Suspension Geometry

Carrying out a full suspension geometry check is measuring and adjusting the angles of the wheels so that they are set to the car manufacturer’s specification.

It’s an important maintenance area for a car, as getting wheel alignment right reduces tyre wear and ensures a car drives true and straight whilst also steering and handling correctly.

On occasion, alignment angles will be altered beyond the manufacturer’s normal specifications to obtain a specific handling characteristic, of use, for example, when setting up a car for track use.

Some cars of older design, like classic Morgans, only have limited adjustments which can be made and these are done at the front wheels.  For cars of more modern design like Lotus Elises or Morgan Aero 8s there are considerable amounts of adjustment that can be done at both front and rear wheels.

For suspension geometry (wheel alignment) there’s 3 key alignment measurements that are taken:

• Toe
• Camber
• Caster

Read on for an explanation of each and how they can affect tyre wear, handling and steering…
 

Toe

Toe is how much the front of the wheel’s point in (Toe In), or point out (Toe Out), versus the back of the wheels.

 

                     

 

A vehicle’s toe is the most important of the alignment settings as regards tyre wear.  Incorrect toe will cause rapid wear to both tyres.  Too much toe-in wears the outside edges of the tyres, whilst too much toe-out wears the inside edges.

Different toe settings can also be used to alter the handling of a car:

• Increased toe-in will reduce oversteer, steady the car and enhance high speed stability
• Increased toe-out will reduce understeer and free up the car especially during initial turn in when entering a corner.

Camber

Camber is the angle at which a wheel leans in or out – if the top of the wheel leans out, camber is positive.  If it leans in, camber is negative.

 

                 

Wheel camber plays an important role in how well a car holds a straight line and how stable it is during cornering.  Positive camber makes steering lighter and a vehicle more inclined to go in a straight line.  Negative camber makes steering heavier and increases the tyre surface area contacting the road when cornering, which improves stability and handling.

Camber also has an effect on tyre wear.  Too much positive camber wears the outside edges of the tyres, whilst too much negative camber wears the inside edges.

The objective for camber angle is normally that which achieves the best compromise between tyre wear and handling.

Changes to ride height will cause changes to camber.  So when checking suspension geometry it may be necessary to ensure the ride height of the car is correct.  When camber changes, it directly affects toe.

Caster

Caster is the angle of the steering pivot centre line (the line drawn between the top and lower ball joints in the diagram below) when viewed from the side of the vehicle.  If the top of the pivot is leaning towards the rear of the car, the caster is positive, if it is leaning forward, it’s negative.

 

                

 

Positive caster tends to straighten the wheels when moving forward, and so enhances straight line stability.

If a car has positive caster, the steering will be heavier but with improved straight line stability, whilst with negative caster the steering will be lighter but the inclination of the vehicle to ‘wander’ will be greater.

Wheel caster helps a driver feel the straight ahead position and so steer a straight line.  It also contributes to the self centring of the steering.

Caster has little or no effect on tyre wear.

Other points to consider

An important first step before checking and adjusting wheel alignment must be to ensure the suspension and steering components are not worn or damaged.  Otherwise, although the correct alignment settings can be obtained when a car is static in the workshop, as soon as the car is subjected to the forces of cornering, or bumps, or travelling at speed, these forces will take up any slack in worn suspension/steering components, and cause the settings to change.

Another basic check before commencing work is to make sure that tyre pressures are correct and the tyres are in good condition and not excessively worn.

Also, depending on the car, it may be necessary to ensure the ride height is correct (as this affects camber and toe) which can involve putting weights in the car to replicate driver and passenger(s).