A four-wheel steering apparatus comprising a front wheel steering device having a front-wheel steering bar movable for steering front wheels, and a rear-wheel steering device having a rear-wheel steering bar mechanically separated from the front-wheel steering device and movable for steering rear wheels. The latter device has a rear-wheel steering bar drive assembly for moving the rear-wheel steering bar, and a rear-wheel steering control system for controlling the drive assembly in accordance with the steering amount of the front wheels to thereby control the amount of movement of the rear-wheel steering bar and control the steering amount of the rear wheels. The rear-wheel steering device comprises a maximum steering amount regulating assembly having a variably regulating member capable of mechanically restricting the maximum amount of movement of the rear-wheel steering bar and adjusting the maximum amount of movement. The maximum amount of movement of the rear-wheel steering bar to be mechanically restricted by the variably regulating member decreases as the vehicle speed increases.

BACKGROUND OF THE INVENTION 
The present invention relates to a four-wheel steering apparatus for use in 
motor vehicles and the like. 
Four-wheel steering apparatus for use in motor vehicles comprise a 
front-wheel steering device and a rear-wheel steering device. Rear-wheel 
steering devices are known which comprise a rear-wheel steering bar 
mechanically separated from the front-wheel steering device and movable by 
an electric motor in accordance with the speed of the vehicle and the 
steering amount of the front wheels for steering the rear wheels (see U.S. 
Pat. No. 4,669,744 and Unexamined Japanese Patent Publication HEI 
1-136876). 
With such rear-wheel steering devices, the relative steering direction of 
the rear wheels (the steering direction of the rear wheels relative to 
that of the front wheels) and the maximum permissible steering amount are 
controlled according to the speed of the vehicle. 
FIG. 9 shows an example of relationship of the vehicle speed to the 
relative steering direction and steering amount (steering angle) of the 
rear wheels. In the drawing, the vehicle speed (km/h) is plotted as 
abscissa vs. the steering amount (deg.) as ordinate. Represented by the 
steering amount of 0 is the neutral state, by the positive steering amount 
a steering direction of the rear wheels which is the same as that of the 
front wheels, i.e., steering of the same phase, and by the negative 
steering amount a steering direction of the rear wheels which is reverse 
to that of the front wheels, i.e., steering of reverse phase. The 
solidline curve A represents maximum permissible steering amounts at 
varying vehicle speeds. When the vehicle speed is low in this case, 
reverse-phase steering is done, and the maximum permissible steering 
amount increases as the vehicle speed decreases. When the vehicle speed is 
high, same-phase steering is done, and the maximum permissible steering 
amount increases to a value and then decreases with increasing vehicle 
speed. Within the range of maximum permissible steering amounts at varying 
vehicle speeds, i.e., within the range surrounded by the curve A and the 
abscissa, the steering amount of the rear wheels is controlled in 
accordance with the steering amount of the front wheels. In either case of 
steering of reverse phase or same phase, the actual steering direction of 
the rear wheels changes with the steering direction of the front wheels. 
Accordingly, it follows that the curve A of FIG. 9 represents maximum 
permissible steering amounts of the rear wheels at varying vehicle speeds 
in either of right and left directions. 
With the rear-wheel steering device described, the amount of movement of 
the rear-wheel steering bar is mechanically (structurally) limited, and 
the foregoing mode of control is effected within the mechanical limits. 
With reference to FIG. 9, the two dot-and-dash lines B and C parallel to 
the abscissa represent steering amounts when the rear-wheel steering bar 
is moved to the right and left mechanical limits. Naturally, these amounts 
are slightly greater than the greatest value of maximum permissible 
steering amounts of the curve A. Mechanically, the rear-wheel steering bar 
is movable by the mechanically limited maximum amount irrespective of the 
vehicle speed, so that the rear wheels can be steered between the line B 
and the line C. 
In the case where the rear-wheel steering device operates normally in its 
entirety, the steering amount of the rear wheels will not exceed the range 
surrounded by the curve A and abscissa of FIG. 9. In the event of a 
runaway of the electric motor, however, the steering amount of the rear 
wheels is likely to greatly exceed the maximum permissible amount at a 
particular vehicle speed since mechanically the rear wheels can be steered 
between the two lines B and C, hence a great hazard especially during 
high-speed running. 
