JPH04129379U - Rear wheel steering mechanism - Google Patents

Abstract

(57) [Summary] [Purpose] To increase the degree of freedom in setting the rear wheel steering pattern and to reduce the size and weight of the rear wheel steering mechanism. [Structure] The outer cam surface of the cam body has a pair of left and right first cam faces that have a substantially cylindrical outer surface shape and whose distance from the rotation axis gradually increases as the rotation angular position increases, and a distance from the rotation axis. a pair of left and right second cam faces each having a substantially cylindrical outer surface shape that gradually decreases as the rotational angular position increases, and the first cam face and the second cam face abut against the cam body. The distance between the axes of the pair of cam followers is formed to be equal to the distance between the axes at the neutral rotational position of the cam body at any rotation angle position of the cam body.

Description

[Detailed explanation of the idea]

0001

[Industrial application field]

The present invention relates to a rear wheel steering mechanism applied to a four-wheel steering vehicle.

0002

[Conventional technology]

In order to obtain optimal driving performance depending on various driving conditions, the rear wheels are rotated in addition to the front wheels. Four-wheel steering devices configured to steer the vehicle are widely used. Four-wheel steering systems generally move the rear wheels forward when the steering angle is small. The wheels are steered in the same direction, that is, in the same phase, while the steering angle is If the value is greater than the fixed value, the rear wheels are steered in the opposite direction to the front wheels, that is, in the opposite phase. is configured to do so. When turning or changing lanes at medium or high speeds, steering operations are generally required. The steering angle is small, and by steering the rear wheels in the same phase at this time, the steering angle caused by centrifugal force is This makes it possible to suppress skidding of the vehicle and improve driving stability. on the other hand, When making a relatively sharp turn such as a U-turn at low speed, the steering angle should be adjusted. At this time, by steering the rear wheels in the opposite phase, the vehicle's turning half is reduced. It is possible to improve the turning radius by reducing the diameter.

0003 Rear rotary wheel described in Japanese Patent Application Laid-open No. 1-103577, filed by the applicant of the present application The rudder mechanism consists of a cam body that is rotationally driven by a drive means, and a cam body that is rotated by the vehicle width. A pair of left and right cam followers facing each other at a fixed interval in the direction, and this pair of cam followers The cam linkage body is configured to include a cam interlocking body that supports the follower and is movable in the vehicle width direction. On the outer periphery of the above cam body, one cam holder is attached when the cam body is rotated from a neutral state. The outer cam surface that pushes the lower outward in the vehicle width direction, and the outer cam surface that sandwiches the rotation axis center. A concave that allows the other cam follower to move inward in the vehicle width direction at the opposing part. A section has been established. The cam body rotates and the above cam section is directed towards one of the cam followers. When the two cam followers meet, the recessed portion faces the other cam follower. Therefore, one When one cam follower is pushed outward in the vehicle width direction by the cam body, the other cam The follower is allowed to move in the same direction as the one cam follower, so these The cam interlocking body that supports the cam follower is moved in the same direction as the one cam follower mentioned above. It will be done. A rear wheel steering member is connected to the cam interlocking body, and the rear wheel is connected to the cam interlocking body. It is steered in a predetermined direction in response to movement in the vehicle width direction.

0004 The outer cam surface formed on the outer periphery of the cam body has a small steering angle. When the vehicle is running, the rear wheels are steered in the same phase as the front wheels, and the steering angle is increased. At peak times, the rear wheels are steered in the opposite phase to the front wheels. The rear wheels are configured to be steered in accordance with the steering angle.

[0005]

[Problem that the idea aims to solve]

By the way, in the above rear wheel steering mechanism, one side of the outer cam surface of the cam body When the steering angle is small, the rear wheels are rotated in the same phase when the steering angle is small. While a recess is provided for steering, the rear wheels can be reversed when the steering angle is large. Corresponds to the steering angle by providing a convex part for turning the phase. The rear wheels can be steered in the same phase or in opposite phases.

[0006] However, in the rear wheel steering mechanism as described above, the amount of steering of the rear wheels is determined by the steering wheel. The steering ratio can only be determined according to the steering angle of the steering wheel (the amount of front wheel steering). The degree of freedom in setting is small, and it can easily accommodate various rear wheel steering patterns. I have a problem where I can't. In other words, the optimal rear wheel steering pattern varies depending on the vehicle's load distribution and road characteristics. Each vehicle is different, and in order to achieve optimal driving performance, the degree of freedom in setting the rear wheel steering pattern is required. This is because it is desirable that .

