JPS63219486A - Four-wheel steering device for vehicle - Google Patents

Abstract

PURPOSE:To suppress the unnecessary operation of a steering ratio converting device by providing a device to detect that the steering ratio is converted to the same phase area when the acceleration control is being performed by an acceleration device or a car speed restoration device, and maintaining the steering ration until the acceleration control is finished. CONSTITUTION:A controller 53 has a steering ratio setting device 56 to find the steering ratio between the front and the rear wheels, and a deciding device to detect that the steering ratio of the front and the rear wheels is converted from the inverse phase area to the same phase area of the steering ratio property in the acceleration control period by either an acceleration device 71 or a car speed restoration device 73. And when the acceleration control is started from the same phase area, a steering ratio maintaining device 58 outputs a signal to maintain the steering ratio at the starting time until the acceleration control is finished, to the steering ratio setting device 56. Therefore, the unnecessary operation of the steering ratio converting device such as a stepping motor 50 in the acceleration control period can be suppressed.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention steers front wheels and rear wheels in accordance with the operation of a steering wheel, and also sets the steering ratio of the rear wheels in advance in accordance with vehicle speed, etc. Regarding a four-wheel steering system for a vehicle that is configured to vary according to a set steering ratio characteristic, in particular a constant speed system that controls one engine output to maintain a target vehicle speed set based on a vehicle speed setting signal. The present invention relates to a four-wheel steering device for a vehicle equipped with a traveling device.

(Prior art) A four-wheel steering system for a vehicle usually improves the turning performance of the vehicle by controlling the steering ratio of the front and rear wheels in opposite phase directions during low-speed turns, and controls the steering ratios in the same phase direction during high-speed turns. The steering angle of the rear wheels relative to the front wheels is changed based on a predetermined steering ratio characteristic that is set in advance according to the vehicle speed, etc., so as to improve the running stability of the vehicle. Publication No. 55-91457).

Further, constant speed traveling devices that maintain the vehicle speed of an automobile at a desired speed set by a driver have been put into practical use.

As shown in, for example, Japanese Utility Model Application Publication No. 58-44552, this constant speed traveling device allows the driver to operate a switch when the vehicle speed reaches a desired speed, thereby changing the vehicle speed to the target of constant speed control. The vehicle speed is set as a value, and the engine output is controlled so as to maintain the actual vehicle speed at the target vehicle speed.

By the way, the above-mentioned constant speed traveling device normally has an acceleration means that gradually increases the target vehicle speed along with the actual vehicle speed in response to the operation of the acceleration switch, and a deceleration means that gradually decreases the target vehicle speed together with the actual vehicle speed in response to the operation of the deceleration switch. and a vehicle speed return means for returning the actual vehicle speed to the target vehicle speed before the release by the driver operating a vehicle speed return switch when the constant speed control state is released. Vehicle speed control by the acceleration means, vehicle speed return means, etc. is slower than acceleration control performed by the driver using an accelerator pedal. For this reason, in a vehicle equipped with the four-wheel steering device and constant speed traveling device, the steering ratio changing device is frequently operated in accordance with the ever-changing vehicle speed during medium speed control by the acceleration device or the vehicle speed recovery device. There is a possibility that the stepping motor or the like that constitutes the steering ratio changing means generates heat. In addition, in order to prevent heat generation in the stepping motor, etc., it is also considered that the steering ratio changing means is inhibited from operating when the acceleration means, vehicle speed return means, etc. are activated, and the steering ratio is maintained at a predetermined steering ratio. However, with this configuration, if acceleration control is executed while the steering ratios of the front and rear wheels are maintained in the opposite phase range, the oversteering tendency during high-speed turns is exacerbated.

<Object of the Invention> The present invention has been made based on the above-mentioned technical background, and is directed to a stepping motor, etc. constituting a steering ratio changing means in a vehicle equipped with a four-wheel steering device and a constant speed traveling device. A vehicle that can suppress unnecessary operation of the engine and prevent its heat generation, and also prevent the steering ratio of the front and rear wheels from being in an opposite phase state during high-speed turns, thereby preventing an oversteering tendency from being promoted. The present invention provides a four-wheel steering device.

