A hub drive has the drive shaft (28) coupled to the hub (30) via two splined profiles (78,64) which are bridged by a sliding coupling ring (80) which is moved between the coupled and uncoupled positions by a servo drive (88) e.g. a solenoid. The clutch is fitted into the front or rear hubs of a vehicle to couple normally free running wheels into the power train. A simple control system for the coupling ring has the servo drive operating against a return spring, which returns the hub drive into the free running setting.