[0007] In addition, in order to steer the rear wheels in the same phase as the front wheels, we have created a small recess on the outer cam surface. This makes machining the cam surface difficult, making it difficult to create a highly accurate cam shape. cannot be formed. Moreover, in order to form the recess on the cam surface, the cam body The outer diameter of the cam body must be set larger than a specified size, making the cam body smaller and lighter. There is also the problem that it is not possible to promote

[0008] Furthermore, when steering the rear wheels to the opposite phase to the front wheels, they will pass through the same phase region. Therefore, there is a problem that the vehicle behavior feels strange. The present invention was devised under the above-mentioned circumstances, and it solves the above-mentioned conventional problems. This solution provides a large degree of freedom in setting the rear wheel steering pattern, and allows the rear wheel steering conditions to be adjusted in various ways. It can be determined according to driving conditions, and the cam body and rear wheel steering mechanism itself The objective is to provide a rear wheel steering mechanism that can be made smaller and lighter. .

[0009]

[Means to solve the problem]

The present invention takes the following technical measures to solve the above problems. That is, the present invention uses an electric motor whose rotation is controlled by a control means. A cam body that is rotationally driven around the axis, on both sides of the cam body, facing in the vehicle width direction with the cam body in between, and A pair of cam fins that are in contact with the outer cam surface of the Follower and As the above pair of cam followers move in the vehicle width direction, In a rear wheel steering mechanism including a cam interlocking body that is moved, The outer cam surface of the above cam body connects the contact points with each cam follower at the neutral rotation position. On one side of the axis, a standard whose radius is the rotation axis center and the length from this point to the above contact point. For a circle, the distance from the rotation axis increases as the rotation angle position from the above contact point increases. Therefore, the pair of left and right first cam faces each having a substantially cylindrical outer surface that gradually increases in size are On the other side of the axis, the distance from the rotation axis to the above reference circle is the rotation from the above contact point. A pair of left and right approximately cylindrical outer surfaces that gradually become smaller as the angular position increases. Each has a two-cam face, and The above-mentioned first cam face and the above-mentioned second cam face are the upper part that comes into contact with the above-mentioned cam body. The distance between the axes of the pair of cam followers is determined at any rotation angle position of the cam body. Even if the distance between the axes is equal to the distance between the axes of the cam body at the neutral rotation position, It is characterized by

0010

[Functions and effects of the idea]

The cam body is rotated from the neutral rotation position, and its outer cam surface is When the lower is pushed, each cam follower is moved in a predetermined direction in the vehicle width direction. Accordingly, the cam interlocking body is also moved in the same direction as the cam follower. This allows the A movement in the vehicle width direction is applied to the rear wheel steering member connected to the wheel linkage body, and the rear wheels are steered. Ru. In the present invention, the input shaft that rotationally drives this cam body is used as a control means. Therefore, the cam body is configured to be rotated by an electric motor whose rotation is controlled. By controlling the rotation of the rear wheels, the steering of the rear wheels can be controlled. There is. That is, the steering of the rear wheels can be freely controlled by the control means, In addition, by simply changing the shape of the cam body, the cam body can push the cam follower. You can also easily change the distance to move. Therefore, setting the rear wheel steering amount The degree of freedom is increased, making it easier to set various rear wheel steering patterns. .

[0011] In addition, in the present invention, the outer peripheral cam surface of the cam body is On one side of the axis connecting the contact points of the cam follower, centering on the rotation axis, connect this and the above contact point. The distance from the rotation axis to the reference circle whose radius is the length of A pair of left and right cylindrical outer surfaces that gradually become larger as the angle of rotation increases. The first cam face of The distance gradually decreases as the rotation angle position from the above contact point increases. A pair of left and right second cam faces having substantially cylindrical outer surfaces are respectively formed. Sunawa In other words, the larger the rotation angle of the cam body in one direction, the larger the rear wheel steering angle. The cam face is formed so as to

