A wheel system for providing one or more protrusions in a ground-engaging surface of the wheel. In some embodiments, the wheel comprises a first hub, a tire mounted on the first hub, a second hub rotatable with respect to the first hub, and system for deforming the tire in a generally radial direction in response to relative rotation between the first and second hubs. The invention also provides a wheel cover comprising an outer surface having one or more protrusions and, when the cover is fitted to a wheel, the outer surface of the cover provides the ground-engaging surface of the wheel. The wheel and wheel cover of the invention are particularly suited for use with prams and similar vehicles in order to impart a rocking motion to the pram during use.