and works very well for 8 ton and also 10 ton soil rollers the solenoid valve sends controlled pressure to either the maximum or minimum displacement control piston not shown in each motor in this embodiment it takes 12 volts to engage minimum displacement and the lack of 12 volts selects maximum displacement the solenoid valve controls can be manually operated by a dash mounted switch not shown the switch can be in the form of a switch valve which depending on preferred action as described controls the sending of the 12 volts to the motors 20 30 in order to switch between turtle rabbit and reverse slope operation if the vehicle for instance a road roller moves forward the operator can manually set the displacement of the front and rear motors 30 20 respectively to the suitable control settings namely the turtle setting or rabbit setting in the embodiment shown in fig5 the front motor 30 is assigned to the front drum of the road roller depending on the desired driving speed the motors 20 30 are manually set at 25 cc per rev for higher speeds with less power or the front motor 30 is set at the lower displacement of 25 cc per rev and the rear motor 20 is set at the higher displacement 75 cc per rev when a lower speed is desired or forward travel of steep slopes because of this unequal distribution of the torques to the front drive 60 and rear drive 50 of the driven vehicle during uphill motion the drive which is relieved of load by the inclination of the plane is driven with less moment for example if a vehicle is moving up a hill the leading drive tends to slip first this tendency.