The steering of a four wheel
vehicle is, as far as possible, arranged so that the front wheels will roll
truly without any lateral slip.
The front wheels are supported on front axle so
that they can swing to the left or right for steering. This movement is
produced by gearing and linkage between the steering wheel in front of the
driver and the steering knuckle or wheel. The complete arrangement is called
the steering system. The steering system essentially consists of two elements-
a steering gear at the lower end of the steering knuckles and steering linkage
.shows a simplified diagram of a steering system.
Fig 1.1 Steering System.
The function
of the steering
system is to convert
the rotary movement
of the steering
wheel into angular
turn of the front wheels. The steering systems also absorb a
large part of the road shocks, thus preventing them from being transmitted to the driver.
Fig 1.1 shows a late model of
steering system. It has worm and roller type steering gear and relay type
steering linkage. When the driver
turns the steering
wheel, the resulting motion
is transmitted down a
steering tube to a steering gearset at
the end of
the steering tube. The gear set changes the direction of
motion, and multiplies the twisting force according to the gear ratio.
Its output shaft rotates to move the pinion arm which transmits the
motion of the steering knuckles through the relay road , idler arm , two tie
rods , two steering arm and the two front wheels. Thus as soon as the driver
puts his hands on the steering wheel the motion of the front wheels is in his
hands. If he wants to turns the vehicle to the left, he turns the steering
wheel to the left, and if he wants to turn the vehicle to the right, he turns
steering wheel to the right, otherwise the steering wheel is in its middle
position and the vehicle is going in a straight line.
REQUREMENTS OF STEERING SYSTEM
For the smooth performance of the system, the steering system of any
vehicle should fulfill the following requirements:
It should multiply the turning effort applied on the steering wheel
by the driver.
It should be to a certain degree irreversible so that the shocks of
the road surface encountered by the
wheels are not transmitted to the driver’s hand.
The mechanism should have self –rightening effect
so that when the driver release the steering wheel after negotiating
the turn , the wheel should try to achieve straight ahead position .
FUNCTIONS OF THE STEERING SYSTEM
The various functions of the steering wheel are
To control the angular motion the wheels and thus the direction of
motion of the vehicle.
To provide directional stability of the vehicle while going straight
ahead.
To facilitate straight ahead condition of the vehicle after
completing a turn.
The road irregularities must be damped to the maximum possible
extent. This should co-exist with the road feel for the driver so that he can
feel the road condition without experiencing the effects of moving over it.
To minimize tyre wear and increase the life of the tyres.
FOUR WHEEL
STEERING
In a typical front
wheel steering system, the rear wheels do not turn in the direction of the
curve, and thus curb on the efficiency of the steering. Normally, this system
has not been the preferred choice due to the complexity of conventional
mechanical four wheel steering systems. However, a few cars like the Honda
Prelude, Nissan Skyline GT-R have been available with four wheel steering
systems, where the rear wheels turn by a small angle to aid the front wheels in
steering. However, these systems had the rear wheels steered by only 2 or 3
degrees, as their main aim was to assist the front wheels rather than steer by
themselves.
With advances in
technology, modern four wheel steering systems boast of fully electronic
steer-by-wire systems, equal steer angles for front and rear wheels, and
sensors to monitor the vehicle dynamics and adjust the steer angles in real
time. Although such a complex 4WS model has not been created for production
purposes, a number of experimental concepts with some of these technologies
have been built and tested successfully.
Compared with a
conventional two wheel steering system, the advantages offered by a 4WS system
include:
Superior cornering
stability.
Improved steering
responsiveness and precision. High speed straight line stability.
Notable
improvement in rapid lane-changing maneuvers.
Smaller turning
radius and tight-space maneuverability at low speed.
Relative Wheel
Angles and their Control.
Fig 1.3 Four Wheel Steering System
