JPH05178231A - Rear wheel steering device for vehicle - Google Patents

Abstract

PURPOSE:To steer left and right rear wheels surely in synchronization, with the compact constitution, by driving a suspension arm supporting member directly or through s link mechanism, through a driving lever by the power of a hydraulic motor. CONSTITUTION:A rear wheel steering device is equipped with a driving mechanism for steering left and right rear wheels in the toe-in direction and toe-out direction by slide-shifting the lateral links 13L and 13R arranged before and behind a rear wheel supporting member, and the driving mechanism is equipped with a driving lever 35 which is swung through a wheel gear part 36 by turning a worm 37 by a hydraulic motor 40. A suspension arm supporting member 34 is directly driven by the swing movement of the driving lever 35, and the lateral link 13R is shifted in the axial direction, and a suspension supporting member 33 is driven through a link mechanism 35 by the swing movement of the driving lever 35, and the lateral link 13L is shifted in the axial direction, and rear wheels are steered.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear wheel steering system for a vehicle which steers left and right rear wheels simultaneously in a toe-in direction or a toe-out direction in accordance with the running state of the vehicle.

[0002]

2. Description of the Related Art Conventionally, as disclosed in, for example, Japanese Unexamined Patent Publication No. 59-23716, tie rods provided on the support portions of the left and right rear wheels are slid in the vehicle width direction by hydraulic cylinders. When driving at high speed, the left and right rear wheels are simultaneously steered in the toe-in direction to improve the running stability of the vehicle, and at low speed, the left and right rear wheels are simultaneously steered in the toe-out direction to improve the steering response when the vehicle turns. There is known a rear wheel steering device which is configured as described above.

[0003]

In the rear wheel steering system having the above-mentioned structure, the left and right hydraulic cylinders for applying steering force to the left and right rear wheels are provided separately, and both hydraulic cylinders are individually controlled. It is difficult to accurately synchronize the movement of the suspension arm, and there are problems that the piping structure for applying hydraulic pressure to both cylinders is complicated and the device becomes large.

In order to solve the above problem, as shown in FIG. 8, a drive lever for slidingly displacing one (13R) of the left and right suspension arms 13L and 13R formed of lateral links or the like for applying steering force to the rear wheels. 61,
A link mechanism 62 for reversing the driving force of the driving lever 61 and transmitting the driving force to the other suspension arm 13L, an intermediate shaft 63 and a first worm gear portion 64 for applying the driving force to the driving lever 61, and the intermediate shaft 63. A drive shaft 65 and a second worm gear portion 66 that transmit a driving force,
A single electric motor 67 that rotates the drive shaft 65 is provided, and the driving force of the electric motor 67 is set to the first and second electric motors.
By transmitting to the suspension arms 13L and 13R via the worm gears 64 and 66, the left and right rear wheels are simultaneously steered in the toe-in direction or the toe-out direction.

When the left and right rear wheels are steered by the single electric motor 67 as described above, the movements of the rear wheels can be reliably synchronized, but on the other hand, the above electric drive is possible. Since the driving force of the motor 67 cannot be set so large, it is necessary to reduce the torque in two stages by the first worm gear portion 64 and the second worm gear portion 66 to increase the torque, which may result in an increase in size of the device. There was an unavoidable problem.

The present invention has been made in order to solve the above-mentioned problems, and a rear wheel steering device capable of reliably synchronizing the left and right rear wheels in a toe-in direction or a toe-out direction with a compact structure. Is intended to provide.

[0007]

The invention according to claim 1 is
By slidingly displacing the left and right suspension arms that support the rear wheels in the vehicle width direction, a rear wheel steering device for a vehicle that symmetrically steers the left and right rear wheels is swingably supported with a support shaft as a fulcrum. A pair of left and right suspension arm support members whose one end is connected to the suspension arm, a drive lever for operating one suspension arm support member, and a drive force of the drive lever is reversed to the other suspension arm support member. A transmission link mechanism and a single hydraulic motor for applying a driving force to the drive lever are provided.

According to a second aspect of the present invention, the driving force of the hydraulic motor is directly transmitted to the drive lever via a worm gear.

According to a third aspect of the present invention, vehicle speed detecting means for detecting the traveling speed of the vehicle is provided, and the steering angle of the rear wheels is set in the toe-out direction during low speed traveling in accordance with the traveling speed of the vehicle detected by the vehicle speed detecting means. The steering angle of the rear wheels is set in the toe-in direction when the vehicle is traveling at high speed, and steering angle setting means for gradually changing the set value according to the vehicle speed is provided.

