JPH0534084U - Rear wheel steering system - Google Patents

Abstract

(57) [Summary] [Configuration] A screw shaft 34, which is eccentric with respect to the input shaft 9, is integrated with the input shaft 9 that rotates around the front-rear axis according to the front wheel steering angle. The rotation of the screw shaft 34 causes the pinion 37 that meshes with the nut 35 that moves in the front-rear direction to rotate about the vertical axis. The swinging member 39 that supports the pinion 37 and is rotatable about an axis in the vertical direction supports the roller 48 rotatably about the axis in the vertical direction. The rotation of the pinion 37 caused by the rotation of the screw shaft 34 causes the rear wheel steering rack 12 to move laterally. Rack 12 for rear wheel steering due to whirling of screw shaft 34
The nut 35 is sandwiched between the left and right sides of the pinion 37 and the roller 48 so that the right and left can move. The nut is used in response to changes in the horizontal distance between the pinion 37 and the roller 48 due to the swinging of the swinging member 39.
Concavities and convexities 35a and 35b are formed on the contact portion of the roller 35 with the roller 48. [Effect] The transmission resistance of the steering operation input is small, the transmission efficiency is good, and desired steering characteristics can be obtained.

Description

[Detailed description of the device]

[0001]

[Industrial application]

 TECHNICAL FIELD The present invention relates to a rear wheel steering device in a steering angle-sensitive front and rear wheel steering vehicle in which steering of rear wheels is performed according to a front wheel steering angle.

[0002]

[Prior Art]

 In front and rear steering vehicles that are sensitive to the steering angle, the rear wheels are required to make complicated movements according to the front steering angle. For example, a rear wheel steering device disclosed in Japanese Patent Publication No. 60-44186 includes a cam that moves according to a front wheel steering angle and a cam follower that is provided on a rear wheel steering shaft. The rear-wheel steering shaft is converted into a vehicle width direction movement via the vehicle, and the rear wheel is steered by the rear-wheel steering shaft movement in the vehicle width direction.

Further, the rear wheel steering device disclosed in Japanese Utility Model Laid-Open No. 60-50078 discloses a rear wheel steering rack that rotates an input shaft according to a front wheel steering angle via a crank mechanism and a planetary gear mechanism. Then, the rear wheels are steered by the movement of the rear wheel steering rack in the vehicle width direction.

[0004]

[Problems to be solved by the device]

 In the case where the steering operation input is directly transmitted to the rear wheel steering shaft by the cam and the cam follower as in the conventional case, the transmission resistance is large and the steering operation cannot be smoothly performed.

Further, a structure in which a steering operation input is transmitted to a rear wheel steering rack by a crank mechanism and a planetary gear mechanism is extremely complicated in structure and is not practical, and the transmission efficiency of the steering operation input is also extremely low. ..

As a solution to such a problem, there is a rear wheel steering device proposed by the applicant of the present invention (see Japanese Patent Application No. 3-157749). This is an eccentricity with respect to the input shaft that rotates around the front-rear direction axis according to the front wheel steering angle and is supported by the vehicle body, and that is integrated with this input shaft and swings by the rotation of the input shaft. Screw shaft, a nut that is screwed to the screw shaft so as to move in the front-back direction by the rotation of this screw shaft, a rack formed on this nut, and this rack moves in the vertical axis center by moving the nut in the front-back direction. A pinion that meshes with each other for rotation, a swing member that supports the pinion and is rotatably supported by the vehicle body about an axial center in the vertical direction, and this swing member enables rotation about an axial center in the vertical direction. It has a supported roller. Then, the pinion is engaged with the rear wheel steering rack so that the rear wheel steering rack moves left and right by the rotation of the pinion, and the rear wheel steering rack moves left and right by the whirling of the screw shaft. A nut is placed between the pinion and the roller, and the nut is sandwiched between the pinion and the roller.

As a result, when the input shaft rotates about the vehicle front-rear direction axis according to the front wheel steering angle, the screw shaft integrated with this input shaft rotates, and the nut screwed onto this screw shaft moves in the front-rear direction. Moving. The movement of the nut rotates the pinion that meshes with the rack formed on the nut, and the rotation of the pinion moves the rear wheel steering rack in the left-right direction.

