Olympic Games 2018: l’X hits the slopes with Martin Fourcade
The performances of biathlete Martin Fourcade were undeniably brilliant, but it was his skis that captured the interest of researchers at the l’X Hydrodynamics Laboratory and the ENS Statistical Physics Laboratory. Since 2015, ski waxing has been the subject of research that seeks to gain a better understanding of its underlying physics.
It is common knowledge for all skiing enthusiasts that the base of the ski is protected by a thin layer of wax that allows it to glide more smoothly. Above a certain level, it is essential for athletes to master the right choice of wax in order to have the best possible chance of victory, since a bad choice could mean losing a race. But from a scientific point of view, there is still a poor level of empirical knowledge concerning ski glide.
However, this is all due to change, as Martin Fourcade and his waxing technicians have inspired researchers at l’X to work on this very subject. Research teams directed by Lyderic Bocquet (LPS) and Christophe Clanet (LadHyX) have been researching this issue since 2015 in an attempt to understand the frictional forces at play during the practice of skiing.
Skiing in the lab
The first objective of this research project was to reproduce the conditions of a ski piste in the laboratory, in order to measure the friction produced when snow and ski rub together. After several unsuccessful attempts in a freezer at ENS, a "ski station" was built at LadHyX inside a refrigerated container. A 12-meter measuring bench and a turntable designed at ENS allowed the teams to perform tribology experiments, that is, measuring the friction between the snow and the ski using force transducers. In doing so, they were able to observe three rules of contact that are found on the slopes, which depend on the amount of water created when the ski rubs against the snow, and therefore on the temperature that is reached during this action:
- If the temperature is too high, at around 0°C, the ski melts a thick layer of water when gliding by. This water exerts a force of attraction upon the skis, increasing friction and thus slowing down the skier.
- Conversely, if the temperature is too low, at under -15°C, the skis rub against the snow without causing it to melt, no lubrication is produced on contact and the skier does not advance effectively.
- The optimal conditions are found in between these two temperatures, when the snow melts just enough to create an extremely fine film of water (≈0.1 µm) that lubricates contact while preventing the skis from sticking.
The next challenge was to measure the thickness of this film of water using a smaller-scale experiment, which was set up at ENS.
Importance of the wax
The wax is used to improve lubrication. With an often hydrophobic nature, it repels water droplets so that they do not attach to the ski and slow it down. There are over one hundred different types of wax. Before a race, the waxing technicians feel the snow to analyze it. In doing so, they can determine five or six waxes that would perform the best for that specific snow type. This selection is then tested out on the slopes to establish which covering would adhere the least to the ski.
Working in collaboration with Martin Fourcade’s wax experts and using the Olympic champion’s own skis, the researchers attempted to reproduce this empirical knowledge in a repeatable and controlled manner in the laboratory, to identify the physical phenomena that make it possible to improve lubrication. This is how they managed to deduce that when the waxers touch the snow, they measure a whole host of other parameters outside of temperature, like humidity and density. "They are excellent experimenters, who have helped us immensely in understanding their practices in order to carry out better analysis of them," explains Caroline Cohen, researcher at LadHyX.
Cohen was not able to tell us everything about which materials proved most effective for lubrication, or those that the research team are planning to use for future tests. But she did explain that the wax may have an additional mechanical effect due to its geometric design. For instance, if there are ridges carved along the length of the ski, they could help water to escape when the layer of lubricant is too thick. On the other hand, if the snow were too cold, ridges etched perpendicularly to the direction of the ski would "scrape" the snow and increase the amount of water, causing more lubrication.
With the research project still ongoing at ENS and LadHyX, Martin Fourcade brought home three gold medals with the French Olympic team at the 2018 Games. In addition to Fourcade’s incredible physical skill, the quality and expertise of his ski waxing—a factor that researchers will soon be able to improve—must also be taken into account in his success.
Physics, Sports and Disabled Sports at LadHyX
At the crossroads of physics, engineering and biomechanics, the "Physics, Sports and Disabled Sports" project—led by Christophe Clanet, Director of Research at CNRS and Director of the l’X Hydrodynamics Laboratory (LadHyX)—has opened up a vast field of studies. L’X has been placed at the forefront of this research thematic, which combines fundamental research with applications for medicine and society. Using sports and disabled sports as sources of unique scientific challenges, this research strives to improve sports materials and strategies, but also to understand athletes’ optimal movements, and the underlying physical laws of such movements. This project, in collaboration with a number of professional athletes, is supported by the École Polytechnique Foundation.