Towing weights: What do they all mean?
Before we hitch up our first caravan, most of us are blissfully unaware of the importance of weight and what it all means to a caravanner.
When you hop in your car and go somewhere, most people don’t give the car’s loaded weight a moment’s thought. Just about any car can handle a few suitcases thrown in the back for a family holiday – unless you want to take a load of bricks with you.
When you start caravanning, though, weight becomes really important, no matter what caravan you have, and you have to know what the weights are and what they mean.
Like most equipment, vehicles and caravans are assigned a weight limit they can cope with. Everything is engineered to withstand a certain weight, with a safety margin, before it fails. Manufacturers do a lot of testing to make sure you are not going to be innocently driving along and have a tyre blow out, a bearing fail or a chassis bend just because there is too much weight on the part.
Engineers and manufacturers realise that vehicle owners can inadvertently overload their vehicle and they allow a safety margin for this. Whatever the maximum weight is, it represents a safe maximum, not the breaking point of the equipment.
In fact, with tow vehicles, the maximum set towing capacity might have more to do with the cooling system’s ability to cope with heavy loads, rather than any risk of the weight of the caravan snapping the vehicle’s chassis in half.
But this doesn’t mean you can take the weights given with a grain of salt. When a vehicle or caravan manufacturer has specified weights, this becomes a legal requirement for the driver to adhere to. As soon as you wheel the rig out on the road, you’re in charge of it and it is assumed you have taken adequate steps to ensure your rig is safe and legal – including not exceeding maximum weights.
If you are randomly weighed by the state roads authorities or have a crash due to overloading, you can’t claim you didn’t know that the vehicle or van was overloaded. You’re supposed to check.
TOW VEHICLE NUMBERS
So what are the weights you need to know about? First up, tow vehicle weights are crucial to know, so you can figure out what van you can tow. Most of this information should be in the owner’s manual.
Your vehicle will have two weights that represent its maximum towing capacity, given as unbraked and braked. ‘Braked’ means what the vehicle can tow with trailer brakes used; this is the most relevant to caravanners, as the ‘unbraked’ amount is only ever up to the legal maximum of 750kg, which (except for some ultralight campers), is well under what a typical caravan weighs.
The third weight relevant to towing is the maximum towball download amount. This means the maximum allowed weight of the coupling pressing down on the vehicle’s towball. Sometimes this is about 10% of the maximum braked towing capacity.
This is the first place people can be tripped up when researching a vehicle and van combination. Not all vehicles run with 10% of total mass with their towball mass maximum; European vehicles in particular can have far less than 10% available to impose on their towball. You might find, for example, that a vehicle with a 2000kg towing maximum has only a 100kg towball download maximum, severely limiting your caravan options.
Check the towbar
The next thing to check is the towbar on the vehicle, to ensure it matches the maximum amount the vehicle can tow. There should be a placard on the towbar stating what the maximum weights are and also in a door jamb of the vehicle. Mostly, this will match the information you’ ve already got on the vehicle’s maximum towing weights, but not always. The Holden Commodore, Ford Falcon and Ford Territory are three common vehicles that offer a few different towbars with different ratings.
Check the towball
The final piece of towing equipment to check on the tow vehicle is the towball. While most are rated to 3500kg, not all are. If you have a 2500kg-rated tow ball, and you’re towing more than that, you’ll strike trouble.
Vehicles all have a maximum axle capacity, and the important one to note is the rear axle’s. Your vehicle, all loaded up with gear in the back and the caravan hitched up, cannot weigh more than the maximum axle mass specified by the manufacturer. Vehicles usually have pretty generous axle ratings, but you have to remember you’ve potentially got several hundred kilos of gear in the back plus the weight of the caravan coupling pushing down on the back axle.
Check the GCM
The next weight to note is the vehicle’s maximum gross combination mass (GCM). That means what the whole rig – vehicle and van – can weigh altogether.
Usually the figure nominated is the maximum weight the vehicle can legally weigh (that is, gross weight, which is its kerb weight plus payload) plus the maximum it can legally tow. It pays to check, though, because in some instances it is lower than this.
CARAVAN WEIGHTS EXPLAINED
Now you’re armed with your vehicle’s information you’re good to go with making some caravan choices.
There are four important caravan weights: Aggregate Trailer Mass (ATM), Gross Trailer Mass (GTM), Tare and Tow Ball Maximum. These weights should be on the caravan information placard, usually found in the front boot, on the A-frame or in the door well.
Aggregate Trailer Mass (ATM)
The ATM is an easy one – it is the most your caravan can weigh, with everything in and on it, as it stands, unhitched. It is the weight that each wheel and the jockey wheel impose on the ground all added together, at the very most.
Gross Trailer Mass (GTM)
The GTM is the maximum weight that the caravan wheels can collectively impose on the ground. In other words, the wheels’ share of the ATM figure (remembering that the ATM is the coupling download weight and the weight of the wheels imposed on the ground added together).
Tare mass is the weight of the caravan as it leaves the factory, without water in the tanks, any gas in the cylinders, any luggage or personal effects whatsoever. It is also without the weight of dealer-fitted extras, such as an awning, air conditioner and so on. If you subtract the Tare from the ATM, this leaves you with the maximum payload weight you can use to load your van.
This is another area where caravanners can and do trip up. The weight of those optional extras fitted to the van after its Tare weight has been measured at the factory can amount to hundreds of kilos and if you’re also filling two 95L water tanks (remembering each litre of water weighs 1kg) you’re suddenly reaching a point where you have little legally permitted payload left for your gear.
The final measurement is towball mass; this weight is now being quoted on caravan compliance plates, but on earlier compliance plates you can work out this figure by subtracting GTM from ATM.
Of course, all this knowledge is not especially useful if you don’t know what your rig weighs. The only way to really know is to take your rig – all loaded up, ready to tour – to a public weighbridge and have it weighed. Then you can be confident that your vehicle and van measure up when it comes to safe towing legally.
The full feature appeared in Caravan World #533, January 2014. Subscribe today for the latest caravan reviews and news every month!