Towing safely with Tough Dog 4x4 Suspension (Image Josh Hanger)
In recent years it has come into focus that many tow vehicles on our roads are overloaded or at least badly loaded. In this article, we will explore one of the solutions — a GVM upgrade. This is a complex subject where we can’t cover all aspects. We aim to introduce you to the important concepts so you can consider the variables important to you when researching a GVM upgrade.
Let’s start with a basic explanation of vehicle weights — for more detailed explanations of the most common terms refer to our 'Caravan safety 101' article. The kerb mass/kerb weight of a vehicle is essentially how much it weighs unloaded (i.e., the tare weight but with a full tank instead of 10L). When you add your load to the kerb mass (including cargo, passengers, bolt-on accessories and the download of the caravan on the towball) you get the loaded weight.
Car manufacturers specify the maximum loaded weight for their vehicles and refer to it as gross vehicle mass (GVM). If you exceed this number, your tow vehicle will not be legal, it will not be insured and there’s a good chance it will be unsafe. Car manufacturers place limits on how heavy their vehicles should be for good reason. Too much weight can put a strain on the vehicle, causing things to break. It makes stopping distances longer and the suspension system can become unbalanced which leads to poor handling.
Third-party companies provide solutions to improve vehicle load-carrying capability and legally upgrade the GVM rating above the original manufacturer limit. Broadly speaking GVM upgrades could be divided into two types. The first is simple replacement suspension kits where the exercise involves swapping out the shock absorbers and springs and sometimes small, related components. The second is more complex conversions where multiple key structures such as the chassis or diff/axles are re-engineered in addition to changing over the vehicle's suspension system.
A GVM upgrade kit
In this article, we are going to focus on simple replacement suspension kits available for dual cab-style utes and the SUV wagons derived from them. Think Ford Ranger/Everest or Isuzu D-MAX/MU-X for example. The engineers and accountants from the suspension companies would no doubt take exception to their suspension kits being described as simple, as there is an enormous number of resources invested in bringing these suspension kits to market. They would point to the complexity required to prove the new setup is safe and reliable through a series of prescribed tests and analyses. It includes ride comfort, braking, swerving, component strength, impact on engine and transmission and suitability of wheels and tyres.
Track testing includes driverless swerving
What to consider when choosing a GVM upgrade
Do I need a GVM upgrade at all?
Hitching up a van places considerably more load on the rear of the car. As a result, it is very common to see the towing vehicle’s rear suspension sag and the front suspension lift higher than normal. This lightening of the front end decreases the contact patch of the front tyres on the road making the car harder to steer and come to a complete stop. If the load imbalance between the rear and front of the tow vehicle is great enough it will make the rig unsafe and will need to be addressed.
This does not necessarily mean you need a GVM upgrade. A stiffer rear suspension or supplementary airbags can be used to help level the ride of the tow vehicle and that may be all you need. It all boils down to whether the total load on the tow vehicle (including the towball download imparted by the caravan) takes you over the GVM limit. The best way to know this is to weigh the rig in its fully loaded, ready-to-tour setup — including passengers. Unfortunately, if you are getting a pre-registration GVM upgrade (explained shortly) you may not have the opportunity to weigh the rig and will instead have to settle for estimating all the loads you will be introducing and add them up.
Installing the front strut (Image John Ford)
One of the best pieces of advice we can give you in this article is if you are not going to be overweight then don’t get a GVM upgrade. GVM upgrades are considerably more expensive than just swapping out some rear springs/shocks and they place major constraints on modifying the vehicle once it is done, as we will see later on.
A GVM upgrade doesn’t fix everything
A simple GVM upgrade often does not change your other load parameters. So don’t assume it will increase your braked towing capacity, towball download capacity or gross combined mass (GCM) rating (meaning the total permissible combined loaded weight of the van and tow vehicle). A simple GVM upgrade often does not upgrade the tow vehicle brakes or transmission cooling systems. So, it’s important to understand that a GVM upgrade is not a magic wand. In fact, we will show it may make some things worse.
A GVM coil leaf kit
Pre-registration versus post registration GVM upgrade
We have learnt that GVM upgrade modifications require extensive testing. Pre-registration GVM upgrades are a way for the testing to be done once on a single car and submitted to a statutory authority for approval. The results are then applicable to all vehicles of the same type thereafter. This process requires strict criteria to be adhered to. The suspension company must be recognised as a second stage manufacturer (SSM). As the name suggests The SSM is now considered the manufacturer of the car and takes responsibility for the vehicle modifications.
