Everyone knows about GVM, or Gross Vehicle Mass. If you don’t, we’ve written a comprehensive guide that covers everything you need to know about 4WD’s and weights here – Towing Capacity; the ultimate guide.
If you don’t want to read that post, the simple explanation is this; GVM is the maximum amount of weight that your 4WD can weigh when you are travelling down the road, and it includes the weight imposed on the tow ball by a trailer (if you are towing).
However, did you know that its possible to be under your GVM, but still illegal because you are over your maximum axle capacities?
What are axle capacities?
Axle capacities are the maximum amount of weight that each axle is capable of carrying on your 4WD. In essence, it refers to the front axles, and rear axles, or in other words the front two wheels and rear two wheels.
For many 4WD’s, you’ll have a front axle capacity of about 1350kg, and a rear axle capacity of around 1700kg. From the factory, your 4WD will weigh less than the factory axle capacities, in its stock standard format. If you’ve added any accessories (bull bars, tow bars, long range fuel tanks, underbody protection etc) then the actual axle weights can be much higher, and easily over what the manufacturer specifies.
How do you find your axle capacities?
Your owners manual should show the axle capacities, or if you’ve had a GVM upgrade it will be with that paperwork, or stamped on the engineering plate.
Weighing your axle weights
If you have access to a weighbridge that is fairly flat on the entry and departure, you can weigh your front axle weight, then GVM and then rear axle weight fairly easy. However, the easier way to do it is to find a mobile weighing service and get them to weigh all 4 wheels individually.
Does a GVM upgrade change your axle capacities?
Some do, and some don’t. For example, the factory GVM on our Isuzu Dmax is 2950kg. With the Dmax GVM Upgrade that we had fitted it increases to 3220, but the axle capacities remain at 1350 and 1860kg. The GVM upgrade in this case is just the sum of the two axle capacities originally specified by Isuzu.
Some GVM upgrades offer a much greater increase though, and to do this they need to adjust the OEM axle ratings. They do this by taking over the responsibility for these parts, and test that they are rated to their new weight capacity.
The Pedders GVM upgrade for our Dmax for example increases the front axle capacity to and the rear axle capacity to . This totals a whopping 3600kg GVM maximum, which is substantial.
Dual cab Utes are notorious for overweight rear axles
If you have a dual cab Ute, you’ll notice that a lot of the room on the rear is behind the rear axle, and that means you have the design working against you in order to maintain a decent rear axle capacity.
Is your front axle overweight?
A lot of 4WD’s are over their front axle capacity by simply adding a bull bar and winch. From the factory, many only have a small amount of capacity left, and when you hang a bull bar off the front, it can put you over very easily. This is without considering secondary batteries, bash plates, winches, additional filtration, catch cans and wiring.
Watch the leverage from tow ball weight
I’m often baffled by the number of 4WD’s out there towing heavy caravans without any upgrades, and loaded up to the hilt. The most scary thing is that many are so far over the rear axle capacity its not funny.
Take our Dmax again, with a 132kg tow ball weight applied from the Reconn R2. Any weight applied to the tow ball so far back has great leverage, and it pushes the rear of the vehicle down and lifts the front up. In our case, 44% of the tow ball weight comes off the front axles, and is applied to the rear axles. This means that despite having a 132kg tow ball weight, we are actually applying around 190kg to the rear axle.
If you have a heavy van, with a tow ball weight of 250 – 350kg you can be applying 500kg + on the rear axle, and that’s going to put you way over the rear axle capacity long before you hit the maximum GVM.
Can your vehicle be under GVM and still over weight?
The answer is yes, you absolutely can, and yes, many people would be. Every single weight factor is individual, and you have to comply with them all, not just one, or two or three.
You need to be under GVM, axle weights, tow ball weight, towing capacity, trailer ATM, GCM and have the correct brakes for the trailer.