JPH03153472A - Rear-wheel steering device and method thereof - Google Patents

Abstract

PURPOSE:To improve the extent of safety at time of rear-wheel steering control by installing a means, regulating a steering angle capable of steering each rear wheel within the specified range, in a linkage device, in such one which transmits the steering force inputted via the linkage and a speed-up gear to these rear wheels after being speeded up. CONSTITUTION:A support part 7 supporting a wheel motor 6 is installed in a lower end of a strut 3 whose upper end is supported on a body frame 1, of a strut type suspension 2. A first cylindrical member 11 is fitted in an upper hole 8 formed in this support part 7 free of rotation, and a knuckle arm 13 is connected to an upper end of the said member 11. This clockle arm 13 is linked up with a rack 16 in mesh with a pinion gear 21 being rotated by a rear-wheel steering electric motor 19 via a relay rod 17 and a tie rod 14. In this case, a lock means 28 as a steering angle regulating means capable of locking the relay rod 17 at the neutral position, is installed in a rack housing 26 housing the rack 16.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a rear wheel steering device and method for a vehicle capable of steering the rear wheels, and particularly to safety of rear wheel steering when a steering mechanism or control device is abnormal. The present invention relates to a rear wheel steering device and method that takes into account performance.

[Prior Art] In recent years, so-called four-wheel steering (4W) has been adopted for automobiles, which steers the front wheels and also steers the rear wheels when the vehicle turns.
S) Vehicles incorporating mechanisms are emerging. According to such a four-wheel steering mechanism, when the vehicle makes a small turn for a lane change etc. while driving at high speed, the front wheels and rear wheels are steered in the same phase, thereby steering the vehicle body inward in the direction of travel. It is possible to generate centripetal force without having to direct it significantly.

Therefore, the shaking of the vehicle body is reduced, so running stability can be improved, and the vehicle can turn smoothly. Also, when driving at low speeds, the front and rear wheels can be steered in opposite phases to make a short turn for parking, etc., or the front and rear wheels can be steered in the same phase to perform parallel movement for parallel parking, etc. By steering the vehicle, it becomes possible to improve maneuverability.

By the way, in order for a four-wheeled vehicle to turn smoothly, it is necessary for all four wheels to have the same turning center.

For this purpose, conventional steering mechanisms employ, for example, a steering link mechanism that includes a knuckle arm and a tie rod.

As shown in Fig. 6, this steering link mechanism is
The tie rod 01 and the knuckle arm 02 are relatively rotatably connected at the tie rod ends O1a, O1a, and the other ends of the knuckle arms 02 are connected to the king pins 03 of the left and right wheels.

And the tie rod 01 of this steering link mechanism
The tie rod 01 is then moved left and right by transmitting turning torque from the steering handle using, for example, a rack and pinion, a worm gear, or a hydraulic cylinder, and the left and right movement of the tie rod 01 causes the knuckle arm 02 to rotate. This allows the left and right wheels to be steered left and right.

[Problems to be Solved by the Invention] However, in such a steering link mechanism, as shown in FIG. 6, once the tie rod 01 and the knuckle arm 02 are aligned, the wheels cannot be steered any further. It disappears. That is, this state is the limit of possible wheel steering, and the limit angle of the steering angle α is usually around 45°. If there is a limit to wheel steering as described above, steering for making a tight turn will be restricted.

Furthermore, even if the steering angle α of the wheels exceeds this limit angle, for example, 90°, if the steering angle is set to 90＠, the wheels may change depending on the driving force from the drive source installed in the vehicle body. A problem arises in that the motor cannot be driven.

Therefore, the present applicant has filed a patent application for a steering mechanism that solves these problems and a vehicle to which the same is applied (Japanese Patent Application No. 165498/1982).

In the steering mechanism of this patent application and the vehicle to which it is applied, when the steering angle of the steering wheel is mechanically transmitted to the wheels via a link mechanism, the speed is increased by a speed increasing gear, and each wheel is independently driven. A drive motor is provided for this purpose.

Therefore, by increasing the speed of the wheels using the speed-increasing gear, it is not only possible to steer the wheels by 90'' or more, but also it is possible to rotate the wheels by the drive motor when the wheels are steered by 90 degrees.

Therefore, if a steering mechanism using such a speed-increasing gear is used for the front and rear wheels, both the front wheels and the rear wheels can be adjusted by 90'.
It can be steered, making it possible to drive the vehicle in a short turn or move it parallel to the side.

