Four-wheel steering system for automotive vehicles

A four-wheel steering system for an automotive vehicle includes a front wheel steering mechanism operable in response to a steering effort applied thereto to steer a pair of dirigible front road wheels, a rear wheel steering mechanism arranged to steer a pair of dirigible rear road wheels in response to steerage of the front road wheels, and a control apparatus arranged to control the rear wheel steering mechanism in such a manner that the rear road wheels are retained in their neutral positions when the front road wheels are steered at an angle less than a predetermined small angle and that the rear road wheels are steered in an opposite direction relative to the front road wheels when the front road wheels are steered at a larger angle than the predetermined small angle. The control apparatus is further arranged to control the rear wheel steering mechanism in such a manner that a changing rate of the rear wheel steering angle per a unit steering angle of the front road wheels increases in accordance with an increase of the front wheel steering angle.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a steering system for automotive vehicles, 
and more particularly to a four-wheel steering system including a front 
wheel steering mechanism operable in response to a steering effort applied 
thereto to steer a pair of dirigible front road wheels and a rear wheel 
steering mechanism arranged to steer a pair of dirigible rear road wheels 
in response to steerage of the front road wheels. 
2. Description of the Prior Art 
In Japanese Utility Model Early Publication No. 60-92669, there has been 
proposed such a four-wheel steering system as described above, wherein the 
rear wheel steering mechanism is controlled to retain the rear road wheels 
in their neutral positions when the turn angle of the steering wheel is 
less than a predetermined small angle and to steer the rear road wheels in 
an opposite direction relative to the front road wheels when the turn 
angle of the steering wheel is larger than the predetermined small angle. 
Under such control of the rear wheel steering mechanism, however, the rear 
wheel steering angle is steppedly increased at a constant changing rate in 
accordance with an increase of the turn angle of the steering wheel. It 
is, therefore, apparent that if the changing rate of the rear wheel 
steering angle relative to the front wheel steering angle was determined 
in a small value, the rear road wheels would not be steered at a desired 
angle even when the front road wheels are steered at a larger angle than 
the predetermined small angle. This deteriorates the smaller turning 
ability of the vehicle. If the changing rate of the rear wheel steering 
angle was determined in a large value to enhance the smaller turning 
ability of the vehicle, the rear road wheels would be steered in excess 
when the front road wheels are steered from their neutral positions. This 
causes a great change in behavior of the vehicle and deteriorates the 
turning ability of the vehicle. 
SUMMARY OF THE INVENTION 
It is, therefore, a primary object of the present invention to provide a 
control apparatus for the rear wheel steering mechanism which is capable 
of enhancing the smaller turning ability of the vehicle and of ensuring 
the stable turning ability of the vehicle without causing any problems 
described above. 
According to the present invention, the primary object is attained by 
providing a four-wheel steering system for an automotive vehicle which 
comprises a front wheel steering mechanism operable in response to a 
steering effort applied thereto to steer a pair of dirigible front road 
wheels, a rear wheel steering mechanism arranged to steer a pair of 
dirigible rear road wheels in response to steerage of the front road 
wheels, and a control apparatus arranged to control the rear wheel 
steering mechanism in such a manner that the rear road wheels are retained 
in their neutral positions when the front road wheels are steered at an 
angle less than a predetermined small angle and that the rear road wheels 
are steered in an opposite direction relative to the front road wheels 
when the front road wheels are steered at a larger angle than the 
predetermined small angle, wherein the control apparatus is further 
arranged to control the rear wheel steering mechanism in such a manner 
that a changing rate of the rear wheel steering angle per a unit steering 
angle of the front road wheels increase in accordance with an increase of 
the front wheel steering angle. 
Under such control of the rear wheel steering mechanism, the rear wheel 
steering angle is maintained at a zero value when the front road wheels 
are steered at an angle less than the predetermined small angle during 
travel of the vehicle at a high speed. When the front road wheels are 
steered at a larger angle than the predetermined small angle during travel 
of the vehicle at a medium or low speed, the rear wheel steering angle 
gradually increases from the zero value in accordance with an increase of 
the front wheel steering angle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 of the drawings, there is schematically illustrated an automotive 
vehicle equipped with a four-wheel steering system which comprises a front 
wheel steering mechanism A arranged to steer a pair of dirigible front 
road wheels FW.sub.1, FW.sub.2 and a rear wheel steering mechanism B 
arranged to steer a pair of dirigible rear road wheels RW.sub.1, RW.sub.2 
in response to steerage of the front road wheels FW.sub.1, FW.sub.2. 