Further, since the screw shaft is eccentric with respect to the input shaft, the rotation of the input shaft causes the screw shaft to perform whirling motion around an axis along the front-rear direction. Due to this whirling of the screw shaft, the nut moves vertically and horizontally. The lateral movement of the nut moves the pinion connected to the nut laterally, and the lateral movement of the pinion laterally moves the rear wheel steering rack that meshes with the pinion.

That is, the left-right direction movement of the rear wheel steering rack is a combination of movement based on rotation of the screw shaft and movement based on whirling of the screw shaft. The amount of movement of the rack based on the rotation of the screw shaft changes linearly according to the front wheel steering angle. The amount of movement of the rack based on the whirling of the screw shaft periodically changes according to the front wheel steering angle. Thus, by adjusting the gear ratio between the pinion and the rack, the amount of eccentricity of the screw shaft with respect to the input shaft, and the like, the desired movement of the rear wheel steering rack can be achieved according to the front wheel steering angle.

However, in the above configuration, the nut and pinion are connected by disposing the nut between the left and right sides of the pinion and the roller and sandwiching the nut by the pinion and the roller. Further, the pinion and the roller are supported by a swing member which is rotatably supported by a vehicle body at an axis center along the vertical direction.

Therefore, the left and right movements of the pinion and the roller due to the whirling of the screw shaft follow an arc locus centering on the swing center of the swing member, so that the left and right movements of the pinion and the roller cause a horizontal gap. Changes. When the horizontal distance between the pinion and the roller becomes small, the pinion and the roller are pressed against the nut with a large force to prevent the nut from moving in the front-rear direction, allowing the rear wheel steering rack to accurately follow the rotation of the screw shaft. Can not be. In addition, if the lateral distance between the pinion and the roller becomes large, a large gap is created between the nut and the pinion, and it becomes impossible for the rear wheel steering rack to accurately follow the whirling of the screw shaft.

An object of the present invention is to provide a rear wheel steering device that can solve the above technical problems.

[0013]

[Means for Solving the Problems]

 The feature of the present invention is that in a front-rear wheel steering vehicle in which the rear wheels are steered by moving the rear wheel steering rack to the left or right in accordance with the front wheel steering angle, the front-rear axis is centered according to the front wheel steering angle. An input shaft that rotates and is supported by the vehicle body, a screw shaft that is integrated with this input shaft and that is eccentric to the input shaft so that it swings by the rotation of the input shaft, and a forward and backward movement due to the rotation of this screw shaft. Nut that is screwed to the screw shaft, the rack formed on this nut, the pinion that meshes with this rack so that it rotates around the vertical axis due to the forward and backward movement of the nut, and the pinion is supported and supported by the vehicle body. A swinging member rotatably supported around an axis extending in the vertical direction, and a roller rotatably supported by the swinging member about the vertical axis. The pinion is engaged with the rear wheel steering rack so that the rear wheel steering rack moves left and right by the rotation of the union, and the pinion and roller are moved so that the rear wheel steering rack moves left and right by the whirling of the screw shaft. The nut is placed between the left and right sides of the nut, and the nut is sandwiched between the pinion and the roller. It is in the point of being formed.

In the straight-ahead steering state, the line segment connecting the rotation centers of the pinion and the swing member is perpendicular to the axis of the rear wheel steering rack, and the line segment connecting the rotation centers of the pinion and the roller. Is preferably parallel to the axis of the rear wheel steering rack.

[0015]

[Action]

 According to the configuration of the present invention, when the input shaft rotates about the front-rear direction axis according to the front wheel steering angle, the screw shaft integrated with the input shaft rotates, and the nut screwed to the screw shaft is rotated. Move back and forth. The movement of the nut rotates the pinion that meshes with the rack formed on the nut, and the rotation of the pinion moves the rear wheel steering rack in the left-right direction.

Further, since the screw shaft is eccentric with respect to the input shaft, the rotation of the input shaft causes the screw shaft to perform whirling motion about an axis along the front-rear direction. Due to this whirling of the screw shaft, the nut moves vertically and horizontally. The nut is sandwiched between the roller and the pinion, and the roller and the pinion are supported by the rocking member. Therefore, the lateral movement of the nut causes the rocking member to rock about the vertical axis, and the pinion also moves. Moves left and right. When the pinion moves in the left-right direction, the rear wheel steering rack that meshes with the pinion moves in the left-right direction.

