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Tech FAQ: All about carbon clinchers

This week, Lennard Zinn answers questions related to carbon clinchers, including the differences between tubulars

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Carbon clinchers for heavier riders

Dear Lennard,
I am considering purchasing a full carbon wheel set, the Bontrager Aeolus D3 5. My concern is with the structural integrity of this wheel, seeing that I am 6 feet and 275 pounds, and also the braking/stopping performance.
— Richard

Dear Richard,
If I weighed 275 pounds, you would not catch me riding any carbon clincher wheels unless I were to ride them only on flat and rolling terrain. I think that because of heat developed during braking, riding a carbon clincher at your weight on mountain descents would be ill-advised.

And while the wheel has no weight limit and has a wider spoke bracing angle for stiffness than most wheels, my experience with riders your weight (my frame building business brings in lots of riders in the 275-pound range) is that wheels not specifically built for riders of your weight will develop fatigue problems within a year. Aluminum rims under heavy riders usually develop cracks at each spoke nipple on the rear wheel. On carbon wheels, spokes often break.

It’s an expensive wheelset, and unless you buy an extended warranty on it, I wouldn’t recommend the investment.
― Lennard

Late vs. gradual braking to prevent blowouts

Dear Lennard,
There has been much talk recently about the potential for heat-related blowouts with carbon clincher rims. While I have never had this problem myself, and have never heard a first-hand report of such an occurrence, I remain concerned. With the advent of SRAM’s hydraulic rim brakes, the potential for heat buildup seems even worse. My question is whether a very late and powerful braking action or an earlier, more gradual braking action will control heat buildup better. I will be doing a seven-day race through the Alps this summer and I’d like to be as prepared as possible for the notoriously steep (upwards of 22 percent), twisting descents. Also, are latex tubes any more or less susceptible to heat?
— John

Dear John,
My experience with doing a lot of testing of carbon clinchers on Flagstaff Mountain above Boulder is that shorter, more powerful braking produces less heat buildup than does prolonged braking. I’ll ask around more about this, because it’s a very good question.

The inner tube type is irrelevant; the tire only explodes because the tire bead blows off the rim when it gets hot enough, and no tube could hold the pressure once it’s no longer constrained within the tire.
― Lennard

Carbon tubulars vs. clinchers

Dear Lennard,
Would please give a brief explanation for the advantages and disadvantages of carbon tubular and clincher wheels? Is there more to it than the weight savings of the former and the easy flat-fixing of the latter?
— Sam

Dear Sam,
The carbon tubular rim is lighter due to the lack of bead walls. The tires corner better because they are round in cross section rather than lobe-shaped like a clincher. They are usually a bit lighter as well, having no beads and being able to use a lighter inner tube. For events and road conditions that warrant it, high-end tubular tires can generally be inflated to higher pressures than clincher tires of the same size. They are harder to pinch-flat than a clincher (due to lower, more rounded rim walls and tougher latex inner tubes). Tubulars are safer to ride when flat, as they are still glued to the rim, whereas a flat clincher can come off of the rim. And unlike a clincher where higher pressure in the tire applies higher outward pressure on the carbon rim bead walls, the tire pressure in a tubular tire has no effect on the rim (other than to compress it uniformly radially inward, thus reducing spoke tension slightly, something that happens with all tire types).

The disadvantage of a tubular is cost and the time and skill required for gluing it on. With enough braking heat, the glue has the potential to melt and allow the tire to come off (remember Joseba Beloki’s horrific crash?).

Clinchers are quicker to install, the tires are generally cheaper than tubular tires, and flats usually require only replacement of the inner tube and not the entire tire.

Clincher rims are heavier, and clincher tires are also a bit heavier than tubulars of similar casing and tread. While heat buildup in any clincher rim can be an issue during extended braking (see the melting glue problem mentioned above), it’s more of an issue with a clincher. The latest carbon clinchers from top brands, when coupled with the brake pads recommended by the manufacturer, are not likely to fail in this circumstance, but many carbon clinchers in the past have done so. Carbon is very strong under tension but not under compression, so asking it to form a rim wall capable of constraining the pressure of tire beads trying to push it outward is a big ask in the first place. Couple that with the fact that at some temperature, any resin holding the carbon matrix together will soften, and you can have rim walls that fold out like limp taco shells under hard braking. Again, this is a thing of the past for the top brands with the correct (and heavily studied) brake pads under riders who are not beyond a certain weight (and this weight will depend on the rims, braking style, the road steepness and curve sharpness, and the ambient temperature).
― Lennard

Tire pressure minimums

Dear Lennard,
I read on Velonews that the Paris-Roubaix riders used 60-90 psi in their tubulars. Is there a safe lower limit for clinchers?

With 23C tires, I would run 90 up front and 105 in the rear. I was trying to go as low as I could without risking a pinch flat. Now that I’m on 25C tires, I would like to go lower. I’m running 100 in the back and 85 up front right now. I like the way the front feels with the lower pressure, and wondered if I could safely go any lower? Is there a point where handling or safety might become an issue?
— Steve

Dear Steve,
It of course depends on rider weight and riding style (whether you un-weight the bike over bumps, etc.). And the bigger the tire, the lower the pressure you can get away with.

Besides the pinch-flat issue, it’s pretty easy to dent a clincher rim (which means it cracks and is broken, since carbon does not bend). So you don’t want to risk running it low enough to do that.
― Lennard