JPS62181962A - Four-wheel steering device for vehicle - Google Patents

Abstract

PURPOSE:To prevent a sudden change of the behavior of a vehicle accompanying the change of characteristics and improve safety by providing a characteristic changeover means for rear wheel steering ratio, a vehicle speed detecting means, and a characteristic change restricting means for changing the rear wheel steering ratio characteristic when the vehicle speed is below the defined value of a low speed or zero according to both of said means. CONSTITUTION:The captioned device has a characteristic changeover means for changing the characteristic of a rear wheel steering ratio, a vehicle speed detecting means, and a characteristic change restricting means for changing the rear wheel steering ratio characteristic when receiving signals from both the means and if the vehicle speed is below the defined value of a low speed or zero. This is to change the characteristic of the steering ratio on condition that the vehicle speed is zero or below a defined speed, seeing that when a vehicle speed is low a change of the behavior of a vehicle accompanying the change of the steering ratio characteristic is also gentle. Accordingly, since the change of the steering ratio characteristic is carried out in a condition that the vehicle speed is a low speed or zero, a sudden change of the behavior of a vehicle accompanying the change of characteristics can be prevented, improving safety.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a four-wheel steering system for a vehicle, and more specifically,
The present invention relates to a device in which a rear wheel steering ratio is controlled based on a predetermined rear wheel steering ratio characteristic.

(Prior Art) Some vehicles have a so-called four-wheel steering system in which both the front wheels and the rear wheels are steered.

In this four-wheel steering, the rear wheel steering ratio, that is, the ratio of the rear wheel steering angle to the front wheel steering angle, is generally controlled based on a predetermined steering ratio characteristic, and a plurality of these steering ratio characteristics are set. It has been considered to change the steering ratio characteristics as appropriate depending on the situation.

However, since changing the steering ratio characteristics involves changes in vehicle behavior, safety measures are required.

In response to this, it has been proposed to provide a delay means, as seen in Japanese Unexamined Patent Publication No. 60-135369.

According to the above proposal, the steering ratio is changed slowly as the steering ratio is changed, thereby preventing a sudden change in the behavior of the vehicle.

(Problem to be solved by the invention) However, as mentioned above, changing the steering ratio characteristics will cause a change in vehicle behavior in any case, so even if that behavior change occurs, it is safe. It is desirable to change the characteristics in a stable condition. In particular, it is necessary to consider safety against malfunctions and malfunctions of sensors or manual changeover switches that command changes in steering ratio characteristics.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a four-wheel steering system in which the steering ratio characteristics are changed only when a safe state is achieved.

(Means and effects for solving the problem) That is, the present invention focuses on the fact that when the vehicle speed is small, the change in vehicle behavior due to changes in steering ratio characteristics is gentle, and The steering ratio characteristic is changed on the condition that the low speed is below a predetermined value.

Specifically, assuming a four-wheel steering system for a vehicle that controls the rear wheel steering ratio based on at least two different rear wheel steering ratio characteristics, as shown in FIG. a characteristic switching means for switching a ratio characteristic; a vehicle speed detecting means for detecting a vehicle speed; and upon receiving signals from the vehicle speed detecting means and the characteristic switching means, when the vehicle speed is lower than a predetermined low speed value or is zero, the rear wheel steering is performed. The structure includes a characteristic change regulating means for changing the ratio characteristic.

(¥Example) Embodiments of the present invention will be described below with reference to the accompanying drawings.

In Fig. 2, IR is the right front wheel, LL is the left front wheel, 2R is the right rear wheel, and 2L is the left front wheel.The left and right front wheels IR and LL are linked by a front wheel steering mechanism A, and the left and right rear wheels 2R1
2L is linked by rear wheel steering a mechanism B.

In the embodiment, the front wheel steering mechanism A includes a pair of left and right knuckle arms 3R13L and a tyro throat 4R14, respectively.
L, and a relay rod 5 that connects the pair of left and right tie rods 4R14L. A steering mechanism C is linked to the front wheel steering mechanism A, and the steering mechanism C is of a rack and pinion type in this embodiment. That is, a rack 6 is formed on the relay rod 5, and a pinion 7 that meshes with the rack 6 is formed on the relay rod 5.
is connected to a handle 9 via a shaft 8.