By the swing of the swing member, the roller moves in the left-right direction together with the pinion. Since the left and right movements of the pinion and the roller follow an arc locus around the swing center of the swing member, the left and right movements of the pinion and the roller change the left and right distances. Corresponding to this change in the left-right spacing, unevenness is formed on the contact part of the nut with the roller. That is, the nut's left-right width becomes smaller at the position where the horizontal distance between the pinion and the roller becomes smaller, and the nut's left-right width becomes larger at the position where the horizontal distance between the pinion and the roller becomes larger. Unevenness is formed on the part. As a result, the pinion is not pressed against the nut with a large force, and there is no large gap between the pinion and the nut, so the rear wheel steering rack accurately follows the rotation and whirling of the screw shaft. Can be made.

The left-right movement of the rear wheel steering rack is a combination of movement based on rotation of the screw shaft and movement based on whirling of the screw shaft. The amount of movement of the rack based on the rotation of the screw shaft changes linearly according to the front wheel steering angle. The amount of movement of the rack based on the whirling of the screw shaft changes periodically according to the front wheel steering angle. Therefore, by adjusting the gear ratio between the pinion and the rack, the amount of eccentricity of the screw shaft with respect to the input shaft, and the like, the desired movement of the rear wheel steering rack according to the front wheel steering angle can be achieved.

In the straight-ahead steering state, the line segment connecting the rotation centers of the pinion and the swing member is made perpendicular to the axis of the rear wheel steering rack, so that the swing of the swing member causes the pinion to move in an arc track. When moving left and right, the change in the longitudinal distance between the pinion and the rear wheel steering rack is minimized. As a result, the change in the amount of meshing between the rear wheel steering rack and the pinion becomes small, and the meshing can be made smooth.

In the straight-ahead steering state, the line segment connecting the rotation centers of the pinion and the roller is made parallel to the axis line of the rear wheel steering rack, so that the rear wheel steering is performed by the pinion based on the whirling of the screw shaft. When pushing the vehicle rack to the left or right, the pushing force can be most efficiently applied from the pinion to the rear wheel steering rack.

[0021]

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

FIG. 3 shows a schematic configuration of a front and rear wheel steering vehicle that controls the rear wheel steering angle according to the front wheel steering angle. The left and right front wheels 1 are connected via a front wheel steering member 2 and a tie rod 3. A rack 4 is formed on the front wheel steering member 2, and a pinion 5 meshing with the rack 4 is interlocked with a handle 6. As a result, when the steering wheel 6 is operated, the front wheel steering member 2 moves in the left-right direction to steer the front wheels 1.

A rack (not shown) is formed on the front wheel steering member 2 separately from the rack 4, and a front end of an input shaft 9 of a rear wheel steering device 8 is attached to a pinion 7 that meshes with the rack. ing. The input shaft 9 is supported by the vehicle body so as to be rotatable about the vehicle longitudinal axis. As a result, when the front wheel steering member 2 moves in the vehicle width direction due to the steering operation input, the pinion 7 rotates, so that the input shaft 9 rotates by an amount corresponding to the front wheel steering angle.

The rear wheel steering device 8 includes a rear wheel steering rack 12, and the left and right rear wheels 10 are connected to each end of the rear wheel steering rack 12 via tie rods 11. The rear wheel steering rack 12 is supported by the vehicle body via a bearing 24 so as to be movable in the left-right direction. The rear wheel steering device 8 includes a conversion mechanism 13 that converts the rotation of the input shaft 9 into the movement of the rear wheel steering rack 12 in the left and right directions.

As shown in FIGS. 1 and 2, the conversion mechanism 13 includes a housing 32 that supports the input shaft 9 via bearings 30 and 31. A dust seal 28 is interposed between the input shaft 9 and the housing 32. The housing 32 is fixed to a component member 33 of the vehicle body. Inside the housing 32, a screw shaft 34 is integrally formed on the outer circumference of the input shaft 9. In this embodiment, the screw shaft 34 is a ball screw. The axis 34c of the screw shaft 34 is eccentric to the axis 9c of the input shaft by an amount indicated by e in the figure.

A nut 35 is screwed onto the screw shaft 34. The nut 35 is a ball nut, and a ball (not shown) is interposed between the ball nut 35 and the screw shaft 34, which is a ball screw. A rack 36 is formed around the nut 35. The axial direction of the rack 36 is along the front-rear direction.

