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.