A spherical ball rolls without slipping along a surface, with uniform velocity v/c (close to 1) with respect to the lab frame. It is thus seen to contract by a factor of [tex]\gamma[/tex], forming a grape-like shape. Why doesn't it wobble as it rolls, like a rolling grape? One can certainly argue from the principle of relativity: a state of absolute rest could be determined by setting up a detector that beeps if the ball experiences a change in maximum distance from the surface, as it would if it rolled like a grape. But this is a principled argument. What is the mechanism?