JPH0592794A - Two-wheel drive system for motorcycles - Google Patents

Abstract

(57) [Abstract] [Purpose] In a two-wheel drive system for a motorcycle, increase in overall driving force, secure wide front wheel steering angle, generation of front wheel drive force when rear wheel slips, unsprung load It is intended to suppress the increase, reduce the weight, and suppress the increase in the front weight. [Structure] The front wheel is equipped with a hydraulic motor that can drive it.
A hydraulic pump is interlocked with the rear wheel so as to rotate in proportion to the rotational speed of the rear wheel, and the hydraulic motor and hydraulic pump are connected by piping to form a closed hydraulic circuit. Further, a bypass passage having a switching valve is provided in the closed hydraulic circuit, and a detection mechanism for detecting the operation amount of the operation member is provided in one or more of various operation members such as a clutch and a brake, and the detection mechanism drives the switching valve. Connected to the mechanism to open the electromagnetic switching valve when two-wheel drive is unnecessary. Furthermore, the hydraulic pump is a variable displacement type, and the power distribution between the front and rear wheels is changed.

Description

Detailed Description of the Invention

[0001]

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

[0002]

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

As shown in FIG. 8, the former is one in which the rear wheel 2 and the 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 interlocked with the output shaft 5 of the transmission 3 via the rear wheel drive drive chain 4, and the front wheel 1
Is mechanically interlocked with the output shaft 5 via drive chains 9, 7, 6, and a constant velocity joint 8 and the like, and a driving force is constantly generated in the front and rear wheels 1, 2. According to this structure, when traveling straight on a slippery road surface, it is preferable. On the other hand, when traveling on a road surface with good grip, the rear wheel 2 tries to pass inside the locus of the front wheel 1. Therefore, the speed of the front wheels 1 starts to slightly increase, and unless the front wheels slip, the driving force of both wheels decreases. Therefore, this type of two-wheel drive system is not very suitable for general driving.

Although the latter non-rigid type does not currently exist, it is generally considered that a passive type equipped with a viscous coupling 10 and the like as shown in FIG. 9 and a differential unit 15 as shown in FIG. There are a differential type provided and an active type having a wet multi-plate hydraulic clutch 20 capable of adjusting a transmission force by changing an operating hydraulic pressure as shown in FIG. 11, but the following problems occur.

[0005]

The passive type using the viscous coupling 10 shown in FIG. 9 or a fluid coupling similar to the viscous coupling 10 does not cause a considerable rotational speed difference between the front and rear wheels 1 and 2. It is difficult to generate the necessary driving force for the front wheels 1.

Next, as shown in FIG. 10, a differential type using a differential gear 15 composed of a planetary gear, a bevel gear, etc. is effective for a road surface with a good grip. If either one slips, the driving force of both wheels may decrease.

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

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

[0009]

It is an object of the present invention to provide a so-called passive type two-wheel drive system in which the overall driving force is increased, a wide front wheel steering angle is ensured, and the front wheel driving force when the rear wheels slip. It suppresses the occurrence of unsprung load, the reduction in weight and the increase in front weight. The object of the invention according to claim 2 is to provide a passive two-wheel drive system in addition to the above objects,
Two-wheel drive is used only when necessary, and the driving force of the front wheels can be cut off when not needed, so that an effective two-wheel drive mode is achieved. The object of the invention according to claim 3 is to be an active type capable of positively changing the power distribution to the front and rear wheels,
The purpose of this active type is to achieve the same purpose as described above.

