pivot pins 122 124 projecting from the upper and lower surfaces of the steering axle 110 are journalled by means of part spherical bearing shells 136 in this way the steering axle 110 can pivot about the vertical axis 126 relative to the skate 100 to produce the desired compound steering movement the skate 100 is itself journalled by means of its pivot pins 101 and 102 onto a body member 140 that forms part of the body of the tractor this allows the skate 100 to pivot together with the steering axle 110 about the fore aft axis 128 which enables the wheels 112 at the opposite ends of the steering axle 110 to move up and down relative to the tractor body when the tractor is moving over uneven ground the pivoting about the fore aft axis 128 is limited in that surfaces 106 on the opposite sides of the plate 103 come into contact with adjustable end stops on the underside of the body member 140 of the tractor the advantage offered by the invention is that even when the skate 100 has moved to a limit position against an end stop the steering axle 110 can still rotate about the vertical axis 126 without the stop surfaces rubbing against one another not only.