However, when attempting to provide such a steering mechanism on the front and rear wheels, the following problems occur.

In other words, 1. For example, when making a small turn, the rear wheels must be steered in the opposite phase to the front wheels, whereas when moving the vehicle in parallel in a desired direction, the rear wheels must be steered in the same phase as the front wheels. need to be steered. Moreover,
Not only does the magnitude of the absolute value of the steering angle ratio of the front and rear wheels need to be changed depending on whether the vehicle is traveling in a short turn or traveling in parallel, but even when traveling in the same corner or traveling in parallel, the steering angle ratio must be changed depending on the magnitude of the steering angle of the front wheels. It is necessary to change the absolute value of

Furthermore, when steering the wheels to change lanes while driving at high speeds, it is not only necessary to set the rear wheels in the same phase as the front wheels in order to prevent the vehicle from shaking.
The steering angle ratio also needs to be set appropriately depending on the heavy speed.

Therefore, when the rear wheels are steered according to the steering angle of the 011 wheels, and when the steering angle of the front wheels is simply transmitted to the rear wheels using a mechanical structure, the structure is designed to meet all of these requirements. What you do is extremely difficult.

Therefore, the present applicant has further developed a rear wheel steering control system in which the rear wheels are steered by a rear wheel steering motor, and the rear wheel steering motor is sent to a control device and controlled based on vehicle speed, front wheel steering angle, etc. We have proposed a method and filed a patent application (
According to the rear wheel steering control method disclosed in Japanese Patent Application No. 63-310482), it becomes possible to meet the above-mentioned requirements.

By the way, when the rear wheel steering is performed by the link mechanism rear wheel steering motor and control device,
If these abnormalities occur, it is conceivable that the rear wheel steering control will not be performed properly.

The present invention has been made in view of the above circumstances, and its purpose is to control the rear wheel steering by limiting the steerable range of the rear wheels even when an abnormality occurs in the steering mechanism and control device. An object of the present invention is to provide a rear wheel steering device and method that can further improve safety in control.

[Means for Solving the Problems] For this purpose, the rear wheel steering device according to the invention of claim 1 has a link mechanism (13, 1
4, 16, 17) and a steering mechanism (25, 49, 50, 5) that increases the speed of the input steering angle and transmits it to the rear wheels (47, 48) through the speed increasing gear (40).
1, 52, 53, 54), wherein the link mechanism (13, 14, 16, 17) regulates a steerable steering angle of the rear wheels within a predetermined range. It is characterized in that a steering angle regulating means (28) is provided.

Further, the invention of claim 2 provides a structure in which the steering angle regulating means (28) is fixedly supported by a recessed fitting part (30) formed in the link mechanism (17) and the vehicle body frame (1). 3
0), and a locking member (32) that can be inserted into and removed from the locking member (32).

Furthermore, in the invention of claim 3, the locking member (32) is provided with a biasing means (57) for biasing the link mechanism (17) in a direction to return the rear wheels to a non-steering position when the rear wheels are steered. It is characterized by

Furthermore, the invention according to claim 4 provides that the control device (53) includes a mode setting means (50) for setting a general traveling mode, a parallel movement mode and/or a small turning mode, and a mode set by the mode setting means. The present invention is characterized in that it includes means for controlling fitting of the locking member into and removal from the recessed fitting portion accordingly.

Furthermore, the rear wheel steering method of the invention of claim 5 provides a link mechanism (1
3, 14, 16, 17) and a speed-increasing gear (40), the rear wheels are steered by increasing the speed and transmitting the steering angle to the rear wheels (47, 48), ,
A general driving mode, a parallel movement mode and/or a small turning mode are set, and when the general driving mode is set, the link mechanism (17) is fixedly supported by the vehicle body frame (1) in the recessed fitting part (30) formed in the link mechanism (17). By fitting the locking member (32), the steering angle at which the rear wheels (47, 48) can be steered is restricted within a predetermined range, and when setting the parallel movement mode and/or the small turning mode, the locking member (32) is inserted into the recess. Fitting part (
The restriction is released by removing the locking member (32) from the locking member (30).

[Operations and Effects of the Invention] In the invention of claim 1 configured as above, the link mechanism (13, 14, 16, 17) includes a steering wheel for regulating the steerable steering angle of the rear wheels within a predetermined range. Corner regulating means (28
), the rear wheels can be steered within a predetermined range. therefore.

