Compliance Plates And Trailer Weights
In December 1988, a national regulation came into effect that limits the mass a vehicle can tow. So it is essential you give consideration to your vehicle’s towing mass and construction prior to purchasing a caravan or camper trailer or having one constructed.
For caravans there is a manufacturer’s compliance plate normally affixed inside the boot. It lists specifications that must not be exceeded. You will need to become familiar with some terminologies to understand the details.
Kerb, tare and payload weights for manufactures will vary depending on options and tray/body type.
Kerb Weight Or Kerb Mass – this is the weight of your tow vehicle as delivered by the manufacturer (with no occupants, payload or accessories) and this should be located in the owner’s manual.
The kerb weight of a caravan (also known as the Tare weight) is the weight of the caravan as it leaves the manufacturer and is usually the weight without water in the tanks or the gas bottles filled. Often manufacturers have their own definition of tare weight so it is good to check with them to be sure. The kerb weight should be on the caravan’s compliance plate.
Gross Vehicular Mass (GVM) – this is the maximum weight the vehicle is allowed to be operated. GVM is the kerb weight plus the payload. Operating a vehicle exceeding its GVM is illegal. It will void the manufacturer’s warranty and likely to void your insurance. The GVM can be found on the vehicle compliance plate normally located under the bonnet of the vehicle on the firewall.
For example: The Toyota Owner’s Manual for the Toyota Prado has a maximum permissible payload of 740 kg and a kerb weight of 2110 kg so the GVM is 2850 kg. Its maximum towing capacity is found in the owner’s manual. In this case it is allowed to tow 750 kg without brakes and 2,500 kg with trailer brakes. (Manufactures sometimes set the maximum towing capacity of a vehicle below the rated limit).
The GVM for motorhomes is specified by the base vehicle manufacturer. It is usually about 4,495kg for the small to mid-size motorhomes which is just under the limit for a Light Rigid (LR) truck licence of 4.5 tonne. (See Motorhomes article).
Road Transport and Department of Transport websites state – If the tow vehicle manufacturer has not specified the maximum towing mass then the maximum towing mass is one and a half (1.5) the unladen mass of the towing vehicle (provided the caravan/trailer is fitted with brakes which are connected and operable) or the unladen mass of the towing vehicle if the trailer does not require brakes.
Payload – the payload of a vehicle is the maximum capacity of the vehicle itself for transport including 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 less (minus), the kerb mass, the total weight of the occupants and any optional equipment (tow bar and tongue, bull bar, winch, roof racks, long range fuel tanks). The figure left is the maximum allowable weight of the cargo which can be placed upon/in the vehicle. We suggest you do a Google search above for payload calculators.
Ball Weight/Tongue Weight – Or Tow Ball Mass (TBM) is the maximum allowable weight to be placed on the tow ball. It is the difference in weight between the caravan/trailer on and off the tow vehicle The ideal weight for this is considered to be 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%. Ball Weights of up to 120 kilograms can be measured with a set of household bathroom scales by resting the trailer coupler on the scale and placing the scale on a box so that the coupler is at its normal towing height. The trailer must be fully loaded and level. For heavier ball weights, place a household scale and a brick that’s as thick as the scale three feet apart as shown in Figure 1. Set a length of pipe on each and rest a beam across the pipes. Re-zero the scale to correct for the weight of the beam and pipe. Securely block the trailer wheels. Rest the trailer jack on the beam as shown, 300mm from the brick and 600mm from the scale.
Ball Weight with Bathroom Scales
To obtain the ball weight, multiply the scale reading by three (3). For greater ball weights, place the scale and brick 1200mm apart, rest the jack on the beam 900mm from the scale and multiply the scale reading by four (4).
Too much ball weight can affect the tow vehicle’s stability, steering and braking whilst too little ball weight can cause the caravan or travel trailer to become unstable and sway.
Another way to calculate the ball load/tongue weight is the difference between the ATM and the GTM will be the ball load/tongue weight.
Weight Of Tow Vehicle – Like the trailer, the tow vehicle has a maximum weight capacity it was designed to tow. Its maximum towing capacity can be found in the owner’s manual.
Tare Mass – the unladen weight of the caravan or camper trailer when not carrying any load. See kerb weight or kerb mass above.
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 (eg: water, luggage, gas being the sum of the GTM plus the weight on the towball).
ATM is measured by placing the fully loaded trailer on a vehicle scale. The entire weight of the trailer should be supported on the scale as shown in Figure 2.
GTM (Gross Trailer Mass) – this is the total permissible mass or weight recommended by the manufacturer of the caravan or trailer fully loaded which includes what you add as a payload (eg: water, luggage, gas) that can be supported by the axles and wheels of the trailer. This does not include the mass supported by the towball. ie. the weight of the trailer fully loaded in its actual hitched up towing condition.
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.
Now, after reading all the above, your head is probably in a spin so to put all this fairly 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 loaded caravan/trailer unhitched (on it’s jockey wheel) with its wheels on the weighbridge this will be 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).