The front wheel steering mechanism A is in the form of a well-known 
steering mechanism which includes a lateral rack bar 11 arranged to be 
axially displaced by operation of a steering wheel 15, a pair of tie rods 
17a, 17b each connected to opposite ends of the rack bar 11 by means of a 
pair of rack ends 16a, 16b, and a pair of knuckle arms 18a, 18b each 
connected to the tie rods 17a, 17b to steer the front road wheels 
FW.sub.1, FW.sub.2 in response to axial displacement of the rack bar 11. 
The rack bar 11 is operatively connected to the steering wheel 15 through 
a pinion gear 12, a lower steering shaft 13a, intermeshed bevel gears 14 
and an upper steering shaft 13b. The lower steering shaft 13a is provided 
thereon with a control valve 21 in the form of a four way valve which is 
responsive to the driver's steering effort applied thereto through 
steering shafts 13a, 13b to control the flow of hydraulic fluid under 
pressure supplied into a hydraulic power cylinder 24 from a hydraulic pump 
22 through a flow dividing valve 23 and a conduit P.sub.1 and to control 
the flow of hydraulic fluid discharged from the power cylinder 24 into a 
fluid reservoir 25 through a conduit P.sub.2. The hydraulic pump 22 is 
driven by a prime mover 26 of the vehicle, and the power cylinder 24 is 
activated under control of the hydraulic fluid to cause axial displacement 
of the rack bar 11 to assist steerage of the front road wheels FW.sub.1, 
FW.sub.2. 
As shown in FIGS. 1 and 2, the rear wheel steering mechanism B includes a 
lateral rod 31 arranged within a housing 32 to be axially displaced, a 
pair of tie rods 33a, 33b each connected to opposite ends of lateral rod 
31, a pair of knuckle arms 34a, 34b each connected to the tie rods 33a, 
33b to steer the rear road wheels RW.sub.1, RW.sub.2 in response to axial 
displacement of the lateral rod 31. The housing 32 is fixedly mounted on a 
body structure of the vehicle. As shown in FIG. 2, the housing 32 is 
formed to contain therein a hydraulic power cylinder 35 for effecting 
axial displacement of the lateral rod 31 and a coil spring 36 applied with 
a preload for biasing the lateral rod 31 toward its neutral position. The 
power cylinder 35 includes a power piston 35a axially movably disposed 
within the housing 32 to form a pair of fluid chambers 35b and 35c. The 
power piston 35a is fixed to the lateral rod 31 to effect axial 
displacement of the lateral rod 31 in accordance with hydraulic fluid 
under pressure selectively supplied into the fluid chamber 35b or 35c. The 
coil spring 36 is assembled in surrounding relationship with the lateral 
rod 31 and supported in place by engagement with a pair of axially spaced 
annular retainers 37a, 37b. The left-hand annular retainer 37a is retained 
in place by engagement with an internal annular shoulder 32a of housing 32 
and is engaged with the power piston 35a through a sleeve member 38a in 
such a manner as to permit leftward movement of the lateral rod 31. The 
right-hand annular retainer 37b is retained in place by engagement with 
the inner end of a closure plug 39 threaded into an end portion of housing 
32 and is engaged with a retainer ring 38b fixed to the lateral rod 31 in 
such a manner as to permit rightward movement of the lateral rod 31. 
The housing 32 is integrally formed thereon with a second housing 41 which 
contains therein a spool valve assembly 42. The spool valve assembly 42 
includes a valve sleeve 42a axially movably disposed within the second 
housing 41 and a valve spool 42b axially movably disposed within the valve 
sleeve 42a. The valve spool 42b cooperates with the valve sleeve 42a to 
selectively supply the hydraulic fluid under pressure into the power 
cylinder 35 from the hydraulic pump 22. The valve sleeve 42a is 
operatively connected at its left end to the lateral rod 31 by means of a 
swingable lever 43. 
