Carbon clincher road rims were once touted as the Next Big Thing in wheels with promises of ultralight weight, unbeatable strength and wildly aerodynamic shaping that just isn't possible in aluminum.
Some examples have come closer to this ideal than others but the biggest Achilles' heel continues to be heat management. And what's lacking is any sort of industry-standard test to ensure to riders that this stuff is actually safe to use. This is not okay.
Flagstaff Road is arguably the most iconic road climb near BikeRadar's US headquarters in Boulder, Colorado. Situated adjacent to the city's western edge, it gains about 2,500ft/760m of elevation in just under 6mi/10km. The average gradient is about 11 percent but it's much steeper in certain sections and there are more than two dozen turns, many of which are tight hairpins that can't be taken at anything approaching full speed (at least not by mere mortals).
If something goes wrong on one of those corners, there's often just a single guardrail and minimal run-off area to (hopefully) keep you from hurtling off the precipitous edge of the road. It's a wickedly fun descent and exciting ribbon of tarmac as a result but also a demanding one – and one that I at times won't tackle on a road bike with carbon clinchers and rim brakes.
The problem is heat. Squeezing brake pads against a rim's sidewall basically transforms your forward energy into friction, which in turn creates heat. Aluminum rims dissipate that thermal energy into the surrounding air pretty well but carbon fiber composite is more of an insulator, holding it in instead of happily letting it go.
Periodically releasing the brakes can quickly bring those temperatures down but that isn't always possible. Plus, some riders are more comfortable with higher speeds than others, and heavier riders carrying more momentum simply need to use their brakes more. At best, those carbon rims can get uncomfortably warm or even hot to the touch.
Pass a certain point, though, and you hit what's commonly referred to as Tg, or the material's glass transition temperature where the rim quickens softens. Meanwhile, the high pressures inside the tire persistently want to push the bead hooks outward. Combining the two events can produce significant sidewall deformation or sudden tire and/or rim blowouts, none of which is desirable when you're hurtling down the side of a mountain on 25mm-wide tires wearing nothing but Underoos.
I've thankfully dodged that bullet so far [knocks on wood] but I know others who weren't so fortunate. In fact, two BikeRadar staffers, Ben Delaney and Nathan Forbes, have had carbon wheels 'peel open' after the rims softened from heat build-up. (Forbes was coming down Flagstaff; neither were hurt.) Levi's Gran Fondo even officially warned against the use of carbon clinchers just a few years ago.
Bikes and components on the whole, though, are thoroughly tested and generally must hold up to accepted standards imposed by third party organizations. The CEN protocol for a fork, for example, is impressively rigorous.
But such testing doesn't exist for carbon clincher rims. Some companies do their own in-house evaluations but protocols vary wildly and there's no reliable certification in which one can put faith. Even worse, many carbon road clinchers currently on the market are produced overseas by unnamed factories and marketed by companies that can sometimes produce no test documentation whatsoever.
I had a frank discussion on the topic with one prominent industry insider – who requested to remain anonymous – and what I heard wasn't encouraging. While there had been talk about establishing a standard test for carbon clincher rims, there were too many dissenting opinions (and too many disparate business interests) to form any sort of consensus. Some companies are confident they could pass whatever test is written while others hold the opinion that carbon clinchers shouldn't exist in the first place. Yet others are flat-out opposed even to the idea as it would instantly put them out of business.
"The industry has been arguing for years on this and there is little to no movement. There isn't a standard that's accepted currently for safety and with the ISO regulations updated every 10 years, there likely won't be for a while," we were told. "The last I heard, the current plan was to go with putting the wheel in a 120°C [about 250°F] oven and seeing it if goes out of true."
If that doesn't sound good enough to you, that's because it isn't. 120°C isn't much hotter than boiling water and nowhere near what an average rider can generate.
"My bigger worry is the low-cost stuff. The way you keep costs low is by not spending a lot of time and effort on this. One company told me straight up that it was just more economically sensible for them to continue to warranty rims."
This is not acceptable by any means – and I'll openly shoulder part of the burden for taking for granted the safety of this technology back in the day, before most of us fully understood what was required to actually make it work. Will I still ride (and test) carbon clinchers? Yes, but that company had damn well better be able to prove to me that I won't cripple myself (or worse) as a result due to a heat-related failure.
I'm not willing to cast myself as a guinea pig because someone decided that saving a few bucks was more important than my safety – and you shouldn't, either. Moving forward, I'll make sure to report on such testing for any carbon clincher wheel I encounter. My wife and I have a new baby girl at home and I like the idea of being able to walk into the door upright at the end of a ride.