A pinion 37 that meshes with the rack 36 is provided. The axis of the pinion 37 extends along the vertical direction. The pinion 37 is supported so as to be movable in the left and right directions. That is, the pinion 37 is rotatably supported by a pair of upper and lower arms (swing members) 38 and 39 via bearings 40 and 41. The arms 38 and 39 are rotatably supported by supporting shafts 42 and 43, which are screwed into the housing 32 and have vertical axes, through bearings 44 and 45. As a result, the arms 38 and 39 swing about the support shafts 42 and 43, whereby the pinion 37 moves in the left-right direction. As shown in FIG. 1, a line segment connecting the rotation center A of the pinion 37 and the swing center B of the arms 38 and 39 is in the left-right direction of the rear wheel steering rack 12 in the straight steering state shown in the drawing. It is defined as a right angle to the axis line.

When the input shaft 9 rotates, the screw shaft 34 rotates, so that the nut 35 screwed onto the screw shaft 34 moves in the front-rear direction. As the nut 35 moves back and forth, the pinion 37 that meshes with the rack 36 formed on the nut 35 rotates about the vertical axis. The pinion 37 is meshed with the rear wheel steering rack 12 so that the rear wheel steering rack 12 moves in the left-right direction by the rotation of the pinion 37.

Since the shaft center 9c of the input shaft 9 and the shaft center 34c of the screw shaft 34 are eccentric, the rotation of the input shaft 9 causes the screw shaft 34 to swirl around a shaft extending in the front-rear direction. Due to the whirling of the screw shaft 34, the nut 35 moves in the vertical direction and the horizontal direction. The pinion 37 and the nut 35 are connected so that the rack 12 for steering the rear wheels is moved in the left-right direction by the movement of the nut 35 in the left-right direction. That is, the roller 48 is rotatably supported by the pair of arms 38 and 39 via a bearing so as to be rotatable about the vertical axis, and the nut 37 is arranged between the roller 48 and the pinion 37 on the left and right sides. The nut 35 is sandwiched between the roller 48 and the pinion 37. As a result, when the nut 35 moves in the left-right direction due to the whirling of the screw shaft 34, the arms 38, 39 swing about the support shafts 42, 43, and the pinion 37 moves in the left-right direction. By the lateral movement of the pinion 37, the rear wheel steering rack 12 that meshes with the pinion 37 moves in the lateral direction. As shown in FIG. 1, the line segment connecting the rotation center C of the roller 48 and the rotation center A of the pinion 37 is parallel to the axis of the rear wheel steering rack 12 in the straight steering state shown. As a result, when the pinion 37 exerts a force to push the rear wheel steering rack 12 to the left or right based on the whirling of the screw shaft 34, the component force in a direction other than the left and right directions is minimized. Can be most efficiently operated from the pinion 37 to the rear wheel steering rack 12.

Since the movement of the pinion 37 in the left-right direction is a movement on an arc locus with the center of rotation B of the arms 38 and 39 as the center, the amount of meshing between the pinion 37 and the rear wheel steering rack 12 is determined. Measures have been taken to prevent the change in. That is, the support yoke 50 and the spring 51 are inserted into the insertion hole 32a formed in the housing 32, and the adjusting screw 52 is screwed into the insertion hole 32a. The support yoke 50 is movable in the front-rear direction, the rear wheel steering rack 12 is pressed against the pinion 37 by the elastic force of the spring 51, and the meshing amount between the pinion 37 and the rear wheel steering rack 12 changes. Is being prevented. By adjusting the screwing amount of the adjusting screw 52 into the insertion hole 32a, the elastic force of the spring 51 can be adjusted so that the pinion 37 and the rack 12 can be meshed smoothly. In this case, as described above, the line segment connecting the rotation center A of the pinion 37 and the rotation center B of the arms 38, 39 is made straight at the right angle to the axis of the rear wheel steering rack 12 in a steering state. The change in the meshing amount between the pinion 37 and the rear wheel steering rack 12 is minimized when the 37 moves on the arc locus. As a result, the fluctuation of the pressing force of the rear wheel steering rack 12 onto the pinion 37 by the support yoke 50 is minimized, so that the rear wheel steering rack 12 and the pinion 37 are smoothly meshed with each other.

