Every day there are arguments, questions and lengthy discussions about what is legal when it comes to towing with a 4WD, and every guide I've ever read has not made things overly simple or clear.
There was an educational exercise done in the Eastern States some time ago, where a heap of vehicles towing caravans and other trailers were weighed in various ways by the transport authority and police. Majority of them were not legal. Only one fine was handed out with the intention of it being an educational exercise, but it proves the fact that people don't understand what needs to be done to be compliant.
So, they jump online, and are met with a barrage of acronyms like GTM, ATM, GVM, GCM, TBM etc etc and soon get lost. Even those who have put a lot of effort into trying to understand it all still get confused; there's a lot of poorly written (and sometimes wrong) information out there.
I'm going to make it really simple. You need to meet 7 things in order to be legal
For the purposes of making this article relevant to you, head over to Redbook, select your vehicle and write down the figures referred to in the below points. Alternatively, use your owners manual to find the information or give your vehicle manufacturer a call with your VIN number handy.
Be under the maximum total weight (vehicle and trailer together)
When your 4WD is attached to your trailer, the total weight of your setup moving down the road must not exceed the Gross Combination Mass (GCM). Every vehicle has a GCM, which you can find in your owners manual, Redbook or by ringing the vehicle manufacturer.
As an example, our Isuzu Dmax has a GCM of 5950kg. As long as our trailer and vehicle do not weigh more than 5950kg we've passed the first test.
It's simply the total weight of your trailer unhitched, and the total weight of your vehicle unhitched, added together.
Your tow ball weight is not considered in the GCM; its a separate concept and anyone who tells you otherwise is full of it.
Be under the vehicles maximum weight
Your vehicle has a maximum weight, which is called the Gross Vehicle Mass (GVM). This is the maximum weight of your vehicle with everything in/on it and includes the tow ball weight.
If we take a 200 series Land Cruiser, you'll see they have a GVM of 3350kg. The empty vehicle weighs 2740kg, which leaves you with 610kg of weight that can be added. Your payload includes everything that weighs the vehicle down; passengers, drawer systems, bull bars, water, food and the weight your trailer applies on the towball.
This is the primary way in which people are overloaded. Normal 4WD's don't have the greatest payloads to begin with, and when you add a heap of accessories and gear on board along with a trailers towball weight, you'll go over the GVM very quickly!
Add 2 passengers, a bull bar, winch, tow bar, fridge and 250kg of towball weight (10% of a 2.5 tonne trailer) and a 600kg payload is pretty much gone already.
Be under the vehicles rated axle capacities
Balancing the load on your 4WD is extremely important. If your vehicle has a payload of 800kg, its assumed that the load is evenly balanced throughout the vehicle. If you put 800kg over the rear axles (or even behind!) you will exceed the maximum weight that your vehicle is rated to carry on the rear axle.
Every vehicle has a maximum weight rating for the front and rear axles. Don't make the mistake of loading up the rear of your vehicle excessively as you risk chassis damage, insurance companies walking away from claims and a very unstable/unsafe vehicle to drive. This is how dual cab utes so often end up with bent chassis.
Be under the vehicles rated towing capacity
Every vehicle comes with a maximum towing capacity. If its 3000kg, you cannot tow a trailer that weighs more than 3000kg. Simple. Point to mention; you can tow a trailer that is rated heavier than the the tow capacity, so long as you don't load it up beyond your tow capacity.
Another thing I will mention is to make sure that the tow bar and tow tongue is also rated to the same (or higher) rating and is suited and attached correctly as per the vehicle manufacturers recommendations. If they have a lower towing capacity than that of your vehicle, that is your limit.
Be under the vehicles rated maximum tow ball weight
Your vehicle will also come with a maximum tow ball weight. This can vary based on the model of the vehicle and the tow bar that you are using, but you cannot exceed the maximum tow ball weight.
In Australia, the guide or general rule of thumb is to have 6 - 10% of the trailers loaded mass on the tow ball; if you have a 3000kg trailer you should have 180 - 300kg on the tow ball. However, make sure your vehicle is rated to take 300kg of down force on the tow ball.
The Mitsubishi Pajero for example, is only rated for 180kg on the towball. Many trailers have obscenely heavy tow ball weights, especially forward and rear for camper trailers.
Be under the trailers maximum weight
Just like your vehicle, your trailer can only weigh a certain amount when fully loaded. The Aggregate Trailer Mass (ATM) is the maximum amount your trailer can weigh when not hooked up to your 4WD. This, and other figures can be found on the nameplate, usually on the draw bar of your trailer. Remove the weight of the empty trailer and you have the amount of weight you can add. For example, our Camper Trailer has an empty weight of 540kg and an ATM of 1200kg, giving it a 'payload' of 660kg.
However, don't just go off the nameplate on your trailer, as there's a pretty high chance it won't be accurate. Some nameplates can be out by a huge amount when the trailer is empty, and people get caught out all the time. This happens because manufacturers aren't always honest, and because of different accessories fitted to each trailer.
Have the right brakes
Every trailer that weighs over 750kg should have brakes fitted. Some are mechanical and operate with the trailer hitch moving in and out. Some are hydraulic, and most camper trailers and caravans these days are electric.
If the brakes are electric, you need a tow unit in your vehicle, like the Redarc Towpro. You cannot legally tow a trailer over 750kg with electric brakes if you do not have a working module installed in your 4WD.
Go and visit a weighbridge
If you don't know what your setup weighs, head to the local weighbridge and check it out. Be prepared for a shock; I guarantee you'll be surprised at the result.
These things have a way of being made more complex than they really are. Take your time to understand them, see a weighbridge and you'll sleep tight knowing you aren't going to have issues with the law and insurance.
Why does any of this matter?
If you've gotten to the bottom of this post, and you still have no idea why its important to be aware of these things, I'll make it short and sweet.
Firstly, its a legal requirement to meet the above criteria. If you don't meet it, you are breaking the law. Beyond that, if your insurance company deems an accident you have was contributed by being over weight, they can legally walk away or reduce your claim. Not good. In fact, seriously not good. Rear end a nice Ferrari or sports car and you could be in a world of financial pain for the rest of your life.
Lastly, ratings are given from an engineering perspective as that is what they are designed to do, and if you push beyond that limit the chances of something going drastically wrong increase substantially.
What if you are over weight?
If you do visit a weighbridge, and find out that you have broken one of the 7 things above, there's a few things that you can do. The first, and most obvious is to ditch as much weight as possible. This may be from your trailer, or from your 4WD. Start with emptying your setup, and re-packing the things that you really need. Remember that a huge amount of weight usually comes from accessories - second tyres, rear bars, bull bars, winches, extra fuel tanks etc all add up, very fast.
If your tow ball weight is excessive, shifting weight around can help dramatically, just do it sensibly.
If you are still over weight, and there is nothing you can do about it, you can look at getting an engineering certificate to carry more weight. This applies to trailers (see your trailer manufacturer first) and 4WD's. A number of shops sell off the shelf kits for vehicle GVM (and sometimes GCM) upgrades.
Independent engineers can certify a myriad of different things too, and are worth consulting.
We found our Dmax was going to be over weight at the end of the build, and approached ARB who fitted an Old man emu GVM upgrade. If you want to know more about it, check the post out - ARB GVM Upgrade.
Do you know what you weigh, and that you are legal?