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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!”

“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.”

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.

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?’

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.

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?

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.

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.