As a result, when the handle 9 is operated to the right, the relay rod 5 is displaced to the left in FIG. The steering wheel is steered clockwise in the figure by an amount corresponding to the amount of operation displacement 9, that is, the steering angle of the steering wheel. Similarly,
When the steering wheel 9 is operated to the left +/J, the left and right front wheels IR and LL are steered to the left in accordance with the amount of displacement of this operation.

Similarly to the front wheel steering mechanism A, the rear wheel steering mechanism B also includes a pair of left and right knuckle arms 1OR, 10L and a tie rod 11R1IIL, and the tie rods 11R, 11L.
In the embodiment, the rear wheel steering mechanism B is configured to include a hydraulic power steering mechanism.

To explain this back-steering mechanism, a cylinder device 13 is attached to the relay rod 12, and the cylinder 13a is fixed to the vehicle body.
A piston 13d that defines the interior of 3a into two chambers 13b and 13C.
is integrated into the relay rod 12. Two chambers 13b and 13c within this cylinder 13a are connected to a control/help 16 via piping 14 or 15. Further, pipes 18 and 19 extending from the reservoir tank 17 are connected to each of the control valves 16, and an oil pump 20 driven by an engine (not shown) is connected to the pipe 18, which serves as an oil supply pipe. The control valve 16 is of a so-called booster valve type (spool type) in which the control rod 21 is a sliding type, and the input portion 21a of the control rod 21 is used as a moving member of a steering ratio changing device E to be described later. The output portion 21b of the control rod 21 is also integrated into the relay rod 12 of the rear wheel steering mechanism B.

In such a power steering mechanism, as is known, when the control rod 21 is displaced to the left in FIG. 2, the relay rod 12 is displaced to the left in FIG. The arms IOR and IOL rotate clockwise in FIG. 2 about their rotation centers 10R' and IOL', and the rear wheel 2R12L is steered to the right.

During this steering, oil is supplied into the chamber 13b of the cylinder device 13 according to the amount of displacement of the control rod 21 to assist in driving the relay rod 12 (boosting effect). Similarly, Control Rondo 21
When the rear wheel 2R12L is displaced to the right in FIG. 2, the rear wheel 2R12L is steered to the left while receiving the boosting action of the cylinder device 13 (oil is supplied to the chamber 13b) according to the amount of displacement. That will happen.

The front wheel steering mechanism A also has a power steering mechanism F like the rear wheel steering mechanism B. This power steering mechanism F includes a cylinder device 65 attached to the relay rod 5 of the 1m wheel steering mechanism A, and while the cylinder device 65a is fixed to the vehicle body, one cylinder 65a has two chambers 65b and 65c. a piston 65 defined in
d is integrated into the relay rod 5. The two chambers 65b and 65c within this sill 65a are connected via piping 66 or 67 to a rotary control/help 68 provided on the shaft 8 of the steering mechanism C. This control valve 68 is connected to a pipe 70 extending from a branch valve 69 connected to the discharge side of the oil pump 20, and a pipe 71 branching from the pipe 19.

Such a power steering mechanism F is /\endor9
The operation force of the power steering mechanism F is boosted (boosted by supplying oil to the chamber 65b or 65c of the cylinder device 65) and transmitted to the relay rod 5.The operation of the power steering mechanism F itself is basically as follows. Since this is the same as the power steering mechanism described above, further detailed explanation will be omitted.

The steering mechanism C and the rear wheel steering mechanism B are linked via the front wheel steering mechanism A and the steering ratio changing device E.

An input rod 22 extends forward from the steering ratio changing device E, and a binion 23 attached to the front end thereof is engaged with a rack 24 formed on the relay rod 5 of the front wheel steering mechanism A. Note that the output lot of the steering ratio changing device HIE is shared by the input portion 21a of the control lot 21 in the control valve 16, as described above.

An example of the steering ratio changing device E will be explained with reference to FIG. In this steering ratio changing device E, the control rod I
The input unit 21a of ``21'' is held slidably in the vehicle width direction with respect to the vehicle body, and its movement axis is shown as ``1''. 3
1, and the base end of the swing arm 31 is pivotably attached to the holder 32 by a pin 33. This holder 32 has a rotation axis 32a that is perpendicular to the movement axis it of the input section 21a! L
It is rotatably held on the vehicle body around 2. The bin 33 is located at the intersection of the two lines fL1 and the intersection 2, and extends in a direction perpendicular to the orthogonal line 9.2. Therefore, the swinging arm 31
By rotating the holder 32, the bin 33 and the movement axis 9. t, that is, the angle of inclination of the swing orbital surface centered on the bottle 33 with respect to a plane (reference plane) orthogonal to the movement axis line 1 is variable.

