A caster wheel assembly 1' for a shopping trolley comprises a support member 2' fixed to a trolley chassis, a carrier member 4' freely rotatable about a vertical axis 5' with respect to the support member 2', and a wheel 6' carried by the carrier member 4' so as to be rotatable about a horizontal axis 7' offset from the vertical axis 5'. A cam follower 24' in the form of two planet wheels 26' rotatably mounted on a lower portion 30 of the support member 2' engages a cam surface 21' having an undulating profile on the carrier member 4' in order to bias the wheel 6' of the assembly towards a direct forward line. In order to steer the trolley to one side or the other of the direct forward line, a sufficient steering force must be applied to the trolley in order to overcome this biasing action, and this overcomes any tendency of the trolley to move in a direction other than the intended direction of travel.