Abstract: A hydraulic gear motor is supported by supporting structure secured to an axle adjacent a ground engaging wheel. The support structure is pivotally mounted upon a bolt or shaft extending upwardly from the supporting structure and the hydraulic motor, together with a sprocket driven thereby, is capable of vertical movement on the bolt or shaft and is held in the uppermost position by means of a detachable lock pin through the bolt or shaft. A length of sprocket chain is secured in the form of a circular track, to adjacent the rim of the wheel and concentric with the rotatable axis thereof. When the hydraulic motor and sprocket are in the uppermost position, the sprocket engages the inner surface of the chain whereby rotation of the sprocket, rotates the ground engaging wheel. A pivotal retaining bracket holds the hydraulic motor in the innermost position when the sprocket engages the chain.