An agricultural machine includes a wheel at each end of a transverse main frame with each wheel being carried by a vertically pivotal link to which an extensible and retractable actuating device is coupled for effecting vertical adjustment of the frame relative to the ground. The upper end of each actuating device is attached to a further link which is vertically pivotally mounted to the frame and has its upward movement resisted by a frame-mounted spring so as to dampen forces imposed on the wheel by the terrain. Each further link may be selectively locked to the frame so as to eliminate the function of the associated spring.