The rear-wheel steering devices include those wherein the rear-wheel 
steering bar is movable hydraulically as by a hydraulic motor or cylinder 
as disclosed, for example, in Unexamined Japanese Patent Publication HEI 
2-63974. The same problem as described above is encountered also with 
these devices. 
SUMMARY OF THE INVENTION 
The main object of the present invention is to provide a four-wheel 
steering apparatus which is operable with safety without the likelihood 
that the rear wheels will be steered much beyond the maximum permissible 
amount when the electric motor or the like operates out of control during 
high-speed running. 
The present invention provides a four-wheel steering apparatus comprising a 
front wheel steering device having a front-wheel steering bar movable for 
steering front wheels, and a rear-wheel steering device having a 
rear-wheel steering bar mechanically separated from the front-wheel 
steering device and movable for steering rear wheels, the rear-wheel 
steering device having a rear-wheel steering bar drive assembly for moving 
the rear-wheel steering bar, and a rear-wheel steering control system for 
controlling the drive assembly in accordance with the steering amount of 
the front wheels to thereby control the amount of movement of the 
rear-wheel steering bar and control the steering amount of the rear 
wheels, the four-wheel steering apparatus being characterized in that the 
rear-wheel steering device comprises a maximum steering amount regulating 
assembly having a variably regulating member capable of mechanically 
restricting the maximum amount of movement of the rear-wheel steering bar 
and adjusting the maximum amount of movement, the apparatus further being 
characterized in that the maximum amount of movement of the rear-wheel 
steering bar to be mechanically restricted by the variably regulating 
member decreases as the vehicle speed increases. 
The rear-wheel steering control system may be adapted to detect the vehicle 
speed and to control the maximum steering amount regulating assembly so 
that the maximum amount of movement of the rear-wheel steering bar to be 
mechanically restricted by the variably regulating member decreases as the 
vehicle speed increases. 
The front-wheel steering device may comprise a hydraulic power steering 
mechanism for steering the front wheels by the rotation of a steering 
wheel and oil pressure in accordance with the rotational torque thereof, 
the power steering mechanism having a vehicle speed sensor pump for 
producing working oil pressure corresponding to variations in the vehicle 
speed, the maximum steering amount regulating assembly being connected to 
a hydraulic circuit of the vehicle speed sensor pump so that the maximum 
amount of movement of the rear-wheel steering bar to be mechanically 
restricted by the variably regulating member decreases as the working oil 
pressure of the sensor pump increases. 
The invention further provides a four-wheel steering apparatus comprising a 
front wheel steering device having a front-wheel steering bar movable for 
steering front wheels, and a rear-wheel steering device having a 
rear-wheel steering bar mechanically separated from the front-wheel 
steering device and movable for steering rear wheels, the rear-wheel 
steering device having a rear-wheel steering bar drive assembly for moving 
the rear-wheel steering bar, and a rear-wheel steering control system for 
controlling the drive assembly in accordance with the steering amount of 
the front wheels to thereby control the amount of movement of the 
rear-wheel steering bar and control the steering amount of the rear 
wheels, the four-wheel steering apparatus being characterized in that the 
front-wheel steering device comprises a hydraulic power steering mechanism 
for steering the front wheels by the rotation of a steering wheel and oil 
pressure in accordance with the rotational torque thereof, the rear-wheel 
steering device comprising a maximum steering amount regulating assembly 
having a variably regulating member capable of mechanically restricting 
the maximum amount of movement of the rear-wheel steering bar and 
adjusting the maximum amount of movement, the maximum steering amount 
regulating assembly being connected to a hydraulic circuit of the power 
steering mechanism so that the maximum amount of movement of the 
rear-wheel steering bar to be mechanically restricted by the variably 
regulating member decreases as the front wheel steering oil pressure 
decreases. 
With the four-wheel steering apparatus of the present invention, the 
maximum amount of movement of the rear-wheel steering bar to be 
mechanically restricted by the variably regulating member decreases during 
high-speed running to obviate the likelihood that the rear wheels will be 
steered by an amount in large excess of the maximum permissible amount 
when the electric motor or the like operates out of control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 to 7 and 8(a) through 8(c) show a first embodiment. 