As shown in FIGS. 2 and 3, the swingable lever 43 is rotatably supported at 
its upper end by means of a support pin 44 to be swung leftward or 
rightward in response to axial displacement of the lateral rod 31. As 
shown in FIG. 3, the support pin 44 is threaded into an adjustable 
fastening nut 45 which is threaded into a periperal wall of second housing 
41. The fulcrum of swingable lever 43 is displaced in a distance .DELTA.L 
from the rotation center of support pin 44. Thus, the fulcrum of swingable 
lever 43 can be slightly adjusted by rotation of the support pin 44 in a 
leftward or rightward direction in the figure. When the support pin 44 has 
been axially displaced by its rotation, the fastening nut 45 is rotated to 
fasten the support pin 44 in place. A screw 46 threaded into the housing 
41 is engaged with the inner end of support pin 44 to retain the swingable 
lever 43 in place. The swingable lever 43 has an enlarged pivot portion 
43a formed with a spherical surface for engagement with a corresponding 
bore 42a.sub.1 in the left end portion of valve sleeve 42a. A pin 47 fixed 
to the lower end of swingable lever 43 is engaged with an annular groove 
48a of a sleeve member 48 fixed to the lateral rod 31. 
As shown in FIGS. 1 and 2, the second housing 41 is provided with an inlet 
port 42c, an exhaust port 42d, and distribution ports 41e, 42f. The inlet 
port 42c is connected to the flow dividing valve 23 by way of a conduit 
P.sub.3, while the exhaust port 42d is connected to the fluid reservoir 25 
by way of a conduit P.sub.4. Disposed between the conduits P.sub.3, 
P.sub.4 is an electromagnetic changeover valve 51 which is provided with a 
solenoid 51a and a spring 51b. (see FIG. 1) When the solenoid 51a is 
deenergized, the changeover valve 51 is maintained in a first condition 
under the load of spring 51b to disconnect the conduit P.sub.1 from the 
conduit P.sub.3. When the solenoid 51a is energized, the changeover valve 
51 is maintained against the load of spring in a second condition to 
provide a direct fluid communication between the conduits P.sub.3 and 
P.sub.4. The solenoid 51a of changeover valve 51 is connected to a vehicle 
battery 53 through a cancel switch 52 of the normally open type. Thus, the 
solenoid 51a is energized only when the cancel switch 52 has been closed. 
The distribution ports 42e and 42f are connected to the fluid chambers 35b 
and 35c of power cylinder 35 through conduits P.sub.5 and P.sub.6, 
respectively. 
The valve spool 42b is connected to a connecting rod 61 to be axially 
displaced by rotation of a cam plate 62. The connecting rod 61 is axially 
movably disposed within the second housing 41 and connected at its one end 
to the valve spool 42b by means of a pin 63. A pin 64 is fixed to the 
other end of connecting rod 61 and engaged through a ball bearing 65 with 
a cam groove 62a formed in the bottom face of cam plate 62. The cam plate 
62 is in the form of a disc plate which is integrally formed at its center 
with upper and lower shaft parts 62b and 62c. The shaft parts 62b and 62c 
are rotatably supported on the second housing 41 through ball bearings 66a 
and 66b to support the cam plate 62 in place. As shown in FIG. 4, the cam 
groove 62a of plate 62 is formed spirally to cause axial displacement of 
the connecting rod 61 in accordance with clockwise or counterclockwise 
rotation of the cam plate 62, as shown by a solid characteristic line in 
FIG. 6. In a condition where the cam plate 62 is maintained in an initial 
position to retain the pin 64 and ball bearing 65 as shown in FIG. 4, the 
connecting rod 61 is retained in a neutral position. Even when the cam 
plate 62 is rotated from the initial position in a clockwise or 
counterclockwise direction at a small angle less than a predetermined 
angle, the connecting rod 61 is still retained in the neutral position. 
When the rotation angle of cam plate 62 exceeds the predetermined angle, 
the rate of axial displacement of the connecting rod 61 per a unit 
rotation angle of the cam plate increases at three steps as shown in FIG. 
6. When the rotation angle of cam plate 62 is maximized, the rate of axial 
displacement of the connecting rod becomes a maximum value. 
