Why are caravan weights and masses so confusing?

Article by · January 9, 2018 ·

Caravan masses and towing weights are two of the most confusing aspects of caravanning – and it’s not hard to see why. To ensure you stay legal when towing, you really need to know a lot of data, and only some of it isn’t actually what’s advertised on your compliance plate or in a sales brochure.

Aggregate Trailer Mass (ATM), for example, is completely irrelevant to your towing limitations, as it’s not an offence to tow a trailer with a higher specified aggregate trailer mass than the maximum braked towing capacity of your car. It’s only an offence to tow a trailer that actually weighs more than the braked towing capacity of your car. For example, you can tow a trailer with an AGM of 3.5 tonnes behind an Isuzu MU-X provided the trailer physically weighs less than three tones.

Other weight listings can be just as confusing. Take the trailer’s axle group loading, for example. In some cases, the axle or axles aren’t rated high enough to carry the full aggregate mass of the caravan themselves (the Coromal Appeal AP554S is one example – its axle loading is 1718kg, while the ATM is 1939kg). The caravan is relying on a certain amount of ball weight to bridge that gap, which means that you can only legally tow the van if the ball is loaded to a certain point (in this case, 202kg at ATM). You’ve either got to sacrifice load capacity (effectively reducing the ATM) or ensure your tow car can legally have that much weight on the ball.caravan weights atm gvm gcm

The most confusing towing weights dilemma of all is the relationship between Gross Combination Mass, tow ball download, trailer mass and Gross Vehicle Mass. Here’s the scenario – you own a Toyota Fortuner which has a kerb mass of 2120kg and a load capacity of 630kg which equals a 2750kg GVM. You’ve filled it to the brim for a trip. It’s an auto, so you are towing a caravan that weighs 2800kg with 300kg of that on the ball. The Fortuner has a gross combination mass of 5545kg. Even in this situation, you’re 5kg overloaded, because although the car can legally tow 2800kg, the two cant ever weigh more than 5545kg. 2750 + 2800 = 5550.

Gross Combination Mass is the maximum specified weight that the car and its load and its trailer can weigh. In reference to caravanning, you’d make sure you were under this limit simply by adding the actual towed weight of your caravan (not the ATM – that’s largely irrelevant), the actual kerb weight of your car, and the actual weight of your load, which includes people and personal effects. Simple enough. In the above situation, if you lose 5kg from the caravan (empty the water tanks a little), you’re set.

In practice, though, the tow ball download stuffs up the simple maths. Because once you drop the tow-hitch onto the car’s tow-ball, that’s taken mass from the caravan and transferred it as load to the car. It hasn’t changed the Gross Combination Mass – that’s still the sum of the caravan’s weight, kerb weight and load weight – but you have just overloaded the Fortuner by 300kg, thanks to the ball weight.towing weights tow ball download

What you have to do now is find 300kg of savings from somewhere – either in the ball weight (you could probably afford to find some there – maybe 50kg by redistributing the load in the van, but lowering the ball weight too much will likely lead to trailer instability) or in the car. But most of it has to be from the car. Even if you take 300kg out of the caravan (thereby giving yourself a spare 300kg in the GCM), the car is still overloaded as long as the ball-weight is more than 0kg, and that just can’t happen. It’s a balancing act.

Essentially, in a vehicle where the GCM is simply an addition of the GVM and maximum braked towing capacity, you could never responsibly or legally tow a caravan of that weight while your car is loaded to its own GVM.

None of this is to say ATM isn’t important, it is! Like GVM, which is the most a vehicle is permitted to weigh while being driven, Aggregate Trailer Mass is the most a caravan, camper trailer, box-, car- or horse-trailer is able to weigh while being towed, and is a mass that’s measured irrespective of the tow ball download. You can’t load up the ball to reduce ATM and if your aggregate trailer mass is more than what your car can tow, you are effectively reducing the potential load-capacity of your caravan.

No wonder towing weights are all so confusing, huh?

If you’ve got a specific question about your own towing masses, weight distribution, limits or loads ask it in the comments below and we’ll answer, or find answers to your questions. The information will help out other caravanners confused by it all.

