 1000 C for resin stability, and it goes completely unchecked. So manufacturers could create anything and say anything, and there was nothing in place to ensure that these products would be safe enough to ride in mountainous terrain. So we wanted to show off our new product while simultaneously creating a metric that would display how rims truly perform under the same braking scenario, and how they would delaminate (if at all).

We reached out to Ryan Mason at Spark Wheel Works to see if they would be interested in this sort of data, and if they would want to be involved. As one of the top custom wheel builders in the country, he saw the value in learning this sort of information. Spark came on board as a sponsor that could provide us with new rims from each brand that they have access to, which is really what allowed us to do this test. We’re a small R&D brand with very little marketing budget. So without Spark, I highly doubt that we would have been able to make this video. It should also be said that we didn’t even know what the results would be when we partnered with Spark. We had planned to release this video regardless of how our rim compared with our competitors, because everyone involved with this project felt as though it was information that customers deserved to know.

We wanted to be absolutely sure that we were testing the heat capacity/dissipation of the rim, and not the tube, rim tape, etc. We didn’t want any premature failures other than the rim itself. In the prototype testing of our own rim we found two extremely interesting bits of information: a nylon rim strip would melt, and a small tube would fail pretty quickly. So the first thing we did was develop our new high temperature tubeless tape, which has a melting point of 915 F. That solved that issue. For the tubes, we saw a massive difference when we switched to a 25-32c Continental tube, as opposed to their 18-25c option. Even though we were testing with 25mm tires, the wider rims allow that tire to have an actual width around 27mm. So the 18-25c tube was too small, and the wall of the tube was failing at sustained temperatures over 200 F. So, as two side notes for customers to take away from this test: make sure you’re using quality rim tape and that you’re using the largest tube possible for your setup. Every rim in this test used our rim tape and the same sized Conti tube.

There are 100 ways that we could do this test, but we wanted to focus on eliminating as many variables as possible so that we could focus on the rim. That meant standardizing the brake pads, and using the same generic carbon pad on every rim. In this case, we used SwissStop Black Prince pads. Of course the times and temperatures will be different with brand specific pads, but we found that as long as the pads are the same across the entire test (regardless of what is used), the actual results of how brands compared with one another remained the same.

We also decided to let the speed of the wheel be a floating variable, and to set the power input from the motor at 1200 watts. This way, the total energy into the rim would be constant, and the speed would simply be affected by whatever the rim manufacturer was doing to their brake track. We felt that this would be more accurate than controlling wheel speed and forcing the motor to do more work on some rims than others.

We also decided to measure the cooling curve of each rim after failure. We just left the software running so that we could measure the time that it took for the rim to cool down to 120 F, so that we could see how quickly heat would dissipate from the rim. This is important because the performance of a rim is more than just its total heat capacity, and it’s unlikely that people will simply hold the brakes all the way down a mountain. They’re going to be on and off of them, so the rim has an opportunity to cool itself between braking. The faster it cools, the better chance that rim has of making it to the bottom. So two rims that delaminate after 3 minutes of constant braking are not necessarily the same under real world situations, and we wanted to display that data as best as possible.

At the end of the day, we were completely shocked at the results of our new rim. We expected one or two other manufacturers get through phase 1 of the test, and we had no idea that our rim would do as well as it did. But I think it’s safe to say that we’ve completely eliminated the risk of a carbon clincher heat failure for all of our rim brake products. We make great disc brake wheels, of course, but I’m personally still a fan of rim brake bikes from an ease-of-use and performance standpoint.

I’d also like to be clear that this was simply one way to do this test in order to collect a specific type of data. We haven’t tested every rim with their brand specific pad, and we only tested one rim of each brand. So results in the field could vary, but we think this test gives a snapshot into the reality of carbon clincher heat transfer and braking performance. And we want to test as many brands as possible, or more rims from the brands that we’ve already included

So if any company feels left out or if anyone wants to be retested, please send them our way (or fly to Sarasota!) and we will do it all again. The more open and all-encompassing this can be, the better it is for the customer. At the end of the day, that’s what it’s all about.

About the author: RoadBikeReview

RoadBikeReview.com is an online community of cyclists who share a passion for the sport. Visitors of the site regularly purchase gear to upgrade their bikes, share inspiring photos of rides, and keep up to date with the latest industry and technology news. Which products perform best? Where to buy them? Where to ride? How to ride better? Cyclists come to RoadBikeReview.com for the answers.


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  • Terrence Bennett says:

    Interesting. However, some wheels (ie. Reynolds and Mavic) require that you use their brake pads to improve performance. I wonder if that would increase or reduce the chance of failure on those wheels.

  • Orrin says:

    I am not going to disc brakes anytime soon because I am not going to spend many thousands on a new bike!!!! It has nothing to do with weight, use, or tradition.

  • Tony says:

    That busted up wheel is a Boyd. That sucks.

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