Four-Wheel Drive Suspension Upgrades Explained

Sam Richards — 4 March 2021
Caravan World catches up with four leading businesses to answer all the questions you’ve had about beefing up your 4WD’s suspension.

WHY IT’S A GOOD IDEA TO CONSIDER AN UPGRADE

In conversation with Kristian Ristell from Ironman 4x4

Ph: 1300 731 137



W: ironman4x4.com

WHAT MAKES THE STANDARD SUSPENSION ON A VEHICLE INADEQUATE? 

A vehicle’s original suspension is always a compromise. It's designed by the manufacturer to be applicable to a range of uses so that it suits every possible owner. In the aftermarket, we tailor suspension to suit the way a person actually uses their vehicle, whether that be setting up their vehicle for load carrying, for greater comfort, or to drive on tougher terrain.

Once you change the weight of a vehicle, the original suspension is no longer well configured to control that weight. If you do not upgrade, and are carrying heavier loads, you can encounter problems with ride height control — that is, where the vehicle sits with a load applied. At a more severe level, the vehicle may be driving close to the bump stops and the impact may be more regular than normal, which can affect the safety of the vehicle.

When we upgrade suspension, we ensure the dynamics for that vehicle are sound, and that there’s proper velocity control so the vehicle is not bobbing and bouncing down the road, nor body rolling any more than it needs to around corners.

IN WHAT WAYS CAN AN UPGRADE IMPROVE THE EXPERIENCE OF DRIVING A VEHICLE?

Upgrading suspension can increase comfort. There’s a lot of people who use big, burly utes these days as working vehicles doubled up as family vehicles. Getting the vehicle to ride well, so it doesn’t upset the passengers, is one thing an upgrade can achieve.

There’s a bit of a misunderstanding that if you upgrade a vehicle’s suspension, it’s going to get stiff and rigid. That’s not true. Quite often, when we increase the practical load carrying of the vehicle with stronger springs and more control in the shock, that vehicle will be far more smoothly balanced and won’t bounce or jump around as much as it did on the original suspension.

There’s also, of course, handling. If you’ve got too much load on a soft spring, you’ll end up with a low natural frequency (level of oscillation) and that makes the vehicle sluggish and unresponsive to drive. You’ll have a lack of support and feeling throughout the vehicle. A well-chosen suspension upgrade can correct that.

Any weight on the tow hitch of the vehicle is applied far back, which has a greater leverage effect on the rear suspension than load directly over the axle. Accordingly, heavy weight at the back can unload the front of the vehicle and create an imbalance. We can fix this by recalibrating the dynamics of the vehicle and ensuring correct ride height.

THE DIFFERENT COMPONENTS THAT GO INTO A SUSPENSION UPGRADE

In conversation with Mark de Prinse from Old Man Emu

Ph: (03) 9761 6622 / 1300 ARB 4×4 

W: arb.com.au / oldmanemu.com.au

WHAT IS SPRING RATE AND WHAT IMPACT DOES IT HAVE ON SUSPENSION PERFORMANCE?

Springs are about carrying weight and determining your ride height. Because spring shape is determined by the vehicle manufacturer, the variables we can change are spring free height and spring rate — together, these factors determine your ride height. By increasing the spring rate, you make a spring stiffer; a stiffer spring will compress less for a given load than a softer spring.

Softer springs will provide a more comfortable ride and better articulation but won’t maintain ride height if heavier weights are applied. Firmer springs carry weight better but if the appropriate weight is not carried, the ride quality can be harsh, and your ride height may be too tall. So, it’s about selecting the correct springs for the job.

WHAT IS A VARIABLE OF PROGRESSIVE SPRING, AS OPPOSED TO A CONSTANT LOAD SPRING? 

A constant load spring, or a linear rated coil spring, basically gives the same spring rate through the entire compression of the spring, whereas variable rate springs are softer in the initial amount of travel and then as the spring compresses, the spring rate increases and becomes firmer.

With coils, progressive rate is achieved through progressive pitching throughout the coil, which creates different spacing between the coils of the spring. Progressive rate can also be achieved through tapered wire which alters the diameter of the coil wire. Variability in a leaf spring is achieved through a primary stage and a secondary stage — the primary stage has an initial spring rate and as the primary comes into contact with the second stage, the second stage provides additional spring rate.