As shown in FIG. 5, a pair of vertically spaced annular grooves 62d and 62e 
are formed in an outer periphery of cam plate 62 to receive therein a pair 
of cables 67 and 68 which are fixed at their rear ends to the cam plate 62 
to rotate the cam plate 62 in response to steerage of the front road 
wheels FW.sub.1, FW.sub.2. The cables 67, 68 are extended outwardly from 
the second housing 41 through holes 41a, 41b and supported on an 
appropriate portion of the vehicle body structure to extend in a forward 
direction. Thus, as shown in FIG. 1, the cables 67, 68 are connected at 
their front ends to the rack ends 16a, 16b of front wheel steering 
mechanism A. As clearly shown in FIG. 5, a spiral spring 71 is mounted on 
the cam plate 62 in surrounding relationship with the shaft part 62c of 
plate 62. The spiral spring 71 has inner and outer ends 71a and 71b which 
are bent upward and engaged with a lever 72 and a stationary member 73, 
respectively. The lever 72 is fixed to the cam plate 62, while the 
stationary member 73 is fixed to the second housing 41. Thus, the cam 
plate 62 is loaded by the spiral spring 71 toward the initial position. 
Hereinafter, the operation of the four-wheel steering system will be 
described in detail. Assuming that the cancel switch 52 is maintained in 
an open position, the changeover valve 51 is maintained in the first 
condition to block the direct fluid communication between the conduits 
P.sub.3 and P.sub.4. Thus, the hydraulic fluid under pressure from pump 22 
is supplied into the inlet port 42c of spool valve 42 through the flow 
dividing valve 23 and conduit P.sub.3 to steer the rear road wheels 
RW.sub.1, RW.sub.2 in response to steerage of the front road wheels 
FW.sub.1, FW.sub.2. When the steering wheel 15 is turned rightward, the 
lateral rack bar 11 is displaced rightward by the driver's steering effort 
applied thereto through the steering shafts 13a, 13b. The rightward 
displacement of rack bar 11 is transmitted to the front road wheels 
FW.sub.1, FW.sub.2 through the tie rods 17a, 17b and knuckle arms 18a, 18b 
to steer the front road wheels in a rightward direction. In this instance, 
the control valve 21 is operated to supply the hydraulic fluid under 
pressure from the flow dividing valve 23 into the left-hand fluid chamber 
of power cylinder 24 therethrough and to discharge therethrough hydraulic 
fluid from the right-hand fluid chamber of power cylinder 24 into the 
fluid reservoir 25. Thus, the power cylinder 24 is activated to assist the 
steerage of front road wheels FW.sub.1, FW.sub.2. 
During the steerage of front road wheels FW.sub.1, FW.sub.2, the cable 68 
is pulled forward in accordance with the rightward displacement of rack 
bar 11 to rotate the cam plate 62 counterclockwise against the load of 
spiral spring 71. Thus, the pin 64 is moved by engagement with the cam 
groove 62a toward the center of cam plate 62 to cause rightward 
displacement of the connecting rod 61. In turn, the valve spool 42b is 
displaced rightward relatively to the valve sleeve 42a to permit the 
hydraulic fluid under pressure supplied from the inlet port 42c into the 
left-hand fluid chamber 35b of power cylinder 35 through the distribution 
port 42e and conduit P.sub.5 and to permit the flow of hydraulic fluid 
discharged from the right-hand fluid chamber 35c of power cylinder 35 into 
the fluid reservoir 25 through the conduit P.sub.6, distribution port 42f, 
exhaust port 42d and conduit P.sub.4. Thus, the power cylinder 35 is 
activated to cause rightward displacement of the lateral rod 31 against 
the load of spring 36. The rightward displacement of lateral rod 31 is 
transmitted to the rear road wheels RW.sub.1, RW.sub.2 through the tie 
rods 33a, 33b and knuckle arms 34a, 34b to steer the rear road wheels 
leftward or in an opposite direction relative to the front road wheels 
FW.sub.1, FW.sub.2. Simultaneously, the rightward displacement of lateral 
rod 31 causes the swingable lever 43 to swing counterclockwise about the 
pin 44, and in turn, the valve sleeve 42a is displaced rightward to block 
the supply of hydraulic fluid under pressure into the power cylinder 35 
and to block the discharge of hydraulic fluid from the power cylinder 35. 
As a result, the power cylinder 35 is deactivated to retain the lateral 
rod 31 in its displaced position thereby to arrest the steerage of front 
road wheels FW.sub.1, FW.sub.2. As is understood from the above 
description, the rear road wheels RW.sub.1, RW.sub.2 are steered in the 
opposite direction relative to the front road wheels FW.sub.1, FW.sub.2 in 
accordance with the rightward displacement of connecting rod 61 caused by 
rotation of the cam plate 62. 
