Going for the glide
Friction – or the lack of it — in cross-country skiing events at Winter Olympic games in Vancouver is a decisive factor in who wins the gold, NTNU researchers explain.
Fully seven of Norway’s 11 Olympic medals to date have been won by residents of the small counties of Nord and Sør-Trondelag, which also happens to be home to NTNU. Among the university’s researchers are experts on the physical demands of cross country skiing, the physics of ski glide (this article), physical training and the aerodynamics of ski jumping.
Wax and skis, skill and endurance
Felix Breitschädel, a PhD candidate at the Norwegian University of Science and Technology, has studied the interplay between the choice of skis and wax that makes a winning combination for skiers.Cross-country skiing takes enormous physical skill and endurance – but it also takes the right skis and the right wax to bring home the gold, as Norway’s elite athletes have learned during the Vancouver Olympic Games.
Cross-country skiing takes enormous physical skill and endurance – but it also takes the right skis and the right wax to bring home the gold, as Norway’s elite athletes have learned during the Vancouver Olympic Games. Felix Breitschädel, a PhD candidate at the Norwegian University of Science and Technology, has studied the interplay between the choice of skis and wax that makes a winning combination for skiers.
The wrong wax, wrong skis or mistakes in preparation of the base of the ski, “might lead to a change for the worse by up to 3 per cent,” he says.
The prep that works best
Cross -country skiers are able to kick and glide because of the way the wax and the physical structure of the ski and its base interact with the snow. When the skier presses down on one ski during a kick, the wax and ski base grip the snow, enabling the skier to push off and glide on the other ski.
Breitschädel, who is in Vancouver with the Norwegian national team, says ski preparation specialists that travel with racing teams have developed a four-step process that helps them decide how the skis should be prepared and what will work best. The steps are:
1) Different skis are tested on the track the day of the race to see what works best.
2) Once a ski itself has been chosen, the prep specialists go to work to create a micro structure on the ski base that will work in specific snow conditions. This structure is tested prior to the race.
3) Just a few hours before the race, the prep specialists have to test different waxes and wax combinations and wax the skis, which are then tested.
4) Just minutes before the race, the base of the ski is fine-tuned.
Coastal weather, mild temperatures
Breitschädel reports that the weather and track conditions at the Whistler Olympic Park in the Callaghan Valley are very special. “The arena is located 10 km west from Whistler, and about 200 km from the Pacific Ocean, and the area gets an average snow fall of 10 m in the surrounding mountains,” he says. “Currently, the average snow depth is 1.2 m at the Nordic area.”
Even though the site is not directly on the coast, it is still affected by coastal weather he says. The average temperature in February has been + 0.6°C, far warmer than the -1.4°C that has been the 4 year February average temperature.
But as long as there is enough snow, why does snow temperature matter to skiers? Breitschädel, says the mild temperatures in combination with regular showers increase the speed at which the snow changes structure, transforming pointy freshly fallen snowflakes into rounded snow grains. Regular freeze-thaw cycles further increase the snow grain size. Clusters of wet and round bonded snow crystals are the consequence.
Ski base grind and structure
Because the ski slides on the snow, the actual amount of surface area on the ski base is one important factor that determines how much friction there is.
If there is too much real surface contact area, the skier will actually experience some suction under wet conditions, but if it is colder, lots of surface area generates enough frictional heat to generate a thin water film for the ski to glide on.
“The ski base structure has to fit to the given snow grain condition,” Breitschädel says. “New snow, with its complex snow crystals, requires a different ski base structure than old transformed snow grains.” That means cold conditions call for fine grinds while coarse grinds are best for wet snow.
A tiny but critical difference
But what of the disappointings results for the Norwegian men’s team in the 15 km freestyle race? After race favourite Petter Northug turned in a disappointing finish, Norwegian media speculated that the wax might have been wrong. Breitschädel says that’s an overly simplistic assessment.
“Waxing is one out of four parameters which affect the total performance of a ski. In addition to the ski characteristic, structure and track conditions, the waxing and the final ski tuning with a manual rilling tool are all important,” he says. Each team carefully guards its wax and ski structuring secrets, but mistakes happen. The 3 per cent decrease in performance wouldn’t make much of a difference for the average skier, he says, but “at such a high level they are crucial and can make the difference whether an athletes wins a medal or not.”