The truck comprises a front part pivotally connected to a rear part via a king pin 14. A steering motor drives gear 12 meshed with gear 18 to cause the steering effect. The front wheels 20 are driven by a motor 30 via differential 32 and clutches 26, 28 are located in the respective half shafts 22, 24. When the steering angle exceeds a predetermined amount, eg 45 degrees, in either direction, the corresponding clutch is actuated to disconnect drive to the wheel which is nearer to the centre about which the truck is turning. Thereafter the outside one of the wheels 20 is driven and the other three are free to turn in either direction or remain stationary as the vehicle is manoeuvred by the driver. The drive may be hydrostatic with a motor in the rear part driving a pump feeding a motor at each driven wheel. At the threshold steering angle, a spool valve cuts the supply to the inner wheel motor. Alternatively, the drive may be electric with an electric motor for each driven wheel, the inner motor being switched out of circuit at the threshold angle. IMAGE
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