will assume its preset ride height of the loading platform and the frame structure when the switch is set to its third position 97 the ride height is adjustable within the second smaller range 21 a and 21 b the third switch 93 can be switched between a first position 98 a second position 99 and a neutral position when the third switch 93 is set to its first position 98 the desired elevation is increased when the third switch 93 is set to its second position 99 the desired elevation is decreased when the third switch 93 is set to its neutral position the first and the second valve means will assume such positions as to allow neither compressed air into or out from the pneumatic suspension means this means that the first the third and the fourth solenoid valves 50 52 and 69 are all set to their respective second positions 50 b 52 b and 71 b the elevation adjustment i e the setting of the desired height level of the loading platform and of the size of the desired distance between the wheel axle and the frame structure respectively within the smaller range when the control unit is set into its driving mode as well as the larger range when the control unit is set into.