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.
Related
Articles:
Hitches & Hitch Receivers
Towing equipment explained including hitches and
adjustable hitch receivers explained. Also
information on pintle hooks, tongue receivers, ball
mount shanks, hitch heads and front mounted hitch /
gooseneck receivers.
Hiring or Renting before you buy Try before you buy gives
you the advantage of discovering what best suits your needs.
This can save you considerably when you eventually buy your own
caravan / rv
Kerb,
tare and payload weights for manufactures will vary
depending on options and tray/body type.
Kerb Weight -
this is the
weight of your tow vehicle
as delivered by the manufacturer 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.
For
example: 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.
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 60mm 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
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.
ATM (Aggregate Trailer Mass) -
the total laden weight of the caravan or camper trailer
which includes the tow ball mass 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 of the caravan or trailer fully loaded
which
includes what you add as a payload (eg: water, luggage, gas)
permitted 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).
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