Since the left and right movements of the pinion 37 and the roller 48 are movements on an arc locus around the rotation center A of the arms 38 and 39, the swinging of the arms 38 and 39 causes the pinion 37 and the roller 48 to move. The left-right spacing of changes. That is, as shown in FIG. 5, in this embodiment, the line segment connecting the rotation center A of the pinion 37 and the swing center B of the arms 38, 39 is made perpendicular to the axis of the rear wheel steering rack 12 and the pinion 37. A line segment that connects the rotation center A of the roller 48 to the rotation center C of the roller 48 is parallel to the axis of the rear wheel steering rack 12. Therefore, the lateral distance W between the pinion 37 and the roller 48 becomes maximum in the straight steering state. From this state, when the arms 38 and 39 are swung to the left and right and the arms 38 and 39 are swung, the lateral distance W ′ between the pinion 37 and the roller 48 is reduced. Corrugations are formed on the contact portion of the nut 35 with the roller 48 in correspondence with the change in the lateral distance between the pinion 37 and the roller 48. That is, as shown in FIG. 1, the portion that comes into contact with the roller 48 in the straight steering state is the convex portion 35a, and the portion that comes into contact with the roller 48 in the left-right steering state is the concave portion 35b. In this embodiment, a step is formed between the convex portion 35a and the concave portion 35b. The step difference δ between the convex portion 35a and the concave portion 35b is determined based on the difference between the lateral distance W in the straight steering direction of the pinion 37 and the roller 48 and the lateral distance W'in the lateral steering state. As a result, the lateral width of the nut 35 becomes smaller at the position where the lateral distance between the pinion 37 and the roller 48 becomes smaller, and the lateral width of the nut 35 becomes larger at the position where the lateral distance becomes larger.

According to the rear wheel steering system 8 having the above-described configuration, the input shaft 9 rotates in accordance with the steering angle of the front wheel 1, whereby the screw shaft 34 rotates and whirling motion occurs. The rotation of the screw shaft 34 moves the nut 35 in the front-rear direction to rotate the pinion 37, and the rotation of the pinion 37 moves the rear wheel steering rack 12 in the left-right direction. Further, the pinion 37 is moved in the left-right direction by the lateral movement of the nut 35 due to the whirling motion of the screw shaft, and the rear wheel steering rack 12 is moved in the left-right direction by the movement of the pinion 37. That is, the left-right movement of the rear wheel steering rack 12 is a combination of movement based on rotation of the screw shaft 34 and movement based on whirling of the screw shaft 34. Therefore, by adjusting the gear ratio between the pinion 37 and the racks 36, 12, the eccentric amount e of the screw shaft 34 with respect to the input shaft 9, and the like, the desired movement of the rear wheel steering rack 12 according to the front wheel steering angle can be achieved. Can be

In FIG. 4, an alternate long and short dash line 60 in the figure shows the relationship between the front wheel steering angle and the rear wheel steering angle when the rear wheel steering rack 12 moves based on the rotation of the screw shaft 34. The rear wheel steering angle changes linearly according to the steering angle. A two-dot chain line 61 in the figure shows the relationship between the front wheel steering angle and the rear wheel steering angle when the rear wheel steering rack 12 moves based on the whirling of the screw shaft 34, and the rear wheel steering angle depends on the front wheel steering angle. The angle changes periodically. The solid line 62 in the figure is a combination of the relationship shown by the one-dot chain line 60 in the figure and the relationship shown by the two-dot chain line 61 in the figure. The front wheel steering angle and the rear wheel steering of the front and rear wheel steering vehicle according to the present invention are shown. Show the relationship with the corner. That is, the front wheel 1 and the rear wheel 10 are steered in the same direction while the front wheel steering angle is small, and when the front wheel steering angle is large, the front wheel 1 and the rear wheel 10 are steered in opposite directions. Then, the steering characteristic becomes favorable as a front and rear wheel steering vehicle.

Even if the left and right distances between the pinion 37 and the roller 48 change due to the swinging of the arms 38 and 39 due to steering, the nuts 35 and the rollers 48 are moved in correspondence with the change in the left and right distances. Since the concavities and convexities 35a and 35b are formed at the contact portions of the pinion 37, it is possible to prevent the pinion 37 from being pressed against the nut 35 with a large force and a large gap between the pinion 37 and the nut 35. As a result, the rear wheel steering rack 12 can accurately follow the rotation of the screw shaft 34 and the whirling of the screw shaft 34, and desired steering characteristics can be obtained.

