August 8, 2022

How to measure the slipperiness of skis

How to measure the slipperiness of skis
How to measure the slipperiness of skisHow to measure the slipperiness of skis

SKIING IS ABOUT to get a little bit harder. For decades, those trying to go slip-sliding away have been able to call on some (legal) chemical help. Fluorinated ski wax hugely reduces friction with the snow and its introduction has revolutionised much of the sport, sending speeds soaring and times tumbling.

But concerns about health and environmental impacts of the fluorocarbon additives have prompted the International Ski Federation, the body that sets rules and governs competitions, to crack down. Top-level skiers this season were stopped from using a wax chemical class called C8, and a fuller ban on all fluorinated help is promised once officials work out a way to quickly detect it on skis.

The coming ban has sent ski manufacturers and racing teams scrambling for alternatives, including new waxes and equipment. But it is surprisingly difficult to test their impact on the speeds of different sets of skis. The most obvious experiments—strap them on and see—are undermined by changes in the weather, snow conditions and the position of the skier.

As those who have tried it will know, skiing is the art of balancing competing forces. For a beginner, friction (or the lack of it) is the hard part. But as racers go faster, the air drag on the body slows them down. It also complicates measurements of the friction of skis in the real world.

Scientists at the Norwegian University of Science and Technology in Trondheim think they have the solution. They built an indoor ski piste, and to go with it a robot skier which can quickly and reproducibly accelerate to nearly 30kph. By sliding the robot down the same snow tracks, but wearing different skis, the researchers can test the frictional resistance of the snow with much greater accuracy and reliability than they could outside on a real ski slope.

The disappointingly prosaic name for such a testing kit is a linear tribometer (tribology being the science of interacting surfaces in relative motion). The 9m-long Norwegian linear tribometer is not the first developed to test full-length skis. But it is the first to do so on real-world, so-called “dendritic” snow.

Linear tribometers in Austria and Finland, two rival skiing countries, test on artificial snow, of the type discharged from cannons currently being used at the Winter Olympics in Beijing. Prepared quickly from water, this kind of artificial snow is made up of tiny balls of ice and has different physical properties from the spiky and sharp crystals of fresh-fallen dendritic snow.

Friction is measured by a coefficient, symbolised by the Greek letter μ (a μ of zero is frictionless). Writing in Tribology International, a journal, the Norwegian researchers say their indoor test track is reliable enough to separate skis running on dendritic snow with performance differences in μ of just 0.001.

What difference would that make? Audun Formo Buene, an organic chemist and keen skier, who leads the Norwegian project, says probably not much for the most rapid downhill racers. Reducing friction is less important to such Alpine-style skiers because their speed makes air drag the dominant force slowing them down.

But for the slower, cross-country events, including the ski-shooting crossover biathlon, a reduction in friction of that size could be the difference between winning a medal or not. A decrease in μ from 0.037 to 0.030 is enough to slash almost 6% from a skier’s finishing time in a 5km race. That’s more than enough to propel a racer from tenth place to the top of the podium.

Dr Buene says it is unlikely that any fluorine-free chemicals will come close to matching the slipperiness of the soon-to-be-banned waxes. He thinks manufacturers should instead investigate how to release the frictional brakes by grinding different microscopic structures into the bits of a ski’s base that are in greatest contact with the snow. And although holiday skiers might not notice the difference of such design changes at first, Dr Buene says, technological changes at the top do have a habit of eventually sliding down to the mass market.