JP3250813B2 - Two-wheel drive system for motorcycles - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a motorcycle and, more particularly, to a two-wheel drive system for a racer that travels on uneven terrain such as a motocross vehicle.

[0002]

2. Description of the Related Art A two-wheel drive system comprises a rigid type in which front and rear wheels are mechanically connected by a constant velocity joint and a chain, and a viscous cup which allows a slight relative rotation difference between the front and rear wheels. It is roughly classified into non-rigid type with a ring etc. interposed.

In the former, as shown in FIG. 8, a rear wheel 2 and a front wheel 1 are mechanically connected by drive chains 4, 6, 7, 9 and a constant velocity joint 8. That is, the rear wheel 2 is mechanically and operatively connected to the output shaft 5 of the transmission 3 via the drive chain 4 for driving the rear wheel.
Is mechanically linked to the output shaft 5 via the drive chains 9, 7, 6, and the constant velocity joint 8 and the like, and the front and rear wheels 1, 2 constantly generate a driving force. According to this structure, when the vehicle travels straight on a slippery road surface, on the other hand, when the vehicle travels on a road surface with good grip, the rear wheel 2 tends to pass inside the trajectory of the front wheel 1. Therefore, the speed of the front wheels 1 starts to slightly increase, and the driving force of both wheels decreases unless the front wheels slip. Therefore, this type of two-wheel drive is not very suitable for general traveling.

[0004] The latter non-rigid type is not present, but is generally considered to be a passive type having a viscous coupling 10 as shown in FIG. 9 and a differential type as shown in FIG. There are a differential type equipped with a wet type multi-plate hydraulic clutch 20 that can adjust the transmission force by changing the operating oil pressure as shown in FIG. 11, but the following problems arise.

[0005]

The passive type using the viscous coupling 10 shown in FIG. 9 or a fluid coupling similar thereto has a considerable rotational speed difference between the front and rear wheels 1, 2. It is difficult to generate the necessary driving force for the front wheels 1.

[0006] Next, as shown in Fig. 10, a differential type using a differential gear 15 composed of a planetary gear or a bevel gear is effective for a traveling road surface having a good grip. If one of the wheels slips, the driving force of both wheels may decrease.

The active two-wheel drive system shown in FIG. 11 includes a wet multi-plate hydraulic clutch 20, a hydraulic oil pump 25 for the clutch, a pressure regulating valve 21, and an oil reservoir tank 2 for hydraulic oil.
2 and a controller 23, etc., and the power distribution of the front and rear wheels 1 and 2 is adjusted depending on the adjustment of the operating oil pressure, so that the front wheel driving force can be positively adjusted. The active type, which uses a hydraulic clutch and actively takes out the necessary front wheel driving force, can be said to be an ideal two-wheel drive with electronic control, but the structure is complicated and the cost is high. Weight also increases considerably. It is estimated that the so-called cost performance is not good.

In the above-described two-wheel drive system regardless of the rigid or non-rigid type, in order to drive the front wheel 1,
A power transmission structure capable of absorbing a change in inclination of the front wheel 1 due to steering and a displacement due to a movement of the front wheel suspension is required. For this reason, as shown in FIGS. 8 to 11, in a conventional driving device or a generally conceivable driving device in which a constant velocity joint 8 and a chain drive are combined, a driving sprocket is provided at a swing center of a front wheel suspension. Although the change in the distance to the front wheel center is eliminated, this method tends to limit the steering angle to a small value due to the limit value of the inclination angle of the constant velocity joint 8. Alternatively, the suspension becomes a swing arm system instead of the conventional telescopic system, and the unsprung weight increases. Further, since the front wheel drive device is concentrated in front of the vehicle, the front half of the vehicle becomes heavy. As a countermeasure, it is necessary to review the positions of all components to adjust the center of gravity.

[0009]

SUMMARY OF THE INVENTION An object of the invention described in claim 1 of the present application is to provide a so-called passive two-wheel drive system, in which the overall drive force is increased, a wide front wheel steering angle is secured, and the front wheel drive force during rear wheel slip is reduced. Generation, suppression of unsprung load, weight reduction, and suppression of front weight increase. A second object of the present invention is to provide a passive two-wheel drive device,
Two-wheel drive is performed only when necessary, and the driving force of the front wheels can be cut off when not necessary, thereby providing an effective two-wheel drive mode. The object of the invention described in claim 3 is that the power distribution to the front and rear wheels can be actively changed,
An object of the active type is to achieve the same object as described above.

