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
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.
tare and payload weights for manufactures will vary
depending on options and tray/body type.
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.
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
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
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.
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
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
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.
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
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.
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 weightincludes 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).
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.
– this is the total permissible mass
recommended by the manufacturer
of the caravan or trailer fully loaded
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
GCM (Gross Combined Mass)
– this is the total mass of the tow vehicle and the
caravan/trailer, with everything loaded in the vehicle and
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).