FIG. 1 schematically shows the construction of a four-wheel steering 
apparatus comprising a front-wheel steering device 1 and a rear-wheel 
steering device 2 for use in motor vehicles. FIG. 2 shows mechanical 
components of the device 2. 
The front-wheel steering device 1 is, for example, of the rack-pinion type 
already known wherein the rotation of a steering wheel 3 is transmitted to 
a front rack bar (front-wheel steering bar) 4 via an unillustrated pinion. 
The rack bar 4 extends laterally and is supported by a front housing 5 so 
as to be movable in the lateral direction (axial direction). The housing 5 
is fixed to the body of a vehicle. The rack bar 4 is connected at each end 
thereof to a front wheel 8 by a tie rod 6 and a knuckle arm 7. The 
steering wheel 3, when rotated, moves the rack bar 4 rightward or leftward 
to steer the front wheels 8. 
The rear-wheel steering device 2, like the front-wheel steering device 1, 
comprises a rear rack bar (rear-wheel steering bar) 11 supported by a rear 
housing 10. The housing 10 is in the form of a tube extending laterally 
and comprises a housing body 12 and a housing tube 13 fitted in the right 
end of the body. The rack bar 11 extends laterally as inserted through the 
housing 10 and is movable laterally as supported by known bush 14 and 
support yoke 58 (see FIG. 3) within the housing 10. The rack bar 11 is 
connected at each end thereof to a rear wheel 17 by a tie rod 15 and a 
knuckle arm 16. The rack bar 11, when moved rightward or leftward, steers 
the rear wheels 17. 
The housing body 12 has an inward flange member 18 secured to the left end 
of its inner periphery and an inward flange 19 at a portion thereof 
rightwardly away from the member 18. An outward flange member 20 is 
secured to the outer periphery of the rack bar 11 at a portion thereof 
close to its left end and corresponding to the inward flange member 18 on 
the housing body 12. A retaining ring 21 is secured to an outer peripheral 
portion of the rack bar 11 corresponding to the inward flange 19 of the 
housing body 12. On the left side of the ring 21 immediately adjacent 
thereto, a stopper ring 22 is fitted around the rack bar 11. Between the 
inward flange member 18 and the inward flange 19 of the housing body 12, 
the rack bar 11 has laterally slidably fitted therearound a left slide 
ring 23 positioned on the right side of and immediately adjacent to the 
outward flange member 20 and a right slide ring 24 on the left side of and 
immediately adjacent to the stopper ring 22. A centering spring 25 in the 
form of a coiled compression spring is fitted around the rack bar 11 and 
positioned between the left and right slide rings 23, 24. A left 
regulating member 26 in the form of a hollow cylinder and extending 
rightward is fixedly provided around the left slide ring 23. A right 
regulating member 27 in the form of a hollow cylinder and extending 
leftward is fixedly provided around the right slide ring 24. While the 
rack bar 11 is not subjected to any lateral force, the centering spring 25 
biases the left and right slide rings 23, 24 away from each other, 
pressing the left slide ring 23 into contact with the outward flange 
member 20 on the bar 11 and the right slide ring 24 with the stopper ring 
22 on the bar 11. At this time, the stopper ring 22 is prevented from 
moving rightward by the retaining ring 21. Further at this time, the left 
slide ring 23 is in pressing contact with the inward flange member 18 of 
the housing body 12, and the right slide ring 24 with the inward flange 19 
of the body 12, and the rack bar 11 is held in a neutral position. 