[0010]

According to the first aspect of the present invention,
A front wheel hub supporting the front wheel is formed in a bottomed cylindrical shape, a hydraulic motor is arranged on the inner peripheral side of the front wheel hub, a motor case is fixed to one front fork, and a motor shaft is fixed to the other front fork. Is rotatably supported, a front wheel hub is fixed to the motor shaft, a hydraulic pump is interlockedly connected to the rear wheel so as to rotate in proportion to the rotation speed of the front wheel hub, and the hydraulic motor and the hydraulic pump are flexibly connected. The closed hydraulic circuit is configured by connecting with flexible or bendable piping. According to a second aspect of the present invention, in the first aspect of the present invention, both pipes are short-circuited between a hydraulic oil pipe and a return oil pipe which constitute the closed hydraulic circuit by connecting a hydraulic pump and a hydraulic motor. A bypass passage is provided, and the bypass passage is provided with a no-load switching valve that opens and closes the bypass passage. Connecting the detection mechanism to the drive mechanism of the switching valve,
The switching valve is opened by detecting the amount of operation when two-wheel drive is unnecessary. According to a third aspect of the present invention, in the first or second aspect of the present invention, at least one of the hydraulic pump and the hydraulic motor is of a variable displacement type, and the front and rear wheel rotational speed detection mechanisms and the accelerator opening are detected. An accelerator opening detection mechanism is provided to increase the front wheel driving force by comparing the difference between the rear wheel rotation speed and the front wheel rotation speed with a reference value determined from the rear wheel rotation speed, the rear wheel angular speed and the accelerator opening. , Or a control circuit for instructing the variable displacement type hydraulic pump or hydraulic motor to determine whether to decrease or maintain the current state.

[0011]

[Function] When the front wheel and the rear wheel are traveling straight on flat ground or the like at the same rotation speed, the hydraulic oil pump that operates according to the rear wheel rotation speed and idles according to the front wheel rotation speed. Since the amount of suction oil of the hydraulic motor to be driven is substantially equal, there is no power transmission between the hydraulic pump and the hydraulic motor in principle, and no driving force is transmitted to the front wheels. When slippage occurs in the rear wheels at the time of turning or the like, the rotation of the hydraulic pump increases, pressure oil is supplied to the hydraulic motor, and the front wheels are driven by the hydraulic motor. According to the second aspect, for example, when the clutch is completely disengaged or when the front brake is on, the switching valve in the closed hydraulic circuit is opened to set the hydraulic oil to be bypassed, so that the front and rear are set. Even when there is a difference in rotational speed between the wheels, the two-wheel drive does not generate front wheel drive force as compared with the case where the driving performance is impaired, and an effective two-wheel drive mode is achieved. In claim 3, the capacity of the hydraulic pump or the hydraulic motor is automatically adjusted, and it is possible to make an active change exceeding the range of the front wheel driving force generated passively only in response to the slip speed, Two-wheel drive traveling suitable for various traveling conditions can be performed.

[0012]

FIG. 1 is a schematic diagram of a hydraulic circuit of a two-wheel drive device according to the first embodiment of the present invention. A hydraulic motor 38 capable of driving the front wheel 1 is provided, and a transmission 3 is provided on a rear wheel 2.
The hydraulic pump 35 is interlocked with the output shaft 5 through the output shaft 5 so that the hydraulic pump 35 rotates in proportion to the rotation speed of the rear wheel 2. The hydraulic pump 35 and the hydraulic motor 38 are connected by a flexible pressure oil supply pipe 41 such as a pressure-resistant rubber hose and a return oil pipe 42 to form the closed hydraulic circuit 18. When the vehicle travels straight on a road surface without a normal slip, the discharge amount of the hydraulic pump 35 driven by the rear wheels 2 is substantially equal to the suction amount of the hydraulic motor 38 that rotates with the front wheels 1.

Therefore, when both the front and rear wheels 1 and 2 are running without slipping, such as when traveling straight on flat ground,
There is no mutual power transfer between the hydraulic pump 35 and the hydraulic motor 38. When the rear wheel 2 slips and a relative rotational speed difference occurs between the front wheel and the rear wheels 1 and 2, a driving force (front wheel driving force) of the hydraulic motor 38 is generated in a direction to return to the original rotational relationship. That is, when the rear wheel 2 slips, a front wheel driving force corresponding thereto is automatically generated.

