Abstract
The sliding performance of cross country skis is mainly influenced by the ability of the ski base to minimize capillary forces and contact area. Whereas, the first condition depends on hydrophobicity, the second one is controlled by the ski grinding structure and the morphology of snow. In this contribution the results of sliding tests with five typical grinding structures will be presented and compared to calculations of the real area of contact. Surface topographies were measured and corresponding roughness features were analyzed by 3D optical microscopy. The measured ski base profiles and the measured grain size distribution of granular snow at −2°C were employed within a bearing model for a rough surface in contact with loose and freely-moving snow grains treated as ice spheres. For the five grinding structures, this model revealed a good correlation of the real area of contact between ski and snow with run times in lab-condition sliding tests. The results indicate that the snow-containing volume of the grinding structure is pivotal for tailoring the sliding behavior.
Highlights
According to the microscopic friction law by Bowden and Tabor Ff = τ Ar (Bowden and Hughes, 1939) there are two entities that determine the magnitude of friction Ff : the shear stress τ as a joint contribution of upper and lower friction body and the real area of contact Ar
The shear stress τ is a complex entity, since it comprises the near-surface shear conditions of the ski base, which can be altered by ski preparation
Τ is strongly influenced by a nanometer-thin water film due to pre-melting of snow. This water film grows upon mechanical contact, mainly by the action of frictional heat. τ cannot be measured and will be treated as a constant in this contribution, as it is well-known that this assumption only holds under well-defined lab-conditions
Summary
According to the microscopic friction law by Bowden and Tabor Ff = τ Ar (Bowden and Hughes, 1939) there are two entities that determine the magnitude of friction Ff : the shear stress τ as a joint contribution of upper and lower friction body and the real area of contact Ar. The shear stress τ is a complex entity, since it comprises the near-surface shear conditions of the ski base, which can be altered by ski preparation. Τ is strongly influenced by a nanometer-thin water film due to pre-melting of snow This water film grows upon mechanical contact, mainly by the action of frictional heat. The real area of contact between the grinding structure with the snow withdraws itself from direct observation. This phenomenon is the inbuilt calamity of tribology as the science of friction, wear, and lubrication
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