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About Brendan Batty

ROAM's fearless editor, Brendan's most often found searching for the next best campsite, or fixing his caravan so it will make it to just one more.

8 Comments

  • comment-avatar

    Another important weight is the Tare. There has been significant misunderstanding around the legal definition and how it is applied. It is important because Tare is used to calculate your maximum payload.

    The legal definition of Tare in VSB-1 is:
    “Tare Mass is the total mass of the trailer when not carrying any load, but when ready for service, unoccupied (if relevant) and with all fluid reservoirs (if fitted) filled to nominal capacity except for fuel, which shall be 10 litres only, and with all standard equipment and any options fitted. This includes any mass imposed onto the drawing vehicle when the combination vehicle is resting on a horizontal supporting plane. (Fluid reservoirs do not include water tanks and waste water tanks fitted to caravans).”

    The key points here are that Tare is measured at the point that the caravan/trailer is ‘ready for service’ i.e. the point of registration. Not at the factory unless it is being supplied directly from the factory to the customer.

    It must include ALL standard equipment i.e. what is advertised in the brochure.

    It must include ALL options i.e. what you included in your contract and paid for as options such as solar panels, extra batteries, awning, extra spare wheel, bike racks etc.

    For a great explanation of ALL the ratings and masses, go here:
    http://docs.wixstatic.com/ugd/74afe1_b5e04665a68442208d9142a2a77b479e.pdf

  • comment-avatar

    David

    Thank you for confirming what I already knew. ” It’s only an offence to tow a trailer that ACTUALLY weighs more than the braked towing capacity of your car”. At the caravan show today I had two salespeople swear that the ATM of the caravan, irrespective of the actual weight of the caravan as towed, could not be greater than the towing capacity of the tow vehicle. I have a 2000kg towing capacity, the caravan I want has a tare of 1740kg and an ATM of 2120kg. I argued that if I loaded the van to a total weight of 1999kg then I could legally tow the caravan. They said I could not and would not sell me a caravan. Both of these poor imbeciles missed out on a $70,000 caravan sale.

  • comment-avatar

    Ralph

    I have a Subaru 2008 tribeca with gvm 2000kg and towball weight 160kg can i tow a caravan with gvm 1600 tare1420 towball 140

    • comment-avatar

      Sean

      Yes. I think you’ll find that the Tribeca has a braked towing capacity of 2000kg and a GVM of 2586kg. Add those two together and you get your GCM of 4586kg. The van is under the cars towing/towball limits being 1600kg/140kg. This leaves you with available payload of around 400kg. Keep in mind as soon as you load the van you could potentially go over the cars towball limits.

  • comment-avatar

    noel wheeler

    I have a 2007 Navara rx manual and tow a 1999 Jayco westport van. Usually with 2 mountain bikes on the A frame and 2 15ft kayaks on the roof rack of the Navara and fridge genny and assorted junk in the back. how do I weigh it on a weighbride as a single unit? or weight car and van seperately? I have never worried before as it tows OK.

  • comment-avatar

    Tony

    I have a 2007 Coromal 618 Lifestyle Tare 1661 GTM 2003 ATM 2140 Fully loaded water & Gas Etc. Van weight unhooked 2200 Ball Weight 160 hooked up to a Toyota Landcruiser (Troopy) GVM 3035 when weighed was 2040 which ment I was 60K over on the Van but they tell me I can take the 160 ball weight off the Van giving me an extra 100k to put in the Van providing I don’t exceed the car wieght of 3035 is that correct

    • comment-avatar

      Sean

      Towing capacity of 3500kg Tony? Your Gross Combination Mass (GVM) will then be 6535kg. If you add your current weights Troopy – 2040 (I would assume your kerb weight is a lot higher??) and Coromal 2200kg your combined weight is only 4240kg. Well under.

  • comment-avatar

    Bill

    We have a ford ranger XLT that can tow 3.5 ton. We pull a Jayco Silverline with a GVM of 2889, a TARE of 2579 and a towball weight of 220kgs. When van was weighed fully loaded with gas, water, food and everything we were taking, disconnected from Ute & on jockey wheel it weighed 3006 kg. We were told we were 117kgs overweight. Should the towball weight have been taken into consideration.


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