Even if something goes wrong with the steering mechanism or control device.

The rear wheels will no longer be steered significantly, and the safety of rear wheel steering will be greatly improved.

Further, in the invention of claim 2, the steering angle regulating means (28)
a recessed fitting portion (30) formed in the link mechanism (17);
and a locking member (3) that is fixedly supported on the vehicle body frame (1) and that can be inserted into and removed from the recessed fitting portion (30).
2), the steering angle regulating means (28) is formed mechanically. Therefore, the steering angle of the rear wheels can be reliably regulated within a predetermined range with a simple structure.

Furthermore, in the invention according to claim 3, the locking member (32) is provided with a biasing means (57) for biasing the link mechanism (17) in a direction to return the rear wheels to a non-steering position when the rear wheels are steered. So,
The link mechanism is urged by the urging means (57) so as to return to the neutral position when the rear wheels are steered. Therefore, when the steering force disappears when there is an abnormality in the steering mechanism or control device, the rear wheels return to the non-steering position by the biasing means (57).

Therefore, rear wheel steering is eliminated and safety is further improved.

Furthermore, in the invention according to claim 4, the control device (53) includes a mode setting means (50) for setting a general traveling mode, a parallel movement mode and/or a small turning mode, and a mode setting means (50) configured to set the mode. The steering angle is automatically regulated according to the set mode. The means (28) become operative. Therefore, forgetting to operate the steering angle regulating means (28) during the normal driving mode is prevented, and safety is further improved.

Furthermore, the rear wheel steering method of the invention of claim 5 provides a link mechanism (1
3, 14, 16, 17) and a speed-increasing gear (40), the rear wheels are steered by increasing the speed and transmitting the steering angle to the rear wheels (47, 48), ,
A general driving mode, a parallel movement mode and/or a small turning mode are set, and when the general driving mode is set, the link mechanism (17) is fixedly supported by the vehicle body frame (1) in the recessed fitting part (30) formed in the link mechanism (17). By fitting the locking member (32), the steering angle at which the rear wheels (47, 48) can be steered is restricted within a predetermined range, and when setting the parallel movement mode and/or the small turning mode, the locking member (32) is inserted into the recess. Fitting part (
By releasing the locking member (32) from 30), the restriction is released, so that in the normal driving mode, the maximum steering angle required in this mode is ensured.The driving stability at high speeds is improved. At the same time, when there is an abnormality in the steering mechanism or control device, rear wheel steering is restricted to within a predetermined range, thereby improving safety. Furthermore, in the downward movement mode and/or the small turning mode, rear wheel steering is not restricted and the rear wheels can be largely steered. Therefore, it becomes possible to freely perform parallel movement and small-turn driving, and it is possible to easily perform parallel parking, parking in a garage, etc., and the maneuverability is improved.

Note that the symbols in parentheses are used only for contrast with the drawings, and do not limit the structure of the present invention in any way.

[Example] Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing one embodiment of the rear wheel steering mechanism in this embodiment.

As shown in FIG. 1, the upper end of a strut 3 of a strut type suspension 2 is supported by a bearing 4 on a vehicle body frame 1. As shown in FIG. Of course, this bearing 4 is not necessarily necessary and can be omitted. The lower end of this strut 3 is swingably supported by the vehicle body frame 1 by a lower arm 5. Further, a support part 7 that supports the wheel motor 6 is provided at the lower end of the strut 3, and a pair of upper holes 8 and lower holes 9, which are eccentric to each other, are formed in the support part 7 in parallel with the struts 3. It is perforated. ”
The second hole 8 has a first cylindrical member 11 having a gear with internal teeth 10.
is fitted and supported by a bearing 12 so as to be rotatable around the central axis a of the upper hole 8 .

A knuckle arm 13 is connected to the upper end of the first cylindrical member 11, and a tie rod 14 is connected to the knuckle arm 13 via a universal joint 15. Furthermore,
This tie rod 14 is connected via a universal joint 18 to a relay rod 17 having a rack 16 formed at a predetermined position. Furthermore, a pinion gear 21 is engaged with the rack 16 of the relay rod 17 and is rotated at a reduced speed by the rear wheel steering electric motor 19 via a reduction gear 20 . In addition, in the figure, the electric motor 19, the gear reducer [20, and the meshing part between the rack 16 and the pinion gear 21 are indicated by A-
The cross section on the left half of line A is the cross section on the right half of line A-A, that is, the cross section is perpendicular to the cross section in FIG. A rear wheel steering angle sensor 25 is disposed at the lower end of the pinion gear 210 rotation shaft 22 via a pair of gears 23 and 24 that are made of external teeth and mesh with each other. By using the pair of gears 23 and 24 in this way, the rotating shaft 22
It is designed to absorb the downward movements of the

Of course, the rear wheel steering angle sensor 25 can also be directly attached to the rotating shaft 22.