[0010]

The invention according to claim 1 is
A hydraulic motor capable of driving the front wheel is provided, and a hydraulic pump is interlocked and connected to the rear wheel so as to rotate in proportion to the rotational speed of the rear wheel. The hydraulic motor and the hydraulic pump are flexible or bendable. It is characterized in that a closed hydraulic circuit is configured by connecting with different pipes. The invention according to claim 2 is the invention according to claim 1, wherein a bypass passage is provided between the hydraulic pump and the hydraulic motor in the closed hydraulic circuit, and a bypass valve for opening and closing the bypass passage is provided. Provided with a detection mechanism for detecting the operation amount of the operation member in one or more of various operation members such as clutches and brakes, the detection mechanism is connected to the switching valve, and the operation amount when two-wheel drive is unnecessary Is detected and the switching valve is opened. The invention according to claim 3 is the same as the invention according to claim 1 or 2,
At least one of the hydraulic pump and the hydraulic motor is a variable displacement type, the rotation speed detection mechanism for the front and rear wheels and the accelerator opening detection mechanism for detecting the accelerator opening are provided, and the rear wheel rotation speed and the front wheel rotation speed are Is compared with a reference value that is determined from the rear wheel rotation speed, rear wheel angular acceleration, and throttle opening to determine whether the front wheel drive force increases, decreases, or maintains the current state. A control circuit for instructing the motor is provided.

[0011]

[Operation] When the front wheels and the rear wheels are traveling straight on flat ground at the same rotation speed, the amount of oil discharged from the hydraulic pump that operates in response to the rear wheel rotation speed and the idle speed corresponding to the front wheel rotation speed Since the amount of intake oil of the hydraulic motor is substantially equal to that of the hydraulic motor, no power is transmitted between the hydraulic pump and the hydraulic motor in principle, and no driving force is transmitted to the front wheels. When the rear wheel slips during turning or the like, the rotation of the hydraulic pump increases and pressure oil is supplied to the hydraulic motor, and the front wheel is driven by the hydraulic motor. In Claim 2, for example, when the clutch is completely disengaged or the front brake is on, the switching valve in the closed hydraulic circuit is opened to set the hydraulic oil to bypass the front and rear. Even when the rotational speed difference occurs between the wheels, the front wheel drive force is not generated as compared with the case where the two-wheel drive impairs the traveling performance, resulting in an effective two-wheel drive mode. In Claim 3, the displacement of the hydraulic pump or the hydraulic motor is automatically adjusted, and it is possible to actively change the front wheel driving force corresponding to only the slip speed and exceeding the range of the passively generated front wheel driving force. It is possible to perform two-wheel drive traveling suitable for various traveling conditions.

[0012]

[First Embodiment] FIG. 1 is a schematic diagram of a hydraulic circuit of a two-wheel drive system according to the present invention. A front wheel 1 is provided with a hydraulic motor 38 capable of driving the front wheel 1, and a rear wheel 2 is provided with a transmission 3.
The hydraulic pump 35 is interlocked and coupled via the output shaft 5 of the hydraulic pump 35, and the hydraulic pump 35 rotates in proportion to the rotational 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 a closed hydraulic circuit 18. Normally, when traveling straight on a road surface without slip, the discharge amount of the hydraulic pump 35 driven by the rear wheels 2 and the suction amount of the hydraulic motor 38 entrained around the front wheels 1 are substantially equal.

Therefore, when both the front and rear wheels 1 and 2 are traveling without slip, as when traveling straight on flat ground,
There is no mutual power exchange 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 and rear wheels 1 and 2, the driving force of the hydraulic motor 38 (front wheel driving force) is generated in the direction of returning to the original rotational relationship. That is, when the rear wheel 2 slips, the front wheel driving force corresponding to the slip automatically occurs.

[0014]

[Embodiment 2] FIG. 2 is a schematic diagram of a hydraulic circuit of a two-wheel drive system according to a second embodiment of the present invention, which 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 that connects (short-circuits) the two pipes 41 and 42 is provided in the closed hydraulic circuit 18, and an electromagnetic switching valve 51 that opens and closes the bypass passage 50 is provided in the bypass passage 50. The electromagnetic switching valve 51 includes a solenoid 51a as a drive mechanism thereof. When the solenoid 51a is not energized, the bypass passage 50 is closed as shown in FIG. 2 so that the closed hydraulic circuit 18 normally operates between the hydraulic pump 35 and the hydraulic motor 38. Maintain oil circulation. On the other hand, when the solenoid 51a is energized, the bypass passage 50 is opened and the pipes 41 and 42 are short-circuited to cut off the oil transmission between the hydraulic pump 35 and the hydraulic motor 38, thereby cutting off the front wheel driving force. Normally, when driving straight on a road surface without slip,
The discharge amount of the hydraulic pump 35 driven by the rear wheels 2 and the suction amount of the hydraulic motor 38 that rotates with the front wheels 1 are substantially equal. The solenoid 51a is connected to various detection mechanisms 47 and 48 for detecting the operation amount of the clutch or the front brake. When the detection mechanism 47 or 48 detects a predetermined operation amount, the solenoid 51a is energized and the electromagnetic switching valve 51 is connected. Is designed to open.

