1. A WHEELED VEHICLE INCLUDING A CHASSIS, A PAIR OF FRONT WHEELS SUPPORTED ON THE CHASSIS BY MEANS OF A FRONT CROSS MEMBER FOR ROTATION ABOUT A FRONT WHEEL AXIS, AT LEAST ONE REAR WHEEL MOUNTED FOR ROTATION ON THE CHASSIS ABOUT A REAR WHEEL AXIS, MEANS TO DRIVE SAID REAR WHEEL, SAID FRONT WHEEL AXIS AND SAID REAR WHEEL AXIS BEING IN PARALLEL VERTICAL PLANES FOR STRAIGHT FORWARD MOTION OF THE TRACTOR, A HORIZONTAL TOOL BAR EXTENDING IN A PLANE PARALLEL WITH SAID WHEEL AXIS PLANES AND IN FRONT OF SAID FRONT WHEEL AXIS, EARTH WORKING MEANS MOUNTED ON SAID TOOL BAR, A TOOL BAR SUPPORT COMPRISING SIDE MEMBERS HAV ING A HORIZONTAL PORTION AND A FORWARD DOWNWARDLY EXTENDING PORTION PIVOTALLY CONNECTED TO SAID CHASSIS BETWEEN SAID FRONT AND REAR WHEEL AXES, SAID PIVOTAL CONNECTION PERMITTING FREE PIVOTAL MOVEMENT OF SAID TOOL BAR SUPPORT RELATIVE TO SAID CHASSIS ABOUT SAID PIVOTAL CONNECTION IN DIRECTION TO RAISE SAID TOOL BAR, LIMIT MEANS MOUNTED ON SAID FRONT CROSS MEMBER, SAID LIMIT MEANS BEING POSITIONED BENEATH SAID HORIZONTAL PORTION OF SAID SIDE MEMBERS TO RESTRICT MOVEMENT OF SAID TOOL BAR BEYOND A PRE-DETERMINED POSITION RELATIVE TO SAID CHASSIS IN A DOWNWARD DIRECTION, AN ARM EXTENDING FORWARDLY FROM SAID TOOL BAR SUPPORT, AND A DEPTH CONTROL MEMBER CONNECTED TO SAID ARM TO CONTROL THE NORMAL OPERATIONAL HEIGHT OF SAID TOOL BAR ABOVE SAID LIMIT MEANS, SAID DEPTH CONTROL MEMBER BEING ARRANGED TO ENGAGE THE GROUND IN FRONT OF SAID TOOL BAR.