When the steering wheel 15 is turned leftward to steer the front road 
wheels FW.sub.1, FW.sub.2 in a leftward direction under activation of the 
power cylinder 24, the cable 67 is pulled forward in accordance with 
leftward displacement of the rack bar 11 to rotate the cam plate 62 
clockwise against the load of spiral spring 71. Thus, the pin 64 is moved 
by engagement with the cam groove 62a in a direction apart from the center 
of cam plate 62 to cause leftward displacement of the connecting rod 61. 
In turn, the valve spool 42b is displaced leftward to permit the hydraulic 
fluid under pressure supplied from the inlet port 42c into the right-hand 
fluid chamber 35c of power cylinder 35 and to permit the flow of hydraulic 
fluid discharged from the left-hand fluid chamber 35b of power cylinder 35 
into the fluid reservoir 25. Thus, the power cylinder 35 is activated to 
cause leftward displacement of the lateral rod 31 thereby to steer the 
rear road wheels RW.sub.1, RW.sub.2 in an opposite direction relative to 
the front road wheels FW.sub.1, FW.sub.2. The leftward movement of lateral 
rod 31 causes the swingable lever 43 to swing clockwise about the pin 44, 
and in turn, the valve sleeve 42a is displaced leftward to block the 
supply of hydraulic fluid under pressure into the power cylinder 35 and to 
block the discharge of hydraulic fluid from the power cylinder 35. As a 
result, the power cylinder 35 is deactivated to retain the lateral rod 31 
in its displaced position thereby to arrest the steerage of rear road 
wheels RW.sub.1, RW.sub.2. 
From the above description, it will be understood that the rear road wheels 
RW.sub.1, RW.sub.2 are steered in response to steerage of the front road 
wheels FW.sub.1, FW.sub.2 in an opposite direction relative thereto. The 
steering angle of rear road wheels RW.sub.1, RW.sub.2 is determined in 
accordance with a displacement amount of connecting rod 61 defined by 
rotation of the cam plate 62, as will be described below. When the 
rotation angle of cam plate 62 is less than a predetermined small angle, 
the connecting rod 61 is maintained in the neutral position to retain the 
rear road wheels RW.sub.1, RW.sub.2 in their neutral positions. (see FIG. 
6) Assuming that the steering wheel 15 is turned at an angle less than 
100.degree. during travel of the vehicle at a high speed, for instance 70 
km/h, the rear road wheels RW.sub.1, RW.sub.2 are retained in their 
neutral positions to ensure the travel stability of the vehicle. 
When the rotation angle of cam plate 62 exceeds the predetermined small 
angle, the connecting rod 61 and valve spool 42b are displaced in 
accordance with the rotation angle of cam plate 62 defined by the steering 
amount of front road wheels FW.sub.1, FW.sub.2. Thus, the rear road wheels 
RW.sub.1, RW.sub.2 are steered at an angle in proportion to the 
displacement amount of connecting rod 61 and valve spool 42b. Assuming 
that the steering wheel 15 is turned at an angle of 
100.degree.-200.degree. during travel of the vehicle at a medium speed, 
for instance 40-70 km/h, the displacement amount of connecting rod 61 
relative to a unit steering amount of front road wheels FW.sub.1, FW.sub.2 
is maintained in a small value, as shown by the solid characteristic line 
in FIG. 6. As a result, the steering angle of rear road wheels RW.sub.1, 
RW.sub.2 changes at a small rate to ensure the stable turning ability of 
the vehicle. Moreover, the steering angle of rear road wheels RW.sub.1, 
RW.sub.2 is controlled in a small value to ensure the travel stability of 
the vehicle. 
When the steering wheel 15 is turned at an angle more than 200.degree. 
during travel of the vehicle at a low speed, for instance less than 40 
km/h, the displacement amount of connecting rod 61 relative to a unit 
steering amount of the front road wheels becomes large at two steps as 
shown by the solid characteristic line in FIG. 6. As a result, the rear 
road wheels RW.sub.1, RW.sub.2 are steered at a large angle to enhance the 
smaller turning ability of the vehicle during low speed travel. When the 
steering wheel 15 is returned to its neutral position, the pulling force 
acting on the cable 67 or 68 is released, and the cam plate 62 is returned 
to the initial position under the load of spiral spring 71 to return the 
connecting rod 61 and valve spool 42b to their neutral positions. Thus, 
the spool valve 42, power cylinder 35 and swingable lever 43 are returned 
to their neutral positions, and the steering angle of rear road wheels 
RW.sub.1, RW.sub.2 becomes zero. 