[0014]

FIG. 2 is a schematic diagram of a hydraulic circuit of a two-wheel drive device according to a second embodiment of the present invention. The hydraulic circuit has the same closed hydraulic circuit 18 as in FIG. 1, and the same parts as those in FIG. It is attached. A bypass passage 50 is provided in the closed hydraulic circuit 18 for communicating (short-circuiting) the two pipes 41 and 42, and an electromagnetic switching valve 51 for opening and closing the bypass passage 50 is provided in the bypass passage 50. The solenoid-operated switching valve 51 includes a solenoid 51a as a drive mechanism thereof. When the solenoid 51a is not energized, the solenoid 51 closes the bypass passage 50 as shown in FIG. 2 and normally operates between the hydraulic pump 35 and the hydraulic motor 38 of the closed hydraulic circuit 18. Maintain the state in which the oil flows. On the other hand, when the solenoid 51a is energized, the bypass passage 50 is opened and the transmission of oil between the hydraulic pump 35 and the hydraulic motor 38 is cut off by short-circuiting the pipes 41 and 42, thereby cutting off the front wheel driving force. Normally, when traveling straight on a road without slip,
The discharge amount of the hydraulic pump 35 driven by the rear wheel 2 is substantially equal to the suction amount of the hydraulic motor 38 that rotates with the front wheel 1. The solenoid 51a is connected to various detection mechanisms 47 and 48 for detecting an operation amount of a clutch or a front brake. When a predetermined operation amount is detected by the detection mechanisms 47 and 48, the solenoid 51a is energized and the electromagnetic switching valve 51 is turned on. Is to open.

FIG. 3 shows an embodiment of the second aspect of the present invention. In FIG. 3, the rear wheel 2 includes an output shaft 5 of the transmission 3 and an output sprocket 30 and a drive chain 4 for driving the rear wheels. And are mechanically interlocked and connected via. The hydraulic pump 35 is disposed above the transmission 3, and is operatively connected to the output shaft 5 of the transmission 3 via the input sprocket 32, the drive chain 6 and the pump driving sprocket 31 of the hydraulic pump 35. Therefore, the hydraulic pump 35 rotates in proportion to the rotation speed of the rear wheel 2. A hydraulic oil reservoir tank 36 is provided at an upper portion of the hydraulic pump 35, and an electromagnetic switching valve 51 having the bypass passage 50 (FIG. 2) is provided at a side portion. The hydraulic motor 38 is connected to the front wheel hub 4
The hydraulic pump 3 is disposed on the inner peripheral side of the hydraulic pump 3 and is provided with the above-described pressure oil supply pipe 41 and return oil pipe 42.
5 to form a closed hydraulic circuit 18. A flexible pressure-resistant rubber hose and a swivel joint are used for both the pipes 41 and 42. Further, oil leaks between the hydraulic motor 38 and the reservoir tank 36.
There is provided a drain pipe 43 for returning the pressure to the drain pipe 43. Since the pipe 43 does not need to have particularly high pressure resistance, it is not necessary to use a pressure-resistant rubber hose.

The detecting mechanisms 47 and 48 are each provided with a clutch 4
5 and the front brake 46. As the clutch detection mechanism 47, for example, a limit switch (not shown) is used, which does not operate when the grip stroke is between 0 and a half clutch, does not energize the solenoid 51a, and operates when the full clutching state is detected. Then, the solenoid 51a is energized to open the electromagnetic switching valve 51. Also, by adjusting the limit switch, the solenoid 51a can be energized even in the half-clutch state. For example, a limit switch (not shown) is also used as the front brake detection mechanism 48. When the brake is off, the limit switch (not shown) is not activated and the solenoid 51a is not energized. When the brake is detected, the solenoid 51a is activated to energize the solenoid 51a. Then, the electromagnetic switching valve 51 is opened.

In this embodiment, regarding the steering angle of the steering wheel 68, the opening degree of the accelerator 69, and the operation amount of the front and rear suspensions, it is considered that there is no situation in which the front wheel driving force is cut off in accordance with the change in the operation amount. Therefore, no detection mechanism is provided for the steering angle, the accelerator opening, and the suspension operation amount. That is, in the entire range of the steering angle, the entire range of the accelerator opening, and the entire range of the operation amount of the suspension, the change in the operation amount does not interrupt the front wheel driving force. However, when it is desired to cut off the front wheel driving force in the desired steering angle range, accelerator opening range or suspension operation amount range, a detection mechanism is provided at each location as appropriate, and the front wheel driving force is cut off when the operation amount is detected in a predetermined manner It can also be configured to do so.

