Aussies love a good trailer set-up. More and more of us are loading up with more gear, especially when we head away for a road trip, and caravans, camper trailers, boats and dirt bikes – not to mention the good old utility trailer for tip runs – just make life that much more convenient.
However, if you have little to no experience with the wide world of towing (there’s literally no requirement for you to have any before buying the biggest van you can afford), there’s a good chance you’re not aware of the rules, regulations and safe practices with lugging something along behind you.
Towing acronyms decoded: GVM, GCM and ATM without the jargon
Here are a few acronyms you’re likely to encounter:
GVM – Gross Vehicle Mass
This is the maximum weight in kilograms your vehicle is allowed to be. It includes passengers, fuel, cargo, tow-ball weight (roughly factored to be 10 per cent of trailer weight) and all modifications such as roof racks and bull bars. Found on rego papers or the compliance plate.
GCM – Gross Combined Mass
The maximum weight your vehicle-trailer combination can be. Found on the compliance plate, in the owner’s manual or on the manufacturer’s website.
ATM – Aggregate Trailer Mass
The maximum weight your trailer can be. Found on the compliance plate.
Just quickly, kerb weight is the vehicle or trailer weight with a full tank of fuel, all fluids onboard but without passengers or cargo. Tare weight is the same but with only 10 litres of fuel. For this article we’ll keep it to kerb weight as it’s the more relevant figure.
What each limit means in practice
The GVM and GCM are relative to kerb weight. If you subtract your kerb weight from your GVM you will have your payload capacity – the amount of weight you can legally carry.
However, keep in mind that additions to the kerb weight – including passengers, bull bars, winches, canopies, roof racks and tow-ball weight – will all need to be factored in before you subtract this number from your GVM.
A quick case study
| Vehicle | Ram 1500 Laramie, dual-cab, 3.0L twin-turbo straight-six petrol |
| Gross Vehicle Mass (GVM) | 3505kg |
| Kerb weight | 2642kg |
| Gross Combined Mass (GCM) | 7711kg |
| Maximum towing capacity | 4500kg ATM (70mm tow ball) |
| Payload | 863kg |
One of the more popular dedicated tow rigs, the Ram is great at towing. With its high power and torque engine and eight-speed auto, it’s basically built for purpose, right? Let’s run some numbers.
We have 863kg of payload to play with.
When we add:
That’s 1000kg of payload, much more than the allowable 863kg, and rendering our dedicated tow rig extremely illegal to operate on our roads.
It gets worse. If you are towing at the maximum ATM of 4500kg, that must be subtracted from our GCM figure of 7711kg leaving us only 3211kg of GVM to use. With the kerb weight of 2642kg removed, we now only have 569kg left, which must account for our ball weight of 450kg.
| 7711kg Gross Combined Mass (GCM) - 4500kg Aggregate Trailer Mass (ATM) = 3211kg Gross Vehicle Mass (GVM) |
| 3211kg Gross Vehicle Mass (GVM) - 2642kg kerb weight = 569kg payload |
| 569kg - 450kg ball weight = 119kg payload |
Yep, that’s just 119kg of legal payload. One of the kids is going to have to do their best Kevin McCallister impression this summer.
Common overloading mistakes
As you can see, it’s all too easy to inadvertently drive while overloaded. However, many people are also unaware of the importance of weight distribution. Check the below video for a simple demonstration.
As you can see, the 'just chuck it in the van' mentality has to change. You’d be amazed how many manufacturers put heavy water tanks (for example) in the worst possible positions.
It also raises the point that the whole 'ball weight being factored at 10 per cent of the trailer weight' is a bit of a stretch too. It has just been the industry standard for so long that we all just get lazy and use it. Every set-up is different and really should be checked before use.
The police are cracking down in a big way on overloaded vehicles, so knowing your weights and then getting them checked at a local weighbridge (tips will often allow you to weigh your set-up for a small fee) or, even better, having them checked by a dedicated caravan weight specialist, can pay for itself in saved fuel and fines.
A good first step is to simply guesstimate your weights. Take stock of everything you’re loading up and include the rough kilogram figure in your tally. Many people who do this are surprised at how much they’re lugging along for the ride.
Other stuff you should know
It’s simple, really: read your policy. Particularly the parts regarding towing. You’ll likely find that they’re pretty hard-line about your car and trailer being road-legal and won’t hesitate to void your policy if you’re overladen, have bald tyres or other defects.
Some policies also require the tow bar to be installed by a licensed fitter, so again, read up or ask the question. The last thing you need in the worst-case scenario is your insurance deciding to sit this one out.
In the past 15 years as an automotive journo and 35-plus years of inveterate car-guy antics, Dex has worked across numerous titles and has even occupied the occasional editor’s chair when nobody was watching. He spends his downtime doing engine swaps (plural) on the nature strip out front and also once ripped a handbrake spin into a perfect car park. His parents remain indifferent.