(Structure of the Invention) The present invention steers front wheels and rear wheels in accordance with the operation of a steering wheel, and changes the steering ratio of the front and rear wheels based on steering ratio characteristics corresponding to vehicle speed. a steering device, a constant speed traveling device that controls the engine output so that the actual vehicle speed matches the target vehicle speed set based on the heavy speed setting signal, and a constant speed traveling device that controls the target vehicle speed of the constant speed control by the constant speed traveling device to the actual vehicle speed. In a vehicle, the vehicle is equipped with an acceleration means for increasing the vehicle speed as the constant speed control state is canceled, and a vehicle speed return means for returning the vehicle speed to the target vehicle speed before the cancellation when the constant speed control state is canceled,
Discrimination means for determining that the steering ratio has changed from the opposite phase region to the same phase region of the steering ratio characteristic when acceleration control is executed by either the acceleration means or the heavy speed return means. and steering ratio holding means for maintaining the steering ratio of the front and rear wheels at the steering ratio at the time when the discrimination signal is output from the time when the discrimination signal is output from the discrimination means until the acceleration control ends. It has been established that

According to the above configuration, when acceleration control is executed by either the acceleration means or the vehicle speed return means of the constant speed traveling device, the steering ratio of the front and rear wheels changes to the same phase region. Since the steering ratio is maintained, unnecessary operation of the steering ratio control means is suppressed, and an oversteering tendency is prevented from increasing during high-speed turns.

(Example) FIGS. 1 and 2 schematically show the configuration of a four-wheel steering system for a vehicle, including front wheels IL, IR and rear wheels 2L.

2R is a front wheel steering mechanism 3 and a rear wheel steering mechanism 12, respectively.
is supported by.

The front wheel steering mechanism 3 includes a pair of left and right knuckle arms 4L.
, 4R, tie rods 5L, 5R, and a relay rod 6 that connects the left and right ties and ronds 5L, 5R. Further, a steering wheel 10 is connected to the front wheel steering mechanism 3 via a rack and pinion type steering device side 7. That is, the steering mechanism 7 includes a rack 8 formed on the relay rod 6, and a steering shaft 11 to which a steering wheel 10 is connected to the upper end and a pinion 9 that meshes with the rack 8 is attached to the lower end. The left and right front wheels 1L and 1R are configured to be steered in accordance with the operation 10.

On the other hand, similarly to the front wheel steering mechanism 3, the rear wheel steering mechanism 12 has a relay rod 15 that connects the left and right knuckle arms 13L, 13R and the tie rods 14L, 14R.
and a hydraulic power steering mechanism 16.
It is equipped with This power steering mechanism 16 includes a power cylinder 17 that is fixed to the vehicle body and has the relay rod 15 as a piston rod. Rooms 17b and 17c
The hydraulic chambers 17b and 17C are connected to a control valve 20 via pipes 18 and 19, respectively. Furthermore, two pipes, an oil supply pipe 22 and an oil discharge pipe 23, leading to a reserve tank 21 are connected to this control valve 20, and a hydraulic pump 24 driven by an engine (not shown) is arranged in the oil supply pipe 22. It is set up. The control valve 20 is a known spool valve type, and includes a cylindrical valve casing 20a integrally attached to the relay rod 15 via a connecting member 25, and a valve casing 20a fitted into the valve casing 20a. The hydraulic pump 24 supplies pressure oil from the hydraulic pump 24 to one hydraulic chamber 17b (17c) of the power cylinder 17 according to the movement of the spool valve to assist the driving force to the relay rod 15. It is something. Note that the relay rod 15 is placed in the neutral position (trailing wheels 2L, 2R) in the power cylinder 17.
Return springs 17d and 17d are attached to bias the steering wheel to a position where the steering angle θR is 0.

The relay rod 6 of the front wheel steering mechanism 3 is provided with a rack 26 at a different position from the rack 8 constituting the steering mechanism 7.
A pinion 27 attached to #JM of a rotating shaft 28 extending in the longitudinal direction of the vehicle body is engaged with this rack 26, and the rear end of this rotating shaft 28 is connected to the rear end via a steering ratio υI@l structure 29. It is connected to a wheel steering mechanism 12.

