Abstract
Previous research has shown that friction between ski and snow can vary substantially due to changes in snow conditions. The variation of friction affects the speed a freestyle skier or snowboarder (athlete) reaches during the in-run of a jump. Athletes risk severe injuries if their take-off speed is not within the right margin to land in the “sweet spot” zone. To reduce the risk of injury, snow park designers and competition managers need to calculate the speed athletes reach during the in-run. However, despite multiple attempts over the last decades, to date no model can predict ski-snow friction from snow physical quantities. Hence, simulations of in-run speeds suffer from insufficient validity. For the first time, this work combines kinematic athlete data and comprehensive snow surface measurements to infer the coefficient of friction of freestyle skis and snowboards across a wide range of snow conditions. Athletes’ point mass kinematics were recorded at more than 200 straight gliding runs with differential global navigation satellite systems. The subjects’ air drag and lift were deployed from wind tunnel measurements. Along with the kinematic data and data from wind measurements, a mechanical model of the athlete was established to solve the equation of motion for the coefficient of friction between ski/snowboard and snow. The friction coefficients for ski (snowboard) ranged from 0.023 ± 0.006 (0.026 ± 0.008) to 0.139 ± 0.018 (0.143 ± 0.017) and could be explained well (Radj2 = 0.77) from the measured snow parameters using a multivariate statistical model. Our results provide a new quantitative tool for practitioners to predict the friction of skis and snowboard on snow of various conditions, which aims to increase athletes’ safety in slopestyle and big air.
Highlights
Ski–snow friction, air drag and gravity are the three forces that regulate speed in skiing and snowboarding
The higher overall snow friction for snowboarders compared to the skiers, statistically significant (p 0.008), was rather small compared to the huge changes in friction caused by different snow conditions
The presented data quantified how strongly snow properties influence the friction of skis and snowboards
Summary
Ski–snow friction, air drag and gravity are the three forces that regulate speed in skiing and snowboarding. The acceleration due to gravity depends on the incline of the slope, while air drag is a function of speed, frontal area, the shape and material properties of the athlete’s clothing, and air density. The most complex force that acts on skiers and snowboarders is the ski–snow friction force. The level of ski-snow friction and the governing mechanisms can change distinctly, as the material properties of the snow change (Nachbauer et al, 1996; Buhl et al, 2001; Fauve et al, 2005). The snow’s physical instability facilitates rapid changes in properties (Szabo and Schneebeli, 2007), and increases the difficulty of describing the mechanics of the frictional contact between snow and ski
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