According to a fourth aspect of the present invention, the front wheel steering angle detecting means for detecting the steering angle of the front wheels and the front wheel steering angle detected by the front wheel steering angle detecting means are equal to or greater than a preset reference value. When it is confirmed, the steering angle of the rear wheels is set in the toe-in direction, and the steering angle setting means for changing the set value stepwise corresponding to the steering angle of the front wheels is provided.

According to a fifth aspect of the present invention, there is provided braking detection means for detecting whether or not the vehicle is in a braking state, and when the braking detection means confirms that the vehicle is in a braking state, the rear wheel A steering angle setting means for setting the steering angle in the toe-in direction is provided.

According to a sixth aspect of the present invention, rotation angle detecting means for detecting the rotation angle of the hydraulic motor is provided, and the steering angle of the rear wheels is controlled while detecting the rotation angle of the hydraulic motor by the rotation angle detecting means. It is provided with a control means.

[0013]

According to the first aspect of the invention, when the driving force of the hydraulic motor is transmitted to the drive lever and the drive lever is oscillated and displaced, the one suspension arm support member is driven by the drive lever. In the opposite direction, the other suspension arm support member is driven by the link mechanism, whereby the left and right suspension arms are slid and displaced in the vehicle width direction, and the left and right rear wheels are simultaneously steered in the toe-in direction or the toe-out direction. It will be.

According to the second aspect of the present invention, the driving force of the hydraulic motor is directly transmitted to the drive lever after being decelerated once by the worm gear.

According to the third aspect of the present invention, when it is confirmed that the vehicle is in the low speed traveling state in accordance with the output signal of the vehicle speed detecting means, the rear wheels are steered in the toe-out direction and the vehicle is When it is confirmed that the vehicle is traveling at high speed, the rear wheels are steered in the toe-in direction, and the rear wheels are steered in the toe-in direction as the vehicle speed increases.

According to the fourth aspect of the invention, when it is confirmed that the vehicle is in a turning state according to the output signal of the front wheel steering angle detecting means, the rear wheels are steered in the toe-in direction. The smaller the turning radius of the vehicle is, the larger the rear wheels are steered in the toe-in direction.

According to the fifth aspect of the invention, when it is confirmed by the braking detecting means that the vehicle is in the braking state due to the operation of the brake pedal or the engine braking, the rear wheels are in the toe-in direction. Will be steered to.

According to the invention described in claim 6, the rotation angle of the hydraulic motor is detected while the rotation angle of the hydraulic motor and the operation amounts of the left and right suspension support members are detected according to the output signal of the rotation angle detecting means. Control will be executed.

[0019]

1 is a front wheel steering device A for steering left and right front wheels 1L, 1R, a suspension device B for supporting left and right rear wheels 2L, 2R, and a rear wheel steering device C according to an embodiment of the present invention. 1 shows an overall configuration of a suspension system of a vehicle including and.

The front wheel steering mechanism A includes a pair of left and right knuckle arms 3L and 3R and tie rods 4L and 4R, a relay rod 5 connecting the tie rods 4L and 4R, and rack teeth formed on the relay rod 5. A pinion 6 meshing with (not shown), a steering wheel 7 and a steering shaft 8 that rotationally drive the pinion 6 are provided. By rotating the steering wheel 7 and slidingly displacing the relay rod 5 in the vehicle width direction, the front wheels 1L, 1R are steered in the same direction.

The suspension device B includes rear wheels 2L,
A rear wheel support member 11 that rotatably supports the 2R, and lateral links 12L, 12R, and 13 that form suspension arms arranged in front of and behind the rear wheel support member 11.
And L and 13R. Further, the rear wheel steering device C
Slides the rear lateral links 13L, 13R on the rear side in the vehicle width direction, thereby
A drive mechanism section 21 for steering the 2R at the same time in the toe-in direction or the toe-out direction, and a hydraulic pump 22 and a hydraulic line 23 for supplying drive hydraulic pressure to the drive mechanism section 21
And a fail safe valve 24 and a switching valve 25 provided on the hydraulic line 23, and a control unit 26 for controlling the fail safe valve 24 and the switching valve 25.

