Technical library for cars specialists

Wheel alignment

DEFINITIONS

- CAMBER -

1- The camber is the angle formed between the wheel centre line and the vertical, or to put it another way, the angle between the stub axle axis and the horizontal.
2- The camber is positive when the wheels converge at the bottom.
3- The camber is negative when the wheels converge at the top.

Note : In the past, camber angles were quite large to compensate for the heavily cambered roads and the vehicle technology at that time (zero offset wheels and vertical king pins).
Nowadays this angle is very near to zero, which is best for minimising tyre wear.
Moreover, on rear wheels there is often quite a large negative camber angle which is designed to improve the vehicle's stability.

4- An excessive camber angle on a wheel will cause a cone effect which will tend to make the vehicle pull to one side : — towards the outside if the excessive camber is positive (Fig.4).
- towards the inside in the excessive camber is negative.
5- A camber defect will show up by uniform abnormal wear on one side of the tyre tread.

- KING PIN INCLINATION (K.P.I.) -

6- This is the angle formed between the vertical and the axis of the hub carrier pivots when looking at the vehicle from the front (Fig.6).
- It has a most important effect on the steering by providing the wheel selfcentering action, especially when the lock angles are small. At higher lock angles the caster angle effect is added which compensates for the reduction in king pin inclination.
- When the hub carrier is moved away from the straight ahead position, it describes an arc which tends to push the wheel towards the ground; by reaction the body rises.
- As soon as the steering wheel is released the weight of the vehicle brings the hub carrier back to its original position, and thus the wheels to the straight ahead position.
- In some vehicles which have very little weight on the steering axle, the self centering effect is improved by the addition of a spring in the rack and pinion gear.

- INCLUDED ANGLE -

7- This is the angle between the hub carrier pivot axis and the axis of the stub axle. It is the sum of the k.p.i. and camber angles plus 90°. This angle is fixed at manufacture and is little used in workshop practice (Fig.7).
- Checking this angle enables the operator to decide whether a difference between left and right side angles of a vehicle is caused by a bent stub axle following an impact.
- ln all circumstances :
The sum : Camber + K.P.I. should be equal on both sides within 1°.
(IP) King pin inclination
(CA) Camber

- KING PIN OFFSET -

8- This is the distance "d" on the ground between the wheel centre line and the axis of the stub axle pivots, when these two lines are produced to the ground at tyre contact (Fig.8).
- The figure for king pin offset is directly related to the included angle and the wheel shape.
- It is useful to reduce the king pin offset to as small a figure as possible in order to reduce braking reactions in the steering ; however this offset helps to provide reversibility in the steering, and a small offset facilitates the turning of the steering during parking maneuvers.

- CASTER -

1- This is the angle formed between the vertical and the hub carrier pivot axis when the vehicle is viewed from the side.
(AA') Steering axle
(d) Caster trail
(ch) Caster angle
2- The caster is said to be positive when the top of the hub carrier is inclined towards the rear of the vehicle.
- The caster is said to be negative when the top of the hub carrier is inclined towards the front of the vehicle. (Negative caster is very seldom used on European vehicles).
- When the top pivot of the hub carrier is inclined towards the rear of the vehicle, the axis of the pivots produced to ground level touches the ground in front of the point of contact of the tyre (Caster trail); when a force is applied to the pivot axis it drags the wheel behind it and so provides the vehicle with directional stability and selfcentering action after turning (the principle of compensation for the reduction in the k.p.i. angle at large lock angles)

Note :
3- The caster angle is greatly affected by changes in the attitude height and longitudinal inclination of the vehicle, especially in the case of overloading at the rear.
- If the vehicle pulls to one side on braking, the cause may be a difference in caster angles from left to right.
- Faulty caster does not show itself in abnormal tyre wear.

Important :
4- A change in the caster angle automatically changes the steering rack height (this may or may not be adjustable, depending on the model).
This is because a change in caster causes a change in the position of the steering arm.

- ACKERMAN EFFECT -

5- ln order to avoid excessive slip by the tyres on cornering, the outside wheel in a corner must describe a larger circle than the inside wheel. This situation is obtained by the angles of the steering arms on the hub carriers. These arms are at an angle to the wheel centre line, the angle being fixed at manufacture.
- A change in these angles can only be the result of an impact (see checking of full lock angles). lt will result in incorrect wheel aligment on turns, with consequent tyre wear and unsatisfactory road behaviour.

- TURNING CIRCLE -

The turning circle is the smallest circle through which the vehicle can turn.

- WHEEL ALIGNMENT -

6- Wheel alignment is the difference in the distance between the front and the rear of the wheels at the height of the hubs.
7- Toe-In is convergence of the wheels towards the front of the vehicle. "A" larger than "B". In this case the toe-in is represented by a + sign.
8- Toe-out is convergence of the wheels towards the rear of the vehicle. "A" smaller than "B". It is represented by a - sign.

Note :
The generally accepted idea that front wheel drive vehicles should have toe-out, and rear wheel drive vehicles toe in, in order to compensate wheel alignment change while moving, is not always true on modern vehicles. The direction of alignment variation is directly related to the position of the steering rack in the body, and the position of the steering arms on the hub carriers in front of or behind the pivot axis.
- Incorrect wheel alignment manifests itself by abnormal tyre wear caused by excessive slip while driving.
- Since wheel alignment is adjusted to compensate the cone effect due to camber :
- positive camber will be compensated for by toe-in.
- negative camber will be compensated for by toe-out.

- STEERING RACK HEIGHT - WHEELBASE - TRACK -

1- Wheel alignment changes with the attitude of the vehicle.
In order to reduce this effect as much as possible it is necessary to set the steering rack at the height which will give the smallest changes possible (Fig.1).
2- The wheelbase is the distance "e" between the perpendiculars dropped from the center of the front and rear axles to the ground (Fig.2).
3- The track is the distance "v" between the tread middle of the two wheels of the same axle (Fig.3).

- ATTITUDE OF THE VEHICLE -

- The attitude is the horizontal alignment of the vehicle in relation to the road surface.
- Various factors affect the attitude and, as a consequence, the steering geometry.
- Centrifugal force which tends to press down the vehicle on the outside of a corner. This is the roll effect which can be attenuated by fitting anti—roll bars to the front and rear suspension which effects load transfer from one side to the other.
- Inertia force :
- on acceleration - it tends to transfer the weight to the rear suspension.
- on braking - it tends to transfer the weight to the front suspension.
This weight transfer effect is known as pitching.
- Aerodynamic lift. This tends to lift the front of the vehicle at high speed. This undesirable effect can be attenuated by means of winglets, spoilers etc. which push the vehicle down towards the ground because of their angles of incidence.
- The weight distribution. The load should be spread as evenly as possible throughout the vehicle.

- SETTING FOR GEOMETRY CHECK - STANDARD RIDE HEIGHT -

In order to take account of changes in the steering angles due to changes in the attitude of the vehicle during use, figures for front geometry are defined during vehicle design at a standard attitude, known as the "setting for geometry check".
This attitude condition is specified by the manufacturer for each model from design stage.
In order to obtain correct adjustment it is essential to adhere to this attitude condition.

CONTROL OF PRESSURE OF THE TYRES

LOW TYRE PRESSURE INDICATOR
Silhouette
The indicator displays the position of the particular sensor.

TYRE PRESSURE MONITORING SYSTEM (TPMS)
TPMS warning light.
- Defect code presence.
- No signal from one (or more) wheel.

LOW TYRE PRESSURE INDICATOR
K220 warning light.
Check the tyre pressure.
- Proper tyre pressure.

Note : To allow the resetting, it is essential to scrupulously follow the reset process.