Further, a rack housing 26 that accommodates the rack 16 is arranged to cover the outer periphery of the relay rod 17.
A telescopic boot 27 is disposed between both ends of the rack housing 26 and the tie rod 4. This rack housing 26 is provided with locking means 28 that can lock the relay rod 17 in its neutral position.

As shown in FIG. 2, this locking means 28 includes an N through hole 29 formed in the rack housing 26 and penetrating to the inner peripheral surface of the housing, and a through hole 29 formed in the relay rod 17 at the neutral position of the relay rod 17. A recessed fitting part 30 facing the rack housing 29, a supporting member 31 fixed to the rack housing 26, and a tip 32a on the short side can pass through the through hole 29 and fit into the recessed fitting part 30, and the supporting member 31 on the long side The end portion 32b of the support member 31 is attached to the bottle shaft 33 and the E ring 34.
an L-shaped lever 32 rotatably supported by a tongue lever 32 disposed between this lever 32 and the support member 31;
is connected to the lever 32 and a spring 35 that constantly biases the tip 32a in the direction of fitting into the recessed fitting part 30,
The tip 32a resists the urging force of the spring 35 and the recessed fitting portion 3
A cable 36 is provided for rotating the lever 32 in a direction to escape from zero. This cable 36 can be pulled manually, or, although not shown, power such as electromagnetic force from a solenoid can be used.

Then, as shown in FIG. 3, when the relay rod 17 is in the neutral state, the tip 32a of the lever 32 fits into the recessed fitting part 30,
when the left and right sides 3 of the tip 32a of the lever 32 are
Relay rod 17 forming recessed fitting portion 30 with 2b and 32c
Gaps c, c are formed between the side walls 17a, 17b. Therefore, at this time, the lever rod 17 can only move left and right within the range of this gap C. The size of the gap C is determined by the small rear wheel steering angle (for example, the maximum The wheel steering angle is set to approximately 5°). Furthermore, when the tip end 32a has escaped from the recessed fitting portion 30, the relay rod 17 can freely move left and right. Therefore, the rear wheel 47.4
8 will be able to steer a lot.

On the other hand, in the lower hole 9 of the support part 7, a second cylindrical member 38 having a gear with external teeth 37 on the upper end side is mounted on a double angular thrust ball bearing 39 and is rotatable around the central axis of the lower hole 9. Fitted and supported. The external teeth 37 of the second cylindrical member 38 are
It is meshed with the internal teeth 10 of the cylindrical member 11. The internal teeth 10 and the external teeth 37 constitute an upward speed increasing gear 40.

A motor case 41 of the wheel motor 6 is attached to the lower end of the second cylindrical member 38 . In that case, the mounting surface 41a of the second cylindrical member 38 in this motor case 41
is formed on an inclined surface perpendicular to the central axis of the cylindrical body. Inside the motor case 41, there are a stator 42 fixed to the motor case 41, a rotor 44 fixed to the motor rotating shaft 43. A planetary reduction gear 45 and the like are provided. The stator 42, the motor rotation shaft 43, and the rotor 44 constitute a travel drive motor.

Further, one end of the motor rotating shaft 43 is fitted to the other end of the output shaft 46, and the motor rotating shaft 43 and the output shaft 46 are disposed on the same axis. A planetary reduction gear 45 is disposed at the fitting portion of both wheels 43 and 46,
Power is transmitted from the motor rotating shaft 43 to the output shaft 46 via the planetary reduction gear 45 in a reduced speed. Furthermore, a wheel consisting of a wheel 47 and a tire 48 is attached to the other end of the output shaft 46. In this way, a wheel motor 6 is constructed in which a driving drive motor and a wheel are combined together. This wheel motor 6 allows the wheel to be rotationally driven even if the wheel is steered through 90 degrees via the steering mechanism. In that case, the center axis of the second cylindrical portion 38 is the rotation center axis for steering the wheels 47°48, and this center axis is arranged parallel to the center axis C of the strut type suspension 2. ing.

Wheel motor 6 and speed increasing gear 40 arranged in this way
Although not shown, the suspension 2 and the steering link mechanism are arranged symmetrically and in the same configuration with respect to the left and right wheels. In that case, the left and right tie rods 14
are connected by one relay rod 17.

FIG. 4 is a diagram showing an example of the configuration of a control system for the rear wheel steering motor 19.

As shown in FIG. 4, a front wheel steering sensor 49, a rear wheel steering sensor 25, a mode changeover switch 50
, a vehicle speed sensor 51 and an accelerator opening sensor 52 are connected to a central processing control unit (CPU) 53 by electrical wiring. The front wheel steering sensor 49 detects the front wheel steering angle, and the rear wheel steering sensor 25 detects the rear wheel steering angle. Further, the rear wheel steering control mode can be set to three modes: a general driving mode, a parallel movement mode, and a small turning mode using a mode changeover switch 50. Furthermore, the vehicle speed sensor 51 detects the vehicle speed, and the accelerator opening sensor 52 detects the accelerator opening.

Further, a control map 54 is connected to the CPU 53. This control map 54 consists of a rear wheel steering angle control map and a rear wheel drive force control map. The rear wheel steering angle control map stores the rear wheel steering angle based on the vehicle speed and the front wheel steering angle. On the other hand, the rear wheel drive force control map stores rear wheel drive force based on vehicle speed and accelerator opening. By using such a control map, each control can be performed quickly and accurately.

Furthermore, a steering motor driver 55 for the rear wheel steering motor 19 is connected to the CPU 53 . Further, a motor driver 56 for driving the rear wheel drive motors 42, 43, and 44 is connected.

The operation of the rear wheel steering mechanism configured as described above will be explained.

Under normal conditions, when the steering mechanism and its control device are normal and the general driving mode is set, the
The lever 32 of 8 rotates the cam of the relay rod 17 by the biasing force of the spring 35, and its tip 32a is fitted into the recessed fitting part 30. Therefore, the relay rod 17 is in a state where it can move left and right within the range of the gap C.

When the mode is set to the general driving mode by the mode changeover switch 50, the central processing unit 53 also selects the control map 54 as the control map for the general driving mode. and,
The central processing unit 53 has a control map 54 for general driving mode.
Based on the front wheel steering angle from the front wheel steering sensor 49, the rear wheel steering angle from the rear wheel steering sensor 25, and the vehicle speed from the vehicle speed sensor 51, the optimum rear wheel steering angle is determined when the vehicle turns at high speed. Determine. Furthermore, the central processing unit 53 instructs the determined rear wheel steering angle to the steering motor driver 55. The steering motor driver 55 drives the rear wheel steering motor 19 so that the rear wheels have the specified rear wheel steering angle. In this case, the gap C allows the rear wheels to be steered to the maximum steering angle necessary for turning during high-speed running. This improves driving stability during high-speed driving in the normal driving mode.

For example, if an abnormality occurs in the steering mechanism and control device due to a failure in the steering motor 19, an abnormality in each sensor, or an abnormality in the CPU 53, and the rear wheels are attempted to be steered far beyond the normal control range, 1 gap C Then, the side walls 32b and 32c of the lever tip 32a and the side walls 17a and 17b of the recessed fitting portion of the relay rod 17 engage with each other. For this reason,
Relay rod 17 does not prevent further left and right movement.
Steering of the rear wheels is now restricted within a predetermined range.

In this way, even if an abnormality occurs in the steering mechanism and the control device, the steering of the rear wheels is restricted, so the safety of rear wheel steering in the normal driving mode is improved.

In the parallel movement mode or the small turning mode, the lever 32 is rotated upward by pulling the cable 36 in both directions manually or with power such as electromagnetic force, and the lever tip 32a is released from the recessed fitting part 30. For this reason,
The relay rod 17 can move freely left and right, and the rear wheels can be largely steered left and right. Therefore, 5) parallel parking, parking in a garage, etc. can be easily performed, and 1) mobility is improved.

FIG. 5 is a partially enlarged sectional view showing another embodiment of the present invention. Note that the same components as those in the above-described embodiments are given the same reference numerals, and their explanations will be omitted.

