a suspension system for improving the ride quality and service life of the tractor used in a tractor - trailer combination . an a - shaped sub - frame is connected at its front apex and with the tractor frame for universal pivotal movement and is rigidly connected at its back end with the rear axle of the tractor . the lift cylinders for the fifth wheel that bears the trailer weight are mounted on the arms of the sub - frame , thus applying the trailer load to the sub - frame rather than to the tractor frame . suspension elements are interposed between the tractor axle and frame and provide the same cushioning effect regardless of whether the tractor is traveling empty or fully loaded . the universal joint mounting of the sub - frame allows tilting of the rear axle and accommodates other irregularities while avoiding undue stresses on the axle . a transverse link applies side loads from the sub - frame to the frame to avoid undue stressing of the spherical bearing that mounts the sub - frame .