JRA with the Angry Asian: The folly of rim-brake carbon clinchers

Alto Cycling has just announced a new carbon clincher rim, which the company claims can handle far more heat than any other rim it’s tested. That’s all well and good, but carbon clinchers have now been around for nearly twenty years. Why are we still talking about braking-related heat failures at all? Shouldn’t the bike industry as a whole have this figured out by now? And why are riders willing to accept anything other than ironclad guarantees that any carbon clincher rim is safe to ride?

On the surface, Alto Cycling’s test protocol was quite grueling. Each wheel was driven by an electric motor at a sustained 1,200-watt output, and 135lb of radial load was applied to simulate bike and rider weight. For the initial 20-minute round of testing, the brake lever was squeezed with a constant 7lb of force. If the wheel survived that, the lever was increased to 9lb, and then the test continued for another 20 minutes.

A teaser of Alto’s heat-failure test. Full version below. 

The same make and model was used for the tire, tube, rim tape, brake caliper, brake lever, and brake pads throughout, and new pads were used for each round of testing. All of the testing was conducted by independent wheel builder Spark Wheelworks.

Catastrophic carbon clincher rim failures like this are truly rare in real-world riding, and it’s unclear if Alto Cycling’s test protocol is a reasonable approximation of what might happen on the road. But that said, heat-related failures are still far more common with carbon fiber clinchers than ones made of aluminum. Photo: Alto Cycling.

Perhaps not surprisingly, Alto reports that its latest carbon clinchers performed the best by a wide margin. What was surprising, however, was that they were the only rims in the test that survived the complete testing cycle without catastrophic failure. Among the also-rans were such notable brands as Zipp, Bontrager, Enve, Mavic, Knight Composites, Roval, Boyd Cycling, and FSE, all of which failed Alto’s test with visually (and audibly) spectacular results.

“Alto tested for 40 minutes, through both phases, without a blowout failure,” said Alto Cycling CEO Bobby Sweeting. “There was some small amount of delamination that we’ll report, but it wasn’t very significant. I was hoping that another brand would come close to this result, but the second best rim only last 5.5 minutes in phase 1, so it does make the test look very biased. So we will openly invite other brands to replicate the test or fly to Sarasota to re-test their products. We’ll even pay for the plane tickets!”

A video showing Alto’s full test. 

Alto should absolutely be applauded for the endeavor, and the fact that its rims survived such an ordeal is impressive and reassuring. But the testing protocol is subject to criticism. For example, only one sample per wheel model was used, which is hardly enough for a statistically valid conclusion. And according to Sweeting, the 1200W input force was chosen more for reasons of practicality, not empirical data collected out in the real world.

“The motor input was simply a number that was able to bring the wheel up to a reasonable speed and input enough power to fail the rims in a short amount of time,” he said. “We didn’t want any of the tests going over ten minutes, ideally, because it would make the video longer.”

Carbon fiber composites rely on the resin matrix to hold the fibers in place. Once that resin gets hot enough to soften, though, the rim quickly falls apart. Photo: Alto Cycling.

Also, the same type of brake pad was used throughout, whereas most wheel companies these days are very specific about what type of pads should be used to minimize heat build-up.

“We went back and forth on the pad issue a lot,” said Sweeting. “What we found in our testing was that heat building is being caused by one thing only: friction. There’s no chemical reaction taking place between the pad and the rim resin, so a brand-specific pad doesn’t mean anything, except that it has a lighter density and results in less friction. So, for example, if we used an Enve pad for the entire test, it would result in the Enve rim lasting 10% longer, but every other rim would also last 10% longer. So the results would be the same; the test would just last longer.

“What we didn’t want to do was have a different pad on every rim, because the data wouldn’t mean anything,” he continued. “We could take a really low-quality rim and test it with a cork pad, and it would last for an eternity. Then we could test an Enve rim with an Enve pad and it might last only five minutes. Does that mean the first rim is of higher quality? It’s impossible to quantify the rim data without eliminating the pad variable, so we had to standardize that in order to get real results.”