In this embodiment as well, the lever 32 is biased toward the relay rod 17 by a spring 35 (not shown). As shown in FIG. 5, in this embodiment, a leaf spring 57 is attached to the tip 32a of the lever 32 by fastening with rivets or the like. This leaf spring 57 is connected to the tip 3 of the lever 32.
When 2a is fitted into the recessed fitting part 30, it comes into contact with the side walls 17a and 17b forming the recessed fitting part 30. Therefore, when rear wheel steering is performed in the normal driving mode and the relay rod 17 moves left and right, the leaf spring 57 is elastically deformed. As a result, when the rear wheels are steered, the restoring force of the leaf spring 57 biases the relay rod 57 in the direction of returning to the neutral position. Therefore, if an abnormality occurs in the steering mechanism or control device while the rear wheels are being steered in the normal driving mode, the rear wheels will be returned to the non-steering position by the restoring force of the leaf spring 57. , the safety of rear wheel steering in normal driving mode is further improved.

Further, the operation in the case where the rear wheels are largely steered when the steering mechanism or control device is abnormal, or in the parallel movement mode or the small turning mode is the same as that in the above-described embodiment, so a description thereof will be omitted.

Note that the present invention is not limited to the above-described embodiments, and various design changes are possible.

For example, instead of the spring 35 that biases the lever 32 to rotate, a compression coil spring may be used.

Further, the insertion and removal of the lever tip 32a into and out of the recessed fitting portion 30 can be made to be linked with the mode switching operation of the mode switching switch 50. In that case, the central processing unit 53 may control the lever 32 in accordance with the mode set by the mode changeover switch 50 so that the lever tip 32a fits into and escapes from the recessed fitting portion 30.

As is clear from the above explanation, in the event of an abnormality in the steering mechanism or control device, the lateral movement of the relay rod is mechanically restricted within a predetermined range, greatly improving safety in rear wheel steering control. I come to do it.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of an embodiment of a rear wheel steering device according to the present invention, FIG. 2 is a partially enlarged perspective view of a locking means in this embodiment, and FIG. 3 is taken along the line ■-■ in FIG. cross section,
Figure 4 is a configuration diagram of the control system in the rear wheel steering device.
FIG. 5 shows the third locking means in another embodiment of the present invention.
FIG. 6, which is a sectional view similar to the one shown in the figure, is an explanatory diagram of a conventional general steering mechanism. 1... Vehicle body frame, 13... Knuckle arm,
14...Tie rod, 16...Rack 17-
...Relay rod, 19...Rear wheel steering motor, 2
DESCRIPTION OF SYMBOLS 1... Pinion gear, 25... Rear wheel steering sensor, 28... Locking means, 30... Concave fitting 3
2...Lever, 32a...Lever tip Il! 4
2. 43. 44... Traveling drive motor, 49...
- Front wheel steering sensor, 50... Mode changeover switch, 51... Vehicle speed sensor, 54... Control map, 53... Central processing unit (CPU), 57... Leaf spring

Claims (5)

[Claims] (1) A rear wheel steering device including a steering mechanism that increases the speed of a steering angle input by a link mechanism and a speed-increasing gear and transmits it to the rear wheels, and a control device thereof, wherein the link mechanism is connected to the rear wheel. A rear wheel steering device comprising a steering angle regulating means for regulating the steering angle at which the wheels can be steered within a predetermined range. (2) The steering angle regulating means is configured by a recessed fitting portion formed in the link mechanism and a locking member that is fixedly supported by the vehicle body frame and that can be inserted into and removed from the recessed fitting portion. The rear wheel steering device according to claim 1. (3) The rear wheel steering device according to claim 2, wherein the locking member is provided with a biasing means for biasing the link mechanism in a direction to return the rear wheels to a non-steering position during rear wheel steering. . (4) The control device includes a mode setting means for setting a general traveling mode, a parallel movement mode and/or a small turning mode, and a mode setting means for setting the recessed fitting portion of the locking member according to the mode set by the mode setting means. 4. The rear wheel steering device according to claim 2, further comprising means for controlling insertion and removal. (5) A rear wheel steering method for steering the rear wheels by increasing the speed of the steering angle input by a link mechanism and a speed increasing gear and transmitting the same to the rear wheels, the method comprising a general driving mode, a parallel movement mode and/or When setting a small turning mode, and when setting a general driving mode, a locking member fixedly supported on the vehicle body frame is fitted into a recessed part formed in the link mechanism, thereby adjusting the steering angle at which the rear wheels can be steered. A rear wheel steering method characterized in that, at the same time, when a parallel movement mode and/or a small turning mode is set, the restriction is released by letting the locking member escape from the recessed fitting part. .