FIG. 3 shows an embodiment of the present invention as defined in claim 2. In FIG. 3, the rear wheel 2 has an output sprocket 30 on the output shaft 5 of the transmission 3 and a drive chain 4 for driving the rear wheels. Are mechanically interlocked with each other. The hydraulic pump 35 is arranged above the transmission 3, and is linked 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 above the hydraulic pump 35, and an electromagnetic switching valve 51 having the bypass passage 50 (FIG. 2) is provided at a side portion thereof. The hydraulic motor 38 is the front wheel hub 4
0 is arranged on the inner peripheral side, and the rear hydraulic pump 3 is provided by the above-mentioned pressure oil supply pipe 41 and return oil pipe 42.
5 and constitutes 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.
Since the drain pipe 43 for returning to the pipe is provided and the pipe 43 does not need to have a particularly high pressure resistance, it is possible to use a pressure resistant rubber hose.

The detection mechanisms 47 and 48 are respectively the 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 between the grip strokes of 0 to a half clutch, does not energize the solenoid 51a, and operates when the entire clutching state is detected. Then, the solenoid 51a is energized to open the electromagnetic switching valve 51. Further, the solenoid 51a can be energized even in the half-clutch state by adjusting the limit switch. A limit switch (not shown), for example, is also used as the front brake detection mechanism 48. When the brake is off, the limit switch is not activated and the solenoid 51a is not energized. When the brake on is detected, the limit switch is activated and the solenoid 51a is energized. 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 amounts of the front and rear suspensions, it is considered that there is no situation in which the front wheel drive force is interrupted in accordance with changes in these operation amounts. Therefore, there is no detection mechanism for the steering angle of the steering wheel, 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 suspension operation amount, the front wheel drive force is not interrupted by the change in the operation amount. However, if you want to shut off the front wheel drive force within the desired steering wheel steering angle range, accelerator opening range, or suspension operation amount range, a detection mechanism is installed at each location as appropriate to shut off the front wheel drive force when a predetermined operation amount is detected. It can also be configured to do so.

FIG. 4 shows a specific structure of the axial plunger type hydraulic motor 38. In FIG.
Of the left and right front forks 52, 53, a motor shaft 56 is rotatably supported at the lower end of the right front fork 53 via a bearing 55, and the rotary shaft 56 has a bottomed cylindrical front wheel. The hub 40 is fixed. Spokes 5 are provided on the outer circumference of the front wheel hub 40.
The rim 59 is fixed via 7 and the brake disc 58 is fixed. Left front fork 5
A valve plate 60 is integrally formed at the lower end of the valve plate 2. The valve plate 60 projects into the front wheel hub 40, and the cylinder block 63 of the hydraulic motor 38 is placed in a pressure contact state at its tip. On the outer peripheral side of the valve plate 60, a bottomed cylindrical motor case 6 is formed so as to cover the cylinder block 63.
1 is fixed, and the outer circumference of the left end portion of the motor case 61 is a bearing 6
The inner periphery of the front hole hub 40 is rotatably supported via the shaft 2, 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 (7 or 9) of bores are formed on the right end surface 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 in each bore. The valve plate 60 is connected to the pipes 41, 42 and 43 and is formed with a kidney port to distribute pressure oil to each piston 65 and generate an axial force in the piston 65. Piston 6
The tip of 5 is pressed against the swash plate 67, and this swash plate 67 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 rotation speed as the cylinder block 63. It has become. This is the piston 6
5 and the swash plate 67 are pressed against each other to prevent excessive slippage. The reaction force of the rotational force generated in the cylinder block 63 is taken up by the motor case 61 holding the swash plate 67. The rotational force of the cylinder block 63 is transmitted to the motor shaft 56 and further to the front wheel hub 40 integrally fixed to the motor shaft 56 to drive the front wheels 1.