While the rack bar 11 is in the neutral position, the right end of the left 
regulating member 26 and the left end of the right regulating 27 are 
laterally at a short distance from each other. When a rightward or 
leftward force greater than the force of the centering spring 25 acts on 
the rack bar 11, the rack bar 11 is movable rightward or leftward by an 
amount corresponding to the distance between the right and left regulating 
members 26, 27. When subjected to the leftward force, the rack bar 11 
moves leftward with the outward flange member 20 brought out of contact 
with the left slide ring 23, whereupon the right slide ring 24 and the 
right regulating member 27 are also moved leftward by being pushed by the 
retaining ring 21 and the stopper ring 22. The rack bar 11 stops moving 
upon the left end of the right regulating member 27 coming into contact 
with the right end of the left regulating member 26 at rest. Conversely, 
when subjected to the rightward force, the rack bar 11 moves rightward 
with the stopper ring 22 brought out of contact with the right slide ring 
24, whereupon the left slide ring 23 and the left regulating member 26 are 
also moved rightward by being pushed by the outward flange member 20. The 
rack bar 11 stops moving upon the right end of the left regulating member 
20 striking against the left end of the right regulating member 27 at 
rest. Thus, mechanically, the rack bar 11 is movable rightward or leftward 
by an amount twice the distance between the right and left regulating 
members 26, 27 in the neutral state. This amount of movement will 
hereinafter be referred to as the "mechanically maximum amount of 
movement" of the rack bar 11, and the corresponding steering amount 
(steering angle) of the rear wheels 17 as the "mechanically maximum 
steering amount." 
The housing body 12 is provided with a rack bar drive assembly (rear-wheel 
steering bar drive assembly) 28 for laterally moving the rack bar 11, a 
first steering amount detector 29 for detecting the steering amount of the 
rear wheels 17, and a second steering amount detector 30 for the same 
purpose. 
The rack bar drive assembly 28 comprises an electric motor 31 serving as a 
drive source, clutch 32 and reduction gear 33. As shown in detail in FIG. 
3, a first casing 34 is fixed to one face of the housing body 12, and a 
second casing 35 is fixed to one face of the first casing 34. A portion of 
the housing body 12, and the first and second casings 34, 35 form a gear 
compartment 36 inside thereof. The motor 31 is attached to another face of 
the second casing 35. Although not shown, the output shaft (motor shaft) 
of the motor 31 is connected to the input shaft of the clutch 32. 
Rotatably supported inside the gear compartment 36 are an intermediate 
gear shaft 37 and a pinion shaft 38 extending in parallel to each other 
and perpendicular to the rack bar 11. A worm 40 fixed to the output shaft 
39 of the clutch 32 is in mesh with a worm wheel 41 fixed to the gear 
shaft 37. A small gear 42 fixed to the gear shaft 37 is in mesh with a 
large gear 43 fixed to the pinion shaft 38. A pinion 44 formed on the 
pinion shaft 38 is in mesh with a rack 45 formed on the rack bar 11. The 
rotation of the motor 31 is transmitted via the clutch 32 to the reduction 
gear 33, which in turn rotates the pinion 44 at a reduced speed to move 
the rack bar 11 rightward or leftward in meshing engagement with the 
pinion. 
As seen in detail in FIG. 3, the first steering amount detector 29 
comprises a first rotation sensor 46, such as a potentiometer, attached to 
the housing body 12. The rotation sensor 46 is connected to one end of the 
pinion shaft 38 of reduction gear 33 of the rack bar drive assembly 28 for 
detecting the rotation of the pinion shaft 38 to thereby detect the 
steering amount of the rear wheels 17. 
The second steering amount detector 30 is shown in detail in FIGS. 4 and 5. 
A second rotation sensor 47, such as a potentiometer, is attached to the 
housing body 12 and has an input shaft 48, to which one end of a lever 49 
is fixed. The lever 49 is formed with a slot 50 extending lengthwise 
thereof. A pin 51 secured to the rack bar 11 has an outer end projecting 
from the outer periphery of the bar 11 and fitted in the slot 50 of the 
lever 49 so as to be movable longitudinally thereof. When the rack bar 11 
moves laterally, the pin 51 also moves in the same direction while moving 
within the slot 50 longitudinally thereof to rotate the lever 49 rightward 
or leftward. The rotation sensor 47 detects the amount of rotation of the 
lever 49 to thereby detect the amount of movement of the rack bar 11, 
i.e., the steering amount of the rear wheels 17. 
The front-wheel steering device 1 has a frontwheel steering amount detector 
52 for detecting the steering amount of the front wheels 8 by suitable 
means. 