The tip of the swing arm 31 and the input section 21a are connected by a connecting rod 34. That is, the connecting member 34 connects to the swing arm 31 via the pole joint 35.
The input section 21a is connected to the input section 21a via a pole joint 36.

Due to the connecting rod 34 as described above, the spacing between the pole joints 35 and 36 at each end of the swinging arm 31 is
It will always be held constant. Therefore, if the hall joint 35 is displaced in the left-right direction in FIG. 3, the input section 21a will be displaced in the left-right direction in FIG. 3 in accordance with this displacement.

The swinging of the swinging arm 31 about the pin 33 is performed according to the operational displacement of the steering a structure C, that is, the steering angle of the steering wheel. Therefore, in the embodiment, the bevel gear A rotating plate 37 consisting of is connected. This rotating plate 37 is rotatably held on the vehicle body so that its rotating shaft 37a is at the moving axis Jomon 1, and the connecting rod 34 is connected to the pole shaft with respect to the eccentric portion of this rotating plate 37. It is slidably penetrated through the int 38.

A bevel gear 39 connected to the input rod 22 is fitted to a rotating plate 37 made of a bevel gear.

With such a rotating plate 37, the swinging arm 31 is moved around the pin 33 by an amount corresponding to the steering angle of the steering wheel, but the axis of the pin 33 and the axis of movement 1 are When the pole joint 35 is tilted, the pole joint 35 is displaced in the left-right force direction in FIG. is transmitted to the input section 21a, and the human power section 21
a will be displaced. The displacement of the pole joint 35 in the left-right direction in FIG.
When the inclination angle of the bin 33, that is, the rotation angle of the holder 32 changes, it is changed (steering ratio change).

In order to change the inclination angle, the rotation shaft 32a of the holder 32
A sector gear 40 as a worm wheel is attached to the worm wheel, and a worm gear 41 meshing with the sector gear 40 is rotationally driven by a stepping motor 44 as an inclination angle changing means via a pair of bevel gears 42 and 43. It has become so.

Here, the above-described swing angle of the swing arm 31 about the pin 33 and the inclination angle of the swing arm 31 (the inclination angle of the pin 33) are determined by the movement axis 9 of the pole joint 35 (input section 21a). The influence on displacement in one direction will be explained. Now, let us consider the swing angle of the swing arm 31 about the pin 33 as δ, the reference plane orthogonal to the O5 movement axis fLt, and the inclination angle that the swing orbital surface of the swing arm 31 makes with the reference plane δ as α.
, the eccentric distance of the pole joint 35 from the pin 33 is r
Then, look at the axis of movement of this pole joint 3! The displacement X in the direction is X=r tan a * sin θ, which is a function with α and θ as parameters. Therefore, if the inclination angle α is fixed at a certain value, X becomes θ
In other words, it is a function of /\depending on the steering angle of the steering wheel, and if the value of this inclination angle α is changed, the value of X will change even if the steering angle of the steering wheel is the same. This change in the inclination angle α naturally results in a change in the steering ratio.

As shown in FIG. 4, this steering ratio change is made based on a steering ratio characteristic that is set in advance using the vehicle speed as a parameter.
A first steering ratio characteristic (hereinafter referred to as Nol characteristic) and a second steering ratio characteristic (hereinafter referred to as N02 characteristic) which is obtained by offsetting this Nol characteristic to the low speed side are set, and as appropriate by manual operation, Switching is performed between the Nol characteristic and the No2 characteristic.

(Hereinafter, blank spaces) Here, in order to force the rear wheels 2R12L to the neutral position, that is, the straight-ahead state, a pair of return springs 13e and 13f are attached to the rear wheel power steering mechanism. Both springs 13e and 13f apply the rear wheel relay rod 12 from left and right opposite directions with equal force. The oil chambers 13b and 13c of the power steering mechanism are connected through a communication passage 46, and an electromagnetic on-off valve 47 is connected to the communication passage 46. As a result, the on-off valve 47
In the closed state, the rear wheel 2R12L is steered against the spring 13e or 13f by the supply of hydraulic pressure to the oil chamber 13b or 13c, and when the on-off valve 47 is opened to make both the oil chambers 13b and 13C at the same pressure. , spring 1
By the action of 3e and 13f, the rear wheel 2R12C is forced to the neutral position. Of course, these springs 13e, 13
The biasing force f is set to a size that allows the vehicle to take a neutral position against the external force received from the rear wheels 2R or 2L during turning.

