Most experts will tell you that good handling is the single most important aspect of car preparation - get it right and driving is a real pleasure, but get it wrong and it can be hell. The pleasures of fast driving are totally lost if it is a constant fight to keep the car under control. The following advice and information is primarily for road cars, since this subject is too broad and complex to include competition preparation.
Lowering and stiffening are the two most common words uttered when discussing suspension upgrades, so lets deal with these first:
Lowering alters (lowers) the centre of gravity and thus makes a car less top heavy which will reduce roll when cornering. Most modern cars today have independent coil or torsion spring suspension. Lowered coil springs are available for numerous models but if your car has torsion bar suspension (usually on the rear only) then they can be adjusted.
On older cars with leaf springs, lowering block kits can be used. The degree of lowering will vary for different cars but it is usually in the region of 20-50mm due to the following limiting factors:
1) Wheel clearance under arches when fully loaded - if you are changing tyre size don't forget to take this into consideration also.
2) Piston rod travel in dampers - bottoming out will destroy dampers very quickly. Shorter length dampers are available, as are competition bump stops.
3) Critical changes to steering / suspension geometry.
Stiffening reduces body movements (rolling, diving, etc.) resulting in a more stable and controllable vehicle, especially in demanding situations. The following items all contribute to firming up the suspension:
1) Increased poundage rate springs.
2) Higher rate dampers / shock absorbers.
3) Larger diameter anti-roll / stabiliser bar.
4} Harder suspension bushes.
Now to put this all into some form of perspective. It does not follow that the stiffer the suspension the better the road holding. Apart from greatly contributing to your driving comfort, these humble components also control the ability of the wheels to stay in contact with the road surface. Under hard driving conditions if you hit a bump and the suspension is too soft it will either bottom out or go into full rebound, resulting in an instant weight transfer and loss of traction and control. Using the same scenario for springs that are too stiff, when you hit the bump instead of absorbing the shock, it will be 'launched' off the bump thus losing ail contact with the road surface and all control will be lost.
Sport suspension kits (uprated dampers and matched springs) supplied by most of the major manufacturers have been specifically designed to take all the above factors into consideration and will give a positive improvement in handling and road holding. Most of the 'sports' shock absorbers have provision for manual adjustment of the damping which enables 'fine tuning' to suit individual requirements.
FWD cars suffer from 2 inherent problems, namely understeer and wheelspin under hard acceleration. This can be compensated to a large degree by having stiffer rear springs and a slightly lower ride height at the front (NB. Understeer is a condition where the car tries to carry on forward after turning the steering wheel, whereas oversteer occurs when the back of the car breaks away and tries to overtake the front). To minimise rear end bump steer, anti-roll bars are a positive solution. Suspension bushes are a major factor in the compliance of the whole system and should be replaced, preferably with uprated items. Harder rubber-bushes used to be the only type available but polyurethane is fast becoming a more popular material due, in the main, to its greater durability and stableness. Gas assisted dampers have superior performance to their hydraulic counterpart due to their instantaneous response and resistance to fade.
And finally, adjustable dampers do have settings other than maximum. Many manufacturers (e.g. Koni) provide this type of adjustment to the damping rate to compensate for wear and restore damper efficiency, thereby giving the damper 2 or 3 additional 'lives'. Setting the damper rates to maximum from new will not only greatly reduce their life it will also have a very negative effect on driver comfort and. in ail probability, do little or nothing to improve handling.
The suspension and steering are inextricably linked, so that any alterations to the suspension will invariably alter the steering geometry as well. This can result in unwanted characteristics e.g. bump steer, excessive tyre wear, etc. We strongly recommend that the steering and suspension geometry are checked and realigned as necessary by a professional.
Progressive coil springs are designed to give a progressively higher spring rate the further they are compressed. When fitting this type of suspension spring always make sure that the end with the closer wound coils is fitted at the top.