Assuming that the cancel switch 52 is closed to energize the solenoid 51a, 
the changeover valve 51 is maintained in the second condition to provide a 
direct fluid communication between conduits P.sub.3 and P.sub.4. Thus, the 
hydraulic fluid discharged from pump 22 circulates into the fluid 
reservoir 25 through the flow dividing valve 23 and conduits P.sub.3, 
P.sub.4. In such a condition, the lateral rod 31 is retained in the 
neutral position under the load of spring 36 to retain the rear road 
wheels RW.sub.1, RW.sub.2 in their neutral positions irrespectively of 
steerage of the front road wheels. 
Although in the rear wheel steering mechanism B the cam groove 62a of plate 
62 is formed to change the steering angle of the rear road wheels relative 
to the turn angle of the steering wheel at four steps, it may be formed to 
change the steering angle of the rear road wheels at five or six steps or 
to smoothly change the steering angle of the rear road wheels. 
The four-wheel steering system may be modified as shown in FIG. 7, wherein 
the rear wheel steering mechanism B is arranged to steer the rear road 
wheels RW.sub.1, RW.sub.2 under control of an electric control apparatus 
C. In this modification, the bevel gears 14 in the front wheel steering 
mechanism A and the cables 67, 68 between the steering mechanisms A and B 
are eliminated. In the modified rear wheel steering mechanism B, an 
electric motor 81 and a power cylinder 82 are provided to cause axial 
displacement of the lateral rod 31. The electric motor 81 is mounted on a 
steering shaft 83 which is provided at its lower end with a pinion 84 in 
mesh with a rack part 31a of the lateral rod 31. The steering shaft 83 is 
provided thereon with a control valve 85 which is responsive to a steering 
torque applied thereto through steering shaft 83 to control the supply of 
hydraulic fluid under pressure into the power cylinder 82 and the 
discharge of hydraulic fluid from the power cylinder 82. The other 
construction and component parts of the steering mechanisms A and B are 
substantially the same as those shown in FIG. 1. 
The electric control apparatus C includes a first sensor 91 for detecting a 
turn angle of the steering wheel 15, a second sensor 92 for detecting a 
steering angle of the rear road wheels RW.sub.1, RW.sub.2, and an electric 
control circuit 93. The first sensor 91 is mounted on the steering shaft 
13 to produce an electric signal indicative of the turn angle of steering 
wheel 15. The second sensor 92 is mounted on the housing 32 of rear wheel 
steering mechanism 3 to detect axial displacement of the lateral rod 31 
thereby to produce an electric signal indicative of the steering angle of 
rear road wheels RW.sub.1, RW.sub.2. The electric control circuit 93 is in 
the form of a microcomputer which is responsive to the electric signal 
from the first sensor 91 to determine a desired steering angle of the rear 
road wheels based on a turn angle of the steering wheel and responsive to 
the electric signal from the second sensor 92 to control the rotation of 
electric motor 81 in such a manner that the steering angle of the rear 
road wheels becomes the desired steering angle. In a practical embodiment, 
the microcomputer is arranged to store therein such a characteristic data 
as shown in FIG. 8 to determine the desired steering angle of the rear 
road wheels. 
Assuming that in operation, the steering wheel 15 is turned to steer the 
front road wheels FW.sub.1, FW.sub.2, the electric motor 81 is operated 
under control of the electric control circuit 93 in such a manner that the 
power cylinder 82 is activated under control of the control valve 85 to 
steer the rear road wheels at the desired steering angle. If the turn 
angle of the steering wheel 15 is less than a predetermined value, for 
instance 100, the rear road wheels are retained in their neutral positions 
since the desired steering angle is determined to be zero as shown in FIG. 
8. When the turn angle of the steering wheel exceeds the predetermined 
value, the rear road wheels are steered at the desired angle in an 
opposite direction relative to the front road wheels since the desired 
steering angle is determined to increase in accordance with an increase of 
the turn angle of the steering wheel. 
Having now fully set forth both structure and operation of preferred 
embodiments of the concept underlying the present invention, various other 
embodiments as well as certain variations and modifications of the 
embodiments shown and described herein will obviously occur to those 
skilled in the art upon becoming familiar with said underlying concept. It 
is to be understood, therefore, that within the scope of the appended 
claims, the invention may be practiced otherwise than as specifically set 
forth herein.