Front, rear wheels 1, as when driving straight on flat ground
When both of them are traveling without slip, the hydraulic pump 35 and the hydraulic motor 38 do not exchange power with each other. Since the rear wheel 2 passes inside the locus of the front wheel 1 during turning, the front wheel speed slightly increases and the amount of oil sucked in by the hydraulic motor 38 becomes larger than the amount of oil discharged by the hydraulic pump 35. As a result, the hydraulic pump 35 There is no possibility that the hydraulic motor 38 and the hydraulic motor 38 suddenly bounce off each other, and in actuality, due to the compression leakage of each part on the hydraulic circuit, the change in the front and rear wheel speeds around the turning has little influence on the driving force. 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, the electromagnetic detection valve 51 is not opened by the clutch detection 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 during attitude control during start-up or during mid-air jumps such as motocross, and it is possible to secure a good start and aerial attitude control. .. On the other hand, when the clutch is "off", the clutch detection mechanism 47 operates to open the electromagnetic switching valve 51 to shut off the front wheel drive force during all clutching, so that the resistance force at the time of pushing is eliminated and pushing is performed. make it easier. Regarding 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, and the front wheel driving force is shut off. That is, when the front brake 46 is turned on,
Since front wheel drive force is not required, the front wheel drive force is always cut off at this time.

[0021]

[Embodiment 3] FIG. 5 is a hydraulic piping diagram of a so-called active type two-wheel drive system to which the invention according to claim 3 is applied, which 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 operatively connected to the rear wheel 2 in order 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, so that the closed hydraulic circuit 18 is supplied with hydraulic oil. ing. 79 is an oil tank.

In FIG. 6, the electromagnetic switching valve 5 is shown as in FIG.
1 is provided with a clutch detection mechanism 47 and a front brake detection mechanism 48, and the front wheel 1 has a front wheel rotation speed N.
The front wheel rotation speed detection mechanism 73 for detecting f is provided, and the input sprocket 32 of the hydraulic pump 35a is provided with the 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 detection mechanism 74 for detecting the accelerator opening. The detection mechanisms 73, 71, 74 are connected to a control circuit 72, and the output side of the control circuit 72 is a variable displacement pump 35a for increasing or decreasing its discharge amount. Connected for.

In the control circuit 72, the rotation speed N of the rear wheel 2 is
r, the angular acceleration dNr / dt of the rear wheel and a certain reference value f (Nr, dNr / dt, V) determined from the throttle opening V and the slip speed Nr-Nf are calculated and compared, and the magnitude relationship between them is determined. It is provided with a comparing / determining means for making a determination and a transmitting means for sending an increase / decrease signal of the discharge amount to the variable displacement hydraulic pump 35a based on the determined result, and when the rear wheels slip as shown in the flowchart of FIG. Slip speed Nr-
When Nf is larger than the reference value f (Nr, dNr / dt, V), a signal for increasing the discharge amount of the hydraulic pump 35a is issued according to the difference from the reference value to increase the forward drive force. On the contrary, 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 slipping due to its excessive driving force. , A signal for reducing the pump discharge amount is issued according to the difference from the reference value, and the forward drive force is reduced. If none of the above, an output signal for maintaining the current discharge amount is generated.

In the conventional example of FIG. 9 described above, assuming that the speed difference between the front and rear wheels 1 and 2 is the slip speed, the front wheel driving force corresponding to the slip speed is introduced to the front wheels. The driving force corresponds roughly to one-to-one, and the driving force was always less than the driving force of the rear wheels. This is because the normal viscous coupling 10 or the fluid coupling does not have the function of changing the output torque beyond the input torque.

