The servicing of skis is still reminiscent of alchemy. One attendant takes the demanding slats to his room (if not actually into his bed), whereas his colleague swears by a cold night in the snow for his boards. Coatings are sometimes gently polished, or in need gone over with a steel brush.

This situation is unsatisfying for manufacturers and irritating for engineers. It just cannot be that better understanding of physical principles will not improve the gliding properties of skis! This was also the view of a research team at the Swiss Federal Institute for Snow and Avalanche Research (SLF) in Davos, of which Lukas Bäurle, ETH materials science researcher, is a member (1). Thus Bäurle, together with the ski manufacturer Stöckli, the wax company Toko, and in a CTI (Swiss Innovation Promotion Agency) project, came to analyze the ski coating-wax-snow system in his Ph.D. work.

Apparatus tests instead of field trials

In their investigations the SLF researchers did not send crack skiers on various skis repeatedly down a test piste in a field trial – the standard procedure – but built a test installation in a low-temperature laboratory at their institute. Here disturbance factors such as people, wind and weather could be eliminated and a greater amount of data collected. The test apparatus, a so-called tribometer, consists of a turntable with an ice track, on which a miniature ski travels. On it the friction between ski and ice can be exactly determined.

The measurement data flowed into a model of the contact zone between the ski coating and the snow. Here Bäurle’s analyses indicated that the decisive factor in gliding is the water film produced by friction between ski and snow.

The wild structure of the snow surface: the model shows how warmth spreads through snow when a ski travels over it. Gray indicates where the temperature has reached 0°, and water has been produced. The temperature of the rest of the snow was raised very little (blue). large

Ski coating tester (tribometer): using this apparatus the friction between ski coating and ice can be measured. The ski is attached to the bracket on the right of the picture, and then travels/generates friction on the rotating ice ring. (Pictures: Lukas Bäurle) large

Here the extremely small friction coefficient of skis, of up to a minimum of 0.03, was not surprising. However, this factor depended strongly on the conditions and is, for example, higher in cold snow. The numerical model predicts that with increasing snow temperature the thickness of the water film will increase and hence friction will decrease.

As little contact with snow as possible

Despite this, a water bath is not the optimal speed surface. Bäurle demonstrated that, next to the water film, the actual also has an influence on gliding behavior. In water the extent of the contact surface would be practically 100%. This presents a disadvantage, because the smaller the contact surface is, the less friction and corresponding loss of speed there will be. This is probably also the answer to why, in certain field trials, women’s skis have done better than men’s, which are longer and have more contact. In terms of field tests, the results of both the laboratory experiments and the SLF team model agreed well with the results of field trials.

Soon in practice?

But what does this mean in practice? In dry, fine-grained snow, according to Bäurle, the ski coating should also be finely structured, as this reduces the contact surface. In old or wet snow with large grains, on the other hand, a coarser coating is recommended. How the optimal ski or wax structure should look needs further testing. One thing is clear, however: the gliding ability of skis is determined fundamentally by physics. How far the chemistry of the coating surface has an influence is currently under investigation.

Asked how quickly this new knowledge will be translated into practice, Bäurle felt that it could happen within months. He speculates that Stöckli may even be preparing its skis for this winter’s Olympics in Turin accordingly. However, while the crack skiers will be competing in Italy, Lukas Bäurle will be busy with another test: his doctoral examinations.