As shown in FIG. 2, the steering ratio control mechanism 29 has a control rod 30 that is slidably held in the vehicle width direction with respect to the vehicle body, and one end of the control rod 30 is connected to the control valve 20. is connected to the spool valve. The steering ratio control system 29 also includes a swing arm 33 whose base end is swingably supported by a U-shaped holder 31 via a support pin 32, and the holder 31 is fixed to the vehicle body. It is rotatably supported by a casing (f in the figure) via a support shaft 35 having a rotation axis perpendicular to the movement axis of the control rod 30. The support pin 32 of the swing arm 33 is located at the intersection of the two axes and extends in a direction perpendicular to the rotation axis, and by rotating the holder 31 around the support shaft 35, the tip of the support pin 32 is supported. The angle of inclination between the axis of movement of the bin 32 and the control rod 30, that is, the plane of the swing locus of the swinging arm 33 centered on the support bin 32, with respect to a plane perpendicular to the axis of movement (hereinafter referred to as the reference plane). The angle of inclination is changed.

Further, one end of a connecting rod 37 is connected to the tip of the swing arm 33 via a ball joint 36, and the other end of the connecting rod 37 is connected to the other end of the control rod 30 via a ball joint 38. The control rod 30 is connected to the end portion, and is configured to displace the control rod 30 in the vehicle width direction in accordance with the displacement of the tip of the swing arm 33 in the vehicle width direction.

The connecting rod 37 is slidably supported by a rotation imparting arm 40 via a ball joint 41 at a portion thereof close to the ball joint 36 . This rotation imparting arm 40 has a support shaft 42 on the movement axis.
#Ji of the rotating shaft 28 is provided integrally with a large-diameter bevel gear 43 rotatably supported via a
A bevel gear 44 attached to the end is meshed with it, and the rotation of the steering wheel 10 is transmitted to the rotation imparting arm 40. For this reason, the steering wheel 10
When the rotation imparting arm 40 and the connecting rod 37 rotate around the movement axis by an amount corresponding to the zero rotation angle, and the swing arm 33 swings around the support bin 32 accordingly, this support When the axis of the bin 32 coincides with the movement axis of the control rod 30, the ball joint 36 at the tip of the swing arm 33 only swings on the reference plane, and the control rod 30 is held stationary. However, if the axis of the bottle 32 is inclined with respect to the movement axis and the swing locus plane of the swing arm 33 is deviated from the reference plane, the ball will be moved as the swing arm 33 swings about the bin 32. The joint 36 is displaced in the vehicle width direction, and this displacement is transmitted to the control rod 30 via the connecting rod 37, and the control rod 30 moves along the movement axis to operate the spool valve of the control valve 20. It is configured to allow That is, even if the swing angle of the swing arm 33 about the axis of the bin 32 is the same, the displacement of the control rod 30 in the left-right direction changes as the tilt angle of the bin 32, that is, the rotation angle of the holder 31 changes. and change.

In order to change the inclination angle of the support bin 32 with respect to the movement axis, that is, the inclination angle of the holder 31 with respect to the reference plane, a sector gear 45 as a worm wheel is attached to the support shaft 35 of the holder 31. A worm gear 47 on a rotating shaft 46 is engaged with the worm gear 47 . Further, a bevel gear 48 is attached to the rotating ring 46, and a bevel gear 49 attached to the output shaft 50a of the stepping motor 50 is meshed with the bevel gear 48.
By operating the stepping motor 50 and rotating the sector gear 45, the inclination angle of the holder 31 with respect to the reference plane is changed to control the steering angle θR of the rear wheels 2L and 2R.
When the sector gear 45 is rotated clockwise when viewed from above the vehicle body from a neutral position where its center line is perpendicular to the center line of the rotating shaft 46 of the worm gear 47, the steering ratio is set to 2 for the rear wheels and 2R. The front wheels 1L and IR are controlled to face in the same direction and in the same phase as the front wheels 1L and IR.

Further, in the casing supporting the holder 31, there are stopper members 51 on the opposite phase side and the same phase side, which are comprised of bins that restrict the rotation range of the sector gear 45 on both left and right sides of the sector gear 45 as the rotation member. 52 is attached, and when the sector gear 45 rotates to the opposite phase side, when the rotation angle from the neutral position is -17.5 degrees, for example, the sector gear 45 comes into contact with the opposite phase side stopper member 51. Further rotation is restricted, and sector gear 4
5 to the same phase side, when the rotation angle from the neutral position reaches, for example, 20 degrees, the sector gear 45 comes into contact with the stopper member 52 on the same phase side and the movement is restricted. . Then, υII! of the stepping motor 50 when the sector gear 45 comes into contact with the stopper member 51 on the opposite phase side! The initial position is the 11th position. Note that 39 is a rod stopper that restricts the maximum movement range of the relay rod 15 in the rear wheel steering mechanism 12.

