Rolling friction is commonly associated with deformation of the rolling object or surface,
particularly if the object or surface is soft. When a hard object rolls on a hard surface, surface
roughness is also a significant factor. Several experiments are described where the effects of
surface roughness on rolling resistance were investigated using very rough rolling objects (gear
wheels), and smooth balls on very rough surfaces. In all cases it was found that the coefficient of
rolling friction increased with rolling speed and surface roughness, and decreased as the ball
diameter increased.