Diagnostics – steering
8.8 Diagnostics – steering
8.7.10 Scrub radius
and tyres
The scrub radius is the distance between the con- tact point of the steering axis with the road and
8.8.1 Systematic testing
the wheel centre contact point. The purpose of If the reported fault is heavy steering proceed as designing in a scrub radius is to reduce the steer-
follows.
ing force and to prevent steering shimmy. It also helps to stabilise the straight-ahead position.
1. Ask if the problem has just developed. Road ative, positive or zero scrub radius as described
It is possible to design the steering with a neg-
test to confirm.
below.
2. Check the obvious such as tyre pressures. Is the vehicle loaded to excess? Check geometry?
3. Assuming tyre pressure and condition is as it
Scrub Description
Properties
should be we must move on to further tests.
radius
4. For example, jack up and support the front
Negative The contact point
Braking forces produce a
of the car. Operate the steering lock to lock.
of the steering
torque which tends to make
Disconnect one track rod end and move the
axis hits the
the wheel turn inwards.The
wheel on that side, and so on.
road between
result of this is that the
5. If the fault is in the steering rack then this
the wheel centre
wheel with the greatest
and the outer
braking force is turned in
should be replaced and the tracking should
edge of the wheel
with greater torque.This
be set.
steers the vehicle away
6. Test the operation with a road test and inspect
from the side with the heav-
all other related components for security and
iest braking producing a built in counter steer
safety.
action which has a stabilis- ing effect
8.8.2 Test equipment
Positive The contact point
A positive scrub radius
of the steering
makes turning the steering
axis hits the road
easier. However, braking
Note: You should always refer to the manufac-
between the
forces produce a torque
turer’s instructions appropriate to the equip-
wheel centre and
which tends to make the
ment you are using.
Chassis systems 187
Figure 8.22 Tracking gauges
Tyre pressure gauge and tread depth gauge
Figure 8.23 Wheel balancer
Often under-rated pieces of test equipment. Cor- rectly inflated tyres make the vehicle handle better,
The main reasons for correct alignment are to stop better and use less fuel. The correct depth of
ensure that the vehicle achieves: tread means the vehicle will be significantly safer
to drive, particularly in wet conditions. ● minimum rolling friction ● maximum tyre mileage
Tracking gauges (Figure 8.22)
● stability on the road
The toe-in and toe-out of a vehicle’s front wheels ● steering control for the driver. is very important. Many types of tracking gauges
are available. One of the most common uses a Diagnosing incorrect alignments is usually just a frame placed against each wheel with a mirror on
matter of examining the:
one side and a moveable viewer on the other. The ● tyres for unusual wear viewer is moved until marks are lined up and the
● wheels for damage
tracking can then be measured. ● steering wheel for position.
Wheel balancer (Figure 8.23)
In addition a road test is usually necessary to This is a large fixed piece of equipment in most
check that the vehicle is not pulling to one side, cases. The wheel is removed from the car, fixed
wandering or weaving. Four basic wheel settings on to the machine and spun at high speed. Sensors
or angles determine whether a vehicle is properly in the tester measure the balance of the wheel.
aligned.
The tester then indicates the amount of weight which should be added to a certain position. The ● Camber is the inward or outward tilt of a wheel
weight is added by fitting small lead weights. compared to a vertical line. If the camber is out of adjustment, it will cause tyre wear on one side of the tyre’s tread.