0012 The above-mentioned cam face according to the invention of the present application has the advantage that when the cam body is rotated in one direction, The distance from the axis to the reference circle only tends to increase or decrease. For this reason, like a conventional cam body, rotation of the cam body in one direction allows the same phase and There are no uneven parts on the cam face to obtain a reverse phase. Therefore, one side of the cam The cam follower can be configured to push the cam follower in one direction over the entire range in which it can be rotated in one direction. can. As a result, the use of the cam surface to steer the rear wheels in-phase or out-of-phase is now possible. The range of use is widened, a large amount of steering can be secured, and the cam lift It is also possible to reduce the slope of the cam and downsize the motor that drives the cam. In addition, by setting the cam face to the above shape, the cam face can be easily machined. It is extremely easy to form a cam surface with high precision. Also, the above In order to obtain conventional antiphase and same phase when rotating the body to one side. Since the unevenness of the cam is eliminated, it is also possible to reduce the reference circle diameter of the cam. It is also possible to make the body smaller and lighter.

[0013] Moreover, in the present invention, the first cam face and the second cam face are means that the distance between the axes of the pair of cam followers that come into contact with the cam body is The distance between the axes at the neutral rotational position of the cam body at any rotational angular position of the body. It is formed so that the distance is equal to Therefore, the distance between the pair of cam followers changes as the cam body rotates. Also, there is no possibility that a force will be generated to widen the spacing between these cam followers. No way. Therefore, it is possible to move the cam interlocking body smoothly and accurately in the vehicle width direction. can. Furthermore, when steering in opposite phases, as with conventional four-wheel steering mechanisms, the same phase range is passed through. This eliminates any discomfort caused by the vehicle's behavior.

[0014]

[Explanation of Examples]

Hereinafter, embodiments of the present invention will be specifically described based on FIGS. 1 to 8. FIG. 1 schematically shows the overall configuration of a four-wheel steering vehicle equipped with a rear wheel steering mechanism according to this embodiment. to show. As shown in this figure, the four-wheel steering device according to this embodiment has a front wheel steering mechanism. 1 and a rear wheel steering mechanism 2. The front wheel steering mechanism 1 is a general rack and pinion type steering gear. When the steering wheel 3 is turned, the rack rod 4 is moved in the width direction of the vehicle, and the movement is The signal is configured to be transmitted to the front wheels 5. The rear wheel steering mechanism 2 is driven by an electric motor 6. After this The wheel steering mechanism 2 is rotatable around an axis in the longitudinal direction of the vehicle, and is connected to the electric motor 6. Therefore, the plate-shaped cam body 7 that is rotationally driven, and the left and right cam bodies arranged on both sides of this cam body 7 A pair of rotating roller-shaped cam followers 8a, 8b, and the cam followers 8a, 8b. is supported in the cam follower support part 9 and is movable in the vehicle width direction in the housing 10. The cam interlocking body 11 is supported by the cam interlocking body 11.

[0015] An electric motor 6 is connected to the cam body 7 via a reducer 12 and a camshaft 13. The rotation of is transmitted. As shown in FIG. 2, the cam followers 8a and 8b are designed to ensure smooth operation of the cam mechanism. It is shaped like a rotating roller to increase the speed. This rotating roller-shaped cam follower A 8a, 8b is a cam interlocking member supported by the housing 10 so as to be slidable in the vehicle width direction. It is supported by the body 11. The cam interlocking body 11 has a frame-shaped cam follower support part 9 in the center, and this The cam followers 8a and 8b are rotatably supported in the cam follower support part 9. . Further, the cam follower support portion 9 is disposed on both sides of the cam follower support portion 9, and the housing 10 is provided with a vehicle. A tie rod is attached to the tip of the slide bars 14, 14 that are supported so as to be slidable in the width direction. A rear wheel 17 is connected via 15, 15 and knuckle arms 16, 16. . The cam body 7 is rotated, and one cam follower is rotated by its outer cam part to move the vehicle width When pushed outward in the direction, the cam interlocking body 11 is moved in the same direction, and the movement is caused by the rear wheel. 17 will be informed. Thereby, the rear wheels 17 are steered in a predetermined direction.

