4WD Suspension Explained

The suspension in a 4WD vehicle is heavier duty than in, say, a Hyundai Excel, so it can cope with corrugations and pot holes, but time takes its toll. Shock absorber and bush failure are the most common fault, but I have also heard of springs breaking because of overloaded vehicles.

When it comes to suspension setup, there are many variations including leaf spring all-round, coil spring all-round and torsion bars and struts.

It’s time to examine the different components and how they might be fixed out in the middle of nowhere.

LEAF SPRINGS

A leaf spring is a simple, longstanding suspension component that dates back to medieval times. An advantage of a leaf spring is it can take heavy loads and withstand rough roads well. The drawback to being able to handle heavy loads is the ride quality suffers, so leaf springs are naturally stiff.

A leaf spring is most likely to fail if the vehicle is overloaded or if the springs are old and were stressed in the past. Leaf spring packs usually have a military wrap, which stops the corner of the vehicle from losing the spring altogether if the main leaf connected to the chassis breaks. Should this happen, the spring pack can be “splintered” with a thick stick or a piece of metal and some wire to tie it together.

COIL SPRINGS

Coil springs are more refined than leaf springs so coil-sprung vehicles offer a comfortable ride and better handling in comparison but they don’t handle heavy loads as well. Just like leaf springs, issues are commonly caused by overloading the vehicle and a dead giveaway is sagging springs.

Although it doesn’t happen often, coil springs can break leaving the body resting on the tyre and no suspension. If this happens, you can tie a log in its place to maintain the vehicle’s ride height so it can be driven. Careful driving to wherever you can get some assistance will minimise further damage to the chassis and body.

TORSION BAR

The torsion bar is essentially a long piece of spring steel mounted between the chassis and axle assembly, which absorbs some of the wheel’s energy, dissipating the rest by twisting on its axis.

The effective spring rate of the bar is determined by its length, cross section, shape and material.

Torsion bars are a popular choice for front 4WD independent suspension setups as they make the ride height easy to adjust. Reasons for failure include metal fatigue and overloading. Little can be done out bush when a torsion bar breaks as it is unlikely you’d find a stick strong enough to splint the bar.

SHOCK ABSORBERS

Shock absorbers control the spring and therefore the ride and handling of the vehicle.

Shocks have hydraulic liquid inside that passes through valves to control the dampening. The liquid gets hot and sometimes overheats, causing the shocks not to work properly. This is called fade.

Failure of the seal around the shaft is another common issue that can cause the hydraulic fluid to leak, leaving the shock with insufficient liquid to work. A shock can also fail when overextended, literally causing it to fall apart.

A shock can be removed if it fails, but something needs to be put in place to stop the spring from falling out. A piece of rope and cable ties would be a remedy, but remember that without a shock, the affected corner of the vehicle will bounce around, so be careful when driving.

BUSHES

Suspension bushes allow different parts of the assembly to move freely, reducing vibration between components.

Bushes are designed to wear out, because it’s cheaper to replace a bush then replace a whole suspension arm. Therefore, bushes should be checked as part of a pre-trip inspection and during general servicing to avoid a foreseeable failure.

SUSPENSION SETUPS

Most vehicle suspension setups, using a combination of the previously mentioned components, fall into one of two categories. The first being a live axle setup, found on vehicles like an 80 Series or Patrol, which can use either coil or leaf springs.

The second is an independent suspension setup, found in the front of most modern 4WDs and in the rear of vehicles like the full independent suspension Pajero. The most popular independent suspension setups use struts, which means the coil spring fits over the shock and is assembled as one unit, but torsion bars are also used.

Most of the time you will have a trouble-free experience with your vehicle’s suspension setup, but things like corrugated roads and overloaded vehicles can cause problems. Some say those who overload their vehicles with spares are more likely to need them so instead, pack wisely, maintain your rig and stick with the major brands as they’re tested in conditions similar to what you’re driving on and are readily available.

Check out the full feature in issue #86 March 2015 of Camper Trailer Australia magazine.