Weight, Masses and Payloads

There are three important figures that you need to be aware of when towing a caravan, the Tare Weight, the Aggregated Trailer Mass (ATM) and the Gross Trailer Mass (GTM). These figures will differ dependent on the caravan manufactures and upon options and tray/body type.

caravan weights

Tare Weight or Tare Mass – this is the net weight of your caravan as delivered from the manufacturer (with no occupants, payload or after-market accessories), usually without water in the tanks or the gas bottles filled. It should be listed in the owner’s manual or is located on the compliance plate for the van.

Gross Vehicular Mass (GVM) – this is the maximum permissible weight of the vehicle in operation. GVM is the tare weight plus the payload (water, gas, equipment, provisions, etc). Operating a vehicle exceeding its GVM is illegal. It will void the manufacturer’s warranty and likely the insurance as well. For example: The Toyota Owner’s Manual for the Toyota Prado lists a maximum payload of 740 kg and a kerb weight of 2110 kg so therefore the GVM is 2850 kg (tare + payload). Its maximum towing capacity is found in the owner’s manual. In this case it is permitted to tow 750 kg without brakes and 2,500 kg with trailer brakes. (For safety, manufactures often list the maximum towing capacity of a vehicle as below the rated limit).

The GVM for motor homes is specified by the base vehicle manufacturer. It is usually around 4,495kg for small to mid-sized motor homes which is just under the limit for a Light Rigid (LR) truck licence of 4.5 tonne.

The Road Transport and Department of Transport websites state that if the tow vehicle manufacturer has not specified the maximum towing mass then the maximum towing mass will be one and a half (1.5) times the unladen mass of the towing vehicle (provided the caravan/trailer is fitted with brakes which are connected and operable) or the same as the unladen mass of the towing vehicle if the trailer does not require brakes.

Payload – the payload of a vehicle is the vehicle’s maximum load capacity, excluding the vehicle’s own mass, while in transit. That is, the maximum allowable combined weight of the occupants, cargo, and optional equipment as well as any accessories such as bull bar, tow bars, winches, and roof racks. The maximum available payload is the vehicle’s GVM minus the kerb mass. The figure remaining is the maximum allowable weight of the cargo which can be placed upon/in the vehicle. There are payload-calculators available for use online.ball

Ball Weight/Tongue Weight – or Tow Ball Mass (TBM) – is the maximum permitted weight to be placed on the tow ball. It is the difference in weight between the caravan/trailer on the tow vehicle and off. The ideal for this is considered around 10% of the weight of a fully-laden caravan or camper trailer. Care should be taken to check with some imported caravans as they may be considerably less than 10%. Too much ball weight can affect the tow vehicle’s stability, steering and braking, whilst too little ball weight can cause the caravan or trailer to become unstable and sway.

One way to calculate the ball load/tongue weight is the difference between the ATM and the GTM.

ATM (Aggregate Trailer Mass) – is the total mass of the caravan or camper trailer when carrying the maximum load recommended by the manufacturer. This weight includes the tow ball or pin mass imposed on the tow hitch plus whatever you add as a payload (e.g. water, luggage, gas being the sum of the GTM plus the weight on the tow ball).

ATM is measured by placing the fully loaded trailer on a vehicle scale.

GCM (Gross Combined Mass) – this is the total mass of the tow vehicle and the caravan/trailer, with everything loaded in the vehicle and van/trailer.

After reading all of this you may feel slightly overwhelmed, so let’s put it simply:

If you are on a weighbridge with your tow vehicle hitched-up to the caravan/trailer and are all loaded up this will give you the Gross Combined Mass (GCM).

If you have the fully loaded caravan/trailer unhitched (on its jockey-wheel) with its wheels on the weighbridge this will give you the Aggregate Trailer Mass (ATM).

With the van/trailer hitched to the tow vehicle with only the van/trailer’s wheels on the weighbridge this will give you the Gross Trailer Mass (GTM).

The difference between the ATM and GTM will give you the Tow Ball Mass (TBM).

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