Fair enough, but what’s most disturbing to me is that any of this is a topic of discussion at all.

HED is one of the biggest names in high-performance road wheels, but despite intense consumer demand, founder Steve Hed was never willing to offer a carbon clincher due to the safety issues. It’s only just recently that the company released the Vanquish full-carbon clincher, but it’s only sold for use with disc brakes; no rim brakes allowed.

Lew Composites debuted the first carbon clincher to the world in 1998. The technology was touted as being the Next Big Thing: lighter and stiffer than aluminum, while also allowing for far more complex and aerodynamic shapes. Naturally, it wasn’t long afterward that most other wheel companies followed suit (and in fact, Reynolds bought Lew Composites soon after the wheels were introduced). But as appealing as the technology sounded on paper — and I’ll freely admit to being one of the doe-eyed cheerleaders of the technology earlier in my journalistic career — it also didn’t take long for people to uncover the associated downsides.

Carbon fiber is an inherently poor friction surface, particularly when wet, and the issue is compounded by the fact that, as a material, it’s also terrible at shedding whatever heat has accumulated. Moreover, while carbon fiber can be incredibly strong, its structural integrity is dependent on the resin matrix staying sufficiently solid to keep the fibers from moving.

This isn’t as big an issue for tubulars, but a carbon clincher rim is under much more stress; tire pressure always wants to pry the rim walls apart. Usually, problems originate at one “hot spot” where the sidewall width is slightly greater than elsewhere on the rim: often characterized as “pulsing” under braking. That spot generates more friction (and, thus, more heat) with each wheel rotation under braking, and since that heat has nowhere to go, the local temperature continues to increase. Once the rim gets hot enough that the resin begins to soften, the rim loses its structural integrity, which is often accompanied by a tire blowout. If a crash doesn’t occur shortly thereafter, you can consider yourself lucky.

The question of how well carbon clinchers can handle intense heat goes away when disc brakes are factored in, but until the technology is truly universal, it’s still something that needs to be considered.

In fairness, carbon clinchers have gotten a lot better since those early days, and the dramatic failures that once littered internet forums are fewer and further between (although they haven’t been entirely eliminated). And whereas braking on carbon rims was once an exercise in faith — pull the lever, and hope for the best — a few standouts, such as Zipp’s Showstopper-equipped models, have demonstrated that it is actually possible to get good braking performance out of a carbon clincher rim in both dry and wet conditions.

But that sort of performance is hardly universal across all carbon clincher makes and models, and there still exists a healthy amount of debate on what constitutes a sufficiently rigorous test.

I recently brought up this subject with fellow tech journalist, Matt Phillips, current test director for Bicycling, and a 22-year veteran at both that title and former sister publication Mountain Bike. While his view of rim-brake road carbon clinchers differs slightly from mine, the conversation sparked a number of interesting discussion topics.

For sure, aluminum can’t match carbon fiber in terms of what can be done in terms of shaping. In particular, current technology makes it impossible (or impractical) to use aluminum for deeper-section aerodynamic profiles, although new rims like the A-Force Al33 are continuing to push that envelope a little.

For example, Alto’s 1,200-watt drive load is double what Enve uses for its testing. Is that too much? Perhaps, particularly given that Sweeting admits that the input load was chosen more to shorten the test duration, not empirical data based on what riders experience in real-life conditions. But what constitutes “real-life” conditions, and whose riding characteristics should be simulated in the lab? How accommodating should rim and wheel companies be of inexperienced and/or heavier riders who might subject wheels to more abuse than average?

“It’s always fascinating to see what people do with equipment in the real world,” Phillips said. “There obviously has to be some cushion, some safety overlap. But I imagine it gets very tricky to figure out where to put that line. Like, do the guys at Zipp assume that most people who buy a $4,000 wheel set are experienced, and have reasonably good braking technique? How much cushion do they build in? And how much do they sacrifice (weight, price, aerodynamics, etc) by building in that safety net?’

