The Split Friction Model: The Isotropic Origin of the Lateral Force in Curling

The deviation of a curling stone with 1–5 rotations in a normal game can be explained with the isotropic nature of friction. Due to isotropy, the lateral force arises as a delta of asymmetric friction forces opposite to the centripetal forces. Considering lubricant accumulation, the quadratic Stribeck-curve on the advancing side is modified to show rather constant values. Below a critical velocity difference of the two sides, this will result in a curl. For few rotations, an exact calculation based on widely accepted experimental measurements explains the observed deviation of roundabout 1 m declining with increasing rotations. The deviation above 5 rotations can be explained considering statics and additional heuristic calculations that involve the established concept Scratch Theory with respect to gyroscopic precession as well as crush probability. All this then yields a graph that also explains the observed increasing deviation up to 2 m for 80 rotations.