Your vehicle’s suspension is the all-important link between its wheels and tires and the load-bearing platform, ie: the chassis or uni-body. Its function is not only to provide a smooth ride for the vehicle’s load (including the occupants) but to maintain maximum contact between the off-road tires and the road surface. It does this by absorbing some of the lateral loads placed on the vehicle during maneuvers.
To absorb these loads, the suspension uses a system of springs and dampers placed in between the wheel mounting hubs and the chassis. On 4WD vehicles the springs come in a variety of designs but for the most part they will be either be coil, leaf or torsion bar types. Less common are air bag or air springs but their use is becoming more widespread with the availability of quality aftermarket kits.
The dampers, or shock absorbers, are normally a telescopic cartridge that hydraulically displaces oil or gas through an orifice to dampen oscillations in the spring.
The springs and dampers can be set up under the vehicle a number of ways but basically they all do the same thing. Traditionally most 4WD vehicles use a ‘live-axle’ suspension set-up. This is where the entire axle assembly including the differential, its housing, the axles, hubs, brakes, wheels and tires are all suspended away from the chassis.
This robust system is great for 4WDs and heavy vehicles as it delivers long wheel travel and the differential housing protects many of the axle components from road damage. Its shortcomings, however, are that because the wheels on either side of the axle are linked by the axle housing they can’t act independently of each other and thus when the wheel on one side drops into a pothole or bounces over a bump, an opposing action is transferred to the wheel on the opposite end of the axle, affecting that wheel’s contact with the road as well as the wheel that hit the obstacle.
Also, the high unsprung weight of the axle with the wheels and tires affects the ride quality and the suspension’s ability to keep the tires on the ground. Examples of 4WDs with live axles both front and rear include Nissan Armadas, Land Rovers and non-independent front suspension Toyota Land Cruisers.
The alternative to the ‘live-axle’ design is independent suspension. With this design, the differential is mounted directly to the chassis and is connected to the hubs and wheels via constant velocity joints and exposed drive shafts. This allows each wheel to act independently of the other and the unsprung weight is reduced because the diff is now mounted to the chassis and there’s no heavy axle housing.
The downside of independent suspension is that most designs offer less wheel travel than a live-axle set-up, but the improvements in ride quality and handling far outweigh this. Examples of full independent suspension include the Mitsubishi Montero, Toyota RAV4, Hummer and Subaru Forester. Many 4WD vehicles have independent front suspension (IFS) with a live rear axle. These include the Toyota Land Cruiser, Tacomas, Nissan Pathfinders and the Lexus LX470 (1998–2007).
Whatever set up your vehicle has, be it leaf, coil or torsion bar sprung, dual live-axle, independent front or full four-wheel independent suspension, the vehicle manufacturer has spent millions of dollars and thousands of man-hours designing and manufacturing it to be just right. Unfortunately what the vehicle manufacturer delivers is always a compromise between ride, handling, space and manufacturing cost. This is why many 4WD owners turn to the aftermarket to customize their vehicle to better suit their individual application.
The choices are many. Heavy-duty springs and shocks for improved load carrying and handling characteristics. Taller suspension components for additional tire to ‘guard and ground clearance. Custom parts for extended wheel travel and articulation. These are some of the modifications owners will initiate to set their 4WD vehicle up to their own specification.
But before you rush out and start modifying your 4WD, there are a few things to consider. Primarily, why do you want to modify the suspension? Is it for additional clearance; improved towing and load carrying; appearance only or is your old suspension clapped out and in need of replacement?
Also, once modified, will your vehicle still be able to perform its day to day duties? That is; will a raised vehicle still fit under your carport, garage roof and local drive-through? Not only is the roof height an issue but raising a vehicle’s suspensions alters the geometry of the suspension and steering components as well as the drive shafts. This can result in wheel alignment problems, driveline vibrations, excessive tire wear and affect the operation of the system components. Speak to the specialists or other owners who have carried out similar modifications to check what ramifications you might expect.
If you’re fitting heavy-duty springs to cope with the weight of heavy loads the first thing you need to determine is just how heavy is your vehicle likely to be. This means loading it up with all the gear, accessories and people you’re likely to be carrying on a trip and hauling it down to the local weigh station to get an accurate fully-laden weight. Armed with the weigh station ticket and the ride height you want the vehicle at, you can then go to your spring manufacturer or suspension supplier and they should be able to supply you the correct springs for the task ahead.
Another point when fitting heavy-duty springs for carrying large loads: these can make the vehicle skittish over holes and corrugations when the vehicle is unladen. If you only load your vehicle up once a year for a big trip and the rest of the time it hauls a relatively light load around town, then you may want to consider having two sets of springs and shocks on hand to get the best performance under the respective conditions. It is not that expensive an option and with a coil-sprung, live-axle vehicle such as a 1990 to 2008 80 Series Land Cruiser, it’s not a difficult job to swap over the combinations.
A competent suspension supplier or online retailer should also be able to recommend and supply the correct shock absorbers to match the new 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. Your best bet is to work with a suspension specialist when selecting aftermarket components.
Aside from the springs and shock absorbers, there’s a host of bushings and ball joints that hold your suspension and steering together. These generally require regular checking and lubrication which should be done as part of your normal vehicle servicing routine.
Once again, when these parts come up for replacement, either genuine manufacturer’s parts or aftermarket units can be used. In the case of bushings, many people opt for hard-wearing polyurethane items from aftermarket suppliers rather than the OEM rubber bushings.