On the multi-scale nature of ski–snow friction in cold conditions

In the Olympic winter sports cross-country skiing and the biathlon, athletes aim to minimise resistive forces such as aerodynamic drag, gravity, and ski–snow friction to enhance performance. Ski–snow friction is complex, involving multiple friction mechanisms that vary depending on snow conditions. In cold environments, where the moisture and water content are minimal, friction is presumably influenced primarily by dry interactions between the ski and snow, particularly through adhesion and abrasion at the micro-scale. Here, we examined ski–snow friction under cold conditions using eight pairs of cross-country skis, with different apparent contact lengths and real contact areas. Our findings revealed that apparent contact length, a macro-scale parameter, had the greatest impact on friction, followed by total real contact area, which is a multi-scale parameter. For snow temperatures below approximately −10 °C, longer apparent contact lengths reduced friction, whereas shorter lengths are more effective above −10 °C. In addition, at −3 °C, minimising the total real contact area was advantageous for reducing friction, while this effect diminished at −8.5 °C. At the coldest tested temperature of −13 °C, a larger total real contact area resulted in the lowest friction. These findings highlight the importance of considering both macro- and micro-scale contact properties for optimising ski performance in different cold conditions.