A deflection resistant combination slip plate and turn plate assembly for support the steered wheels of a vehicle on a precision planar surface resistant to deflection and distortion. The slip plate and the turn plate are supported on an underlying surface by bearing assemblies, and establish a upper planar surface with the turn plate disposed in a cutout region of the slip plate. Each plate is configured for limited translational movement relative to the base plate surface on which it is supported, while the turn plate is further configured for rotational movement about a central axis. Translational movement of the plates is synchronized. Spacing between the slip plate and turn plate is minimized, and supporting means are provided to maintain the slip plate and turn plate in vertical alignment under load.