The control unit 26 includes vehicle speed detecting means 27 for detecting the traveling speed of the vehicle and front wheels 1L, 1L.
The running state of the vehicle is determined according to the detection signals input from the front wheel steering angle detecting means 28 for detecting the steering angle of R and the braking detecting means 29 for detecting the braking state of the vehicle, and the running state is determined according to this running state. It has a steering angle setting means 30 for setting the steering angle of the rear wheels 2L, 2R, and outputs a control signal corresponding to the steering angle set by the steering angle setting means 30 to the switching valve 25 to operate it. As a result, the supply / discharge state of hydraulic pressure to / from the drive unit of the drive mechanism unit 21 is controlled.

As shown in FIGS. 2 to 4, the drive mechanism section 21 has a casing 3 attached to a cross member 9 of a vehicle body composed of an upper panel 9a and a lower panel 9b.
1, a pair of left and right support shafts 32 whose upper and lower ends are rotatably supported by a casing 31, left and right suspension arm support members 33 and 34 swingably supported by the support shafts 32, and a right side Drive lever 35 protruding forward from the front end of the suspension arm support member 34, a worm 37 meshing with a wheel gear portion 36 formed at the tip of the drive lever 35, and the worm 37 The rotating shaft 38 that rotates and the rotating shaft 38
Has a hydraulic motor 40 connected to one end thereof via a coupling 39.

The suspension arm support member 33,
34 is a boss portion 41 formed integrally with the support shaft 32.
From the peripheral surface of the boss portion 41, the lateral link 13L,
13R, and a bearing portion 42 provided in a protruding manner on the rear side of the vehicle body. The tip ends of the lateral links 13L and 13R are connected to a mounting member 43 formed in the bearing portion 42. Are connected via. The connecting member 44 includes a spherical portion 45 provided at the tip end portions of the lateral links 13L and 13R, a spherical bearing portion 46 fitted on the spherical portion 45, the spherical portion 45 and the spherical bearing portion 4 described above.
A cover 47 for covering the connecting portion 6 and the spherical bearing portion 4
6 and a screw shaft 48 projecting from the front end surface of the bearing 6, and a nut 49 is screwed to the front end of the screw shaft 48 inserted into the mounting hole 43 of the bearing portion 42. It is fixed to the suspension arm support members 33 and 34.

The left and right suspension arm support members 33 and 34 are swing arms 50 and 51 extending in parallel with each other, and a link member 52 connecting the tip ends of the swing arms 50 and 51. Link mechanism 53
The link mechanism 53 inverts the driving force of the right suspension arm support member 34 and transmits it to the left suspension arm support member 33. That is, the left swing arm 50
Is provided so as to project from the boss portion 41 of the suspension support member 33 toward the rear inner side of the vehicle body, and includes the right swing arm 51.
Is projected from the boss portion 41 of the suspension support member 34 toward the front inside of the vehicle body, and when the right swing arm 51 is rotationally driven in the clockwise direction in the figure by the drive lever 35, the left side The swing arm 50 is rotationally driven counterclockwise by the link member 52.

The hydraulic motor 40 is composed of a conventionally well-known vane motor, gear motor, plunger motor, or the like that continuously rotates according to the hydraulic pressure supplied from the hydraulic line 23, and has a compact structure to generate a large driving force. is there. At the end of the rotary shaft 38 located on the opposite side of the installation portion of the hydraulic motor 40, a biasing member 54 composed of a compression spring for pushing the rotary shaft 38 to the drive lever 35 side, and the rotary member 38 are provided. A rotation angle detecting means 55 including a rotary encoder for detecting the rotation angle of the shaft 38 is provided.

When hydraulic pressure is supplied to the hydraulic motor 40 of the drive mechanism portion 21 in response to a control signal output from the controller 26 in the above-described structure and the hydraulic motor 40 rotates, the rotational force thereof rotates via the coupling 39. Axis 38
And the worm 37 is driven to rotate together with the rotary shaft 38. Along with this, the drive lever 35 and the right suspension arm support member 34 are pivotally displaced about the support shaft 32, and the driving force thereof is transmitted to the left suspension arm support member 33 via the link mechanism 53. The left suspension arm support member 33 is the right suspension arm support member 3 described above.
Swing displacement in the opposite direction of 4.

Further, the connecting member 44 is responsive to the swing displacement of the left and right suspension arm supporting members 33, 34.
Also, the suspension arms including the lateral links 13L and 13R are slidably displaced in the axial direction, that is, the vehicle width direction in opposite directions. Then, when it is confirmed that the amount of movement of the lateral links 13L, 13R has reached the set value in accordance with the detection signal output from the rotation angle detection means 55 to the controller 26, the rotation motor 40 is detected.
Is stopped, whereby the rear wheels 2L, 2R are symmetrically steered to a predetermined angle.