The exact components that are used in the testing then become the prescription for their GVM kit. The SSM must assign unique part numbers for each component and only these parts can be used. Fit-out can only be conducted by an installer approved by the SSM. The vehicle is then registered as new with the upgraded GVM rating.
The upside to this approach is it is somewhat of a cookie-cutter system streamlining processes and eliminating the need for case-by-case engineering certification of individual cars. The downside is changing any of the SSM components voids the GVM upgrade. This means, for example, you can’t use any other shocks when they need replacing or try a different kind of spring if you don’t like the ride. This also includes putting the system back to stock. An independent engineer is required to effectively ‘un-GVM upgrade’ it and the relevant road registration authority records need to be updated.
GVM upgraded Tough Dog vehicle pulling the Signature Rugrat X hybrid (Image Capture Factory)
As the suspension mods happen before registration it is often not practical to create a fully loaded set-up to run over scales to get an accurate weight. Therefore, there is some estimating and guesswork involved in selecting the most suitable configuration for your needs which doesn’t always make it easy to get it right.
An administrative advantage is the GVM upgrade is recognised nationally for registration purposes if the vehicle moves interstate.
A post-registration GVM upgrade is exactly what it says. A third-party automotive engineer is required to sign off on the modifications on a case-by-case basis. Using an SSM pre-tested kit (as per pre-registration described above) makes the process simpler, but the engineer still needs to approve the whole car. This can get messy as there may be modifications other than the GVM upgrade which the engineer deems to be non-compliant preventing the car from being approved. The upside is the vehicle can present in its final modified format allowing accurate weights to be taken, giving the best chance of selecting the most suitable suspension set-up.
The administrative disadvantage is the post registration certification is not recognised nationally. You can drive the car in any state but transferring registration to another state requires the car to be re-certified.
GVM rating
This is the main number you will see used to describe a GVM upgrade — it’s simply the new rating available expressed in kilograms. The interesting thing is suspension manufacturers will often offer more than one GVM upgrade option. It’s important to do the maths on your loads and select the most suitable one for your needs. Keep reading to learn about spring rates because bigger is not always better!
We mentioned getting a GVM upgrade does not mean you automatically get a GCM upgrade. If you don’t have a GCM upgrade and utilise your increased GVM you effectively decrease the mass available for a van. Just for easy maths. If the original ratings of a vehicle were GCM 6000kg and GVM 3000kg the available gross trailer mass (GTM) would be 3000kg. If a vehicle GVM upgrade of 3500kg was fully utilised and the 6000kg GCM still applied the available GTM would now be 2500kg. Often a good way to look at a GVM upgrade is it is a way to equip the tow vehicle with the accessories you would like (extra weight) rather than making it possible to tow a larger van.
Be aware that increasing your GVM may move your vehicle into another weight classification class. In some states, this could impact on registration cost and tolls. It wouldn’t apply for GVM upgrades to regular dual cab utes but with some bigger vehicles, you may require a light truck licence.
The GVM rating is very important, but you can’t stop there because all of the numbers below need to be considered as well. SSMs do not always publish these numbers so it is important to ask for them.
Axle re-rating
This is the most curious and often neglected rating when owners consider a GVM upgrade. Original vehicle manufacturers specify a load limit for front and rear axles. When a GVM upgrade is applied, the larger load can exceed the original front and/or rear axle limit. This is overcome by the second stage manufacturer ‘re-rating’ the axle/s. In many instances, this does not involve modifying the axles. Instead, the SSM engineer determines that the axle/s are capable of carrying more than the vehicle original manufacturer stated. The SSM is prepared to do this because they understand the original manufacturer allows ample safety margins plus, they do their own computer modelling to determine it is OK.
Sophisticated track testing undertaken by the second stage manufacturer (SSM)
When assessing axle ratings there are a number of important variables to consider. Vehicles are designed to carry loads in the rear but only have limited capacity to add extra load on the front. When SSMs upgrade axle ratings they place more emphasis on the rear. SSMs offer increases on the front axle rating ranging from ample to minimal to none. This may not be a problem, but it can be if you add a bullbar, winch, front bash plates, roof tray and a couple of ample-sized Aussies in the front seats. Rear seat passengers also contribute to front axle loading.
