May The Force Be With You

Nick Rainbow (1998)

Any conversation regarding cars eventually turns to the topic of how they handle. Intrigued by this, I decided to search the World Wide Web for more information. Here is some of what I’ve discovered…

Tyres & Suspension

These systems work on and through each other and interact to create a handling system. I have found it easier to understand by looking at these components separately, and consider how they react to the various forces involved. A point to note before I go on – the footprint of a typical tyre, regardless of size, is equivalent to the area the size of a postcard. That is smaller than the cover of the club magazine.

The Forces Involved

Lateral force, or centrifugal force is the one most people are familiar with. It’s the force that wants you to keep going in a straight line (through the cones), the force pulling you away from the centre of a turn.

Cornering Force is the opposing lateral force that the tyres create when you turn the wheel into a corner. By completing the turn, cornering force has overcome lateral force. The important thing here is that tyres, and only tyres, can generate cornering force, suspension systems can only affect how tyres generate and share that cornering force.

Slip Angle Tyres can only generate cornering force in two ways. The first is through slip angle, this is simply the difference between the direction the tyre is travelling and the direction the tyre is pointing. The greater the slip angle (the more you turn the steering wheel) the greater the cornering force (up to a maximum point, after which it decreases…. the car’s in a skid).

Camber Thrust The second way the tyres generate cornering force is by camber thrust. Positive camber is when the top of the wheel leans outwards; negative camber is when the top of the wheel leans inwards. Camber thrust is the force that moves the tyre in the direction it is leaning. Like slip angle, the greater the camber angle, the greater the camber thrust (cornering force) generated in the direction the tyre is leaning.

There are several areas of adjustment that can be made. However before you start, make a note of your current set-up, so if you get distracted (e.g. who’s on pole for the next GP) or you are not happy with the outcome of your efforts, you can use this base data to return the car to its original state.

Camber

Camber is a big factor in the performance of suspension systems and, for optimum handling a car’s outside tyres should have zero camber during cornering, so that the total tyre footprint is in contact with the road.

Modern suspension systems are designed to help keep the tyre tread flat to the road even during suspension movement, with the aim of keeping camber near zero under normal driving conditions. Since the suspension is jointed to the chassis, any body roll causes a tilting of the suspension and results in camber change.

This change, if not counteracted by suspension geometry, results in positive camber on the heavily loaded outside wheel during cornering, this in turn will reduce the amount of cornering force generated. If this loss is greatest at the front, an understeer effect occurs (we go through the cones front first). If greatest on the rear, the result is oversteer (we go through the cones rear first). The only way to counteract roll induced positive camber, without modifying the suspension system, is to start with sufficient static negative camber (i.e. then the car is stationery on level ground) to end up with near zero camber at maximum cornering levels.

Warning: Setting up with excessive static camber can cause:

• A serious reduction in straight line braking.
• Tendency to high speed wander.
• Excessive bump steer.
• Accelerated wear on the inside tyre shoulder in normal driving.

Toe

Toe is highly critical suspension setting, and has a significant effect on handling. It is the measurement between a pair of opposite wheels – front or rear – as seen from above Zero toe is when the wheels are parallel, toe-in is when the front of the wheels are closer together, toe-out when they are further apart.

Ideally, maintaining zero toe would allow the suspension to do its job correctly. However, there is a certain amount of compliance built into the suspension system, which allows movement of the suspension components within the bushings to absorb shocks, and static toe is added to compensate. As a rule, less toe-in or even slight toe-out on the front will improve the way the car turns in, inducing an oversteering condition on entering the corner. However, it will tend to adversely affect straight-line tracking. Less toe-in on the rear of independent rear suspension cars can also improve turn-in and reduce understeer. However, be warned that less toe-in can also lead to abrupt oversteer, especially in power off conditions. For this reason avoid toe-out at the rear.

Corner Weights

Big improvements have been made to a car’s handling when the corner weights have been set correctly. This is when you equalise the cars weight of each wheel with the car loaded normally (driver, half a tank of fuel, correct ride height, etc.) You do this with the help of a friend using a corner weight gauge with the car on a flat surface. The idea is to balance the weight across the same axle using the spring platform on the shock absorbers to adjust the weight on each wheel. The platform needs to be raised to increase the weight on a wheel and lowered to reduce the weight, bearing in mind to keep the ride height as low as possible to reduce roll induced positive camber.

As a guide 1 turn of the spring plate = approx. 5lb loading.

Tyre Pressures

Tyre pressures will also have a substantial effect on handling and should always be at least the minimum recommended figure, start with 18 psi and increase/decrease at 2 psi intervals.

Springs

Fitting stiffer springs will reduce the cars body roll and the need for large amounts of static negative camber. A maximum of 25% uprating from standard is a good compromise between improved handling and a reduction in ride comfort. After a change of springs, you will need to adjust the damper settings to find the optimum setting.

Finally, remember to do all your testing on closed circuit and not on the public highway or you could find this seriously damages your driving licence!