The stepping motor 50 acts as a steering ratio changing means, and as shown in FIG.
It is configured to be

That is, this controller 53 includes a vehicle speed sensor 54.
A vehicle speed detecting means 55 detects the vehicle speed according to a detection signal of the vehicle, and a steering ratio setting means 56 determines the steering ratio of the front and rear wheels corresponding to the vehicle speed based on a preset steering ratio characteristic, which will be described later. According to the command signal output from the controller 65 of the constant speed traveling device and the detection signal of the vehicle speed sensor 54, the steering ratio of the front and rear wheels changes as described above during acceleration control by either the acceleration means 71 or the vehicle speed return means 73. Determining that the steering ratio characteristic has changed from the anti-phase region to the same-phase region 5
7, when the acceleration control is started from the opposite phase region, the steering ratio at the time when the discrimination signal is output from the discrimination means 57 is maintained until the acceleration control is terminated; When the acceleration control is started from the same phase region, the steering ratio holding means 58 outputs a control signal to the steering ratio setting means 56 to maintain the steering ratio at the time when the acceleration control starts until the acceleration control ends. and a motor filltl that controls the stepping motor 50, which acts as a steering ratio changing means, in response to an output signal from the steering ratio setting means 56.
1 means 59.

Further, as shown in FIG. 4, the constant speed traveling device has a controller 65 connected via a main switch 64 to a power supply circuit 63 led from a power supply 61 via an ignition switch 62. Accelerator switch 66 as a vehicle speed setting means, "-stroke switch 6"
7 and a resume switch 68, and a signal from the vehicle speed sensor 54, and the controller 65 outputs control signals to six actuators such as solenoids.

The accelerator switch 66 is a switch that also functions as a set switch, and is turned ON when the desired vehicle speed is reached when the vehicle speed detected according to the signal from the vehicle speed sensor 54 is within the range of, for example, 40 to 100 rVn/h. By operating the vehicle speed setting function, the vehicle speed at that time is set as the target vehicle speed of the constant speed traveling device, and by keeping this ON state, the target value increases along with the vehicle speed during this time, and when it is OFF, this increase The vehicle has an acceleration function that performs constant speed driving control based on the set target vehicle speed. Further, the coast switch 67 is used when decelerating while traveling at a constant speed, and when it is turned on, the target vehicle speed decreases along with the vehicle speed. Furthermore, when the constant speed driving control is canceled due to a cause other than the OFF operation of the main switch 64, the resume switch 68 is activated.
By turning this on, the vehicle speed is returned to the target vehicle speed before the release.

Then, based on the operation of these switches 66, 67, and 68 and the signal from the vehicle speed sensor 54, the actuator 6 is activated.
9, acceleration means 71 and deceleration means 72 which output control signals to maintain, accelerate, decelerate, or return the vehicle speed. An I speed return means 73 is provided within the controller 65.

Further, an output line 74 from the controller 65 is connected to the controller 53 of the four-wheel steering system, and a command signal indicating that acceleration control is being executed by the acceleration means 71 or the vehicle speed return means 73 is sent to the output line 74.
4 to the controller 53 of the four-wheel steering system.

Further, in this constant speed traveling device, a clutch switch 7 is provided as a constant speed 1i+1 III canceling means for canceling the constant speed control state between the power supply circuit 63 and the controller 65.
5 and a brake switch 76 are arranged in parallel,
These switches 75, 76 are respectively connected to the clutch l1
It is turned on at 11152 hours and when the brake is applied, and a release signal is input to the controller 65.

In the above-described configuration, the control operation by the controller 65 for constant-speed control l ingestion will be explained based on the flowchart shown in FIG. 5. The above ignition switch 62
When the control operation is started by turning on the main switch 64, first, in step S1, it is determined whether or not the accelerator switch 66, which is a set switch that starts the constant speed control operation, has been turned on, and the constant speed control is started. Determine whether or not it has been done. When it is confirmed that constant speed control has started, a target vehicle speed VS is set in accordance with a detection signal output from the vehicle speed sensor 54 in step S2.

Next, in step S3, the accelerator switch 6
It is determined whether or not the ON state of 6 continues and acceleration control by the acceleration means 71 is started.

When it is confirmed that the acceleration υ control has started, a command signal indicating that the acceleration control state is in effect is outputted to the controller 53 of the four-wheel steering system in step S4, and the actuator 69 is activated in step S5. When activated, acceleration i+ll1l is executed to increase the target vehicle speed Vs as well as the actual vehicle speed.