The present invention is not limited to the above embodiment.

For example, in the above-described embodiment, the unevenness at the contact portion of the nut 35 with the roller 48 is formed by the convex portion 35a and the concave portion 35b having a step, but the unevenness may be a continuous curved surface so that no step is formed. . Further, in the above embodiment, a ball screw is used as the screw shaft 34, but an acme screw may be used, and the nut 35 may be an acme nut instead of the ball nut.

[0037]

[Effect of the device]

 According to the rear wheel steering system of the present invention, the rear wheel steering rack moves in the vehicle width direction based on the rotation of the screw shaft due to the rotation of the input shaft and the whirling of the screw shaft due to the rotation of the input shaft. Since the rear wheels are steered by the movement of the steering rack in the vehicle width direction, it is possible to reduce the transmission resistance of the steering operation input compared to the case where the steering operation input is transmitted to the rear wheels only by using the cam. Moreover, the structure is simplified as compared with the case where the planetary gear mechanism and the cam mechanism are used, and it is practical and the transmission efficiency of the steering operation input can be improved.

Further, even if the horizontal distance between the pinion and the roller changes due to the rocking of the rocking member, the pinion may be pressed against the nut with a large force, or a large gap may occur between the pinion and the nut. It is possible to obtain desired steering characteristics.

Further, by making the line segment connecting the rotation centers of the pinion and the swinging member perpendicular to the axis of the rear wheel steering rack, the meshing between the pinion and the rear wheel steering rack can be made smooth.

Further, by making the line segment connecting the rotation centers of the pinion and the roller parallel to the axis of the rear wheel steering rack, the steering operation input can be efficiently transmitted to the rear wheel steering rack.

[Brief description of drawings]

FIG. 1 is a plan sectional view of a main part of a rear wheel steering system according to an embodiment of the present invention.

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

FIG. 3 is a structural explanatory view of a front and rear wheel steering vehicle according to an embodiment of the present invention.

FIG. 4 is a diagram showing a relationship between a front wheel steering angle and a rear wheel steering angle of a front and rear wheel steering vehicle according to an embodiment of the present invention.

FIG. 5 is an operation explanatory view of a rear wheel steering device according to an embodiment of the present invention.

[Explanation of symbols]

 1 Front Wheel 9 Input Shaft 10 Rear Wheel 12 Rear Wheel Steering Rack 34 Screw Shaft 35 Nut 35a Convex 35b Recess 36 Rack 37 Pinion 38, 39 Arm 48 Roller

Claims (2)

[Scope of utility model registration request] 1. A front-rear wheel steering vehicle in which rear wheels are steered by lateral movement of a rear-wheel steering rack according to a front-wheel steering angle, wherein the front-wheel steering vehicle rotates around a front-rear axis according to the front-wheel steering angle and is supported by a vehicle body. Input shaft, a screw shaft that is integrated with the input shaft and is eccentric to the input shaft so that it swings when the input shaft rotates, and a screw shaft that moves in the front-back direction when the screw shaft rotates. A nut to be fitted, a rack formed on this nut, a pinion that meshes with this rack so as to rotate about the vertical axis center by the forward and backward movement of the nut, and a pinion that supports this pinion and is centered on the vertical axis by the vehicle body. The swing member is rotatably supported, and the roller is rotatably supported by the swing member about an axis in the vertical direction. The pinion is meshed with the rear wheel steering rack so that the wheel steering rack moves left and right.
A nut is arranged between the left and right of the pinion and the roller so that the rack for steering the rear wheels moves left and right by the whirling of the screw shaft, and the nut is sandwiched by the pinion and the roller, and the rocking member swings. A rear wheel steering device characterized in that a contact portion of a nut with a roller is formed with irregularities corresponding to a change in a lateral distance between the pinion and the roller. 2. A straight line connecting the rotation centers of the pinion and the rocking member at right angles to the axis of the rear wheel steering rack in the straight steering state, and a straight line connecting the rotation centers of the pinion and the roller. The rear wheel steering system according to claim 1, wherein the rear wheel steering rack is parallel to the axis of the rear wheel steering rack.