1. IN A VEHICLE HAVING A CHASSIS SUPPORTED ON A RUNNING GEAR COMPRISING GROUND ENGAGING TRACTION MEMBERS SUCH AS WHEELS OR RUNNERS, A LOAD BEARING FRAME MOUNTED FOR LIMITED RELATIVE VERTICAL MOVEMENT ON SAID CHASSIS, MEANS FOR MAINTAINING A PRESSURIZED AIR CUSHION BENEATH SAID VEHICLE PROVIDING SUPPLEMENTARY SUPPORT LIFTING SAID LOAD BEARING FRAME RELATIVE TO THE CHASSIS AS THE VEHICLE MOVES ALONG THE GROUND, AND MEANS ACTUATED BY LIFT OF SAID FRAME RELATIVE TO THE CHASSIS FOR AUTOMATICALLY CONTROLLING THE AIR PRESSURE IN SAID CUSHION TO LIMIT THE DEGREE OF SAID SUPPLEMENTARY SUPPORT FOR MAINTAINING THE TRACTION MEMBERS IN CONTACT WITH THE GROUND.