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Suspension Upgrade

Most tow vehicles come with factory standard suspension and that suspension may need to be upgraded to take the weight of your caravan, 5^th wheeler, travel trailer or camper trailer.  Suspension upgrades are also required with the installation of long range fuel tanks given the weight of the extra fuel.

Firstly, check with your state Department of Transport or Road Transport Authority if there are any regulations for raising the suspension on a vehicle.  For example, from 1/8/09 laws were introduced in New South Wales making it an offence to raise or lower a car’s suspension without proper approval.  Previously in NSW a car could be raised or lowered by up to 5cm without approval and by up to 15 cm with approval however from 1/8/09 any vehicle that is raised or lowered will have to carry a certificate stating that the modifications conform to safety standard requirements.

Then, think about what additions  you are likely to add to your vehicle such as winches, long range fuel tanks and the ball or pin weight of the RV you are likely to tow so you get the right suspension upgrade to suit you.

The vehicle suspension comprises springs, shock absorbers as well as linkages such as wishbones, control arms and rods that connect the vehicle to its wheels.  Torsion bars, commonly found holding up independent front suspension, are straight round bars held to the frame of the vehicle at one end and the lower wishbone at the other use the twisting properties along the bar as a spring force.  All these components are designed to allow the wheels to go up and down and maintain their road contact angles.

Springs

Springs are the devices that absorb shocks and the spring rebound is controlled by shock absorbers (often referred to as ‘dampers’).   Most springs today are powder coated to assist resist corrosion as well as for aesthetics.

Leaf springs comprise several layers of metal leaves which are bound together so they operate as a unit.  The number of design variables is quite large with variations in leaf width and thickness.  You can obtain a ‘two stage’ leaf pack with a smooth ride on the primary stage when the vehicle is unladen and when laden additional support is provided with the secondary stage employed.

Coil springs are the most common type of suspension and they expand and compress to absorb the motion of the wheels.  Constant and variable rate coils are available.  Variable rate coil springs provide a reasonable balance between load carrying capacity and ride quality.  They provide a variable rate of resistance to help reduce side sway, sagging and unequal weight distribution in your vehicle and provide a more comfortable ride than leaf springs.

Given a tow vehicle will do more laden trips you would fit springs with a higher rate to the rear.  The coils can vary both in length and the number of spirals.  The ‘spring rate’ will determine how firm or soft your ride is.  The ‘spring rate’ is measured in newtons per millimetre (N/mm) with 10 newtons being about 1 kilogram this being the units of load per units of distance so springs with a higher rate will not compress as much as the standard springs.

When the spring height of a coil-sprung vehicle is increased the diff housing is pushed away from the body and, as the diff housing pivots from the suspension arms, the wheelbase of the vehicle is then shortened slightly or the front diff pulled back a bit and the rear a bit forward. With that occurring both have rotated slightly so the linkages and bushes don’t sit like they used to.  The angles of the uni-joints and driveshaft will also have changed.  Your suspension specialist will make sure brake lines and brake proportioning valves are adjusted to suit the lift, rectify the wheelbase, castor and stresses on misaligned bushes.

When selecting quality suspension coils you will see terms such as:-

‘Shot peened’ – which is a process of work hardening and pre-stressing a material to create a harder and more durable material and

‘Scragging for Quality Control’ - which is where the coil spring is compressed beyond its yield point to set up residual stresses thereby increasing the elastic limit of the spring.

Most shock absorbers (discussed in more detail below) are speed sensitive and laden touring tow vehicles driven on corrugated roads will benefit from heavy duty or multiple shock absorbers.  Heavy duty or multiple shock absorbers help reduce shock fade which occurs with the heating of the shock’s oil beyond a reasonable temperature by rapid compression and rebound of the suspension.

By raising the suspension the transfer case is raised relative to the differentials and that then increases the prop shaft angle and increases the work demanded from the universal joints each end.  Once the rear suspension has been upgraded the front torsion bars can be wound up to match the suspension height to make the vehicle look even.  However, winding up the torsion bars will then restrict the amount of drop your front wheels may have.

Toyota Landcruiser 100 series have independent front suspension (IFS) and adjusting the height of the vehicle will put more stress or load on the front torsion bar mounts.  Accessories such as bull bars, extra batteries and compressors added to the front of the vehicle will add extra weight and subsequently extra load to the torsion bars and their mounts.  A specific problem to the LC 100 is cracking of the lower suspension A-arm and this is caused by the suspension bottoming out and hitting the bumpstop hard.  Fitting of a brace between the torque arm and the control arm is recommended.  Braces are available from Pedders and ARB.

Bushes

All suspension arms that pivot, as the wheels move up and down, run on rubber, polyurethane or nylon bushes. These bushes assist to insulate the body of the vehicle from harsh vibrations.  As they wear vibration starts to creep in and the suspension arms are no longer in the ideal position to hold the wheels due to the extra movement of the worn bushes.  As a result, handling performance suffers and tyre wear increases.

Polyurethane bushes are generally stiffer and last longer than the rubber bushes and work very well when lubricated.

Air Bags or Air Springs

Pneumatic airbags can be used as a way to carry loads such as caravans, 5^th wheelers and trailers without having to fit stiff springs.  Air springs are preferred by some as they are adjustable by inflating or deflating the air bag.  Air suspension kits are available to assist coil or leaf suspensions.  Air bags can be mounted inside existing open coil spring suspension and accessories are available to inflate and deflate the bags from the comfort of the inside cab of your vehicle.

Some brand names for air springs are Firestone Coil-Rite, Air Bag Man and Polyair Springs.  Firestone Ride-Rite kits are available for leaf suspension.

Polyair bags have a low air pressure requirement and can be filled with air from a hand pump or service station air hose.  The manufacturer states they are made from highly durable space age urethane material and when properly inflated will provide years and years of trouble free service.

Steering Stabilisers

Steering stabilisers are basically a shock absorber for the vehicle’s steering system specifically designed to operate horizontally.  Although some 4WD vehicles have factory fitted steering stabilisers aftermarket kits are recommended if your vehicle is fitted with over-sized tyres or a winch.  Fitted to live-axel 4WDs steering stabilisers dampen jolts and wheel shimmying on rough roads or hard impact like potholes and help stop the steering wheel jolting out of your hands.  

When raising a coil sprung live-axle vehicle an adjustable Panhard rod is likely to be required.  When a coil sprung live-axle 4WD is raised the body is shifted to one side and Panhard rods are used to correct the track of the vehicle. Adjustable Panhard rods are fitted to stop side to side axle movement.  A Panhard rod attaches to the axle on one side of the vehicle and the frame on the other side and may be fitted to the front and rear of a vehicle.

Suspension upgrades should be completed by a professional.  Suspension kits are available from Snake Racing, Superior Engineering, ARB, TJM, Opposite Lock and other 4WD specialists.

Popular modifications to improve ground clearance for 4WDs including raising the suspension can have other implications and if an engineer’s certificate become necessary very few insurance companies will provide cover for your vehicle.

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