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Suspension

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There is an unfortunate but realistic fact about purchasing a new 4WD - it won't come fitted with a suspension system designed to carry the kind of load you'll pack for a serious outback trip. In this article we explain why it is important to consider fitting a new suspension system and what is involved. Suspension will certainly improve handling and hence saftey of driving with heavy loads and will dramatically improve the comfort of the ride. You can forget about corrugations with a good suspension system - you'll feel like you are gliding over them!

What is a Suspension System?

A suspension system is made up of many components that work together, to assist in softening the undesirable effects of the surface that a vehicle is travelling on. It works by delicately balancing these effects, whilst still providing the driver with the necessary road response for driving, steering and braking safely.

The two main aspects of a suspension system include the springs and shock absorbers that help control the suspension movement and the network of control rods, linkages and bushes that connect the wheels to the vehicle. This network is intricately designed to be rigid, whilst allowing vertical movement for the wheels.

Suspension gives all the wheels of the vehicle more chance of making contact with the surface especially when driving over bumpy uneven tracks. When you travel at speed, these effects are amplified and driving over smaller track imperfections could now be quite noticeable. A good suspension system will dramatically help suppress these effects.

Springs

Springs are an amazing device that can store energy when it’s altered from its original state. The spring can then release this energy when the load is removed and therefore returning it to its original state.

Springs are used in vehicles to help absorb changes in the terrain such as bumps or potholes that would otherwise jolt the vehicle. At the time a wheel enters a pothole for example, the spring releases energy at the time the wheel moves into the hole. The spring would then reabsorb this energy as the wheel exits the hole.

The spring rate is the measurement of a spring’s energy storage and is measured in terms of how much force is required to deflect it by a given amount.

Linear or Fixed-rate Behaviour

Is when the spring compresses at a constant rate or measurement to the increase in load placed upon it. For example, if 50kg were placed on a spring and it moved 1cm. The spring’s rate would be 50kgs/cm and an additional 50kg would result in a 2cm compression.

Rising-rate Behaviour

This spring rate is more exponential and is highly suitable for vehicle applications. For example, if 50kg were placed on a spring and it moved 1cm, it may take 150kg for it to move another centimetre. This spring behaviour may allow substantial compressions for medium loads but without fully compressing under heavy loads. The downside to a linear rate spring is that the spring may be rigid enough for heavy loads but too rigid for light loads.

Suspension Arms and Linkages

The suspension arms and linkages all form the core framework of the suspension system. They all work together to serve a common purpose and that’s to keep everything locked in place, whilst providing the support and leverage where necessary. The design of this framework is very important in ensuring that the suspension system operates as intended and in general, the linkages be sturdy and rigid as possible to eliminate undesired movement.

Shock Absorbers

Shock absorbers, also known as dampers or shockies are used to help slow down or dampen the movement of the suspension system. They consist of a piston moving inside an oil or gas filled tube and have various holes and spring loaded valves to control its resistance to movement. Unlike a spring, a shock absorber will keep moving with the force that was applied until the force has stopped or reached its energy limits. When this force is exhausted, the shock absorber will attempt to return to a fully extended position however the weight of the vehicle will limit this travel. In a correctly setup suspension system the shock absorber should be centered to the travel of the device.

Pivot Points

Pivot points, which are joints between moving components need to have either restricted movement or be able to rotate freely in the desired way. There are two main types used in suspension systems and they are bushes and bearings.

Bushes

These components, also known as bushings are made of rubber, nylon or polyurethane and are generally used between joints where vibration needs to be absorbed. These bushes are used to help absorb road shock, reduce noise vibration and harshness. They are used to prevent steel on steel and minimise movement, which is the main cause of vibration transference, which in turn leads to steel degradation. Many of these bushes are designed to slightly flex to allow for a certain amount of linkage misalignment.

Bearings

Metal spherical bearings, also known as rose joints are commonly used to allow free rotation. They are more expensive than bushes; however, they provide a very stiff and solid joint. These components don’t offer the joint the impact absorption of rubber, nylon or polyurethane bushes.

Leaf Springs

The leaf spring, which is the simplest and oldest suspension system, serves a dual purpose – to locate the axle and to provide the suspension. Both ends of the spring's length are attached to the chassis with a bush and a through-bolt. The spring then passes under (although, on some occasions over) the axle and is usually attached to it with U-bolts. Leaf springs are usually made up of multiple leaves, which slide against each other as the spring flexes. The spring’s length changes as it flexes, so shackles are required to take up this change. There are three main types of leaf springs: quarter-elliptical, semi-elliptical and parabolic.

Quarter-ellipticals

Quarter-elliptical leaf springs have been around for many years and were used in the Austin 7s of the 1920s. A quarter-elliptical suspension system is literally half of a semi-elliptical leaf spring and its use in vehicles is not that common anymore. These springs usually work by having one end, which is often the thickest part of the leaf-stack bolted to the chassis and the free end attached to the differential.

Semi-elliptical

This type of suspension is by far the most commonly used spring – especially in heavy vehicle applications such as trucks and trailers. In regards to passenger vehicles, it would be ideal to have a soft suspension for a comfortable ride. This is true to a degree, but unfortunately not very practical because a very soft suspension will compress too easy with large bumps and potholes. 4WDs that are going off-road will no doubt experience these conditions, so it’s vital to have a balance between comfort and rigidity. What is required is a rising-rate leaf spring that gets stiffer the more it is compressed. Extra helper leaves, which are normally straighter and smaller than the standard leaves is what achieves this. As the load increases, and the standard leaves become near parallel with the helper leaves, the helper leaves will then come into play to take some of the load.

Parabolic

The parabolic leaf spring is a more modern version of the standard semi-elliptical leaf spring. This design consists of a thickness variation in the leaf which follows a parabolic arc. In other words, it’s thicker at the center and thinner towards the ends. The advantages of this system, is there is more contact between the springs at the ends and the center, providing less inter-leaf friction as they slide. Having less friction, results in a more supple and flexible spring that reacts smoother to small loads, whilst still retaining the stiffness needed to take heavier loads.

Advantages & Disadvantages of Leaf Springs

Below is a list of advantages and disadvantages in regards to leaf springs:

Advantages:

When the spring is attached to the axle and chassis, the spring itself can hold everything in place minimising the complications of additional linkages and support bars.

Friction as the leaves slide across each other, provides frictional resistance to movement which aides in stopping the spring bouncing out of control.