Suspension Geometry
Confused by camber, caster, toe-in and whether wheel alignment is worthwhile? Follow our guide to suspension mathematics for classic cars.
We cut through the mystery surrounding caster angle, camber, wheel alignment and suspension mathematics.
The ride quality, handling and stability of a car can be adversely affected if factors such as the alignment and angles of the road wheels are incorrect. So it’s worthwhile knowing what’s involved in setting up your car’s suspension geometry, even if you intend to hand the job over to a specialist.
A good starting point for ensuring your car’s suspension geometry is correct is to check the tyre pressures regularly and have the wheel alignment (tracking) checked at least once a year. Tyre pressures are important because they not only affect the grip under cornering, accelerating and braking, but also the ride height (useful if you can alter the ride height), which in turn can make a mess of other suspension settings. Having the wheel alignment checked at regular service intervals can save on uneven tyre wear, making it potentially cheaper than having to renew tyres.
Wheel alignment isn’t particularly sophisticated in most cases. It concerns the direction of the road wheels, which for the majority of cars, should be almost pointing straight ahead (when the steering wheel is straight for the front wheels). Some cars recommend a small amount of toe in or out, depending on whether the wheels are driven or not. Toe in/out is measured in degrees and minutes, millimetres or inches. Imagine looking at the car from above, and if the leading edge of a pair of wheels is pointing towards each other, this is called toe-in. This is usually recommended for the front wheels of a rear wheel drive car. The opposite, toe-out, is where the leading edges of the wheels are pointing away from each other, which is usually recommended for the front wheels on a front wheel-drive car.
The angle of the toe in/out is usually altered via the track rod ends on the front of a vehicle, which connect the end of the steering rack or steering links to the part of the hub (steering arm) that helps to move the wheels from side to side. The track rod ends have a threaded section and lock nut, which allows adjustments to be made to change the amount of toe.
At the rear of some vehicles, toe in and out can be adjusted via a track control arm, which is similar in design to a track rod end. These are found on sports cars such as the Mk1 Toyota MR2 and MGF/ TF.
Budget for upwards of £30-£40 to have the wheel alignment checked by a specialist. Some mainstream wheel and tyre
specialists offer free wheel alignment checks, but charge to make adjustments. DIY tracking kits are also available from around £50 or more, but in most cases, you’ll need a flat surface to check the measurements.
Caster
The amount of caster at a front road wheel is an important aspect of suspension geometry that can be checked at the same time as the wheel alignment. It’s measured in degrees and represents the angle to which the steering pivot axis is tilted forward or rearward from vertical, when viewed from the side of the wheel. It’s usually measured as an angle of the road wheel when the steering is turned 20 degrees left and right from straight-on: the difference between the two figures is the amount of caster. Up to four degrees of positive caster is ideal for many classic cars. Negative caster makes a car unstable when driving in a straight line. Caster can usually be adjusted when the tracking is adjusted, or if adjustable tie-rods are fitted.
The camber of each road wheel can help or hinder the ride quality and handling of a car and scrub tyres if it’s wrong. It represents the vertical angle of a road wheel when positioned on a level surface. Measured in degrees, one degree of negative camber for instance, means the top of the wheel is angled further in to the car than the bottom of the wheel by one degree.
Camber can be measured using a spirit level type of gauge (bubble or digital) fitted against a flat surface on a road wheel, or across the rim.
Cars with MacPherson strut suspension may feature a means of adjusting the top mounts or one of the mounting bolts where the strut is secured to the upright, which in turn changes the camber of the road wheel.
The MX-5 has camber adjustment via the inner mounting bolts for the lower suspension wishbone arms. The Mini has no means of adjusting camber as standard, but camber brackets for the rear radius arms and longer front lower arms are available in adjustable and fixed lengths.
Balancing
There are a number of DIY checks that can be conducted to measure how well-balanced your car is. If there’s a suitable jacking point at the front and rear of the car which is in the centre (e.g. MGF and the rear diff of many RWD cars), then raise the car with a trolley jack and measure the ground clearance at the opposite corners. This helps to measure the corner weighting of the car. It’s not as accurate as using weighing scales, but it can help and if you have ride height adjustable suspension, it can be altered.
Another budget method is square measuring, which entails creating a rectangle with 90 degree angles and equal opposite sides around the car using axle stands, fishing wire and straight lengths of wood or metal. The car must be on level ground and the fishing wire or straight lengths of wood or metal needs to pass by the centre of each wheel. Once this has been set up (which may take a long time), measurements can be made for wheel alignment and to see how square the centre of each wheel is in relation to the rest of the car. Laser equipment is more accurate, but not as cheap or educational.
Suspension geometry is a fascinating subject that isn’t solely for race car engineers, but it does require time and patience to achieve worthwhile results.