A Model of Ice Friction for Skeleton Sled Runners

A numerical model of ice friction for the runners of a skeleton sled has been devised. The skeleton runner consists of a standard stainless steel rod of approximately 16 mm diameter. Two grooves are machined into the trailing half of the runner. The distance between the grooves is approximately 1mm, giving rise to a spine or blade at the centre of the runner. The skeleton sled has no apparent mechanism for steering. Hence, steering is accomplished by the athlete, using either lateral air drag forces (tilting the helmet), dragging a toe, or by attempting to make the spine of the runner "dig into" the ice more on one side than the other. Until now, the physics of the latter steering mechanism has been poorly understood. It has been assumed that when the spine "digs into" the ice, the ice friction increases. Our numerical model calculates the details of the contact footprint of the runner on the ice. It also considers frictional heating, heat conduction into the ice and lateral squeeze flow in order to calculate the ice friction coefficient, assuming fully lubricated friction conditions. The model suggests that skeleton sliding can occur in two regimes. The first is one where the sides of the spine do not contact the ice. The second occurs when the spine "digs into" the ice and the sides of the spine contact the ice. By exploring the second regime, we have shown that, as the contact area between the sides of the spine and the ice increases, the ploughing force increases, in accordance with the traditional explanation of steering. However, the shear stress force in the lubricating layer also increases, resulting in a significantly higher ice friction coefficient for the runner with the longer spine contact. This result provides scientific evidence to support the athlete's experience, that by engaging more of the spine by torqueing the frame of the sled, it is possible to steer the sled, using the differential ice friction on the left and right runners.