Further, if it is confirmed that the acceleration control has not been started as a result of the determination in step S3, it is determined in step S6 whether or not the coast switch 67 has been operated and deceleration control has been started. If it is confirmed that the deceleration control has started, then in step $7, deceleration control is executed to reduce the target vehicle speed Vs as well as the actual vehicle speed, and then the process returns to step S3.

Further, if it is confirmed that the deceleration control has not been started as a result of the determination in step S6, the constant speed control state is canceled by operating a constant speed control canceling means such as the clutch switch 75 in step S8. Determine whether or not. As a result of this determination, if it is confirmed that the constant speed control state has been released, it is determined in step S9 whether or not the resume switch (return switch) 68 has been turned on. As a result of this determination, if it is confirmed that the resume switch 68 has been turned on,
In step 5111, the actual vehicle speed is read out according to the detection signal of the vehicle speed sensor 54 at the current time, and step S11
In this step, it is determined whether the actual vehicle speed V is larger or smaller than the target vehicle speed VS set in step S2. If it is confirmed that this actual vehicle speed V is smaller than the target vehicle speed VS,
After outputting a command signal indicating that the vehicle speed return means 73 is in an acceleration control state to the controller 53 of the four-wheel steering device in step Stz, vehicle speed return control is performed to match the actual vehicle speed ■ with the target vehicle speed VS in step S. Execute. In addition, in step S11 above, the actual vehicle speed ■
If it is determined that the target vehicle speed Vs is larger than the target vehicle speed Vs, the process moves directly to step S13, and vehicle speed return control is executed.

Further, if it is confirmed that the resume switch 68 is not turned on as a result of the determination in step S9, the constant speed control state is canceled in step S(). Then, in step S5, a timer or the like is activated. It is determined whether or not the set predetermined time "S" has elapsed, and if the resume switch 68 is not turned on even after the predetermined time TS has elapsed, the above-mentioned Return to step S1.

Furthermore, as a result of the determination in step S8 above, the constant speed v1
If it is determined that the 1211 state has not been released, normal constant speed control is executed in step S16, and then the process returns to step S3 to repeat the above flow.

Next, the control operation by the controller 53 of the four-wheel steering system will be explained based on the flowchart shown in FIG. When the above control operation starts, first step ST7
It is determined whether or not a command signal indicating that the vehicle is in the acceleration control state is inputted from the controller 65 of the constant speed traveling device, and as a result of this determination, it is confirmed that the vehicle is not in the acceleration control state. In step S18, normal steering ratio control is executed in accordance with the steering ratio characteristics stored as a data table in a storage unit (not shown). In other words, as shown in Fig. 7, the steering ratio characteristics described above change the steering ratio of the front and rear wheels depending on the vehicle speed, and when the vehicle speed is low, the steering ratio of the front and rear wheels changes in order to improve the turning performance of the vehicle. As a result, the steering ratio of the front and rear wheels becomes negative, and the vehicle speed decreases to, for example, 55!
When R/h is reached, the steering ratio becomes O, and a two-way steering state is established in which the steering angle of the rear wheels is set to 0 regardless of the steering angle of the front wheels. Additionally, when the vehicle speed increases further, the rear wheels are steered in the same direction as the front wheels in order to improve wheel grip when turning and increase driving stability, resulting in a positive steering ratio between the front and rear wheels. The stepping motor 50 is controlled as follows.

If it is confirmed in step ST7 that the vehicle has entered the acceleration control state, it is determined in step S whether or not the start point of the acceleration control is in the opposite phase region of the steering ratio characteristic. Distinguish based on vehicle speed. As a result of this determination, if it is confirmed that the acceleration control has been started from the opposite phase region, the steering ratio is controlled in accordance with the above-mentioned steering ratio characteristics in step S, and in step 821, the steering ratio of the front and rear wheels is controlled. It is determined whether the steering ratio has become 0 or more, and the steering ratio is controlled to change in the same phase direction as the vehicle speed increases until the steering ratio becomes O.

As a result of the determination in step 321, the vehicle speed is, for example, 5! If it is confirmed that the steering ratio has changed from the anti-phase region to the same-phase region, the process is performed in step 822 until it is confirmed in step S23 that the acceleration control is completed. The operation of the stepping motor 50 is stopped to maintain its steering ratio.

In addition, the above step S? If it is confirmed in step 9 that the acceleration control has started from the same phase region, the process moves directly to step 822, and the steering ratio is maintained until the acceleration control is finished.

In this way, the acceleration means 71 or the vehicle speed return means 7
When executing the acceleration control in step 3, it is determined that the steering ratio of the front and rear wheels has changed from the opposite phase region to the same phase region, and the steering ratio is fixed at this point, as shown in Fig. 8. When the acceleration tllIIIl is started from the opposite phase region, the steering ratio is controlled according to the steering ratio characteristic until the steering ratio of the front and rear wheels becomes 0. Then, the steering ratio is maintained at 0 until the acceleration control is terminated after quickly exiting the opposite phase region, and the steering angle of the rear wheels is set to O regardless of the steering angle of the front wheels. The vehicle enters a two-wheel steering state. Therefore, unnecessary movement of the stepping motor 50 can be suppressed and heat generation thereof can be prevented, and running stability can be maintained in a good state.

That is, in the acceleration υItill state, the steering ratio is maintained when the two-wheel steering state is entered, so that it is possible to prevent a decrease in running stability due to the front and rear wheels being in an opposite phase state during high-speed turns. , unnecessary movement of the stepping motor 50 can be suppressed. Then, when the acceleration control is completed and the holding state of the steering ratio is released, the stepping motor 50 is activated so that the steering ratio corresponds to the actual vehicle speed, and the steering ratio control is promptly executed. Therefore, the stepping motor 50 can be operated efficiently and heat generation can be prevented.

Note that in the above embodiment, a case has been described in which the steering ratio is maintained at O during acceleration control from the opposite phase region, but it is also possible to maintain the steering ratio at a predetermined value in the same phase region. You may.

(Effects of the Invention) As explained above, the present invention determines that the steering ratio has changed from the opposite phase region to the same phase region when executing acceleration control by the acceleration means or vehicle speed recovery means of the constant speed traveling device. Since the steering ratio is maintained from this point until the end of the acceleration control, unnecessary movement of the steering ratio changing means during the acceleration control can be suppressed and heat generation can be prevented. This has the advantage that it is possible to prevent the oversteering tendency from increasing during high-speed turns and maintain the running stability of the vehicle in a good state.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing an embodiment of a four-wheel steering device for a vehicle according to the present invention, FIG. 2 is a schematic perspective view of the above-mentioned steering device, and FIG. 3 is a block diagram showing the functions of the controller of the above-mentioned steering device. , FIG. 4 is a circuit diagram showing the configuration of the constant speed control device, and FIG. 5 is a circuit diagram showing the configuration of the constant speed control device.
FIG. 6 is a flowchart showing the control operation of the constant speed control device, FIG. 6 is a flowchart showing the control 211vJ operation of the four-wheel steering device, FIG. 7 is a characteristic diagram of medium speed and steering ratio, and FIG. 8 is a flowchart showing the control operation of the constant speed control device. It is a time chart showing the effect of the present invention. 1L, 1R...front wheel, 2L, 2R...rear wheel, 1o.
... Steering wheel, 57 ... Discrimination means, 58
...Steering ratio holding means, 71...Acceleration means, 73.-
・Vehicle speed return means. Patent applicant Mazda Co., Ltd. Representative Patent attorney Etsushi Kotani
Patent Attorney Chief 1) Masayuki Patent Attorney Yasuo Itaya Figure 2 n 19 4Ll Figure 7 Figure 8

Claims (1)

[Claims] 1. A four-way steering device that steers the front and rear wheels in response to steering wheel operations and changes the steering ratio of the front and rear wheels based on steering ratio characteristics corresponding to vehicle speed, and a vehicle speed setting signal. a constant speed traveling device that controls engine output so that the actual vehicle speed matches a target vehicle speed set based on the above; In a vehicle equipped with a vehicle speed return means for returning the vehicle speed to the target vehicle speed before the release when the constant speed control state is released, acceleration control is executed by either the acceleration means or the vehicle speed return means. a discriminating means for discriminating that the steering ratio has changed from an anti-phase region to an in-phase region of the above-mentioned steering ratio characteristic, and from the time when a discriminating signal is outputted from the discriminating means until the above-mentioned acceleration control ends. A four-wheel steering system for a vehicle, comprising a steering ratio holding means for holding the steering ratio of the front and rear wheels between the front and rear wheels at the steering ratio at the time when the discrimination signal is output.