[0016] In this embodiment, the cam profile of the cam body 7 is formed as follows. . As shown in FIG. 2, the cam profile of the cam body 7 according to this embodiment is Connect the contact points 18a and 18b with each cam follower 8a and 8b at the neutral rotational position of the body. The rotation axis O is centered on one side (upper side) of the axis L, and the contact points 18a and 18b are connected to the rotation axis O. The distance from the rotation axis O to the reference circle C whose radius is the length of The left and right sides gradually increase as the rotation angle position from 8a and 18b increases. A pair of first cam faces 19a, 19b each having a substantially cylindrical outer surface shape are placed on the other side (lower side) of the axis. side), the distance from the rotation axis O with respect to the reference circle C is the contact points 18a, 18. A pair of left and right approximate circles that gradually become smaller as the rotation angle position from b increases. Second cam faces 20a and 20b are formed in the shape of cylindrical outer surfaces.

[0017] Therefore, the cam body 7 rotates from the neutral rotation position shown in FIG. As shown, the first cam face 19a faces the cam follower 8a on the right side in the figure. and the left cam mount that faces the cam follower 8a and the cam body 7 in the vehicle width direction. The second cam face 20b faces the follower 8b. in this case, The distance between the first cam face 19a and the rotation axis O is the radius of the reference circle C. Because it is larger than the length, the right cam follower 8a is larger than the first cam face 1. 9a to the right. On the other hand, the second cam face 20b is As the rotation angle position of the cam body 7 increases with respect to the quasi-circle C, it gradually decreases. Since the other cam follower 8b is formed so as to be This allows the cam interlocking body 11 to move in the vehicle width direction. and the rear wheels 17 are steered. On the other hand, the cam body 7 is rotated in the opposite direction. 5, the left cam follower 8b moves to the left first cam face. While the right cam follower 8a is pushed to the left by the cam follower 8a, the right cam follower 8a is Movement is allowed and the rear wheels 17 are steered in the opposite direction.

[0018] In addition, the first cam faces 19a and 19b, which function as cam surfaces, have a rotation angle position. It is formed so that the distance from the rotation axis O gradually increases as the size increases. Therefore, as shown in Fig. 6, the rotation angle of the rear wheel increases as the rotation angle of the cam body 7 increases. The amount of rudder increases. Further, the steering direction of the rear wheels 17 is determined by controlling the rotation direction of the cam body 7. It can be changed by In other words, changing the direction of rotation of the cam body 7 Accordingly, the rear wheels 17 can be freely steered in the same phase and in the opposite phase.

[0019] In this embodiment, the pair of first cam followers 8a and 8b are opposite to each other. The cam faces 19a, 19b and the second cam faces 20a, 20b are In the rotation operation of the arm body 7 in one direction, the distance from the axis O to the reference circle C is There will only be an increasing trend or a decreasing trend. Therefore, unlike the conventional cam face, By rotating in one direction, there are no uneven parts to obtain the same phase and opposite phase. As shown in FIGS. 2 and 6, the entire unidirectional rotatable range of the cam body 7 is The lowers 8a and 8b can be formed to be pushed in one direction. Therefore, when the cam body 7 is rotated by a certain angle, the cam followers 8a and 8b move. This allows for highly accurate steering control of the rear wheels. becomes possible. Also, a cam for steering the rear wheels 17 in the same phase or in an opposite phase. The range of use of the surface is widened, and the maximum amount of steering of the rear wheels can be increased. , the slope of the cam lift is reduced and the size of the electric motor 6 that drives the cam body 7 is reduced. It is also possible to

[0020]Furthermore, since there are no irregularities required to obtain opposite phase and same phase as in the conventional case, processing of the cam face becomes extremely easy, and a highly accurate cam surface can be formed. Moreover, since there is no need to form an inflection point on the cam surface as in conventional cam bodies, it is also possible to reduce the diameter of the reference circle C of the cam body 7, making the cam body 7 smaller and lighter. You can also do that. Moreover, the distance h between the axes of the first cam faces 19a, 19b and the second cam faces 20a, 20b of the pair of cam followers 8a, 8b that abut the cam body 7 from left and right is such that the cam body 7 The cam body 7 is formed to be equal to the center-to-axis distance h ₀ at the neutral rotational position of the cam body 7 at any rotational angular position. Therefore, as the cam body 7 rotates, the distance between the pair of cam followers 8a, 8b does not change, and no force is generated to widen the distance between the cam followers 8a, 8b. Therefore, the cam interlocking body 11 can be moved smoothly and accurately in the vehicle width direction. Furthermore, when steering to opposite phases as in the conventional four-wheel steering mechanism, since the same phase region is no longer passed through, the vehicle does not behave unnaturally.

