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A mower having a cutting deck for cutting grass has four drive wheels arranged in a square or rectangular configuration on the mower's frame. Four wheel drive and four wheel steering are employed with the drive wheels. The four wheel drive is provided by a hydraulic drive system that employs a pair

A mower having a cutting deck for cutting grass has four drive wheels arranged in a square or rectangular configuration on the mower's frame. Four wheel drive and four wheel steering are employed with the drive wheels. The four wheel drive is provided by a hydraulic drive system that employs a pair of identical serial circuits which have their own pumps with each pump serially sending its output through a rear drive motor and a front drive motor that are catty-corner to one another on the frame, i.e. are on opposite sides of the motor. Various flow crossover paths and flow recirculation paths are provided in the serial circuits to ensure the drive motors receive adequate flow in maximum turns of the mower. The flow recirculation paths around a pair of drive motors at one end of the frame are shut off during reverse and during braking so that all drive motors retain hydraulic braking in these conditions.

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1. A mower with four wheel drive, which comprises: (a) a frame supported for rolling over the ground by four ground engaging drive wheels comprising a pair of laterally spaced front drive wheels and a pair of laterally spaced rear drive wheels;(b) a grass cutting device carried on the frame; and(c)

1. A mower with four wheel drive, which comprises: (a) a frame supported for rolling over the ground by four ground engaging drive wheels comprising a pair of laterally spaced front drive wheels and a pair of laterally spaced rear drive wheels;(b) a grass cutting device carried on the frame; and(c) a hydraulic drive system for propelling the drive wheels, which comprises: (i) a plurality of individual hydraulic drive motors for rotating the drive wheels with one drive motor being operatively connected to each of the drive wheels such that the plurality of drive motors comprise a pair of front drive motors for the front drive wheels and a pair of rear drive motors for the rear drive wheels;(ii) a hydraulic fluid supply circuit that comprises: a pair of identical serial flow circuits, wherein each serial flow circuit comprises a hydraulic pump that serves as a source of pressurized hydraulic fluid, the pump having an outlet that is directed in a serial manner first through one rear drive motor and then through one front drive motor for one pair of rear and front drive wheels on opposite sides of the frame, the front drive motor having an outlet that is serially connected by a return line back to an inlet of the pump; anda crossover path between the outputs of the pumps, wherein the pump output crossover path contains an orifice that is sized to allow a maximum of substantially enough flow to crossover from one pump output to the other pump output such that whatever rear drive motor for the rear drive wheel is on an outside of a maximum turn receives adequate flow to permit smooth rotation of the rear drive wheel on the outside of the maximum turn without scrubbing or skidding of the rear drive wheel. 2. The mower of claim 1, wherein the front drive motor in each serial flow circuit is provided with a return recirculation path protected by a check valve which check valve opens to allow hydraulic fluid leaving the front drive motor through the outlet thereof to flow back to an inlet of the front drive motor to permit the front drive motor to speed up during turns of the mower if the front drive wheel to which the front drive motor is coupled is on an outside of the turn. 3. The mower of claim 2, wherein the return recirculation paths around the pair of front drive motors are shut off when the mower is being operated in reverse or when the mower is being braked by the operator retarding the speed of rotation of the drive motors such that hydraulic braking is maintained on the front drive wheels. 4. The mower of claim 3, wherein an on-off valve is placed in each return recirculation path. 5. The mower of claim 4, wherein the on-off valve is automatically placed in an open position during forward motion of the mower and is automatically placed in an off position during reverse motion of the mower and during braking of the mower without the need for operator intervention. 6. The mower of claim 5, wherein the on-off valve is piloted to an open position by output pressure of one of the pumps in one of the serial circuits when the mower is being operated in a forward direction which output pressure is sufficient to open the on-off valve against the bias of a spring and against pressure in the return line in the return recirculation path, the bias of the spring and the pressure in the return line in the return recirculation path becoming stronger and overpowering the output pressure when the mower is in reverse and is being braked. 7. The mower of claim 1, further including a four wheel steering system for pivoting each of the drive wheels about a substantially vertical pivot axis with respect to each wheel with the front drive wheels pivoting about their pivot axes in a direction opposite to that of the rear drive wheels. 