Four-wheel alignment
8.8.3 Four-wheel alignment
● Caster is the degree that the car’s steering axis Standard front wheel alignment is simply a way
is tilted forward or backward from the vertical of making sure the wheels are operating parallel
as viewed from the side of the car. If the caster with one another, and that the tyres meet the road
is out of adjustment, it can cause problems at the correct angle. Four-wheel alignment makes
with self-centring and wander. Caster has lit- sure that the rear wheels follow the front wheels
tle effect on tyre wear. in a parallel path.
● Toe refers to the directions in which two wheels Different manufacturers set different specifica-
point relative to each other. Incorrect toe causes tions for the angles created between the suspen-
rapid tyre wear to both tyres equally. Toe is the sion, steering, wheels and the frame of the vehicle.
most common adjustment and it is always When these angles are correct, the vehicle is prop-
adjustable on the front wheels and is adjustable erly aligned.
on the rear wheels of some cars.
188 Advanced automotive fault diagnosis
Figure 8.24 Laser alignment digital readout
Figure 8.25 Laser alignment scale
● Offset is the amount that the rear wheels are
8.8.4 Test results
out of line, or off set, with the front. Ideally, each rear wheel should be exactly in line with
Some of the information you may have to get the corresponding front wheel.
from other sources such as data books or a work- shop manual is listed in the following table.
Laser based equipment is now almost always used to set alignment (Figure 8.24). The front wheels are placed on turntables and measuring equipment
Test carried out
Information required
connected to each of the four wheels. In simple
Tracking
The data for tracking will be given as
terms, a laser is used to ‘draw’ a perfect box around
either an angle or a distance measurement.
the vehicle so that readouts on displays or scales
Ensure that you use the appropriate data
show how much the wheel positions differ from
for your type of test equipment. Distance
this ‘perfect’ box. In addition, spirit levels are used
will be a figure such as 3 mm toe-in, and
for some measurements. angle one such as 50⬘ toe-in (50⬘ means
50 minutes).The angle of 1° is split into 60
Before checking and adjusting any wheel
minutes, so in this case the angle is 50/60
alignments setting, it is good practice to check
or 5/6 of a degree
the suspension and vehicle ride height (Figure
8.25). Any bent or broken suspension component A simple measurement which should be in
Pressures
bars.You will find, however that many places
will mean that setting alignment will be at best
still use PSI (pounds per square inch).As in
difficult and at worst pointless!
all other cases only compare like with like.
Chassis systems 189
Figure 8.26 Tyre faults
Tread depth The minimum measurement (e.g. 1.6 mm
affected by operating conditions. Tyre pressure
over 75% of the tread area but please note the current local legal requirements)
should always be adjusted when the tyre is cold and be checked at regular intervals.