Mike and I both found it remarkable that United Health Care was using Enve carbon clinchers on the mountain stages of the recently completed US Pro Cycling Challenge (aka Tour of Colorado). Carbon clinchers have a nasty reputation for warping or otherwise self-destructing on treacherous mountain descents. Why would UHC risk certain doom in order to save a few grams over their deeper, wider, and heavier carbon tubulars?
I boil it down to two circumstances. One, the roads they used were pretty wide open. Even Andy Schleck was able to sustain an attack on one of the bigger descents, and he's got a well deserved rep as a horrible descender. Second, pros don't brake. Put a pack of pros on a non-technical and fast downhill, and they can leave their brakes at home. That's the critical point right there.
In general, our experience with carbon clinchers has been great. They've been our most popular wheels by some measure, and in the however many thousands of reliable and faster-than-they-might-have-otherwise-been miles people have used them thus far, there have been two incidents. Both came while the user was very actively trying to go as slowly as possible down a multiple mile, steep descent. I know firsthand than in one of these instances, another wheel from another (very much more expensive and highly touted for its heat resistant resins and other wondrous features) brand failed at the same place at the same moment. The weaknesses inherent in carbon clinchers aren't avoided by any brand, no matter the marketing story. They are avoided by appropriate use.
We've been doing our best to educate people on their shortcomings and benefits (the way I worded that is significant), and have come so far as to create a set of terms and conditions for purchasers of our wheels, which simply make people aware of the limitations inherent to the entire category of full carbon clinchers.
Take a look at this cross section of a clincher, and compare it to the tubular section.
As you can see, the tubular rim supports itself by having a closed section shape, while the clincher rim has an open shape where the tube and tire hook into the rim. These unsupported extensions have two functions - they keep the tire hooked into the rim by resisting its outward force, and they comprise the brake track. The brakes push in and the tires press out.
Under normal circumstances, clinchers don't really leave much to worry about. The static force of the tire is easily overcome by the rim sidewalls, and braking forces are well within the sidewall strength as well. Most clincher rims come with a recommended max tire pressure. For most aluminum rims (we're talking road bikes here), it's in the range of 140-160 psi. So applying a 160 psi force to the sidewall shouldn't deflect it. Most carbon clinchers have a much lower pressure limit - most prominent brands are in the 120 or so range. The reason behind this is somewhat complicated - read on.
Carbon is strong stuff. Pound for pound, it's stronger than most stuff in most measurable dimensions. It has a couple of liabilities compared to aluminum though, and these come into play here. First is that it's not pliable - it doesn't bend before it breaks. This isn't a huge huge deal, but there are those incidents where a partial failure is a notable improvement over a total failure. The bigger thing is that aluminum sheds heat incredibly well. Aluminum is broadly used in industrial applications where you need to get rid of heat. Take a warm can of... soda... and throw it in a cooler full of ice. The outside of the can will quickly feel ice cold even though the... soda... inside is still warm. That turns out to be a big deal.
Brakes generate heat. For every unit of kinetic energy (movement) you have, slowing it down by braking will create heat. Brakes work by friction, friction creates heat. The more you need to slow down, the more heat you create. Steeper grades, heavier riders, and higher speeds all increase the amount of friction, and thus heat, you'll need to create to slow down. A worst case scenario would be a heavy rider trying to go slowly down a steep grade, having slowed down from a higher speed initially. The initial deceleration creates heat, the steep grade and rider's weight contribute to the need to keep generating friction, and heat, in order to maintain a slow speed. So you have heat constantly being generated. The real kick in the pants comes when you realize that because of the slow speed, you are now losing the benefit of air cooling the rim's surface. There's just no place for the heat to go, so it builds up.
Now, back to the tire pressure. Air expands as it heats. The more heat, the more expansion. Since aluminum rims are better at shedding heat, this isn't such an issue since the heat doesn't stick around long enough to expand the air in the tubes that much. Carbon rims, however, keep heating the air inside the tube, which eventually causes the air pressure to rise. And it keeps going. At some point, something's gotta give - either the tube melts and explodes, or it gets overcome by pressure and explodes, or it pushes the rim's brake track wall until it gives way.
While that's going on, the heat is actually affecting the structural make up of the carbon. What we call "carbon" is actually a matrix of carbon fibers in plastic resin. The resin holds the fibers in place, and the fibers reinforce the resin. Carbon itself, for all intents and purposes, doesn't really burn. Plastic, on the other hand, isn't quite as heat resistant. Carbon parts are molded and cured using heat and pressure. Resin systems trade off durability for heat resistance. That's a bit of a simplification, but the more heat resistance a resin has, generally the more brittle it is. Parts are commonly cured between 150 and 350 degrees fahrenheit. Manufacturers and composites suppliers are constantly working to increase heat resistance while maintaining durability.
Cure temp becomes the functional limit of the what temperature the part can bear in a peak. Continuous heating limits are lower. In our worst case scenario above, the brake loads are quite capable of producing 250 degrees or more. When that critical temperature is reached, or when a sub-critical temperature is maintained for long enough, the part is subject to failure. In minor cases the carbon matrix will slightly soften and the outward pressure from the tube will bulge the brake track (this is as close as you'll get to seeing carbon take a permanent bend - but what's really happened is that it's been inadvertantly remolded). In catastrophic cases, the brake track can splinter quite aggressively as the fibers are no longer held in place by the resin.
A big variable in this whole mix is brake pads. Different pad compounds are better or worse at shedding heat, and some quite simply reduce heat buildup by not working that well. Cork pads are like this - they work sort of well, but their smooth surface and non-grabbiness limit the amount of heat they'll allow to build up simply because the amount of friction (and thus heat) they create is limited. A lot of manufacturers are working with different compounds that work at that intersection of actually stopping you without generating a ton of heat. It should be noted that EVERY heat related failure I've become aware of this year (ours as well as other brands) have had the common thread of Swissstop Yellow pads. They are fine (and perform well) in normal conditions, but when heat is a risk they are decidedly not the answer. Using the pads that are supplied with your rims (no matter which brand of rims you have) is your surest bet to avoiding a heat related failure, and to maintaining your warranty coverage should you have an issue. (Read our Carbon Wheels Terms and Conditions for more on this.)
Note that overheating isn't purely limited to the world of carbon clinchers. Tubulars, both aluminum and carbon, can get hot enough that the glue holding the tire to the rim melts and fails. Aluminum clinchers can get hot enough to pop the tube.
Good braking technique is important as well. Riding the brakes causes heat to build up more rapidly than feathering the brakes. Alternating front and back allows one to cool while the other is working. Clamping on the brakes and staying on them is asking for trouble.
Carbon clinchers can be a great option. We have a set in our household that's got about 4500 miles on it this year, including me plodding around all sorts of crappy roads last winter and all of the early season races, and then my wife stole them and has used them on all sorts of occasions including the very hilly and dirt road-featuring Killington Stage Race. There are, however, times and places when they're inappropriate. If you're going to need to go down big, long hills slowly, choose another option. For the majority of riding that most of us do, and with some awareness of their limitations, carbon clinchers are a great choice.