WHAT ROLE TO SHOCK ABSORBERS PLAY? 

Shock absorbers control the motion of the vehicle, taking into consideration the carried mass and spring rate. In other words, it controls the rate of compression and rebound caused by spring rate and vehicle/load mass.

In our range, we have a twin tube shock absorber which is called the Nitrocharger Sport. It’s a velocity-sensitive shock absorber that is tuned by our engineers, which means the shock shaft speed (how fast the shock goes in and out) and our valve tuning determine how much damping the shock offers.

Our other type of shock is the BP-51, a bypass or a position-sensitive damping shock absorber. With this, the level of damping depends on the position of the piston. Bypass offers you the best of both worlds. At ride height, you have extreme comfort and then, as you use more travel, damping increases to give you increased control when needed, such as when you hit an unexpected wash-out.

HOW A SUSPENSION UPGRADE IS FINE-TUNED TO SUIT A CUSTOMER

In conversation with Mike Davison from Lovells Suspension

Ph: (02) 9820 6800

W: lovellsauto.com.au

HOW IS A SUSPENSION UPGRADE TAILORED TO THE LOAD A VEHICLE WILL BE CARRYING?

Put simply, the heavier the load, the higher the spring rate should be. We have springs at various levels, from our standard height springs which have slightly higher spring rate than OE, up to extra heavy-duty for if you’re regularly fully laden and/or towing with considerable ball weights (150kg and above).

If a person says they will be consistently loading their vehicle to GVM, we’d select springs to work with that application. But if they took every bit of load out and drove, particularly in rough terrain, that vehicle would not be set up for it. With the high spring rates, the height would be greater without load, plus, the unladen ride would be uncomfortable and unforgiving.

We have progressive rate coil springs to cover a range of different loads, but it is a bit of a trade-off whenever you are selecting a spring. For that reason we generally recommend selecting a moderate rate with a raised design within the ADRs and state regulations, with mid-range comfort that is built to carry a moderate but not massive load. Airbags and height-adjustable struts are alternative solutions to permit more variable loads.

WHAT IS A GVM UPGRADE AND HOW IS IT DONE?

Occasionally a GVM upgrade may be required when towing an RV with a large ball weight. A GVM upgrade is generally achieved through beefing up of the entire suspension system after rigorous testing has been undertaken to address all affected ADRs, FEA and CAD modelling of axle assemblies, Sine with Dwell testing on ESC equipped vehicles, chassis design calculation and lab testing, plus dynamic stability testing to SAE Standards, to name a few of the test regimes. GVM upgrades are easier and cheaper to carry out prior to vehicle registration.

IS IT ALRIGHT FOR ME TO MIX AND MATCH MY FAVOURITE BRANDS OF SPRINGS AND SHOCK ABSORBERS?

Shock absorbers are designed around springs, so it’s important to go with one brand. If you go for an unknown and unmatched spring and shock absorber system, there is no guarantee the valving or controls of that shock absorber will control the spring rate, or that the extended and compressed lengths of the shock will match the spring or application.

HOW TO MAKE THE MOST OF YOUR SUSPENSION UPGRADE

In conversation with Andrew Goddard from Cruisemaster

Ph: (07) 3624 3821

W: cruisemastertpc.com.au

HOW DO AIRBAG SUSPENSION UPGRADES WORK?

The most common airbag upgrade is the helper bag, which works with coil or leaf springs. With some coil spring vehicles, you even have the option of completely replacing coil springs with airbags. Airbags help by filling the volume within the coil spring, thereby adding resistance to the movement in line with air pressure, and effectively increasing the spring rate. By doing this, an airbag helps maintain the same ride characteristics and height independent of load or lack thereof. 

You can adjust pressure via outlets at the back of your vehicle with the same fitting as your tyres, or use on-board inflation, which can be manually adjustable or automatic.

IS IT BEST TO DRIVE OVER CORRUGATIONS SLOWLY, OR TO 'FLOAT' OVER THEM AT HIGHER SPEED?