FIG. 4 shows a specific structure of an axial plunger type hydraulic motor 38. In FIG.
A motor shaft 56 is rotatably supported at the lower end of the right front fork 53 of the left and right front forks 52 and 53 via a bearing 55. The rotary shaft 56 has a bottomed cylindrical front wheel. The hub 40 is fixed. Spokes 5 are provided on the outer periphery of the front wheel hub 40.
The rim 59 is fixed via the rim 7 and the brake disc 58 is fixed. Left front fork 5
A valve plate 60 is integrally formed at a lower end of the valve plate 2. The valve plate 60 protrudes into the front wheel hub 40, and a cylinder block 63 of the hydraulic motor 38 is placed at a leading end of the valve plate 60 in a pressed state. A motor case 6 having a bottomed cylindrical shape is provided on the outer peripheral side of the valve plate 60 so as to cover the cylinder block 63.
1 is fixed, and the outer periphery of the left end of the motor case 61 is a bearing 6.
2, the inner periphery of the front hole hub 40 is rotatably supported, and the small diameter boss portion 61a at the right end is fitted to the outer periphery of the motor shaft 56.

The cylinder block 63 is fixed to the motor shaft 56, and an odd number (seven or nine) of bores are formed on the right end face of the cylinder block 63 at equal intervals in the circumferential direction. Each bore extends in the axial direction, and a piston (plunger) 65 is inserted therein. Pipes 41, 42, and 43 are connected to the valve plate 60, and a kidney port is formed. The pressure oil is distributed to each piston 65 to generate an axial force on the piston 65. Piston 6
The tip of 5 is in pressure contact with the swash plate 67, which converts the axial force of the piston 65 into a rotational force. The swash plate 67 is supported on the right end wall of the motor case 61 via an inclined needle bearing 66 so that the swash plate 67 can freely rotate with respect to the motor case 61 at substantially the same rotational speed as the cylinder block 63. It has become. This is piston 6
This prevents an excessive slip from occurring in the press-contact portion between the swash plate 5 and the swash plate 67. The reaction force of the rotational force generated in the cylinder block 63 is held by the motor case 61 that holds the swash plate 67. The rotational force of the cylinder block 63 is transmitted to the motor shaft 56 and further transmitted to the front wheel hub 40 integrally fixed to the motor shaft 56 to drive the front wheels 1.

As in the case of straight running on flat ground, front and rear wheels 1,
When both of the vehicles 2 travel without slip, there is no mutual power transfer between the hydraulic pump 35 and the hydraulic motor 38. At the time of turning, the rear wheel 2 passes inside the trajectory of the front wheel 1, so that the front wheel speed slightly increases, and the suction oil amount of the hydraulic motor 38 becomes larger than the discharge oil amount of the hydraulic pump 35. As a result, the hydraulic pump 35 There is no danger that the hydraulic motor 38 and the hydraulic motor 38 will collide with each other. Actually, the compression leakage of each part on the hydraulic circuit has little effect on the driving force when the turning speed is changed before and after the turning. Clutch detection mechanism 4
7, when the clutch 45 is operated between the grip stroke 0 and the half clutch, the clutch detection mechanism 47
Does not operate, so that the electromagnetic switching valve 51 is not opened by the clutch detecting mechanism 47. That is, the clutch is "on"
Alternatively, the vehicle is placed in the "half-clutch state", and the front wheel drive force is not interrupted at the time of starting control or at the time of aerial jumping such as motocross, so that a good starting and aerial posture control can be ensured. . On the other hand, when the clutch is "off", the clutch detection mechanism 47 is activated during all clutching to open the electromagnetic switching valve 51 and cut off the front wheel driving force. make it easier. With respect to the front brake detection mechanism 48, when the front brake 46 is turned on, the detection mechanism 48 operates to open the electromagnetic switching valve 51, thereby cutting off the front wheel driving force. That is, when the front brake 46 is turned on,
Since no front wheel driving force is required, the front wheel driving force is always shut off at this time.

