A device including a torsion bar rotatably mounted on the vehicle frame transversely thereof and a pair of rock arms extending integrally from the opposite ends of the torsion bar at right angles thereto and connected with a right and a left wheel, respectively. An abutment arm is fixed to the torsion bar at the middle thereof and engageable with a stop pad provided on the vehicle frame, when either or both of the wheels are moved down, to restrain further downward movement thereof. Thus, the spring rate of air springs associated with the respective wheels is in effect increased as the loads thereon are reduced, particularly in a lower load region.