Technical aspects to improve performance in cross-country skiing
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Beside the athletes themselves, a number of factors influence performance in crosscountry skiing. Ski technicians typically have five possible adjustments to make to the given snow, track and weather conditions. These are: (1) to choose skis with certain desired mechanical properties, (2) to apply a suitable grind to the ski base, (3) to change the material of the running surface, (4) to apply a combination of various waxes and (5) to decide the fine-tuning of the surface texture with a hand tool. Extensive testing is their major approach prior to important competitions. This thesis presents experimental studies on such methods in the field and laboratory, and is divided into four studies: I Investigation of mechanical ski properties of classical and skate skis in laboratory tests. More than 800 racing skis from the Norwegian cross-country, biathlon and Nordic combined teams have been analysed in the laboratory in the period 2009 to 2011. The study showed that men and women chose skis with similar stiffness characteristics for classic ski in the cold category whereas women selected ski which required 5 % higher load to compress the camber down to 0.2 mm. Several significant differences in ski characteristics were found for skating skis of male athletes between the three Nordic national teams of cross-country, biathlon and Nordic combined. The ski characteristics of the teams were mostly determined by the ski brand’s specific ski characteristics. This finding was also in line with the documented changes due to temperature change. Temperature changes can result in increased or decreased stiffness, but in comparison with differences in characteristics between ski brands, the magnitude of these changes is on a much smaller scale II The resetting process of the base material after a treatment with manual rilling tools were investigated. All tested skis were tested in the laboratory and in the field. The major goal of this study was the development of a new grinding machine for crosscountry skis from scratch. A comparison between traditional stone grinding and the newly developed grinding method is presented. Initial studies showed that the newly developed method resulted in a lower coefficient of variance of the average mean roughness of the produced structure. A reproducibility within smaller tolerances is in strong demand by ski technicians. III A new inertial measurement unit (IMU) -based sensor was developed and tested in a ski tunnel. During the gliding tests, several systems were tested and compared to each other. By using a low-cost IMU and correction methods it was possible to produce reasonable estimates and distinguish between good and bad skis with a difference in the kinetic coefficient of friction of around 0.01. The accuracy needs to be improved by a factor of ten in order to meet the precision requirement and provide a system which can distinguish between skis with similar glide performance. IV The characteristics of two coatings and their contribution to a reduced coefficient of friction were studied. In addition, the structural changes of the ski base material due to the waxing process and subsequent skiing were studied. The Fluorine content, both in the base material and in the wax was given special attention. Field and laboratory experiments on the ski base materials, with and without waxes, were conducted. The study showed that products containing fluorine contributed to a lower surface energy and hence a more water-repelling surface with a higher static water contact angle. Carrying out direct comparisons between the outdoor field tests and the lab tests in this study proved challenging, since the product with highest measured coefficient of friction from the laboratory tests and lowest contact angle performed best in the final gliding test after 34.1 km of skiing. Aside from this, there were many similarities. These four studies resulted in seven publications, P1-7, which are attached in the appendices to this thesis. The work in this thesis resulted in the development of a new grinding machine for crosscountry skis, which opens up for further development in the field of surface treatments. In addition a new device to measure ski characteristics has been developed, which was used in the second half of Study I to measure ski characteristics.