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.
- Leaf springs can be easily designed to take heavy loads
- The simple design of the leaf spring system is not as expensive as other systems
Disadvantages:
- The friction that is generated from inter-leaf sliding can also work against the intended use
- As the spring gets older and rust starts to settle in, the springs don’t slide across each other as smoothly and therefore creating a stiffer spring
- Leaf springs need to be well looked after for maintaining good performance
What to Look For?
Below is a list of some of the things to look out for when choosing a quality leaf spring suspension system:
- Tapered Leaf Ends - To reduce inter-leaf friction & to improve pressure distribution in the bearing area
- Shot Peened & Scragged Tested - To ensure longevity and reduce stress
- Leaf Inserts – Assists in a further reduction of friction
- Bolt Clips & Military Wraps – This helps prevent leaf spread & provide extra safety
- Bolt Clip Liners – Assists to reduce noise and friction.
- Inter-Leaf Graphite Coating - Assists in a further reduction of friction
Coil Springs
A coil spring is basically a torsion bar that is twisted into a spiral. Similar to a torsion bar, which works in storing energy by twisting, a coil will store this twisting energy by compressing it down.
A coil’s stiffness is related to multiple factors such as the diameter of the coil and the diameter and the overall length of the steel used to manufacture the coil. Since a coil is basically a curled torsion bar, a 400mm coil with 10 winds will be stiffer than an 800mm coil with 20 winds. This is because the length of steel used to make the 800mm coil will be twice as long in its straight form and therefore being easier to twist.
Main Advantages
Coil springs are very popular suspension systems in off road vehicles. Since a coil spring doesn't have the friction losses that a leaf spring does, a smooth and predictable suspension system can be designed. These springs are often mounted between a lower suspension wishbone and either the chassis or a chassis outrigger. In 4WD applications, the coil spring can work very well with a telescopic shock absorber, giving greater suspension values and clearance in true off-
road conditions.
What to Look For
Below is a list of some of the things to look out for when choosing a quality coil spring suspension system.
- Shot Peened - To relieve stress of the outer surface of the coil. The coil can then operate under higher fatigue & repeated load conditions to ensure longevity
- Scragged for Quality Control – Scragging is achieved by compressing the coil beyond its yield point to set up residual stresses. Doing this will increase the elastic limit of the spring
- Powder Coated – Helps resist corrosion. Also used for aesthetics
Air Springs
With an air spring attached to a compressor, you get both a rising-rate spring with almost infinite adjustability. Air can be pumped into the spring or released to help level the vehicle depending on various driving conditions such as driving in rough terrain and driving with heavy loads. Automated air suspension systems are electronic and usually have these features:
- Load monitoring
- Automatic traction control
- Automatic pre-set levels. (e.g. parallel to the road surface, including uneven loads)
4WD Applications
The advantage of using air springs especially in 4WDs is you can raise and lower the suspension at will. The vehicle can have a raised suspension dedicated for those uneven bumpy tracks and a lowered suspension for driving on the tar. Some 4WDs such as the latest Range Rovers have a more sophisticated system which can raise or lower each corner individually to keep the vehicle level over rough terrain.
Air Spring Add-ons
An aftermarket option is the adjustable add-on air spring. These air systems can be installed to work with coil and leaf springs and can either be permanently connected to a mounted compressor or adjusted with a separate pump. These systems are relatively cheap and are designed to assist in levelling the vehicle when it’s heavily loaded.
Suspension Considerations
There are many things to think about before rushing out and buying a suspension upgrade.
Some of these considerations are outlined below:
- Standard suspension supplied by the manufacturer is designed to carry the load of the vehicle in it's standard format. This may be 8 persons or 4 people and some luggage. Outback 4WD travellers generally carry 2 - 4 people, extra fuel, recovery equipment, water and the list goes on. This stuff is extremely heavy and not what the original manufacturer made the vehicle to carry.
- Overloaded, under capacity suspension will not ride very well and is dangerous.
- How will the set-up ride when the vehicle is unladen? Super heavy suspension may give a hard ride when unloaded but perfect when fully loaded for the big trip.
- How will you be using the vehicle? If you only travel outback 4 weeks a year then you will need a system that will ride well in the city and have the flexibility to give good performance when you are outback.
- When you change the springs or coils it is advisable to change the shock absorbers at the same time. If lifting your vehicle with new springs then longer travel shocks will be required.
- Some vehicles will ride at a different height on each side of the vehicle. A lot of new suspensions system are designed with different heights in the springs and these are to go in the vehicle in a certain way.
- Your ride and comfort are everything - do not skimp on your choice of suspension.
Suspension Installation
While you can often fit aftermarket suspension yourself, it is highly recommended that you have this job professionally done. On average, it takes around two hours to install a new system with the aid of hydraulic lifts etc. One of the things to look at and tell the installers about is where your load is going to be and how much you are thinking of carrying. Some systems can be configured independently.
Before Coil Spring Installation:
After Coil Spring Installation:
New Suspension Laws for NSW
From August 1st 2009, vehicle owners in
New South Wales will be limited to raising or lowering their suspension by no more than five centimetres, and all modifications will need approval from Roads and Traffic Authority (RTA) engineers. Before this time (August 1st 2009), a vehicle could be raised or lowered by up to five centimetres without approval and by up to 15 centimetres with approval.
NSW Roads Minister - Hon. Michael John Daley says - “it’s about saving lives”. “Raising or lowering a vehicle's height can put the driver, passengers and other road users at risk”. “It can affect handling, braking and safety features such as electronic stability control."
Any vehicle that is raised or lowered after August 1st 2009 will have to carry a certificate stating that the modifications confirm to safety standard requirements. For more information, please click the PDF document:
Vehicle Standards Information (VSI) No. 50
TIP |
It is important to take heed and comply with new laws, as failure to do so, can void insurance claims, etc. |
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Each Topic contains a range of detailed and useful articles on products, services and issues. This section is full of practical tips, ideas, photos and factual information. Each topic has links to relevant Manufacturers, Retailers and Operators that can be found in our Directory; plus links to relevant Posts in our Forum; and links to related Trader listings.
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