Further, the sector gear 40 driven by the steering motor 44 has both swinging stroke ends connected to the same phase side stopper 48 and the opposite phase side stopper 49 (J 3 VJ
(Reference). The rotation range of the stepping motor 44 that is safe over the entire swing range of the sector gear 40 (from the stroke end on the same phase side to the stroke end on the opposite phase side) is r580J in terms of the number of steps.

In FIG. 2, 51 is a control unit composed of, for example, a microcomputer, and this control unit 51 includes:
A signal from a vehicle speed sensor 53 is input, and an ON/OFF signal from a steering ratio characteristic changeover switch 54 is input. Here, the steering ratio characteristic changeover switch (SW) 54 constitutes a characteristic changeover means for changing over the steering ratio characteristic, and the switch 5W54 is ro F FJ.
When it is rONJ, it means the selection of the Nol characteristic, and when it is rONJ, it means the selection of the No2 characteristic. Further, the control unit 51 outputs an output to the stepping motor 44 and the on-off valve 47.

Next, the content of control by the control unit 51 will be explained based on the flowcharts shown in FIGS. 5 to 9. In this embodiment, the steering ratio characteristic changeover switch (S
Even if the switching operation W) 54 is performed, the characteristics are only changed when the vehicle speed is below a predetermined value Vo, and an extremely low speed value is set as the predetermined value VO. In addition, in this embodiment, in consideration of the possibility that "step-out" (deviation between the number of steppings and the corresponding actual positional relationship) may occur in the stepping motor 44, the reference positioning is adjusted at any time, that is, "initial motor position". ”
It is recommended that you do this. In the embodiment, this "motor position initialization" is performed by bringing the sector gear 40 into contact with the opposite phase stopper 49, and this time is the origin position with the stepping number "0", and the motor is driven from this origin position. The stepping number should be set to the motor position "MP" at that time, and this "motor position initialization" should be performed at the start of control (immediately after the engine starts) and every time the vehicle speed becomes zero. be. Also, the flowchart shown in this embodiment.

Two types of flags, "flag 1" and "flag 2", are used, and the meanings of each flag are as follows.

①Flag l This flag is used to distinguish whether or not "motor position initialization" is in progress. "0" means initialization has ended, and "1" means initialization is in progress.

(?) Flag 2 It is set to "1" when "motor position initialization" is executed once, and is used to perform "motor position initialization" only once every time the vehicle speed decreases from 5g to zero. It is something that can be done.

Based on the above, the entire story will be explained in each episode according to the flowcharts shown in Figures 5 to 9, but for the sake of explanation,
The interrupt processing for the main routine as shown in FIG. 5 (FIGS. 6 to 8) will be explained first.

Interrupt processing l (Fig. 6) The interrupt routine shown in Fig. 71S6 is for driving the stepping motor 44 to set the steering ratio according to the vehicle speed based on the Not characteristic or No2 characteristic, and is set by a timer. An interruption is made to the main routine of the 7fSs diagram at every predetermined time interval (for example, every lomsec if the stepping motor 44 is to be driven at a rate of 100 steps per second). In the figure, rCPJ is the target stepping number required to achieve the steering ratio determined based on the No1 characteristic or No2# characteristic shown in FIG. 4, and rMPJ is the opposite phase side stopper as described above. Sector gear 40 from the origin position when 49 is the origin position
The rocking position (steering position of the rear wheels 2R12L) is shown by the number of steps.

Based on the above, first, in step 541, it is determined whether the target stepping aCP and the current position MP match, and if CP=MP, the rear wheels 2R12L are driven according to the predetermined steering ratio characteristic. Therefore, in step S42, the stepping motor 4
4 to prevent the stepping motor 440 from generating heat. After this, in step S43, a timer is set to the predetermined time specified by step S43 in preparation for the next interrupt.