The steering control of the rear wheels 2L, 2R will be described with reference to the flowchart shown in FIG. When the steering control is started, the traveling speed of the vehicle body, that is, the vehicle speed V is read according to the output signal of the vehicle speed detecting means 27 (step S1), and the detected value of this vehicle speed V and the preset value shown in FIG. Based on the one characteristic diagram, the steering angle Vt of the rear wheels 2L and 2R is read and set by the steering angle setting means 30 (step S2). In the second characteristic diagram, the rear wheels 2L, 2R are steered in the toe-out direction when the vehicle speed V is in a low region, and when the vehicle speed V becomes fast to some extent, the rear wheels 2L, 2R.
R is steered in the toe-in direction, and its steering angle Vt is set to increase stepwise in response to an increase in vehicle speed V.

Next, the steering angle θ of the front wheels 1L and 1R is read in accordance with the output signal of the front wheel steering angle detecting means 28 (step S3), and the detected value of the front wheel steering angle θ and the preset value shown in FIG. Based on the second characteristic diagram shown in (1), the steering angle θt of the rear wheels 2L, 2R is read and set by the steering angle setting means 30 (step S4). The second characteristic diagram shows that the rear wheels 2L and 2R are steered in the toe-in direction when the front wheel steering angle θ is equal to or greater than a preset predetermined value, and the steering angle θt increases the front wheel steering angle θ. It is set to gradually increase in response to.

Then, it is determined whether or not the vehicle is in the braking state according to the output signal of the braking detecting means 29 (step S5). If the result of this determination is YES, the preset toe-in direction is set. Steering angle B of rear wheels 2L, 2R
The steering angle setting means 30 reads and sets t (step S6). Then, the above steps S2, 4, 6
Steering angles Vt, θ of the rear wheels 2L, 2R read at
The final steering angle Tt of the rear wheels is calculated by the steering angle setting means 30 by adding t and Bt (step S7), and a control signal corresponding to the steering angle Tt is sent to the switching valve 2 as described above.
5 (step S8).

As a result, when the vehicle is running at low speed, the rear wheels 2L and 2R are steered in the toe-out direction, the vehicle is running at high speed, and the front wheels 1L and 1R are steered. When the vehicle is in the braking state, the rear wheels 2L and 2R are steered in the toe-in direction.

As described above, the suspension arm support members 33 and 34 of the drive mechanism 21 are driven by the hydraulic motor 40 which is rotationally driven according to the hydraulic pressure supplied from the hydraulic line 23 and can generate a large driving force. Since the rear wheels 2L and 2R are configured to be steered, the worm 3 provided between the rotary shaft 38 and the drive lever 35 is provided.
7 and the worm gear consisting of the wheel gear portion 36, the rear wheels 2L, 2R can be simply reduced by one stage.
It is possible to obtain a driving force capable of appropriately steering the vehicle. Therefore, unlike the case of using an electric motor, it is not necessary to adopt a complicated structure in which the driving torque is reduced in two steps in order to increase the driving torque, and the entire structure of the rear wheel drive mechanism 21 can be simplified. it can.

Further, as described above, the driving force of the hydraulic motor 40 is applied to the suspension arm support member 3 by the worm gear composed of the worm 37 and the wheel gear portion 36.
When configured to be transmitted to the hydraulic motors 3, 34, the external force acting on the rear wheels 2L, 2R during turning or the like is applied to the hydraulic motor 40.
Is not transmitted to the rear wheel 2L,
It is possible to prevent the 2R from being steered. That is, a large reaction force is input to the rear wheels 2L and 2R from the road surface,
Even when this reaction force is transmitted to the drive lever 35 via the lateral links 13L, 13R and the suspension arm support members 33, 34, the drive lever 35 cannot rotate the rotary shaft 38, and therefore the rear wheels. 2L and 2R are not steered.

In the above embodiment, the biasing means 54 for biasing the rotary shaft 38 toward the drive lever 35 is provided at the end of the rotary shaft 38.
Since the worm 37 and the wheel gear portion 36 are provided,
Can always be properly meshed with each other, and the lateral link 13L, which is caused by the backlash at this meshing portion,
It is possible to absorb the deviation of 13R in the vehicle width direction and effectively suppress rattling of the rear wheels 2L, 2R.

