Why you should get your caravan rig weighed by an expert

Weight and how it is distributed is one of the most misunderstood aspects of towing. That’s a major issue because weight and its distribution has fundamental legal and safety implications.

You cannot – repeat absolutely cannot – just hitch up any old vehicle to any one old caravan or trailer, bung your family and luggage onboard and head off.

You need to be aware of limits on:

Sadly, there’s just no easy way to go about making sure you’ve got your weights right. It’s a case of studying the numbers and coming up with a mathematical solution. So, let’s work through the basics.

The tow vehicle comes with a whole heap of weight specifications you need to know about to set it up for towing.

Kerb weight: This is what it weighs when its sitting standard with a full tank of fuel and fluids but no passengers or accessories.

Gross Vehicle Mass: This is the kerb weight plus maximum payload.

Payload: This is the maximum weight in addition to kerb weight the vehicle can carry – people, luggage, accessories and so on.

Braked towing capacity: This is how much weight the vehicle can legally tow – caravan or trailer – if it’s fitted with its own braking system. Without brakes the limit is usually no higher than 750kg and sometimes a lot less, even zero.

Maximum Tow Ball Mass: The maximum weight that can be placed on the tow hitch. In many popular towing vehicles, the downball weight is 10 per cent of the maximum braked towing capacity. This figure is deducted from the payload when towing. (Don’t worry there’s a practical explanation of all this coming).

Axle weights: These are the maximum weights that can be applied to the front and rear axles of the tow vehicle. It is entirely possible to have complied on overall towing weight limits yet be overloaded on the rear axle. This is illegal and the police do test for it.

Gross Combined Mass: This is the maximum legal permissible weight of the vehicle and caravan/trailer combined, including payload (so fuel, people etcetera). This number is calculated by the vehicle manufacturer and takes into account engine and transmission performance, the strength of the suspension and the braking system capabilities.

The key things you need to know about here are:

Aggregate Trailer Mass: Abbreviated to ATM, this is the maximum weight your caravan/trailer can be prior to being towed. This includes any luggage, water in the tanks and so on. It’s the equivalent to the tow vehicle’s GVM.

Gross Trailer Mass: This the mass of the trailer minus the tow ball download (see below).

The number stated on the caravan or trailer’s VIN plate is the maximum it can weigh when attached to your vehicle. If you get pulled over and weighed by the police, this is the number they will be looking for.

Tare weight: This is the weight of the caravan as it leaves the factory before it has any fluids, accessories or luggage added.

Tow Ball Download: This is the amount of weight the caravan/trailer exerts on the tow hitch of the vehicle. This is a variable weight with no legal clarity, unlike the car’s tow ball limit.

OK, we’ve chucked a bunch of definitions at you here, which has likely prompted some head scratching. But here’s the good news, we’re going to run through a practical example to sort this all out with the help of Richard Jarvie, the proprietor of ‘Pull Your Weight’.

For a reasonable fee, Richard, his scales and his computer software will come to you and test out whether your load is legal or not – in terms of both weight and its distribution. Richard performs the same testing as the police or insurance companies would do if you’re pulled over or involved in an accident.

“At least 80 per cent of caravans are over-weight,” says Richard. “The most overloaded I have had over my scales is 1.2 tonne.”

“That means more chance of having mechanical issues and if you have an accident your insurance won’t cover you.

“If you get pulled over by the police for a random check and you’re overweight it’s a tow truck job. You’re not going anywhere.”

Richard is worth listening to. Not only is he a long-time caravanner, but he’s also in the employment of an emergency service. He knows the rules.

So, let’s do our sums.

Our tow vehicle is a Mitsubishi GSR Special Edition, valued beyond $60,000 plus on-road costs. This is a classic Aussie tow vehicle, being a dual cab 4x4 ute powered by a turbo-diesel engine.

The key numbers are:

Our caravan is a JB Caravans Gator X EV. This is a two-berth, single axle off-road van, which measures up at 6.9 metres long and costs around $130,000. It’s about as big as caravans get before they swap to dual axles.

Okay, so let’s take this step by step.

First off, we must make sure the Triton can legally tow the Gator X. Its braked towing capacity is 3500kg and the caravan weighs a maximum 3000kg. Tick.

The next step, with Richard’s scales, is to establish how much our vehicles actually do weigh, so we can dial in the specific numbers for our rig. With all four of its wheels on the scales and the caravan attached, the real-world GVM (total weight) of our Triton spits out at 2556kg including down ball weight. Two scales under the wheels of the Gator X and its GTM is revealed as 2458kg. That’s more than 300kg under its limit.

“If you get pulled over by the police they will look for GTM, then they will do the GVM and finally the combination,” explains Richard. “Then they will do axle load.”

The combined mass is 5014kg, so we are 1236kg under the 6250kg GCM Mitsubishi specifies for the Triton. Plenty of room to work with. Deduct the 2556kg real-word GVM from the Triton’s 3200kg manufacturer maximum GVM and our vehicle payload comes out at a very useable 644kg.

FYI, at maximum 3500kg towing with a 350kg tow ball mass the Triton can carry a payload of 584kg. The bigger the van the less weight you can carry.

Richard’s next job is axle weights and with the help of the scales, he establishes both the front and back are well under their limits. If there is going to be a problem, usually it’s the rear axle because it absorbs the tow ball weight and luggage that is stacked into the load box.

It can get a bit tricky because the rear axle also absorbs weight transfer from the front axle to the rear axle when applying weight to the tow ball. It adds approximately 1.5kg to every 1kg of down ball weight. It can be even more when what’s called an ‘extended hitch’ is used.

The one concern Richard has is there isn’t enough weight actually going through the hitch on our set-up.

The golden rule is a tow ball weight that’s 8-12 per cent of ATM. In our case it’s 8.72 per cent and Richard wants it heavier. From his extensive experience, a 30-50mm downward angle on the draw bar helps with caravan stability.

His recommendation is a 20-litre jerrycan full of water placed in the draw bar storage box. That increases the ball weight to 9.4 per cent. You can also experiment with load adjustment in the tow and towed vehicles.

“Anything below 8.5 per cent and there’s a chance of sway,” explains Richard. “When you get up over 11-12 per cent there’s a chance of porpoising, so it’s the other way.

“Ten per cent is ideal.”

Calculations all done it’s a thumbs up from Richard and we’re ready to start our caravanning adventure legally and safely.

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