When it is determined in step S41 that CP=MP is not true, in preparation for driving the stepping motor 44, the supply to the stepping motor 44 is determined. After increasing the ff flow (cancelling the current town), it is determined in step S45 whether or not it is CPOMF. And C
When it is determined that P>MP is not true, the current position of the stepping motor 44 is located on the same phase side as the target stepping number CP, so in step S46, the stepping motor 44 is directed to the opposite phase side and performs l stepping. Only drive. Then, in accordance with the operation of this "l stepping", the current position MP is updated by one stepping in step S47, and then the process moves to step S43. Conversely, when it is determined in step S45 that it is CPOMF, the stepping motor 44 is driven by one step toward the same phase in step 948, the current position MP is updated in step 349, and the current position MP is driven in step 343.
Move to.

Interrupt processing 2 (Fig. 7) This interrupt processing is performed because the vehicle speed sensor 53 is supposed to generate a pulse as the speedometer meter cable rotates. ), the main routine of FIG. 5 is interrupted. The vehicle speed sensor 53 is, for example, a 20 pulse sensor (a sensor in which the number of pulses generated when the meter cable rotates once is 20).
Therefore, the number of pulses generated when traveling 1 km in 42 rows is r12740 pulses. Such pulses generated by the vehicle speed sensor 53 are sequentially counted and integrated in step S51, and are stored as PCN.

Block 1 [Star A 2 [East side]] This interrupt processing is performed by directly calculating the cumulative count pulse number explained in the above interrupt processing 2 (Fig. 7) at the vehicle speed (km/h).
), the relationship between the vehicle speed sensor 53 and the meter cable set as described above 2, 2.
At 82,575m5ec''27j, an interrupt is made to the main routine shown in FIG.
), in step 353, 18 'Q count l' (fi P
ON is cleared.

Note that FIGS. 7 and 8 are merely examples of vehicle speed detection, and vehicle speed can be detected by any conventionally known appropriate means.

Main routine (Fig. 5) First, the entire system is initialized in ST, and in step S2, CP=0, MP=5.
80. Set flag L=rlJ and steering ratio characteristic (TNO) to Not characteristic. That is, as is clear from the explanation of FIG. 6 mentioned above, setting CP=0 means forcibly performing the process from step S45 to step S46 so that the sector gear 40 reaches the opposite phase stopper. The purpose is to return the sector gear 40 until it contacts the sector gear 49, that is, to perform "initialization of the motor position." Setting 1 MP = 580 means that no matter where the sector gear 40 is currently, if you return it by 580 stepping, the reverse will always occur. This is because it can be brought into contact with the phase side stopper 49 and returned to the original position. In addition, the steering ratio characteristic (T N O) is
The reason why this Nol characteristic is set as a characteristic is that this Nol characteristic is considered to be a basic characteristic of four-wheel steering.

After that, in step S3, after checking and checking the characteristics of the steering ratio, which will be described later, the process moves to step S4.
It is determined whether the flag l is "1". In this step S4, since flag 1 is initially set to rlJ in step S2, the process moves to step S5. In this step S5, it is determined whether or not CP=MP, but if CP=MP, then step S3
Therefore, a loop returns to 11f and step S5, and during this loop, the stepping motor 44 shown in FIG. 6 is driven (MP approaches rOJ), and eventually CP=MP. . And this C
When P=MP, "motor position initialization" is completed, and flag 1 is set to "0" and flag 2 is set to "1".

When it is determined in step S4 that flag 1 is not rlJ, it is determined in step S7 whether or not the current vehicle speed is zero.

In this determination, when it is determined that the vehicle speed is not zero, that is, it is determined that the vehicle is running, the CP
depending on the current vehicle speed, the Nol characteristic or N
It is set to a value corresponding to the steering ratio determined based on the o2 characteristic. After this, in step S9, flag l and flag 2 are both set to rQJ, and step S
Return to 3.

Further, when it is determined in step S7 that the current vehicle speed is zero, flag 2 or "
0" is determined.

When flag 2 is not "0", that is, when it is "1", it is assumed that "Since the stepping motor 44 has not been driven after motor position initialization j, it is unnecessary to perform this "motor position initialization" again. , directly return to step S3. In addition, when flag 2 is determined to be "0" in step 310, "motor position and initialization" is performed.
The process moves to step S12, and in this step Sll, CP=O, MP=5802 lag 1=rl'' is set, and the above-mentioned step S4. "Motor position initialization" is performed through step S5.