[0021]

Embodiment 3 FIG. 5 is a hydraulic piping diagram of a so-called active two-wheel drive device to which the invention of claim 3 is applied. The hydraulic piping diagram has the same closed hydraulic circuit 18 and electromagnetic switching valve 51 as in FIG. The same parts as those in FIG. 2 are given the same numbers. A variable displacement hydraulic pump 35a is interlocked with the rear wheel 2 to positively change the power distribution between the front and rear wheels 1 and 2. A charge pump 78 is connected between the pressure oil supply pipe 41 and the return oil pipe 42 via check valves 75 and 76 and an oil cooler 77 to supply hydraulic oil to the closed hydraulic circuit 18. ing. 79 is an oil tank.

In FIG. 6, as in FIG.
1 includes a clutch detection mechanism 47 and a front brake detection mechanism 48 connected to the front wheel 1.
The input sprocket 32 of the hydraulic pump 35a is provided with a rear wheel rotation speed detection mechanism 71 for detecting a sprocket rotation speed proportional to the rear wheel rotation speed Nr. Is provided with an accelerator opening detecting mechanism 74 for detecting an accelerator opening. Each of the detecting mechanisms 73, 71, 74 is connected to a control circuit 72, and the output side of the control circuit 72 is connected to a variable displacement pump 35a to increase or decrease its discharge amount. Connected for.

The control circuit 72 includes a rotational speed N of the rear wheel 2.
r, a reference value f (Nr, dN) determined from the angular acceleration dNr / dt of the rear wheel and the accelerator opening (throttle opening) V
(r / dt, V) and the slip speed Nr-Nf are calculated and compared, and a comparison discriminating means for discriminating the magnitude relationship between them is provided. A transmission means is provided, and the slip speed Nr-Nf is set to a reference value f (Nr, dNr / dt,
When V is larger than V, a signal for increasing the discharge amount of the hydraulic pump 35a is issued according to the difference from the reference value, and the forward driving force is increased. On the other hand, when the front and rear wheel rotational speed difference Nr-Nf is a negative value and its absolute value is larger than the reference value, such as when the front wheel is running with a slight slip due to its excessive driving force, etc. And a signal for decreasing the pump discharge amount in accordance with the difference from the reference value, thereby reducing the forward driving force. If neither of the above cases is satisfied, an output signal for maintaining the current ejection amount is generated.

In the conventional example of FIG. 9 described above, it is assumed that the speed difference between the front and rear wheels 1 and 2 is a slip speed, and a front wheel driving force corresponding to the slip speed is guided to the front wheels. The driving force roughly corresponded on a one-to-one basis, and the driving force was always lower than the driving force of the rear wheels. This is because the ordinary viscous coupling 10 or fluid coupling has no function of changing the output torque to the input torque or more.

On the other hand, in the structure of this embodiment shown in FIG.
The front wheel driving force can be controlled to be increased, decreased or maintained as it is, and the front wheel driving force can be positively adjusted to a range exceeding the rear wheel driving force by using a variable displacement hydraulic pump. That is, according to this embodiment, the power distribution between the front and rear wheels can be continuously changed, and the point at which power transfer starts and can be continuously moved. For example, when the rear wheel is immersed in mud or the like, large power is distributed to the front wheel side.

[0026]

[Other Embodiments] Instead of the pressure-resistant rubber hoses, the pipes 41, 42 and 43 may be flexible metal pipes having swivel joints at a plurality of locations. In the third aspect of the present invention, the hydraulic motor may be a variable displacement type instead of the hydraulic pump being a variable displacement type, or both may be a variable displacement type.

[0027]

As described above, according to the first aspect of the present invention, the front wheel driving force is generated by a combination of a hydraulic pump and a hydraulic motor. (1) Front and rear wheels generated by cornering and the like. The slight difference in rotation speed between the front and rear wheels is hardly restrained by the hydraulic pump or the hydraulic motor due to the compression leakage of the hydraulic pump or the hydraulic motor, so that the driving force is hardly affected. (2) When a slip occurs on the rear wheel, a driving force is generated on the front wheel much earlier than a conventional viscous coupling or a fluid coupling due to a fluid shear force or a viscous resistance. (3) A flexible pipe is used between the rear wheel-side hydraulic pump and the front wheel-side hydraulic motor with a pressure-resistant rubber hose or a flexible metal pipe having a swivel joint, so that the conventional combination of a constant velocity joint and a chain drive is used. In comparison, there is almost no restriction on the steering angle, and a wide steering angle can be secured. (4) The flexible piping enables the use of a conventional telescopic suspension, thereby suppressing an increase in unsprung load. (5) The hydraulic motor 38 can be stored compactly, and the front wheel hub 40 is
8 serves as a protective cover, and can protect the hydraulic motor 38 from external mud and dust.

