Your caravan’s plate lists at least three weights: Tare, ATM and GTM. Your vehicle’s owner’s manual lists more: towing capacity, tow ball limit, GVM and usually GCM. Each one measures something different, and your loaded rig has to be under every limit at the same time.
The caravan’s weights
Tare
Tare is the weight of the caravan empty, with its standard equipment and any factory options fitted. The water and grey water tanks are counted empty.
Anything added after it left the factory is not part of Tare. An awning, extra gas bottles, a TV or a new mattress all count as load.
ATM (aggregate trailer mass)
ATM is the most the caravan is allowed to weigh fully loaded, as set by the manufacturer. It is measured with the caravan unhitched and resting on its jockey wheel, so it includes the weight that sits on the tow ball when you hitch up.
Payload
Payload is ATM minus Tare. It is how much you can pack.
A caravan with a Tare of 2,300 kg and an ATM of 2,800 kg has 500 kg of payload. Water uses it up quickly because a litre weighs a kilogram. Two full 95-litre tanks add 190 kg before you have packed any food, clothes or gear.
GTM (gross trailer mass)
GTM is the most the caravan’s own wheels can carry when it is hitched and fully loaded. It leaves out the weight on the tow ball, because the vehicle carries that part. That is why a caravan’s GTM is usually lower than its ATM.
Tow ball weight
Tow ball weight is the downward load the caravan puts on the tow ball when hitched. RAC WA suggests aiming for about 10% of the caravan’s loaded weight, a little more or less where your vehicle’s limits allow.
Measure it with a tow ball scale. Too little weight on the ball can make the caravan sway. Too much puts extra load on the vehicle’s rear axle.
The vehicle’s weights
Towing capacity
The heaviest trailer your vehicle is rated to tow, from the owner’s manual. For a caravan with brakes, use the braked towing capacity.
Tow ball limit
The most weight allowed on the tow ball. Check the figure for the vehicle and the figure for the towbar and coupling. The lower one applies.
GVM (gross vehicle mass)
The most your vehicle can weigh fully loaded: passengers, fuel, gear and, once the caravan is hitched, the tow ball weight. Some passenger cars list it as maximum loaded vehicle mass (MLVM).
GCM (gross combination mass)
The most your loaded vehicle and loaded caravan can weigh together. Not every vehicle has a GCM. If yours does, it is in the owner’s manual, and you must stay under it.
The lowest limit applies
Transport for NSW gives this example. A vehicle has a towing capacity of 3,000 kg, the towbar and coupling are rated to 3,500 kg, and the caravan’s ATM is 2,500 kg. The most that combination can tow is 2,500 kg, because the lowest figure applies.
Your loaded caravan must be under all of these:
- the towbar and coupling rating
- the vehicle’s towing capacity
- the caravan’s ATM
- the rated capacity of the tyres
GCM can lower the figure again. Say your vehicle has a GCM of 6,000 kg and weighs 2,900 kg loaded. That leaves 3,100 kg for the caravan, even if the towing capacity is 3,500 kg.
How to check your own rig
Brochure weights won’t tell you what your rig weighs once it is packed. Weigh it.
- Pack the vehicle and caravan the way you travel, with water in the tanks and the gas bottles full.
- At a public weighbridge, weigh the caravan unhitched on its jockey wheel. Compare it with the ATM.
- Weigh the caravan’s wheels only, while it is hitched. Compare that with the GTM.
- Weigh the vehicle only, with the caravan hitched. Compare that with the GVM.
- Weigh the whole rig together. Compare that with the GCM.
- Measure the tow ball weight with a tow ball scale and compare it with the tow ball limit.
Some businesses bring mobile scales to you if a weighbridge isn’t handy. Weigh again whenever you add something heavy, and remember that grey water and toilet waste add weight during a trip.
Fines and insurance
Transport for NSW says overloading a caravan is unsafe and illegal. You can be fined, and it may affect your insurance.
To run your own numbers, use the tow checker for towing capacity and tow ball weight, and the GCM checker for GCM and GVM.