There is no one size fits all answer — it truly depends on the terrain. If going slower over corrugations, you tend to use more of your suspension travel as you’re riding up and down each corrugation; the shocks are having to work harder. This makes them more prone to shock fade — a loss of damping properties resulting from the shock absorber’s oil heating up and thinning, and the oil aerating. In a shock, you’ve got a piston full of holes and some form of deflecting disc which provides resistance to the piston moving through to the oil. As that oil thins and aerates, it offers less resistance, so there’s less damping.

Shock fade typically affects dual tube shock absorbers more. In a dual tube you have a free surface between the oil and the nitrogen gas. As it goes up and down on corrugations, the gas goes through the oil and causes the reduction in resistance. Comparatively, on a monotube shock absorber, you’ve got a physical piston, a barrier between the gas and the oil, plus, the gas is at higher pressure. 

On the other hand, when you’re going fast, you’re aiming to stay on top of the corrugations and for the wheel not to drop into each rut individually, so the suspension is moving less. The risk is, when you try to brake or steer, you may find you don’t have traction. I’ve been on some where you can find a sweet spot where it rides well and maintains steering, and others where at speed you’re all over the place. In those areas, you need to be safe. A  safer way to improve the experience in the cab is to drop tyre pressures.

SHOULD A CARAVAN'S SUSPENSION SYSTEM BE MATCHED TO THE VEHICLE'S?

Higher performance vehicle suspensions ride better on rough roads and can cause you to travel faster. Whatever you’re towing has to keep up with that, so it’s important to make sure your trailer’s suspension is up to the job. If the RV’s suspension isn’t as good, there might be more transmitted force from the trailer to the tow vehicle. But, being all nice and isolated in the car, it’s more likely you’re going to break something on your RV.

At Cruisemaster, we offer retro-fitting of suspension to existing RVs. Provided the chassis, framing, construction, interior fittings and a few other factors are up to the job, we can generally undertake a retro-fit. But it’s important to know, you can’t take a road-based caravan, slap offroad suspension on it and call it an offroad caravan. 

EVERYTHING IN MODERATION

GVM, which stands for Gross Vehicle Mass, is the highest specified weight your vehicle, with all its load and accessories included, is safely and legally permitted to reach, and includes the ball weight if you’re towing. GCM, or Gross Combined Mass, refers to the maximum allowable weight of your vehicle and your trailer combined.

Upgrading your vehicle’s GVM will permit you to carry greater loads on the vehicle, including heavier ball weights, which might allow you to tow your dream van. But a GVM upgrade will not change your vehicle’s GCM. 

Those wanting to tow RVs that exceed their vehicle’s current GCM parameters may be able to get a GCM upgrade, depending on the regulations within their state. Some second stage manufacturers can undertake them, while others are against them. Ironman 4x4 do not undertake GCM upgrades and explained some of the difficulties surrounding them:

“When we increase GVM, we can ensure the vehicle is tested and complies to the right kind of performance and level,” Kristian Ristell says. “Whereas with GCM, there’s a lot of question marks about the trailer itself.”

“We don’t know whether that trailer is in good or bad condition, whether it’s suitable to tow that sort of load, or whether it’s practically capable. With GCM, you can’t really get any verification on the trailer side of things, or regarding the capabilities of the driver, so it becomes a much more complex scenario. There’s no control over a few factors, which is a bad thing in our books.”

On a related note, the manufacturers we talked to for this article acknowledged the limits of suspension upgrades. You cannot keep paying more to carry more, ad infinitum. Some weights, for some vehicles, will be simply impossible. For example, the axle capacities of a vehicle may be a limiting factor in how much you can carry. As Cruisemaster explained:

“The axle capacities of a vehicle are often overlooked,” Andrew Goddard says. “We’ve done some calculations and proved it and on the LandCruiser 200, if you’ve got 350kg ball weight on the tongue at the back, it actually puts 500kg onto the rear axle, which pretty much puts it on its axle capacity.”

“Even with a GVM upgrade, if you then put a set of drawers in your LandCruiser with a long range tank, then you put your caravan on the back, you’re going to be over your rear axle capacity.”

If you think you may need to visit Upgradetown to tow your dream combination, have a chat with an aftermarket suspension brand before purchasing to spare yourself some heartache.

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