[0021] As shown in FIG. An electric motor is arranged so as to be rotatable around a direction axis and is interlocked with the camshaft 13. It is rotationally driven by the motor 6. Further, this electric motor 6 is controlled by a control system configured by a microcomputer. Rotation is controlled by stage 21. As shown in FIG. 7, the control means 21 includes a vehicle speed sensor 22 and a steering angle The rotation of the electric motor 6 is controlled based on the control information sent from the sensor 23, etc. The steering control of the rear wheels 17 is then performed. In this example, from the vehicle speed and steering angle, the Calculate lateral G (lateral acceleration) in real time while driving, and adjust according to the magnitude of lateral G. It is configured to control the steering direction and steering angle of the rear wheels 17.

[0022] Lateral G refers to the lateral acceleration that acts on the center of gravity of the vehicle, and this can be approximately expressed as a function of the steering angle (θ) from the neutral position of the front wheels and the vehicle speed (V) as follows: Can be done. G=θ×(V ² /L)/(1+K×V ² ) (Here, L is the size of the wheel base and K is a constant for correction.) As can be clearly seen from this relational expression, the lateral G If the front wheel steering angle is constant, the larger the vehicle speed is, the larger the front wheel turning angle is, and if the vehicle speed is constant, the larger the front wheel turning angle is.

[0023] The control means 21 controls the driving of the electric motor 6 and the rotation of the cam body 7 in a running state. The rotation of the rear wheels 17 can be controlled according to the situation to achieve optimal driving performance. Do the rudder. An example of this will be explained along the flowchart shown in FIG. First, the vehicle speed information from the vehicle speed sensor 22 and the steering angle information from the steering angle sensor 23 are The steering angle information is read (s101, s102), and from this the lateral G is calculated. is calculated (s103). Then, depending on the magnitude of this lateral G, etc., the rear wheel 17 is moved to a predetermined position. The vehicle is steered by a predetermined angle in the direction. When the lateral G is 0.1 G or less (YES in s104), the steering angle ( The steering angle here refers to the rotation of the steering wheel itself, not the steering angle of the front wheels. stomach. The front wheel steering angle is the steering rotation angle divided by the overall gear ratio. . ) is greater than a predetermined value (for example, 240°) (YES in s105), the rear wheel 17 to the opposite phase (s107), and on the other hand, when the magnitude of lateral G is 0.1 G or less ( s104) and the steering angle is smaller than a predetermined value (s1 05 (NO), and if the lateral G is greater than 0.1G and less than 0.2G (NO in s104, YES in s106), so-called 2WS that does not steer the rear wheel 17 (s108). Furthermore, if the size of lateral G is larger than the above case (NO in s106), the rear wheels 17 are steered in the same phase, and in this case, the magnitude of the lateral G is Depending on the steering angle (s109, s111, s113), the amount of steering of the rear wheels 17 is also controlled. (s110, s112, s114, s115).

[0024] That is, when the lateral G is smaller than a predetermined value, the rear wheels 17 are steered to the opposite phase, When the lateral G is larger than a predetermined value, the rear wheels 17 are steered to the same phase. This is horizontal G If it is small, it is generally when turning at low speeds, and at this time the rear wheels By steering 17 to the opposite phase, maneuverability of the vehicle can be improved. This is because it is possible. On the other hand, lateral G is generally large when turning, etc. at medium or high speeds. At this time, by steering the rear wheels 17 in the same phase, the vehicle can be prevented from skidding. This is because it is possible to quickly change direction while suppressing this and increasing driving stability. It is. In this case, the larger the lateral G, the larger the steering amount, which is The larger the lateral G, the stronger the tendency of the vehicle to skid, and in order to suppress this, This is because the amount of steering of the rear wheels 17 required for this purpose also increases.

