Chapter 5 Four Wheel Steering 5.0 Introduction · PDF fileChapter 5 Four Wheel Steering ... an...

Lecture Notes – MAT 1093 Prepared By : Mohd Fairus Jamid Mathematics 1 MAT1012

05/07/2011 Revision date Semester 3

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



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.



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.



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



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



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.



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.

Chapter 5 Four Wheel Steering 5.0 Introduction · PDF fileChapter 5 Four Wheel Steering ... an...

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