A loud bang was the last thing Eberhard W.* heard bevor his race bike developed a momentum of its own while he was happily dashing down a 16-percent-downhill. Braking? No way. All of the 36-year-olds tries to get his bike under control again, failed. He crashed. When he got up again, his whole body was covered with abrasions, the bike was completely demolished, shift/brake-levers, handlebar, pedal, derailleur and saddel scratched. The cause of this horror-crash: The flanges of his carbon-clincher-rims had broken down on ten centimeters of length. The tube had, as a result, pressed the tire outwards and had bursted.
The back carbon-wheel of Klaus H.'s* race bike found its end on a downhill as well. After a 120-kilometres-tour through the Alps, he was dashing down a road with twelve percent of grade, when the brake-levers started pulsating in his hands and the bike started jolting. The hobby-cyclist only just managed to get his bike to a halt. When examining the back wheel, he found something strange: The rims curled like wet cardboard. The cyclist could only continue his ride at walking speed.
As different as the damages of the two wheels may seem, as clear is what caused them: heat. The warmth produced on the rim during braking had become so high that the epoxy resin, in which the carbon fibres are embedded, had started to melt. The carbon fabric layers lost cohesion, the wheel its form. With tubeless tires, first the resin parts shift, the brake surfaces get rubbed down unevenly and, in some cases, the whole rims curl. This is annoying, but at least it signals the cyclist early enough that there is something wrong with the bike before he crashes. With clincher-wheels, however, tube and tire may press the softening rims outwards until the tube is uncovered - and bursts. This process can be very well reproduced in standardized laboratory tests.
Nevertheless: When the two cyclists informed the respective importer of their damaged wheels, they did not at all get a kind apology and some new wheels. Independently of each other, they had to argue with the importer for several months and were accused of not having used the wheels according to their intended purpose. After all, it was explained to them, the wheels had been tested by professional riders under toughest conditions and there had never been any problems.
Of course, also manufacturers know that heat is a problem to carbon wheels and have meanwhile come to experimenting with resins as matrix for carbon fibres which resist up to 300 degree Celsius - a temperature at which tube, tire and brake pads would long have melted away. The problem remains, however, that carbon rims just do not conduct heat very well due to the material - no matter, which resin gums up the layers. It doen't matter either, whether the surface is even and glossily painted or whether the fabric is slightly roughened. The carbon rims, which often do not run parallel to each other, can cause the brakes to rub, squeal and make it very difficult to dose: One time, they bite very well, another time not at all - an insecurity factor. The brake pads wear down rapidly, in wet conditions, they often do not work at all. Thereby, brake pads for carbon rims are already specially designed. Ordinary pads for aluminium rims would usually not resist the strain and melt away just like that.
Brake pads and the wheel's brake-surface-structure therefore have to be synchronised very carefully. Not all wheel manufacturers, however, have special pads made, but only give recommendations.
No question: The trend-material carbon has, in the past few years, allowed for many frame, fork and parts developments which were of (weight)advantage for most race cyclists. With wheels, the disadvantages outweigh the advantages quite often - and the alleged high-tech-accessory quickly becomes a dangerous anachronism.
* Names are known to us, they will not be mentioned, however, due to current legal proceedings.
Author: Dirk Zedler
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