“Where is the line between safe and unsafe?” he continued. “Is it a reasonable expectation to assume carbon rim brake clinchers should never fail, no matter the user, condition, or circumstances?”

Aluminum is already a superior braking surface as compared to carbon fiber, and recent developments in surface treatments have expanded that gap in performance even further.

Unquestionably, consumer demands for ever-increasing performance has driven every bicycle and component company to stray closer and closer to that razor-thin line. This isn’t just for carbon clinchers, either: bicycle and component weights continue to drop across the board, while performance continues to increase, often at the expense of durability. Structural failures on any part of a bicycle are never a good thing, but they can carry particularly disastrous consequences when it comes to heat-related carbon clincher rim failures.

Almost by definition, those incidents occur at high speeds and on steep and/or long descents, and the concurrent tire failures yield near-instantaneous losses of control. Even worse, this usually happens with little-to-no advance warning.

Three years ago, in a column I wrote for BikeRadar, I called on the industry to develop a universal testing and certification protocol for all rim-brake carbon clinchers sold to the public; it still hasn’t happened. In the meantime, carbon clincher rim safety is still largely an unknown as far as I’m concerned. Has a company done its due diligence in terms of testing? How rigorous was that testing? How closely do those testing conditions mimic how people ride?

Also three years ago, Velo magazine published a feature characterizing carbon clinchers that characterized them as being, “safe under anything but extreme riding conditions.” Exactly what riding conditions constitute “extreme”?

Zipp’s “Showstopper” sidewalls feature a roughened surface, embedded silicon carbide ceramic particles, and molded-in grooves that are designed to boost both dry- and wet-conditions braking performance. It works remarkably well, but it’s not a technology that’s widespread among carbon clinchers in general.

I don’t have the answers to any of these questions. It’s also important to note that Alto’s test results don’t necessarily mean that every other carbon clincher is unsafe. It could very well just be that Alto’s test is more demanding than it needs to be, and there are countless carbon clinchers already out on the roads that seem to suit people’s needs just fine.

But in the process of touting the safety of its own wheels, Alto is also calling into the question the safety of the other wheels that were tested, especially given the seemingly huge disparity in heat capacity.

That all said, I find the mere fact that carbon clincher safety is still a major discussion point deeply troubling, especially given that they’ve now been around for nearly two decades. It’s one thing for a product or technology to continually improve over time; it’s another for proponents of that product or technology to still be learning how to make them safe for people to use.

Alto Cycling says its new carbon clinchers are ultra-tolerant of heat, and the company should be applauded for continuing to pursue advancements in that area. But that said, the fact that this is a topic at all is concerning. Photo: Alto Cycling.

Consider this: like HED, Mavic strongly resisted consumer demands for an all-carbon clincher for well over a decade, and it was only just four years ago that the French company released its Cosmic Carbone 40C. Declared by Mavic to be the “first reliable carbon clincher,” it actually used an aluminum tire bed so as to meet the company’s stringent self-imposed braking tests; it was only in March 2016 that a full-carbon clincher followed.

That said, I can think of no other product in cycling (barring specific items like helmets, etc.) where safety is widely touted as a marketing tool, not a given.

Granted, much of this will fall by the wayside if disc brakes become the norm in road cycling. But even if it does, there are countless bikes with rim brakes that are already on the road, and will continue to be for decades to come.

I love that smaller companies like Alto are pursuing this sort of thing. And I love that bigger companies have also continued to make improvements in terms of safety.

But it isn’t enough. Safety should always be a given, not a feature.

JRA is an acronym well-known to bike shop employees, usually applied to customers submitting warranty claims that are clearly invalid (“I was just riding along when my top tube dented!“). It’s in part an homage to James Huang’s long tenure as a shop mechanic, but also the title we’ve given to the collection of random musings that will regularly be published here on CyclingTips. Most — but not all — of them will tech-related, but either way, they’ll reflect what’s been on his mind and what he’s been thinking about when he’s just riding along.