The SSM issues a new plate with the upgraded GVM rating
Here’s the trickier variable — your tow vehicle is like a seesaw. We have already touched on when you put additional weight on the rear of the car the front becomes lighter and vice versa. For caravanners, this is particularly important because you can very easily have the scenario where your front axle loading is fine when the caravan is hitched. But when you unhitch the load on your front axle is too high even with a GVM upgrade. Therefore, it is important to work through all this with your suspension provider.
Spring rates
Spring rates are easier to understand but you still need to be careful. A GVM upgrade is always going to include switching to stiffer springs for better handling when carrying heavy loads. The catch is springs that are too stiff make the car handling worse when it is unloaded. At a minimum, stiffer suspension can make for an uncomfortable ride. At worst it can make the car unsafe to drive as the suspension can’t react to bumps causing wheels to bounce up, leading to a lack of traction and potential loss of control of the vehicle.
A spring rate test
Suspension manufacturers usually (but not always) offer a choice of spring rate. At the front, there is often a heavier upgrade which allows for a bullbar and accessories and a lighter upgrade for vehicles without a bullbar. For the rear, again there is usually a choice of a lighter or heavier spring upgrade. An example of a lighter rating could be a 300kg constant load and a heavier rating a 600kg constant load. As the name suggests the rating is based on the load that will always be on the vehicle. In keeping with what we learned above, bigger is not always better. The spring rate choice is further complicated for caravanners because the load on the rear of the car can vary by hundreds of kilograms depending on whether the van is hitched or not. This issue is sometimes resolved by choosing a lighter-rated spring upgrade suited to non-towing loads and including airbags as part of the overall suspension system. When a caravan is going to be hitched the airbags are inflated and they serve to assist in carrying the extra load.
Airbags can be used to supplement primary springs when under full load
Shock absorber type
Many suspension manufacturers provide a choice of shock types as part of the suspension upgrade. These are not based on load ratings but trading off cost, simplicity and performance. Without getting technical, shocks can be broadly categorised into twin tube and monotube. Twin tubes are more cost-effective and robust but don’t dissipate heat as effectively which limits performance in extreme conditions. However, twin tube shocks can be further divided into nitrogen gas charged and foam cell with the latter offering improved heat dissipation. Monotube shocks cost more and have greater oil capacity for maximum heat dissipation, but they can be less reliable with an increased risk of leaking and stone damage. The most premium monotube shocks feature remote reservoirs for even more oil capacity and heat dissipation. You can also usually adjust the ride characteristics to suit road conditions and loads. Some shocks (both twin and monotube) have varying degrees of height adjustment to level the vehicle.
A GVM upgrade usually increases the ride height
Lift
The suspension kit will usually lift the car 20–50mm — the amount varies from manufacturer to manufacturer. It is also impacted by how much load is sitting on the springs. The max lift is generally 50mm, as this is the legal limit possible without specific modification approval for height. Critically, lifts beyond 50mm also start to impact on driveline and suspension geometry necessitating further modifications. Some suspension kits close to 50mm call for new upper control arms, diff drops and brake line adjustments which all add cost.
Wheels and tyres
Getting a GVM upgrade can impact your wheels and tyres. Your original tyres or possibly even your rims may not have sufficient load rating for the increased load your vehicle can now carry necessitating them to be changed. The SSM may also mandate that the tyre diameter cannot exceed the original manufacturer specification, rendering it illegal to run taller tyres. There are also rules that apply to how much you can raise a car. Given that the suspension will have lifted your car to some degree this could limit your capacity to run a taller tyre too.
Cooling
In our research, we came across SSM GVM/GCM upgrade kits that include adding a transmission cooler. So, this is another variable to take into consideration.
GVM upgrade shocks are large in diameter than original for greater oil capacity and cooling
The wrap
A GVM upgrade can greatly improve your tow vehicle setup or it may not be the right solution for you. It is important to ask all the right questions, starting with do you really need a GVM upgrade or not? If you cross that bridge, work closely with the suspension provider to consider a suitable GVM rating, front and rear axle rating, spring rating, shock absorber type, suspension lift, suitable wheels and tyres and transmission cooling. Happy travels!
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Operation 200 Part 1: Legal and Loud
Expert advice for how to maintain your tow vehicle
A guide to choosing a dual cab ute for towing
A guide to tow vehicle GVM upgrades