Four-wheel drive vehicle

1. A four-wheel drive vehicle comprising:

a drive source which produces torque forming a driving force of the vehicle;

a driving force transmission system which transmits the torque of the drive source to a pair of front wheels and a pair of rear wheels via a plurality of gear mechanisms;

a clutch provided in the driving force transmission system and controlling an amount of the torque transmitted to the rear wheels; and

a pressing mechanism which presses the clutch by transforming a part of the torque transmitted to the rear wheels into axial thrust against the clutch,

wherein in the driving force transmission system, a first drive shaft is connected between an input side of the clutch and a first differential unit which splits the torque to the pair rear wheels, a second drive shaft is connected between an output side of the clutch and one of the rear wheels, and gear ratios of the plurality of gear mechanisms are set to prevent reversal of a direction of a differential rotation between the first drive shaft connected to the input side of the clutch and the second drive shaft connected to the output side of the clutch even when a steered angle of the front wheels changes to a maximum value while the four-wheel drive vehicle is traveling forwards.

2. The four-wheel drive vehicle according to claim 1 , wherein the driving force transmission system comprises:

the first differential unit which includes a first differential case;

a second differential unit which splits the torque to the pair front wheels, and which includes a second differential case;

a propeller shaft provided between the first and second differential units;

a first gear mechanism which connects the first differential unit and the propeller shaft; and

a second gear mechanism which connects the second differential unit and the propeller shaft,

wherein the clutch is disposed between the first differential unit and one of the rear wheels, and

wherein gear ratios of the first and second gear mechanisms are set such that a rotation speed of the first differential case of the first differential unit relative to a rotation speed of the propeller shaft becomes smaller than a rotation speed of the second differential case of the second differential unit relative to the rotation speed of the propeller shaft.

3. The four-wheel drive vehicle according to claim 1 , wherein the pressing mechanism includes an electromagnetic coil.

4. The four-wheel drive vehicle according to claim 1 , wherein the pressing mechanism includes a first cam, a second cam, and spherical cam followers interposed between the first cam and the second cam, and

the first cam and the second cam rotate relative to each other, and the cam followers roll on cam surfaces of the first cam and the second cam by the relative rotation, whereby the axial thrust is produced.

5. The four-wheel drive vehicle according to claim 1 , wherein a gear ratio of a first gear mechanism which connects the first differential unit and a first end of a propeller shaft is greater than a gear ratio of a second gear mechanism connected to a second end of the propeller shaft.

6. A four-wheel drive vehicle comprising:

a drive source which produces torque forming a driving force of the vehicle;

a driving force transmission system which transmits the torque of the drive source to a pair of front wheels and a pair of rear wheels via a plurality of gear mechanisms;

a clutch provided in the driving force transmission system and controlling an amount of the torque transmitted to the rear wheels; and

a pressing mechanism which presses the clutch by transforming a part of the torque transmitted to the rear wheels into axial thrust against the clutch, wherein the pressing mechanism includes an electromagnetic coil, and

wherein in the driving force transmission system, gear ratios of the plurality of gear mechanisms are set to prevent reversal of a direction of a differential rotation between a first drive shaft connected to an input side of the clutch and a second drive shaft connected to an output side of the clutch even when a steered angle of the front wheels changes to a maximum value while the four-wheel drive vehicle is traveling forwards.

7. A four-wheel drive vehicle comprising:

a drive source which produces torque forming a driving force of the vehicle;

a driving force transmission system which transmits the torque of the drive source to a pair of front wheels and a pair of rear wheels via a plurality of gear mechanisms;

a clutch provided in the driving force transmission system and controlling an amount of the torque transmitted to the rear wheels; and

a pressing mechanism which presses the clutch by transforming a part of the torque transmitted to the rear wheels into axial thrust against the clutch, wherein the pressing mechanism includes a first cam, a second cam, and spherical cam followers interposed between the first cam and the second cam, the first cam and the second cam rotate relative to each other, and the cam followers roll on cam surfaces of the first cam and the second cam by the relative rotation, whereby the axial thrust is produced, and

wherein in the driving force transmission system, gear ratios of the plurality of gear mechanisms are set to prevent reversal of a direction of a differential rotation between a first drive shaft connected to an input side of the clutch and a second drive shaft connected to an output side of the clutch even when a steered angle of the front wheels changes to a maximum value while the four-wheel drive vehicle is traveling forwards.