How do I set up suspension on my car?

SUSPENSION - SPRINGS & DAMPERS - DO NOT CONFUSE SPRING RATE AND POSITION OF ADJUSTABLE SPRING SEATS

The spring rate is measured in lbs/inch. Thus a 500 lbs spring will compress 1/2 inch if a load of 250lbs is placed on the spring, 1 inch with a load of 500lbs, 2 inches with a load of 1000lbs and so on.

The length of the spring will control the ride height. If the ride height is low then screwing the spring seats up will raise the ride height. (The spring rate cannot change) as you move the spring seat up the body the tube in the shock absorber extends. The load on the spring has not changed (the weight of the car is unchanged) and the length of the spring does not change. The position of the spring seat has changed and as the spring length has not changed the position of the top mount has moved up.

If you run out of adjustment on the spring seat then a longer or shorter spring can be substituted but keeping the same rate.

This analysis fails if the damper runs out of length. A simple way to check that the damper has sufficient travel is to remove the spring from the damper, fit the damper to the car and with a jack lift and lower the car observing how the damper travels. Ideally on full bump the bump stop on the damper should prevent the chassis from bottoming on the road. On full droop the damper has sufficient travel to allow the wheel to fall to a reasonable position. Ideally on full droop there should be some pre-tension on the spring so that it does not rattle loose on the seats.

On full bump if the bump stops do not prevent the chassis from bottoming it is possible to position the bump stop with 3mm nylon shims that clip on the damper piston.

If on full droop the spring does rattle loose helper springs can be fitted. These flat section springs have a very low rate and as soon as the weight of the car settles on the spring the helper spring collapses to a flat coil. Without the weight of the car the helper spring exerts just enough load to keep the main spring and seats in place.

As well as controlling the ride height the position of the spring seat also affects the load on that corner of the car. This is why competition cars have their corner weights adjusted either on corner weight scales or with a corner weight checker.

The weight on each corner needs to be a proportional spread of the total car weight.

SUSPENSION - CORNER WEIGHTS AND HOW TO ADJUST

As already discussed the position of the spring seat controls the ride height. It is possible to adjust the spring seat such that the ride height is greater on one corner. This means that this corner is carrying a greater proportion of the car weight. In fact what happens is that diagonal corners carry a greater proportion of the weight and the other diagonal corners a lesser proportion.

If the proportion is unequal the car is not balanced and may not brake in a straight line and may corner better in one direction better than the other.

Correctly adjusting the corner weights will remove the problem.

First the weight on each wheel is measured. This can be done with electronic scales or a corner weight gauge. The anti-roll bars must be disconnected when taking these measurements.

Total TW = LF+RF+LR+RR (LeftFront + RightRear etc)

% Front = (LF+RF)/TWx100
% Rear = (LR+RR)/TWx100
To check %front + %rear = 100

%Right = (RF+RR)/TWx100
%Left = (LF+LR)/TWx100
To check %right +% left = 100

Then calculate what the ideal weights should be

LF = TW x %Front x %Left
RF = TW x %Front x %Right
LR = TW x %Rear x %Left
RR = TW x %Rear x %Right

It is not necessary to measure the weights in any particular unit, all that is required is a comparison.

The spring seat positions are then adjusted to give the calculated weights. When this has been done the ride heights can be adjusted by moving equal turns on the adjuster seats. The corners have to be adjusted in pairs, either side or front or rear to keep the proportions correct. (not diagonally)

Finally the anti-roll bars can be reconnected ensuring that they have no preload.