A loud bang - that was the last thing that Eberhard Wickert* (name changed by the editors) heard before his bike went out from under him while he was happily whizzing down a 16 per cent descent. Brakes? Not a chance. All of the 36-year-old's attempts to get his bike back under control failed. He crashed. When he got back on his feet, his body was covered in abrasions, the bike was completely demolished, and the brake lever, handlebars, pedals, rear derailleur and saddle were scratched. The cause of the horror crash, which fortunately turned out to be a minor one: The rim flanges of his carbon clincher tyre rim were torn over a length of ten centimetres. The inner tube had then pushed the tyre outwards and burst.
The rear carbon wheel in Klaus Hildenbrand's* (name changed by the editors) racer also came to an abrupt end on a descent. After 120 kilometres of touring the Alps, he was hurtling down a twelve percent steep road when the brake levers in his hands began to pulsate and the bike started to stutter. The amateur cyclist only just managed to bring his racing bike to a halt. Looking at the rear wheel, he discovered something strange: the rim flanks were curling like damp cardboard. The cyclist was only able to continue the descent at walking pace.
As different as the damage to the two wheels was, the cause was clear: heat death. The heat generated on the rim during braking had become so great that it melted the epoxy resin in which the carbon fibres of the rim are embedded. The layers of carbon fibre lost their cohesion and the wheel lost its shape. In tubular tyre rims, the resin parts shift first, the braking surfaces are rubbed off unevenly and in some cases the entire rim flanks curl. This is undoubtedly annoying, but usually signals to the cyclist in good time that something is wrong with the wheel before a crash occurs. With clincher rims, on the other hand, it can happen that the tube and tyre push the softening rim flanks outwards until the tube becomes free - and bursts. A crash is then almost inevitable, especially if there is a bang on the front wheel. This behaviour can be easily reproduced in standardised test bench tests.
Nevertheless, when the two cyclists reported the damage to their wheels to the respective importer, they did not receive an apology and new wheels. Instead, they had to argue with the importers for several months and were accused of having used the wheels improperly. After all, they were told, the wheels had been tested by professional cyclists under the toughest conditions and there had never been any problems.
The latter may be true, the former is certainly false - and the test bench tests provide explanations for both: Even if professional cyclists ride such wheels, these are not the toughest conditions imaginable. Racing cyclists are usually very light, they ride downhill on car-free roads and have a well-honed riding technique: they brake little, late and with pinpoint accuracy and are capable of very high cornering speeds as they hurtle downhill. The situation is often completely different for amateur athletes. Every extra kilo of rider weight means more heat released during braking - and only a few recreational cyclists are as small and light as the majority of professional cyclists. With very few exceptions, recreational cyclists also have to reckon with car traffic around every bend. Together with the significantly lower riding experience and lack of experience at the limit, this means that amateur cyclists brake much longer and harder before bends on descents. It is not uncommon for them to even roll down entire mountain passes with constant braking. The result: even more heat is generated on the rims.
Manufacturers are of course also aware that heat development during braking is a problem for carbon wheels and are now experimenting with resins as a matrix for the carbon fibres that can withstand temperatures of up to 300 degrees Celsius - a temperature at which the inner tube, tyres and brake pads would have long since melted away. However, the problem remains that the carbon material simply does not conduct heat well - regardless of which resin is used to bond the layers. It doesn't matter whether the surface has a shiny, smooth finish or the fabric is slightly roughened. The rim flanks, which often do not run exactly parallel to each other, also cause the brakes to rub and squeal frequently and make them extremely difficult to modulate: sometimes they grip too tightly, sometimes not at all - an uncertainty factor. The brake pads wear out quickly and often almost completely fail to work in wet conditions. The brake pads are specially designed for carbon rims. Normal brake pads for aluminium rim flanks would generally not be able to withstand the loads and would melt away.
The brake pad and the structure of the wheel braking surface must therefore be precisely matched. However, not all wheel manufacturers have special pads made, but only recommend certain products. Although this recommendation should always be followed, it is no guarantee of the best possible braking performance. The search for a pad that harmonises better with your own carbon wheel can make sense, even if it is a long and cost-intensive process. On the other hand, we are not yet aware of any combination of carbon rim and brake pad whose braking performance is on a par with classic aluminium rim flanks.
So why carbon wheels at all? Because the best possible aerodynamics can hardly be achieved with other means, i.e. materials. The freely mouldable material carbon is ideal for this - if it is not only to be streamlined but also as light as possible. With carbon wheels for clincher tyres, however, the weight advantage is more of a theoretical nature: Campagnolo's "Hyperon Ultra C" wheelset, for example, weighs 1,397 grams with rim tape. Mavic's "R-Sys" wheels with aluminium rims undercut this by 50 grams, while the "Super" wheels from Tune with 1.1 aluminium rims from DT Swiss - with better braking performance - are less than ten grams heavier.
The carbon round wheels only become noticeably lighter as tubular rims. A few carbon wheelsets, such as Tune's "Skyline", weigh just under 900 grams. However, this will not be a major advantage for most amateur riders. With a body weight of 75 kilograms and a road bike weighing nine kilograms (ready to ride with bottles, pump and inner tube bag), switching to super-light carbon wheels saves around 280 grams, or 0.34 per cent of the system weight. This is not noticeable when accelerating - and on long climbs, most riders do not gain as much time as they can lose when descending due to the significantly poorer braking behaviour.
There's no doubt about it: in recent years, the trend material carbon has enabled many developments in frames, forks and components that have been beneficial (in terms of weight) for most road cyclists. When it comes to wheels, however, the disadvantages often outweigh the advantages - so that, if you're unlucky, you're riding ticking time bombs.
Comment: Dangerous fashion
Admittedly: Nothing changes the character of a road bike more than the wheels. Carbon wheels stylise every everyday bike into a competition machine, they are light and make the racer agile and light-footed. However, the actual weight saving for amateur athletes is so small that it is only relevant for very light riders and even then only on long climbs. But when the long climbs come down again, or when the roads are wet - or both - carbon wheels are generally out of place due to their poor braking behaviour. This is a challenge for the manufacturers: There is an urgent need to improve and test the braking behaviour. It would be a step forward if weight limits were set for riders of carbon wheels so that the wheels would not fail even under unfavourable conditions. As long as this is not the case, I would advise all athletes who are not as light as professional cyclists and do not have their riding skills to skip this fashion.
Dipl.-Ing. Dirk Zedler, bicycle expert