Further, according to the vehicle speed detected by the vehicle speed detecting means 27, the rear wheels 2L, 2R are used when the vehicle is traveling at a low speed.
Steering in the toe-in direction and rear wheels 2L, 2R during high speed running
Since the steering wheel is configured to be steered in the toe-out direction, it is possible to enhance the turning ability when traveling at low speed and the traveling stability when traveling at high speed. When the steering angle Vt in the toe-in direction during high-speed traveling is set to increase stepwise according to the vehicle speed as shown in FIG. 6, it is difficult to precisely control the rotation angle, and the electric drive There is an advantage that appropriate control corresponding to the vehicle speed can be executed while compensating for the drawback of the hydraulic motor 40 that the response is lower than that of the motor. Note that, when the vehicle is in a low speed traveling state, the steering angle Vt of the rear wheels 2L, 2R steered in the toe-out direction may be set to increase stepwise as the vehicle speed V decreases.

Further, as shown in FIG. 7, rear wheels 2L, 2R
The steering angle θt for steering the vehicle in the toe-in direction is increased stepwise in accordance with the steering angle θ of the front wheels 1L, 1R detected by the front wheel steering angle detection means 28. It is possible to prevent the tail swing motion of the vehicle due to a large lateral force acting on the rear wheels 2L, 2R, and effectively improve the running stability.

Further, when it is confirmed by the braking detecting means 29 that the vehicle is in a braking state, the rear wheels 2L, 2R are steered in the toe-in direction based on a preset steering angle Bt. According to this, it is possible to effectively prevent a decrease in traveling stability due to the braking operation.

The rotation angle detecting means 5 comprising a rotary encoder for detecting the rotation angle of the rotary shaft 38.
5, while detecting the rotation angle of the rotary shaft 38, the rear wheel 2
When the steering state of L, 2R is configured to be controlled by the control means including the controller 26, the rear wheel 2L is added to the final steering angle Tt obtained by adding the steering angles Vt, θt, Bt. , 2R can be appropriately controlled so that the hydraulic motor 40 can be steered.

Further, in the above-described embodiment, the mounting holes 43 are formed in the bearing portions 42 of the suspension supporting members 33 and 34 which are driven by the drive lever 35 and are swingably displaced about the supporting shaft 32 as a fulcrum to form a suspension arm. By inserting the screw shaft 48 of the connecting member 44 provided at the tip end portions of the lateral links 13L and 13R and fixing the screw shaft 48 by screwing a nut 49 into the tip end portion, the suspension arm supporting member 33, 34 and the lateral links 13L and 13R are connected to each other. Therefore, before assembling the lateral links 13L and 13R into the vehicle body,
A spherical joint including the spherical portion 45 and the spherical bearing portion 46 can be arranged between the support member 44 and the lateral links 13L and 13R.

Therefore, the lateral link 13L,
When assembling 13R to the vehicle body, the assembling work performed in a limited space can be simplified. Further, since the block member constituting the spherical bearing portion 46 can be arranged at a position apart from the support shaft 32 serving as the swinging fulcrum of the suspension arm support members 33, 34, the block member is driven by the drive mechanism. Part 21
There is an advantage that it can be effectively prevented from interfering with the casing 31.

[0042]

As described above, according to the present invention, since the suspension arm is slid in the vehicle width direction by the hydraulic motor to steer the rear wheels, the driving force of the hydraulic motor is reduced in two steps. Therefore, the left and right rear wheels can be reliably synchronized and steered in the toe-in direction or toe-out direction without employing a complicated structure such as transmission to the suspension arm.

When the driving force of the hydraulic motor is directly transmitted to the worm gear that drives the drive lever, the rear wheel steering device can be made compact, and the rear wheel steering device can be used when turning. There is an advantage that the external force acting is surely prevented from being transmitted to the hydraulic motor to protect the hydraulic motor, and at the same time, the steering of the rear wheels due to the external force is prevented.

When the vehicle is traveling at a low speed, the rear wheels are steered in the toe-out direction, while the vehicle is traveling at a high speed, the rear wheels are steered in the toe-in direction, and the steering angle of the rear wheels is gradually changed according to the traveling speed of the vehicle. If it is configured to change, it is difficult to precisely control the rotation angle, and the suspension arm is appropriately slid and displaced while compensating for the drawback of the hydraulic motor that the response is lower than that of the electric motor. There is an advantage that the turning performance of the vehicle can be enhanced during traveling and the traveling stability can be enhanced during high-speed traveling.

When the front wheels are steered and the vehicle is in a turning state, the rear wheels are steered in the toe-in direction and
When the steering angle of the rear wheels is changed in accordance with the steering angle of the front wheels, it is possible to effectively prevent the vehicle body from swinging back and forth to lower the running stability when the vehicle turns. You can Furthermore, when it is confirmed that the vehicle is in a braking state by operating the brake pedal, the rear wheel is steered in the toe-in direction. There is an advantage that it is possible to effectively prevent a decrease in traveling stability due to deceleration.

Then, the rotation angle of the hydraulic motor is detected by the rotation detecting means such as a rotary encoder to confirm whether the steering angle of the rear wheels has reached the predetermined steering angle set by the steering angle setting means. According to the configuration in which the steering control of the rear wheels is executed, the steering angle of the rear wheels is accurately matched with the steering angle set according to the vehicle speed, the steering angle of the front wheels and the operating state of the brake. be able to.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram showing a suspension system of a vehicle equipped with a rear wheel steering device.

FIG. 2 is a plan view showing an embodiment of a rear wheel steering system according to the present invention.

FIG. 3 is a perspective view showing a main part of the rear wheel steering device.

FIG. 4 is a perspective view showing a state in which a cross member of a vehicle body is viewed from the rear.

FIG. 5 is a flowchart showing a steering control operation of rear wheels.

FIG. 6 is a characteristic diagram showing a relationship between a steering angle of rear wheels and a traveling speed of a vehicle.

FIG. 7 is a characteristic diagram showing a relationship between a steering angle of rear wheels and a steering angle of front wheels.

FIG. 8 is a view corresponding to FIG. 3 showing a comparative example of the rear wheel steering system.

[Explanation of symbols]

1L, 1R front wheels 2L, 2R rear wheels 13L, 13R lateral link (suspension arm) 26 control unit (control means) 27 vehicle speed detection means 28 front wheel steering angle detection means 29 brake detection means 30 steering angle setting means 33, 34 suspension arm support Member 35 Drive lever 36 Worm wheel portion 37 Worm 40 Hydraulic motor 53 Link mechanism 55 Rotation angle detection means

Claims (6)

[Claims] 1. A rear wheel steering system for a vehicle that symmetrically steers the left and right rear wheels by slidingly displacing the left and right suspension arms supporting the rear wheels in a vehicle width direction. A pair of left and right suspension arm support members that are movably supported and one end of which is connected to the suspension arm, a drive lever that operates one suspension arm support member, and a drive lever of this drive lever that reverses the drive force of the other. A rear wheel steering system for a vehicle, comprising: a link mechanism that transmits a suspension arm support member; and a single hydraulic motor that applies a driving force to the drive lever. 2. The rear wheel steering system according to claim 1, wherein the driving force of the hydraulic motor is directly transmitted to the drive lever via a worm gear. 3. A vehicle speed detecting means for detecting the traveling speed of the vehicle is provided, and the steering angle of the rear wheels is set in the toe-out direction during low speed traveling in accordance with the traveling speed of the vehicle detected by the vehicle speed detecting means, thereby traveling at high speed. 2. A rear wheel steering system for a vehicle according to claim 1, further comprising steering angle setting means for setting the steering angle of the rear wheels in the toe-in direction and for gradually changing the set value in correspondence with the vehicle speed. apparatus. 4. A front wheel rudder angle detecting means for detecting a rudder angle of front wheels, and a case where it is confirmed that the front wheel rudder angle detected by the front wheel rudder angle detecting means is equal to or more than a preset reference value. The steering angle setting means for setting the steering angle of the rear wheels in the toe-in direction and for gradually changing the set value in accordance with the steering angle of the front wheels is provided. The rear wheel steering system according to claim 2 or claim 3. 5. A braking detecting means for detecting whether or not the vehicle is in a braking state, and a steering angle of the rear wheels in the toe-in direction when the braking detecting means confirms that the vehicle is in the braking state. The rear wheel steering system according to claim 1, 2, 3 or 4, further comprising: a steering angle setting means for setting. 6. A rotation angle detecting means for detecting the rotation angle of the hydraulic motor is provided, and a control means for controlling the steering angle of the rear wheels is provided while detecting the rotation angle of the hydraulic motor by the rotation angle detecting means. The rear wheel steering system for a vehicle according to claim 3, 4 or 5, wherein: