PURPOSE: To provide a device which increases the ground contact pressure of a drive shaft tire when a vehicle starts.

CONSTITUTION: A vehicle 1A has a drive axle 31 and a driven axle 32 supported by a suspension 100 to a frame 10. The suspension 100, a trunnion type, supports the axles 31 and 32 through the bracket 110, leaf spring 120 and torque rod 130 fixed to the frame 10. Between the bracket 140 provided integrally with the drive axle 31 and the frame 10, air spring 150 is provided. The air spring 150 has a diaphragm 152, piston 154 and upper supporting section 156, and the piston 154 is mounted on the bracket on the drive axle side. The upper supporting section 156 is supported by the lower face of the frame 10 through a suitable bracket. When the vehicle starts, compressed air is supplied to the air pressure 150 to increase the ground contact pressure of the drive axle tire 31a and increase its thrust force, assisting the start of the vehicle.