The rear-wheel steering device 2 has a rear-wheel steering control system 
54 for controlling the motor 31 and the clutch 32 of the rack bar drive 
assembly 28 in accordance with the outputs of a vehicle speed sensor 53, 
the front-wheel steering amount detector 52, and the first and second 
steering amount detectors 29, 30. 
The control system 54 comprises a rear-wheel steering amount control 
circuit 55 and a fail safe circuit 56, which in turn comprise an 
electronic circuit or computer. 
The steering amount control circuit 55 controls the amount of rotation of 
the motor 31 according to the vehicle speed and the steering amount of the 
front wheels 8 to thereby control the amount of movement of the rack bar 
11, i.e., the steering amount of the rear wheels 17. More specifically, 
from the vehicle speed detected by the sensor 53 and based on the 
relationship shown in FIG. 9, the circuit 55 recognizes the steering 
direction and the maximum permissible steering amount of the rear wheels 
17 at the vehicle speed to control the motor 31 within the range of the 
maximum amount according to the steering amount of the front wheels 8. At 
this time, the steering amount of the rear wheels 17 is detected by the 
first steering amount detector 29 and fed back to the control circuit 55. 
Further the fail safe circuit 56 always compares the output of the first 
steering amount detector 29 with the output of the second steering amount 
detector 30. If a difference greater than a predetermined value occurs 
therebetween, the circuit 56 interprets this as indicating a trouble 
developing in detecting the steering amount of the rear wheels 17, 
whereupon the circuit disengages the clutch 32 and deenergizes the motor 
31 so as not to steer the rear wheels 17. 
The rear-wheel steering device 2 has a maximum steering amount regulating 
assembly 57 for mechanically restricting the maximum amount of movement of 
the rack bar 11 within the range of the mechanically maximum amount of 
movement. 
FIGS. 6, 7, and 8(a) through 8(c) show an example of construction of the 
maximum steering amount regulating assembly 57. 
The wall of the housing body 12 is formed with a vertically elongated 
rectangular guide recessed portion 59. Laterally elongated slots 60 and 69 
are formed respectively in the wall of the recessed portion 59 adjacent to 
the rack bar 11 and in the corresponding portion of the housing tube 13. A 
variably regulating member 61 in the form of a rectangular thick plate is 
vertically slidably fitted in the recessed portion 59. The regulating 
member 61 has a regulating groove 62 extending downward from its upper 
end, V-shaped and having a lateral width gradually decreasing downward. A 
rack plate 63 in the form of a rectangular thick plate is fixed as by 
unillustrated pins to the outer surface of the regulating member 61. The 
rack plate 63 has a smaller lateral width than the regulating member 61 
and is formed with a rack 64 on the side face thereof recessed from the 
regulating member 61. The housing body 12 is externally provided with a 
step motor 65 opposed to the recessed portion 59 and having a motor shaft 
66 extending into the recessed portion 59. A pinion 67 fixed to the shaft 
66 is in mesh with the rack 64 of the rack plate 63. The rack plate 63 and 
the regulating member 61 are movable upward and downward within the 
recessed portion 59 by the rotation of the motor 65. The rack bar 11 has 
fixed thereto a pin 68 for regulating the amount of movement thereof. The 
pin 68 has one end projecting from the outer periphery of the rack bar 11, 
extending through the slots 69, 60 in the walls of the tube 13 and the 
housing body 12 into the recessed portion 59, and fittable in the groove 
62 of the regulating member 61. 
When the regulating member 61 is in a lowered limit position as seen in 
FIG. 8(a), the pin 68 is positioned above the regulating member 61, with 
the result that mechanically, the rack bar 11 is movable by the 
mechanically maximum amount without being restrained by the regulating 
member 61. When the regulating member 61 is in a raised limit position as 
shown in FIG. 8(c), the pin 68 is positioned as fitted in the bottom of 
the groove 62 of the regulating member 61. The groove width of this 
portion is only slightly larger than the diameter of the pin 68, so that 
the pin 68 is almost immovable laterally, rendering the rack bar 11 also 
almost immovable laterally. When the regulating member 61 is in a 
vertically intermediate position as shown in FIG. 8(b), the pin 68 is in 
an intermediate portion of the groove 62 of the regulating member 61, and 
the pin 68 and the rack bar 11 are laterally movable by an amount 
dependent on the groove width of this portion. Accordingly, the maximum 
amount of lateral movement of the rack bar 11 is mechanically restricted 
by controlling the vertical position of the regulating member 61. 
The fail safe circuit 56 controls the step motor 65 so as to bring the 
regulating member 61 at a higher position as the vehicle speed increases, 
whereby the maximum amount of movement of the rack bar 11 is mechanically 
restricted so as to decrease as the vehicle speed increases as indicated 
in the broken curves D and E in FIG. 9. Consequently, even if the motor 31 
of the rack bar drive assembly 28 operates out of control, the actual 
steering amount of the rear wheels 17 is restricted to a small value. The 
maximum amount of movement of the rack bar 11 is restricted to a small 
value especially during high-speed running. This assures high safety. 
FIGS. 10 to 16 show a second embodiment. 
FIG. 10 schematically shows the construction of a four-wheel steering 
apparatus for use in motor vehicles which is similar to the first 
embodiment. FIG. 11 shows mechanical components of the rear-wheel steering 
device 2 thereof. FIGS. 12 to 16 show components of the device in detail. 
Throughout FIGS. 1 to 16, like parts are designated by like reference 
numerals. 
The front-wheel steering device 1 comprises a hydraulic power steering 
mechanism. 
This mechanism is a known one as disclosed, for example, in Unexamined 
Japanese Patent Publication HEI 2-169371, and will be described below 
briefly although not shown in detail. The front housing 5 front rack bar 4 
has at an intermediate portion thereof a piston slidable in contact with 
the inner peripheral surface of the cylinder portion 90. On the other 
hand, an input shaft connected to the steering wheel 3 is connected by a 
torsion bar to an output shaft connected to the pinion, and a hydraulic 
control valve 91 and a hydraulic reaction portion 92 are provided between 
these shafts. A hydraulic pump 93 connected to an oil tank T communicates 
with the two oil chambers of the cylinder portion 90 via the hydraulic 
control valve 91. The front wheels 8 are steered by the rotation of the 
steering wheel 3 and oil pressure in accordance with the rotational torque 
thereof. The hydraulic reaction portion 92 has connected thereto a vehicle 
speed sensor pump 94 in communication with the oil tank T via a fixed 
orifice 95. The vehicle speed sensor 94 is similar to the one disclosed, 
for example, in Examined Japanese Utility Model Publication SHO 60-38219 
and produces working oil pressure corresponding to variations in the 
vehicle speed, such that the working oil pressure is great during 
high-speed running or is small during low-speed running. The vehicle speed 
sensor pump 94 controls the hydraulic reaction portion 92 so that the 
hydraulic pump 93 produces a great auxiliary steering force during 
low-speed running or a small auxiliary steering force during high-speed 
running. 
The second embodiment includes a maximum steering amount regulating 
assembly 96 having the following construction (see FIGS. 12 to 16). 
The upper wall of the housing body 12 is integrally formed with a 
rectangular parallelepipedal block 74 which is elongated longitudinally of 
the vehicle. A solid cylinder 75 extends forward from the front end of the 
block 74 integrally therewith. A guide recessed portion 76 in the form of 
a rectangular groove is formed in the upper side of the block 74. A 
cylinder bore 77 extends through the front portion of the block 74 and the 
cylinder 75 from the recessed portion 76 to the front end face of the 
cylinder 75. Laterally elongated slots 78 and 79 are formed respectively 
in the upper wall of the housing body 12 at the bottom of the recessed 
portion 76 and in the corresponding portion of the tube 13. 
The upper opening of the recessed portion 76 is closed with a closure 80. A 
variably regulating member 81 in the form of a rectangular parallelepiped 
having a shorter length than the recessed portion 76 longitudinally of the 
vehicle is fitted in this portion 76 and is slidable forward and rearward. 
A plunger 82 projecting forward from the front end of the regulating 
member 81 is fixed thereto. The plunger 82 is intimately fitted in the 
cylinder bore 77 from behind and is slidable forward and rearward. A seal 
83 is provided in the cylinder bore 77 for sealing off the clearance in 
the bore 77 around the plunger 82. A spring cavity 84 is formed in the 
rear end face of the regulating member 81. A coiled compression spring 85 
for biasing the regulating member 81 forward is fitted in the cavity 84 in 
bearing contact with the rear wall of the recessed portion 76. 
The regulating member 81 is formed in the bottom of its rear portion with a 
regulating groove 86 extending forward from the rear end of the member 81. 
When seen from below, the groove 86 is V-shaped and has a lateral width 
gradually decreasing forward. A pin 87 for regulating the amount of 
movement of the rack bar 11 is fixed to the bar 11 and projected upward 
from the top of the bar 11 into the recessed portion 76 through the slots 
79, 78 of the tube 13 and the housing body 12, and is fittable in the 
groove 86 of the regulating member 81. 
The cylinder bore 77 has a threaded tapered front end portion 88 for 
piping. This portion is held in communication with the hydraulic circuit 
of the vehicle speed sensor pump 94 of the front-wheel steering device 1 
by a suitable pipe. 
When the oil pressure supplied to the cylinder bore 77 is small, the 
regulating member 81 is held by the spring 85 in an advanced limit 
position in contact with the front wall of the recessed portion 76, with 
the rear end of the member 81 positioned to the front of the pin 87 as 
seen in FIGS. 12 to 14. Accordingly, the pin 87 is positioned to the rear 
of the groove 86 of the regulating member 81 and is greatly movable 
laterally, consequently rendering the rack bar 11 also greatly movable 
laterally. When the oil pressure supplied to the cylinder bore 77 
increases, the force pushing the plunger 82 rearward increases, moving the 
regulating member 81 rearward from its advanced limit position against the 
force of the spring 85 as shown in FIG. 15. This positions the pin 87 in 
the groove 86, rendering the pin 87 and the rack bar 11 laterally movable 
only by an amount dependent on the width of the groove portion where the 
pin is positioned. The amount of rearward movement of the regulating 
member 81 increases as the oil pressure increases in corresponding 
relation therewith. As the amount of rearward movement of the regulating 
member 81 increases, the width of the groove portion where the pin 87 is 
positioned decreases, also decreasing the maximum amount of lateral 
movement of the pin 87 and the rack bar 11. When the oil pressure supplied 
to the cylinder bore 77 further increases, moving the regulating member 81 
to a retracted limit position in contact with the rear wall of the 
recessed portion 76 as seen in FIG. 16, the pin 87 fits in the front end 
of the groove 86. Since the groove width of this portion is larger than 
the diameter of the pin 87 only slightly, the pin 87 is almost immovable 
laterally, making the rack bar 11 also almost immovable laterally. 
While the vehicle is at a stop or while it is running at a low speed, the 
working oil pressure of the vehicle speed sensor pump 94 is small, and the 
oil pressure supplied to the regulating assembly 96 is also small. 
Consequently, the regulating member 81 advances, increasing the maximum 
amount of movement of the rear rack bar 11 to be mechanically restricted 
by the groove 86 and therefore permitting the rear wheels 17 to be steered 
through a large angle. 
An increase in the vehicle speed increases the working oil pressure of the 
sensor pump 94 and also increases the oil pressure to be supplied to the 
regulating assembly 96. As the vehicle speed increases, accordingly, the 
regulating member 81 moves rearward to decrease the maximum amount of 
movement of the rear rack bar 11 to be mechanically restricted by the 
groove 86. As a result, even if the motor 31 runs away or operates out of 
control, there is no likelihood that the rear wheels 17 will be steered by 
an amount greatly in excess of the maximum permissible steering amount. 
FIGS. 17 to 22 show a third embodiment. 
FIG. 17 schematically shows the construction of another four-wheel steering 
apparatus for use in motor vehicles which is similar to the first 
embodiment. FIG. 18 shows mechanical components of the rear-wheel steering 
device 2. FIGS. 19 to 22 show components of the device in greater detail. 
Throughout the drawings showing these embodiments, like parts are 
designated by like reference numerals. 
The front-wheel steering device 1 has a hydraulic power steering mechanism. 