On the other hand, in the structure of this embodiment of FIG.
It is possible to control the front wheel drive force by either increasing, decreasing or maintaining the present condition, and positively adjust the front wheel drive force to a range exceeding the rear wheel drive force 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 is started can be continuously moved. For example, when the rear wheels are soaked in mud, etc., a large amount of power is distributed to the front wheels.

[0026]

[Other Embodiments] As the pipes 41, 42 and 43, bendable metal pipes having swivel joints at a plurality of positions may be used instead of the pressure resistant rubber hoses. Further, in the invention described in claim 3, the hydraulic pump may be a variable displacement type instead of the variable displacement type, or both may be a variable displacement type.

[0027]

As described above, the invention according to claim 1 of the present application generates the driving force of the front wheels by the combination of the hydraulic pump and the hydraulic motor: (1) Front and rear wheels generated by cornering or the like With respect to a slight difference in rotational speed, the front and rear wheels are hard to be restrained from each other due to compression leakage of the hydraulic pump or the hydraulic motor, and the driving force is rarely affected. (2) When slip occurs on the rear wheels, driving force is generated on the front wheels much earlier than in the conventional case where viscous coupling or fluid coupling due to fluid shear force or viscous resistance is used. (3) Since flexible piping is used between the hydraulic pump on the rear wheel side and the hydraulic motor on the front wheel side by a flexible metal tube with a pressure resistant rubber hose or swivel joint, it is possible to combine the conventional constant velocity joint and chain drive. In comparison, steering angle restrictions are almost eliminated, and a wide steering angle can be secured. (4) In addition, the flexible pipe allows the conventional telescopic suspension to be used, and an increase in unsprung load can be suppressed.

In addition to the above effects, the invention according to claim 2 shuts off the front wheel drive force by opening a switching valve (for example, an electromagnetic switching valve) when front wheel drive force is not required, for example, when the clutch is off or the front brake is on. Therefore, it is possible to prevent an unnecessary front wheel drive force from being generated and to realize an effective two-wheel drive mode.

According to the third aspect of the present invention, the front wheel driving 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 to distribute the power to the front and rear wheels. Is an active type that can be positively and continuously changed, so that in addition to the above effects, it is possible to generate a front-wheel drive force of an effective magnitude according to the traveling situation.

[Brief description of drawings]

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

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

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

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

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

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

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

[Explanation of symbols]

 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

Claims (3)

[Claims] 1. A hydraulic motor capable of driving the front wheel is provided, and a hydraulic pump is interlockingly connected to the rear wheel so as to rotate in proportion to the rotational speed of the rear wheel, and the hydraulic motor and the hydraulic pump are flexible. A two-wheel drive system for a motorcycle, characterized in that a closed hydraulic circuit is configured by connecting with a flexible or bendable pipe. 2. A bypass passage is provided between a hydraulic pump and a hydraulic motor in a closed hydraulic circuit, a no-load switching valve for opening and closing the bypass passage is provided, and various operating members such as a clutch and a front brake are provided. One or more of the above-mentioned switching valves are provided with a detection mechanism for detecting the operation amount of the operating member, and the detection mechanism is connected to the drive mechanism of the switching valve to detect the operation amount when two-wheel drive is unnecessary, and the switching valve The two-wheel drive system for a motorcycle according to claim 1, wherein the two-wheel drive system is opened. 3. At least one of a hydraulic pump and a hydraulic motor is of a variable displacement type, and a rotation speed detection mechanism for front and rear wheels and an accelerator opening detection mechanism for detecting an accelerator opening are provided, and a rear wheel rotation speed. The difference between the front wheel rotation speed and the front wheel rotation speed is compared with a reference value that is determined from the rear wheel rotation speed, the rear wheel angular acceleration, and the throttle opening to determine whether the front wheel drive force is increased, decreased, or maintained. A two-wheel drive system for a motorcycle according to claim 1 or 2, further comprising a control circuit for instructing a hydraulic pump or hydraulic motor of the type.