In addition to the above effects, the invention according to the second aspect opens the switching valve (for example, an electromagnetic switching valve) when the front wheel driving force is not required, for example, when the clutch is off or the front brake is on, and shuts off the front wheel driving force. As a result, unnecessary front wheel driving force is prevented from being generated, and an effective two-wheel drive mode can be realized.

According to the third aspect of the present invention, the front wheel drive force is generated by the combination of the hydraulic pump and the hydraulic motor, and the hydraulic pump or the hydraulic motor is of a variable displacement type, and the power is distributed to the front and rear wheels. Is an active type that can be positively and continuously changed, and in addition to the above-described effects, it is possible to generate a front wheel driving force of an effective magnitude according to the traveling situation.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a hydraulic circuit of a two-wheel drive device to which the invention of claim 1 is applied.

FIG. 2 is a schematic diagram of a hydraulic circuit of a two-wheel drive device to which the invention described in claim 2 is applied.

FIG. 3 is a schematic perspective view of a motorcycle to which the invention described in claim 2 is applied.

FIG. 4 is an enlarged sectional view showing an example of a hydraulic motor.

FIG. 5 is a schematic diagram of a hydraulic circuit of a two-wheel drive device to which the invention described in claim 3 is applied.

FIG. 6 is a schematic perspective view of a motorcycle to which the invention of claim 3 is applied.

FIG. 7 is a flowchart showing a discharge amount control of the hydraulic pump according to the third aspect of the present invention.

FIG. 8 is a schematic perspective view of a conventional example.

FIG. 9 is a schematic perspective view of a conventional example.

FIG. 10 is a schematic perspective view of a conventional example.

FIG. 11 is a schematic perspective view of a conventional example.

[Description of Signs] 1 Front wheel 2 Rear wheel 35 Hydraulic pump 35a Variable displacement hydraulic pump 38 Hydraulic motor 41 Piping 42 Piping 47 Clutch detection mechanism 48 Front brake detection mechanism 50 Bypass passage 51 Electromagnetic switching valve 71 Rear wheel rotation speed detection Mechanism 73 Front wheel rotation speed detection mechanism 74 Accelerator opening detection mechanism

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B62M 19/00 B60K 17/10

Claims (3)

(57) [Claims] 1. A front wheel hub supporting a front wheel is formed in a bottomed cylindrical shape, a hydraulic motor is arranged on an inner peripheral side of the front wheel hub, a motor case is fixed to one front fork, and A motor shaft is rotatably supported by the front fork, a front wheel hub is fixed to the motor shaft, and a hydraulic pump is operatively connected to the rear wheel so as to rotate in proportion to the rotation speed of the front wheel hub. A two-wheel drive system for a motorcycle, characterized in that a closed hydraulic circuit is formed by connecting a pump with a flexible or bendable pipe. 2. A hydraulic pump and a hydraulic motor are connected to each other.
The pressure oil piping and return oil piping that constitute the closed hydraulic circuit
In between, a bypass passage that short-circuits both pipes is provided, a switching valve for no-load that opens and closes the bypass passage is provided in the bypass passage, and an operation amount of the operation member is provided to at least one of various operation members such as a clutch or a front brake. Equipped with a detection mechanism to detect,
2. The motorcycle according to claim 1, wherein the detection mechanism is connected to the drive mechanism of the switching valve, and the switching valve is opened by detecting an operation amount when two-wheel drive is unnecessary. Two-wheel drive system for cars. 3. A method according to claim 1, wherein at least one of the hydraulic pump and the hydraulic motor is of a variable displacement type, and a front and rear wheel rotational speed detecting mechanism and an accelerator opening detecting mechanism for detecting an accelerator opening are provided. The difference from the front wheel rotation speed is compared with a reference value determined from the rear wheel rotation speed, the rear wheel angular speed and the accelerator opening to determine whether to increase, decrease or maintain the current state of the front wheel driving force. 3. The two-wheel drive system for a motorcycle according to claim 1, further comprising a control circuit for instructing the hydraulic pump or the hydraulic motor.