are operated only when the vehicle is running and the cab tilt device 66 is used only when it is parked therefore the same fluid source 62 for cab tilt can be efficiently utilized both when the vehicle is parked and when it is running in addition no separate fluid source for the hydraulic cylinder means of the control apparatus is required so the structure is simplified the fluid source 62 for the cab tilt device 66 need not be used as a fluid source for the hydraulic cylinder means and any suitable fluid source may be used to move the piston of the hydraulic cylinder means the magnitude of the clamping force may be continuously or stepwise varied in accordance with the level of the oil pressure in the power steering assembly 74 the clamping force may be regulated by manual control 86 of the pressure of the compressed oil by the control valve 60 the speed of the vehicle may be also detected in addition to the oil pressure for power steering in this case the pressure of the compressed oil may be controlled in association with the vehicle speed as described above in this invention the increase in the power steering oil pressure at a turn is detected to control the friction between the leaf springs the detecting system can be of an extremely simple structure as compared with that in.