[0025] Also, when the lateral G is within a certain range, the rear wheels 17 should not be steered at all. However, this means that when the lateral G is within the specified range as shown above, This is because directionality is better if the rear wheels 17 are not steered. Furthermore, the lateral G The size is a predetermined value (including 0), and the steering angle is also less than a predetermined value (below 0). (including YES in s104, NO in s105), that is, if the vehicle is traveling almost straight Even in such a case, the rear wheels 17 are not steered. If the magnitude of the lateral G acting on the vehicle changes during a turn, the electric power will be adjusted accordingly. The rotation of the dynamic motor 6 and cam body 7 is controlled to perform rear wheel steering depending on the situation. At the same time, when there is no longer a need for rear wheel steering, the amount of steering is set to 0, As a result, the cam body 7 is returned to the neutral rotation position. That is, the rear wheels 17 are Returns to neutral state of 0. Note that the standard value of lateral G shown in the above flowchart is approximate. By further subdividing the conditions and determining the rear wheel steering conditions accordingly, driving performance will be improved. can increase In addition, the amount of rear wheel turning in the opposite phase is large when the steering angle is large. By increasing the size as the speed increases, maneuverability in the low speed range is improved. further improves.

[0026] In this embodiment, the cam body 7 is controlled by the control means 21. Since it is configured to be rotationally driven by the electric motor 6, the steering of the rear wheels 17 is , can be freely controlled by the control means 21. Also, the shape of the cam body 7 By changing the shape, the distance by which the cam body 7 pushes and moves the cam followers 8a and 8b is increased. Equality can also be easily changed. Therefore, you can freely set the rear wheel steering pattern. As a result, various rear wheel steering patterns can be easily set. As described above, the rear wheel steering mechanism according to this embodiment allows the rear wheel steering pattern to be set. It has a large degree of freedom, allowing rear wheel steering conditions to be determined according to various driving conditions. At the same time, the cam body 7 and the rear wheel steering mechanism 2 can be made smaller and lighter. A steering mechanism can be provided.

[0027] The scope of the present invention is not limited to the embodiments described above. The first cam faces 19a, 19b and the second cam faces 20a, 20b The shape is not limited to the example, and can be changed depending on the amount of rear wheel steering, etc. can. In addition, the rear wheel steering conditions must be set in accordance with various conditions other than the above-mentioned lateral G. Can be done. In addition, as the electric motor 6, various motors such as a DC motor and a stepping motor can be used. can be adopted.

[Brief explanation of drawings]

FIG. 1 is a diagram schematically showing the overall configuration of a four-wheel steering vehicle in which a rear wheel steering mechanism according to the present invention is adopted.

FIG. 2 is a schematic diagram of the cam body according to the present invention viewed from the direction of arrow A in FIG. 1;

FIG. 3 is a schematic diagram for explaining the action of the cam body and cam follower according to the present invention.

FIG. 4 is a schematic diagram for explaining the action of the cam body and cam follower according to the present invention.

FIG. 5 is a schematic diagram for explaining the action of the cam body and cam follower according to the present invention.

FIG. 6 is a diagram showing the relationship between the cam rotation angle and the rear wheel steering angle.

FIG. 7 is a system block diagram of the rear wheel steering mechanism according to the present invention.

FIG. 8 is a flowchart showing an example of rear wheel steering conditions according to the present invention.

[Explanation of symbols]

6 Electric motor 7 Cam body 8 Cam follower 11 Cam interlocking body 19a, 19b First cam face 20a, 20b Second cam face 21 Control means

Claims (1)

[Scope of utility model registration request] Claims: 1. A cam body rotatably driven around an axis by an electric motor whose rotation is controlled by a control means; While contacting the outer cam surface,
a pair of cam followers supported movably in the vehicle width direction;
In the rear wheel steering mechanism including a cam interlocking body that is moved in the vehicle width direction in the same direction as the pair of cam followers move in the vehicle width direction, the outer peripheral cam surface of the cam body is
On one side of the axis connecting the contact points with each cam follower at the neutral rotation position, there is a reference circle whose radius is the center of the rotation axis and the length between this point and the above contact point, and the distance from the rotation axis is as follows. A pair of left and right first cam faces each having a substantially cylindrical outer surface that gradually increases in size as the rotational angle position from the contact point increases is placed on the other side of the axis line at a distance from the rotation axis with respect to the reference circle. The cam has a pair of left and right second cam faces each having a substantially cylindrical outer surface shape that gradually becomes smaller as the rotational angular position from the contact point increases, and the first cam face and the second cam face are The distance between the axes of the pair of cam followers that come into contact with the body is formed to be equal to the distance between the axes at the neutral rotational position of the cam body at any rotational angular position of the cam body. A rear wheel steering mechanism.