This mechanism, which is of the known type, has, for example, the following 
construction although not shown in detail. The front housing 5 is locally 
formed with a hydraulic cylinder integral therewith, and the front rack 
bar 4 is provided at an intermediate portion thereof with a piston 
slidable in contact with the inner peripheral surface of the cylinder. On 
the other hand, an input shaft connected to the steering wheel 3 and an 
output shaft connected to the pinion are interconnected by a torsion bar. 
A hydraulic control valve 71 is provided between these shafts. A hydraulic 
pump 72 communicates with the two oil chambers of the cylinder via the 
hydraulic control valve 71. The front wheels 8 are steered by the rotation 
of the steering wheel 3 and oil pressure in accordance with the rotational 
torque thereof. 
The third embodiment includes a maximum steering amount regulating assembly 
73 having the following construction (see FIGS. 19 to 22). Since this 
assembly 73 is similar to the maximum steering amount regulating assembly 
96 of the second embodiment, like parts are designed by like reference 
numerals throughout the drawings showing these embodiments, and the 
different feature only will be described. 
The regulating member 81 is formed in the bottom side of its front portion 
with a regulating groove 86 extending from the front end of the member 81 
rearward. When seen from below, this groove 96 is V-shaped and has a 
lateral width gradually decreasing toward the rear. 
The threaded tapered front end portion 88 of the cylinder bore 77 is held 
in communication by a suitable pipe with a suitable portion, where the 
steering oil pressure of the front wheels acts, of the hydraulic circuit 
of the power steering mechanism of the frontwheel steering device 1, e.g., 
with the discharge side of the hydraulic pump 72. 
When the oil pressure supplied to the cylinder bore 77 is small, the 
regulating member 81 is held by the spring 85 in an advanced limit 
position in contact with the front wall of the recessed portion 76 as seen 
in FIGS. 19 to 21, with the pin 87 fitting in the rear end of the groove 
87. Since the groove width of this portion is larger than the diameter of 
the pin 87 only slightly, the pin 87 is almost immovable laterally, 
rendering the rack bar 11 also almost immovable laterally. When the oil 
pressure supplied to the cylinder bore 77 increases, the force pushing the 
plunger 82 rearward increases, moving the regulating member 81 from the 
advanced limit position rearward against the force of the spring 86 as 
shown in FIG. 22. This brings the pin 87 to an intermediate portion of the 
groove 86, making the pin 87 and the rack bar 11 laterally movable by an 
amount dependent on the groove width of this portion. The amount of 
rearward movement of the regulating member 81 increases as the oil 
pressure increases in corresponding relation with the pressure. As the 
amount of rearward movement of the regulating member 81 increases, the 
groove width of the portion where the pin 87 is positioned increases to 
increase the maximum amount of lateral movement of the pin 87 and the rack 
bar 11. 
While the steering wheel 3 is not manipulated, the front wheel steering oil 
pressure of the power steering mechanism of the front-wheel steering 
device 1 is very small, and the oil pressure to be supplied to the 
regulating assembly 73 is also very small. As a result, the regulating 
member 81 is located in its advanced limit position or in the vicinity 
thereof, and the maximum amount of movement of the rear rack bar 11 to be 
mechanically restricted by the groove 86 is very small. Accordingly, even 
if the motor 31 runs away, there is little or no likelihood that the rear 
wheels 17 will be steered. 
When the steering wheel 3 is manipulated while the vehicle is at a stop or 
while it is running at a low speed, the oil pressure for steering the 
front wheels 8 is great, and the oil pressure supplied to the regulating 
assembly 73 is also great. Consequently, the regulating member 81 moves 
rearward, increasing the maximum amount of movement of the rear rack bar 
11 to be mechanically restricted by the groove 86 and therefore making it 
possible to steer the rear wheels 17 greatly. 
At an increased vehicle speed, the front wheel steering oil pressure when 
the steering wheel 3 is manipulated is smaller, and the oil pressure to be 
supplied to the regulating assembly 73 is also smaller. As the vehicle 
speed increases, therefore, the regulating member 81 moves forward to 
decrease the maximum amount of movement of the rear rack bar 11 to be 
mechanically restricted by the groove 86. Accordingly, even if the motor 
31 operates out of control, it is unlikely that the rear wheels 17 will be 
steered by an amount greatly in excess of the maximum permissible steering 
amount.