Characteristic switching check (Fig. 9) As mentioned above, in this embodiment, the change in the steering ratio characteristic between the No. 2 characteristic and the No.
) The following conditions apply.

That is, in step S21, it is determined whether the current vehicle speed is less than or equal to a predetermined value (Vo).
o) Step S2 for the first time under the condition that
Move to 2 and switch characteristics.

The ON/OFF state of switch (SW) 54 is read. In the next step S23, the characteristic changeover switch 54 is set to "
ON” is determined, and this step S2
3, when it is determined that the characteristic changeover switch 54 is "ON", that is, when switching to the No. 2 characteristic is selected, the process moves to step S24, and in this step S24, the steering ratio characteristic is changed to the No. 2 characteristic. is set to

Accordingly, the target stepping aCP is set to a value based on the No. 2 characteristic in the same phase direction.

On the other hand, if it is determined in the step S23 that the steering ratio characteristic is "rOFF", that is, if the change to the Nol characteristic is selected, the process moves to step S25, and in this step S25, the steering ratio characteristic is set to N014. ．． Set to IF. Accordingly, the target stepping number CP is changed to < (i (i) based on the Nol characteristic in the opposite phase direction. In this example, N
Since the OL and No. 2 characteristics are set to have almost the same steering ratio, there is only a slight change in vehicle behavior due to changes in characteristics.
Or it doesn't happen.

For this reason, the steering ratio characteristic can be changed in a safe manner at Y island, but even if there is an erroneous operation of the changeover switch 54, for example, if the vehicle speed or a predetermined extremely low speed (Vo
When the value exceeds , the characteristics are not changed, and safety can be improved from this point of view as well.

Although the embodiments have been described above, the present invention is not limited thereto, and includes, for example, the following cases.

(υThe predetermined value Vo mentioned above may be set to zero.

(?) The actuator for changing the steering ratio is not limited to the stepping motor 44, but may be any suitable one such as a DC motor.

(2) When the control unit 51 is configured by a computer, it may be of either a digital type or an analog type.

(4) Manually operated steering ratio characteristic change switch (
Instead of SW) 54, the steering ratio characteristic may be switched using an output signal from a road surface condition detecting means, for example, a p-sensor. As the characteristics in this case, the steering ratio characteristics shown in FIG. 10 will be set.

(4) As for the steering ratio characteristic, it may be possible to switch between front wheel steering and four wheel steering (Fig. 11), or
In a low speed range, the vehicle may be steered largely in the same phase direction for convenience when parking (see Fig. 12). Of course, when the characteristics shown in FIGS. 11 and 12 are set, it is preferable to switch by manual operation.

(Effects of the Invention) As is clear from the above description, according to the present invention, since the steering ratio characteristics are changed under the condition that the steering ratio characteristics are low or zero, there is no sudden change in the behavior of the vehicle due to the change in characteristics. can be prevented and safety can be improved.

[Brief explanation of drawings]

FIG. 1 is an overall configuration diagram of the present invention. FIG. 2 is an overall plan view showing one embodiment of the present invention. FIG. 3 is a skeleton diagram illustrating the rear wheel steering mechanism. FIG. 4 is a characteristic diagram showing an example of steering ratio characteristics. 5 to 9 are flowcharts showing control examples of the first embodiment. 10 to 12 are diagrams showing modified examples of steering ratio characteristics. A: Front wheel steering mechanism B: Rear wheel steering mechanism C Ni-steering mechanism E: Steering ratio changing device IR, IL: Front wheels 2R, 2L: Rear wheel 9: Handle 44 Steering motor 51: J control unit 54: Characteristic changeover switch (characteristic changeover means) Figure 5 Figure 68

Claims (1)

[Claims] (1) A four-wheel steering system for a vehicle that controls a rear wheel steering ratio based on at least two different rear wheel steering ratio characteristics, comprising: characteristic switching means for switching the rear wheel steering ratio characteristics; and detecting vehicle speed. Vehicle speed detection means; Characteristic change regulating means that receives signals from the vehicle speed detection means and the characteristic switching means and changes the rear wheel steering ratio characteristic when the vehicle speed is less than a predetermined low speed value or is zero. A four-wheel steering device for a vehicle characterized by: