A variable track width vehicle axle has an outer axle casing (12) which supports two drive shafts (13) which are driven at their inner ends by drive means (23) and carry wheel attachment means (13a) at their outer ends. Each drive shaft including first and second parts (13a,13b) which are in screw-threaded engagement with each other so that rotation of one part relative to the other changes the effective length of each shaft and thus the track width of the axle. A clutch means (16) is also provided which in a first position connects the two parts (13a, 13b) of each shaft to prevent relative rotation and in a second position disconnects the two parts of each shaft to allow relative rotation therebetween and rotates one of the screw threaded parts relative to the other part (13b) via the drive means (23) to change the effective length of each drive shaft.