GVM Upgrades; how much is too much?

Weights have become a major topic in the 4WD, Caravanning and Camping world today, and for good reason. It’s almost impossible to set a 4WD up for exploring this great country without it being over weight.

Do you know what your 4WD weighs? If you don’t, its worth putting it on a weighbridge, and being prepared for a nasty shock. The weight adds up extremely fast, and a vehicle that is over weight has all sorts of nasty ramifications, including making your 4WD illegal.

Enter the world of GVM upgrades; a common modification for all different types of 4WD vehicles. These are fantastic, but one has to wonder how much is too much?

You see, every 4WD is designed to a certain level, and while many of the better quality units will go beyond their original design intention there comes a point where the likelyhood of failure is greatly increased.

In stock form, a lot of 4WD’s will take a beating. However, if you install tyres that are bigger, increase the engine power by 50% and then give them the same beating off road its almost guaranteed that you will have something break within your drive line.

Engineering is done to a certain level, and you need to think about this when modifying your 4WD.

What is a GVM Upgrade?

In short, every vehicle on the road has a maximum weight it can be, known as the GVM, or Gross Vehicle Mass.

You cannot exceed this (although many do) without having an unroadworthy vehicle, and essentially voiding your insurance, warranty and potentially putting you in a very risky legal position if someone is injured in relation to your vehicle.

A GVM upgrade is a second stage modification done by an approved 4WD shop/engineer which increases the GVM of your vehicle. For example, our Dmax GVM was 2950kg, and we’ve increased it to 3220kg by fitting an ARB Old man Emu GVM Upgrade.

In many cases (ours included) this is just a set of new springs and shock absorbers, as part of a nationally recognised kit. 

Some GVM upgrades change more than suspension, but its not normally the case. The GVM Upgrade Cost varies from a few hundred dollars for an engineers certificate to around $7000 for a full, top of the range suspension kit and GVM upgrade (or more if you are getting extended chassis/diff replacements etc).

GVM Upgrades and towing capacity

If you are towing, have a read of the post we wrote on Towing capacity and legalities; a simple guide to keep you legal. This covers the 7 things you need to ensure are right before calling your setup legal.

For the purposes of this section though, if you increase your GVM usually your towing capacity has to reduce in order to maintain the original GCM.

Axle ratings

An interesting factor that often gets overlooked with GVM upgrades is the capacity of each axle. Your vehicle manufacturer not only gives out a GVM, but they give out an axle rating too.

This is the maximum weight you can apply to each axle (front and rear), which together are usually just a bit over your GVM, to give you some wiggle room in how it is loaded.

This is to ensure that when you load your vehicle, its done evenly. For example, many dual cab Utes have around a 1000kg payload, but if you drop that in the tray all the weight is sitting on the rear axle, and you’ll be over the rear axle rating, and thus illegal. 

Some GVM upgrades also change the axle ratings, and many do not. The GVM Upgrade we have had installed doesn’t, and the sum of both OEM axle ratings added together is the new GVM upgrade.

Isuzu have left some lee way in their weight distribution options, so you can have some flexibility, as many OEM’s do. 

How much is too much?

4WD vehicles are designed for a certain job, and many will do it comfortably, and even beyond what they were designed for to a certain extent. However, if you keep piling the weight on and take a vehicle off road, something is going to break. How much weight in addition to the factory GVM is too much?

Our Dmax GVM Upgrade gives us an extra 270kg of payload. It’s a fair chunk, but its not a massive amount and it stays within the axle ratings of the OEM.

You can get GVM upgrades that give you 500kg extra, and that’s a huge amount especially on a dual cab Ute where you already have a lot of weight behind the rear axles.

It’s possible to be under GVM and over your axle ratings

Just because you have a GVM upgrade and are under it doesn’t mean you are legal; if you haven’t distributed the weight correctly you can still be illegal.

If you tow a heavy trailer, this is especially easy to do as the tow ball weight adds a lot onto the rear axle. If you have a large overhang on the rear (like a lot of dual cab utes) then the leverage applies even more on the rear axle. 

If you aren’t sure about this, have a read of the Mobile Weighing post that we wrote, which shows how our Isuzu Dmax with a GVM upgrade, a light weight tow ball weight and only a 2200kg camper trailer nearly made us illegal.

Too much weight without the right engineering means failures

With the world of social media so prominent, we see failures in the 4WD industry on a daily basis. There are certainly some interesting failures happening now though, many of which occur on vehicles with GVM upgrades.

Bent chassis are one, particularly on dual cab utes (Is your Dual cab likely to bend?), but there have been differential housings snapped in half which is a little unusual, along with lots of other strange failures.

At the end of the day, when you upgrade one component, the stress is passed onto the next weakest one in line.

You can’t keep piling the weight on and achieve the same reliability without extensive engineering changes. Driving style plays a huge role in ensuring your vehicle lasts, and even more so if you have a heavy 4WD.

In a modern world where everyone brings everything including the kitchen sink, these failures are becoming more and more common, and its not good!

Weight is the enemy

Anything you can do to keep the weight down is a good thing. It puts more stress on everything, and makes you use more fuel. If you are looking at getting a GVM upgrade, think very carefully about how far your manufacturers design is going to stretch, before something lets go!

If you have to modify your normal 4WD so much in order to take what you want, perhaps you’ve started with the wrong vehicle from the beginning. Is your 4WD suitable, or do you need a truck?.

There are some pretty neat looking 4WD’s out there today that have had well and truly more than their initial purchase price dumped into them, and in many cases they still don’t do exactly what you’d get out of the box from a truck or yank tank.

By the time you add up the GVM Upgrade cost, along with all of the other accessories you add on, you can be getting up to some pretty serious figures, and it still probably won’t be exactly what you want.

What do you reckon? Have the 4WD aftermarket engineering guys gone completely mad? Obviously there’s a market for them, but perhaps things will start to reign in now that everyone is becoming aware of what is legal and what isn’t!

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