KEY POINTS:
Under normal circumstances, you buy a 4WD vehicle because you want to go off-road. I know it's not always the case but, for those who want to, you must have good suspension.
The more ground clearance you have, the more varied the ground you can cover, which is
one of the reasons why suspension modifications are usually the first on the list of things to change when you get yourself a new 4WD and want to go bush.
However, as soon as you start to alter the standard suspension settings, you have to start making decisions.
For instance, raising your suspension will not only give you better ground clearance it will also raise the centre of gravity of the vehicle. And this means the vehicle will have a lot more body roll when cornering, lowering the speeds you can take corners and making the ride less comfortable.
Fitting a stiffer suspension for better off-road grip and load-carrying ability can also lead to a worsening in general ride quality.
The vehicle's suspension is the link between wheels and the chassis. Its function is not only to provide a smooth ride but to ensure the tyres keep in contact with the road. To do this, it uses a system of springs and shock absorbers (dampers) placed between the suspension components and the chassis.
The purpose of the shock absorbers is to limit the bounce of the springs. If there were no shock absorbers, each time you went over a bump your vehicle would just keep bouncing along like a rubber ball.
On 4WDs, there are steel springs or air suspension. Coil springs allow more wheel travel than the other types but need extra links between the axle and bodywork to ensure the suspension moves up and down, and not side to side, or backwards and forwards.
Leaf springs (these days usually only at the back) are strong and locate the axle well in the backwards and forwards plane, but are unsophisticated and give a worse ride than coils.
Torsion bars are tough, easy to adjust and don't take up a lot of vertical space in the vehicle. However, they are prone to losing their spring more rapidly than other types and, when they reach their limits, can break instead of just bottoming out.
Air suspension is just that; air bags are placed between the chassis and the suspension components in place of springs and perform the same function. This type of suspension is usually allied to a sophisticated pumping system which can lower or raise the suspension as well as making it harder and softer.
There are three main types of suspension. The first, and most common, is a live axle where the 4WD has two solid axles, each suspended on its own springs and shock absorbers.
This type of suspension is preferred by those who want to go into rough territory, because it has few components, and all of them strong and heavy, and it's more robust. Secondly, because the whole vehicle lifts when the suspension rides over an object, it ensures that ground clearance remains more constant at all times.
However, the disadvantage is that ride quality is usually not as good as more modern suspensions.
Off-road vehicles with independent suspension don't have axles as such. Instead each differential is mounted on to the underneath of the body or chassis, and there are drive shafts which are attached by universal joints between the differential and the wheels. The universal joints (also known as constant velocity or CV joints) allow the shafts to move up and down with the suspension, and in front, allow the wheels to turn side to side. Each wheel has its own suspension, either strut (a combination spring and shock absorber), wishbones, or swing arms, complete with shock absorbers and other links to locate it precisely.
The main benefit is a smoother ride because the differential is attached to the body and each wheel has the ability to respond to bumps independently, without passing shocks on to the other side of the vehicle.
The disadvantage is that because each spring compresses individually as a wheel passes over an object the vehicle doesn't necessarily lift, so there's a stronger possibility of the underneath of the vehicle touching the ground.
Before you start changing things, remember the vehicle manufacturer has spent millions ensuring that it's perfect for the job it was designed to do. So only change it if you want to use the vehicle for something quite different to what it was intended for.
Once modified, check your vehicle can still perform day-to-day duties. If you raise the suspension, it may alter the geometry of the suspension and steering components, as well as the drive shafts and the wheel alignment and driveline. You have to ensure you get the right shock absorbers to match the springs. This is most important as the extended and closed lengths of the shocks need to match the extension lengths of the springs and the valving needs to match the spring rate.