Should You Upgrade Your Tow Vehicle Suspension?

Most tow vehicles come with factory standard suspension and that suspension may need to be upgraded to take the weight of your caravan, fifth 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.

So, should you upgrade your vehicle’s tow suspension? You should check first with your state Department of Transport or Road Transport Authority before modifying your vehicle. In New South Wales, it is illegal to modify a car’s suspension without proper approval. 

By reading this article, I can assume that you need help to decide. Vehicle modification can be a daunting task. So, read on to learn more.

Should I Modify My Vehicle?

In the years prior to 2009, a car could be raised or lowered by up to 5cm without approval and by up to 15 cm after complying with regulations. However, with the passage of a new law, 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 vehicles 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.

However, you won’t be able to upgrade the suspension on electronically controlled suspension systems. Vehicles with an electronically controlled suspension system first introduced in 1995, such as for the Range Rover that had electronically adjustable airbags on its live axles. Later, 

The Land Rover Discovery 3 with its ‘Terrain Response’ system allowed the drive to select a driving condition and then select the best suspension setting for that condition automatically raised or lowered the vehicle as well as adjusted the traction control actuation and diff lock actuation. These should work effectively without an upgrade.

What Is A GVM?

The Gross Vehicle Mass (GVM) is the maximum weight that your towing vehicle can handle. So, the gear you are going to include, as well as what gear you intend to load onto your 4WD, needs to be seriously considered in any vehicle suspension upgrade. 

If you exceed the vehicle’s GVM, it becomes a legal problem and it is unlikely your insurer will cover you in the event of an accident. The other alternative if the GVM is exceeded on your vehicle is to check with the manufacturer of the vehicle to see if they offer a GVM upgrade so the vehicle compliance plate can be stamped with the new heavier GVM. 

What Are the Parts of a Vehicle’s Suspension?

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 the 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.

What Are 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. The different types of springs include:

• Leaf springs: comprised of 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: 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.

Things That You Should Remember About Springs

There are a lot of things that you should remember when choosing the right springs for your vehicle. So, here is a list of some of the important ones:

1. You should remember to fit springs with a higher rate to the rear of the towing vehicle because of the heavy load of the RV. 
2. 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.
3. 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.
4. 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.
5. Most shock absorbers 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.
6. 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.

The 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 bump stop hard. Fitting of a brace between the torque arm and the control arm is recommended. Braces are available from Pedders and ARB.

Another solution, used by ATS Geelong, is to put the lift in but lower the front differential by about 25 mm by using a new bracket. By doing this it reduces the angle the CV joints need to work at thereby reducing the chance of breakage. A wheel alignment should be done to complete this exercise.

What Are Shock Absorbers?

Shock absorbers (or dampers as they are known) control the spring rebound or bounce so it is important the shock absorber is the right length and has enough ‘travel’ in compression and rebound to provide the dampening effect. (It is really the spring that absorbs the shock). A piston moves inside a gas or oil filled the cylinder and the hydraulic fluid is forced through tiny holes in the piston head as it compresses and rebounds.

Standard factory shock absorbers are often smaller in diameter hence they contain less oil so they don’t have the ability to transfer the heat as effectively as say an aftermarket large bore shock absorber. Some shock absorber brand names are Old Man Emu, Monroe, Bilstein, Nitrocharger, Koni Shocks, and Rancho shock absorbers.

What Are Bushes?

All suspension arms that pivot, as the wheels move up and down, run on rubber, polyurethane or sometimes called 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.

What Are Airbags or Air Springs?

Pneumatic airbags can be used as a way to carry loads such as caravans, 5th wheelers, and trailers without having to fit stiff springs. Air springs are preferred by some as they are adjustable by inflating or deflating the airbag. Air suspension kits are available to assist coil or leaf suspensions. 

Airbags 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.

What Are 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 becomes necessary very few insurance companies will provide cover for your vehicle.

Related Questions

What Affects Towing Capacity?

Towing capacity is how much the truck can pull, while payload capacity is how much it can carry. These can depend on such factors as the truck’s configuration, its chassis, its engine and transmission, its rear axle ratio, and its weight, and there can be considerable range over a truck’s full lineup.

Do Airbags Improve Ride?

While airbags can help, the better option for improved suspension is to get new and improved leaf springs. Depending on the size of your truck, you might need to add leaves to your springs to get the needed lift and smoothness from your vehicle. After all, trucks are built to carry heavy loads.

What Are the Best Shocks for Towing?

• Bilstein (24-186742) 5100 Series Rear Shock Absorber. …
• Bilstein 24-187367 5100 Series Front Shock. …
• Monroe MA822 Max-Air Adjust Shock Absorber. …
• Bilstein BE5B514H0 6″ Rear Lifted Truck Shock Absorber. …
• Monroe 58640 Load Adjusting Shock Absorber. …
• KYB 565102 MonoMax Gas Shock.

What Is Included in A Towing Package?

Many of us will be using our old reliable vehicle to tow, but if you’re thinking of buying new, you might want to consider a factory tow package. … This means they design the engine, transmission, brakes, frame, electrical system, and cooling system to handle the extra weight of the towed load.