INTRODUCTION: Roller skis are used in training and competition to imitate cross-country skiing during the snow-free season, where different wheels are used for classic and skate roller skiing. Roller-ski wheels are typically categorised based on rolling resistance, with race and standard wheels commonly used in competition. The experience among athletes and coaches is that the rolling resistance coefficient (µR) of the roller-ski wheels varies not only between manufacturers but also within the same series of wheels. However, this has been sparsely investigated in previous studies [1][2]. The difference in µR can significantly influence the race performance [2]. Therefore, the aim of the study was to determine the µR of various types of roller-ski wheels from different manufacturers. METHODS: Measurements of the rolling resistance of unused and pre-heated [1] roller-ski wheels were carried out with newly developed equipment, consisting of an electrically powered spinning Ø60-cm aluminium cylinder. To record the forces acting on the roller-ski wheel, the wheel was mounted to a fork attached to a 3-D load cell (NET Force/Torque sensor Mini58, ATI Industrial Automation, Apex, USA). µR was estimated as the ratio between the tangential and the normal forces measured by the load cell. The measurements were performed at a cylinder periphery speed of 20 km/h with a normal force of 300 N. A sample size of eight wheels of each type was measured, including both race and standard wheels for skate and classic styles. The wheels were all non-ratcheted and sourced from manufacturers Marwe, Swenor, SWIX and FF ski. One-way ANOVA and Bonferroni post hoc tests were used to determine any significant differences (P<0.05) in µR between the different manufacturers. RESULTS/DISCUSSION: Significant differences in µR were found between all manufacturers for the tested skate wheels (P<0.001) and classic wheels (P<0.001), except for Marwe race vs Swenor race classic wheels (P = 0.058, ns), see Fig 1. The range of variation in µR between means for standard wheels was 0.011, 52% of mean (Skate) and 0.008, 41% (Classic), and for race wheels 0.005, 46% (Skate) and 0.001, 13% (Classic). Further measurements need to be made to study whether µR for different types of wheels is influenced by different normal forces and speeds. CONCLUSION: The results show that there are large differences in µR between different manufacturers of equivalent types of wheels, which can be decisive for the roller skier`s performance.
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