Abstract: An apparatus is disclosed for the evaluation and diagnosis of tire contributions to annoying erratic motions of a vehicle, which motions are induced when the vehicle travels over longitudinally grooved pavement. The apparatus utilizes a vehicle the forward end of which is supported on a laterally movable carriage, with freedom to yaw, and the rear end of which carries a loaded test tire and an axially spaced companion tire on respective curcumferentially grooved and circumferentially smooth tire driving means. Sensor means are provided to measure transverse or angular motions or displacements generated as the test tire moves laterally across the grooved drive surfaces in response to lateral movement of the carriage.