Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 1
Chapter 5
Four Wheel Steering
Introduction of electrical steering assisted
System components, construction and operation
Steering troubleshooting, diagnosis, and service procedure
5.0 Introduction
If a car with conventional front wheel steering is parallel parked at a curb
between two vehicles, this car may be driven from the parking without hitting the
car in front if the front wheels are turned all the way to the left Figure 5.1A. When
the same car is equipped with 4WS and the rear wheels steer in the same
direction as the front wheels at low speed, the car will not steer out of the parking
space without striking the vehicle in front Figure 5.1B.
When the car in the same parking space has a 4WS system that steers
the rear wheels in the opposite direction to the front wheels at low speed, the car
steers out of the parking space with plenty of distance between the vehicles
parked in front Figure 5.1C. When the rear wheels steer in the opposite direction
to the front wheels, the rear wheels steer toward the curb. This action causes the
right rear tire to strike the curb if the car is parked close to the curb. Therefore,
the maximum rear steering angle must be considerably less than the maximum
front wheel steering angle to help prevent this problem. A car with 4WS has a
smaller turning circle, or turning radius, compared to a vehicle with conventional
front wheel steering. This improves maneurability while parking.
The rear wheel steering in a 4WS system may be controlled in relation to
vehicle speed or the amount of steering wheel rotation. At low vehicle speeds or
with a considerable amount of steering wheel rotation, the rear wheels are
steered in the opposite direction to the front wheels. When the vehicle is
Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 2
operating at higher speeds or with a small amount of steering wheel rotation, the
rear wheels are steered in the same direction as the front wheels.
When a vehicle is cornering at higher speeds, centrifugal force tends to
move the rear of the vehicle sideways. This action causes the rear tires to slip
sideways on the road surface. This process may be called sideslip. The vehicle
speed and the severity of the turn determine the amount of sideslip. If sideslip is
excessive, the car may spin around, causing the driver to lose control. When the
4WS system steers the rear wheels in the same direction as the front wheels at
higher speeds, sideslip is reduced and vehicle stability is improved. The higher
speed same-direction steering angle is considerably less than the low-speed
opposite-direction steering angle.
Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 3
5.1 Electronically Controlled Four-Wheel Steering
5.1.1 System overview
Some cars are equipped with an electronically controlled 4WS system. On
the electronically controlled 4WS system, there is no mechanical connection
between the front steering gear and the rear steering actuator. This rear steering
actuator is now controlled by a 4WS control unit in the trunk behind the left rear
seat Figure 5.2. the 4WS control unit in the electronically controlled system uses
steering wheel rotational speed, vehicle speed, and front steering angle
information to calculate and control the rear steering angle.
5.1.2 Rear steering actuator
The rear steering may be compared to an electric steering gear. This
actuator contains an electric motor that drives a steering rack through a ball
screw mechanism Figure 5.3. Conventional tie-rods are connected from the
steering actuator to the rear steering arms and spindles. A return spring inside
the actuator moves the rear wheels to the straight-ahead position when the
ignition switch is turned off or when a defect occurs in the 4WS system. A main
rear wheel angle sensor and a sub rear wheel angle sensor are mounted on top
of the rear steering actuator.
Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 4
5.1.3 Input sensors
Input sensors in the electronically controlled 4WS include the following:
i. Main rear wheel angle sensor in the rear steering actuator.
ii. Sub rear wheel angle sensor in the rear steering actuator.
iii. Main steering wheel angle sensor in the steering column under the
combination switch.
iv. Sub front wheel steering angle sensor in the front rack and pinion steering
gear.
v. Conventional rear ABS wheel speed sensors.
vi. Conventional vehicle speed sensor (VSS).
5.2 Four-Wheel Steering System Operation
5.2.1 4WS control unit operation
When the engine is running, the 4WS control unit continually receives
information from all the input sensors. If the steering wheel is turned, the 4WS
control unit analyzes information from the vehicle speed sensor, main steering
wheel angle sensor, sub front wheel angle sensor, main rear wheel angle sensor,
sub rear wheel angle sensor, and the rear wheel speed sensors. The 4WS
control unit calculates the proper rear wheel steering angle and then sends
Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 5
battery voltage to the rear steering actuator motor to provides this rear steering
angle Figure 5.4.
Battery voltage is sent to the rear steering actuator motor through two heavy-duty
output transistors. One of these transistors conducts current during a right turn,
whereas the other transistor is activated during a left turn. The main rear wheel
angle sensor and the sub rear wheel angle sensor send feedback signals to the
4WS control unit, indicating the proper rear steering angle has been supplied.
5.2.2 4WS operating characteristics
When the vehicle speed is less than 18mph (29km/h), the rear wheels
immediately begin to steer in the opposite direction to the front wheels if the
steering wheel is turned Figure 5.5. maximum rear steering angle is 6 degree at 0
mph. The rate of rear steering angle decreases in relation to vehicle speed, and
at 18mph (29km/h), the rear steering angle is almost zero.
When the vehicle speed increases above 18mph(29km/h), the rear
wheels steer in the same direction as the front wheels through the first 200
degree of steering wheel rotation. The rear steering angle reverts to the opposite
phase if the steering wheel is rotated more than 200 degree in the vehicle speed
Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 6
range. When the vehicle speed is 60mph(96km/h) and the steering wheel rotation
is 100 degree, the rear wheels steer about 1 degree in the same direction as the
front wheels. If the steering wheel is rotated 500degree slowly at this speed, the
rear wheels are steered about 1 degree in the opposite direction to the front
wheels.
5.2 Quadrasteer Four-Wheel Systems
A Quadrastee system is available on some light-duty trucks and SUVs.
The Quadrasteer system is a four-wheel (4WS) system that improves low speed
maneuverability, high speed stability, and towing capability. The Quadrasteer
system has an electronically powered rear steering system. Vehicles with
Quadrasteer system require a higher output alternator because of the additional
electrical load of the rear wheel steering actuator motor.
Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 7
5.2.1 Rear steering control module and driver slect switch
The rear wheel steering control module is mounted on the frame under
the rear of the vehicle Figure 5.6. this module controls all the rear wheel steering
functions. A 125A mega fuse in an underhood fuse holder supplies voltage to the
control module at all times Figure 5.7. Battery voltage is also supplied to the control
module from the 4WS fuse, and the control module receives ignition voltage the 10A
ignition fuse.
The rear wheel steering mode select switch is mounted in the instrument
panel Figure 5.8. the driver uses the rear wheel steering select switch to select 4WS,
2WS, or 4WS trailer mode. Each time the driver selects a rear wheel steering mode,
an input signal is sent from the mode select switch to the control module Figure 5.9.
Light emitting diode (LED) indicators in the switch inform the driver regarding the
selected rear wheel steering mode.
Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 8
Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 9
Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 10
Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 11
Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 12
Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 13
Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012
05/07/2011 Revision date Semester 3 Page 14
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