8. The mower of claim 1, wherein the hydraulic fluid supply circuit further includes a crossover path in the serial circuits at a location between the front and rear drive motors in the circuits, the series crossover path comprising a fixed orifice and bi-directional pressure relief valving that is sized to allow an excess increment of flow in the serial circuit feeding the drive motor for the front drive wheel on an inside of the maximum turn to be diverted into the serial circuit feeding the drive motor for the front drive wheel on the outside of the maximum turn. 9. The mower of claim 8, wherein the front drive motor in each serial flow circuit is provided with a return recirculation path protected by a check valve which check valve opens to allow hydraulic fluid leaving the front drive motor through the outlet thereof to flow back to an inlet of the front drive motor to permit the front drive motor to speed up during maximum turns of the mower if the front drive wheel to which the front drive motor is coupled is on the outside of the maximum turn. 10. The mower of claim 9, wherein the return recirculation paths in the serial circuits are shut off except during forward, unbraked motion of the mower. 11. The mower of claim 9, further including a crossover path between the inlets of the pumps, wherein the pump inlet crossover path has an orifice that permits return flow in the serial circuits returning to the pumps to crossover from one serial circuit to the other serial circuit to balance the return flows to the inlets of the pumps. 12. A mower with four wheel drive, which comprises: (a) a frame supported for rolling over the ground by four ground engaging drive wheels comprising a pair of laterally spaced front drive wheels and a pair of laterally spaced rear drive wheels;(b) a grass cutting device carried on the frame; and(c) a hydraulic drive system for propelling the drive wheels, which comprises: (i) a plurality of individual hydraulic drive motors for rotating the drive wheels with one drive motor being operatively connected to each of the drive wheels such that the plurality of drive motors comprise a pair of front drive motors for the front drive wheels and a pair of rear drive motors for the rear drive wheels; and(ii) a pair of serial hydraulic flow circuits, each serial flow circuit directing pressurized hydraulic fluid in a serial manner through one front drive motor and one rear drive motor;(iii) wherein one drive motor in each serial flow circuit is provided with a return recirculation path protected by a check valve which check valve opens to allow hydraulic fluid leaving the one drive motor through an outlet thereof to flow back to an inlet of the front drive motor to permit the one drive motor to speed up during turns of the mower if the drive wheel to which the one drive motor is coupled is on an outside of the turn; and(iv) wherein the return recirculation paths in the pair of serial flow circuits are shut off when the mower is being operated in reverse or when the mower is being braked by the operator retarding the speed of rotation of the drive motors such that hydraulic braking is maintained on the drive wheels whose motors have the return recirculation paths. 13. The mower of claim 12, wherein the front and rear drive wheels powered by the front drive motor and rear drive motor in each serial flow circuit are on opposite sides of the mower frame. 14. The mower of claim 13, wherein each serial flow circuit sends the pressurized fluid first through the rear drive motor and then through the front drive motor in a forward direction of motion of the mower frame. 15. A mower with four wheel drive, which comprises: (a) a frame supported for rolling over the ground by four ground engaging drive wheels comprising a pair of laterally spaced front drive wheels and a pair of laterally spaced rear drive wheels;(b) a grass cutting device carried on the frame; and(c) a hydraulic drive system for propelling the drive wheels, which comprises: (i) a plurality of individual hydraulic drive motors for rotating the drive wheels with one drive motor being operatively connected to each of the drive wheels such that the plurality of drive motors comprise a pair of front drive motors for the front drive wheels and a pair of rear drive motors for the rear drive wheels;(ii) a hydraulic fluid supply circuit that comprises: a pair of identical serial flow circuits, wherein each serial flow circuit comprises a hydraulic pump that serves as a source of pressurized hydraulic fluid, the pump having an outlet that is directed in a serial manner first through one drive motor for a drive wheel on a first end of the frame and then through one drive motor for a drive wheel on an opposite end of the frame for one pair of rear and front drive wheels on opposite sides of the frame, the drive motor for the drive wheel on the opposite end of the frame having an outlet that is serially connected by a return line back to an inlet of the pump; anda crossover path between the outputs of the pumps, wherein the pump output crossover path contains an orifice that is sized to allow a maximum of substantially enough flow to crossover from one pump output to the other pump output such that whatever drive motor for the drive wheel on the one end of the frame that is on an outside of a maximum turn receives adequate flow to permit smooth rotation of the drive wheel on the one end of the frame on the outside of the maximum turn without scrubbing or skidding of the drive wheel on the one end of the frame on the outside of the maximum turn.