To ensure the safety of everyone travelling
Australia’s roads and highways, there are established guidelines for towing.
These legal requirements must be complied with to avoid consequences such as a
loss of warranty or insurance, as well as increased risk of harm to not only
you but your family and others on the road.
For the purposes of this guide, the trailer
reference will refer to caravans instead.
ATM - Aggregate Trailer Mass (total weight)
ATM refers to the total aggregate mass of any
fully-loaded caravan that is unhitched and free standing, and includes things
such as luggage, water supplies, and fuel stores. Your caravan has an ATM
stamped on its Compliance Plate. That weight limit is the upper weight that is
legally allowed by the company that manufactured your caravan. That cannot
exceed the towing ability of your tow vehicle. You can confirm your ATM by
visiting any local weigh bridge.
GTM - Gross Trailer Mass (total weight less tow ball weight)
GTM is the weight on your caravan’s axle while
loaded and attached to your vehicle – minus the weight of the car, of course.
As a general rule, the car assumes ten percent (at least) of the caravan load
weight (your ATM) while it is connected. That means that GTM is always less
than ATM. To comply with the legal requirements, your GTM cannot be higher than
the legal rating assigned to the axle brakes fitted. In the event that the GTM
exceeds that rating, then you will need larger brakes to come into compliance with
the requirements.
TARE (nett weight)
Your Tare weight is best understood as the
default weight of your caravan as it came from the factory. That is, how much
it weighed before you filled it up with water tanks luggage, food, fuel
storage, and everything else.
GVM - Gross Vehicle Mass (total vehicle weight)
GVM is a maximum weight established by the
manufacturer of your vehicle and covers the weight of only your tow vehicle,
fully loaded.
GCM – Gross Combination Mass (total vehicle and caravan weight)
The GCM is also established by the vehicle
manufacturer, but represents the combined weight of both the caravan and the
loaded towing vehicle.
TW – Tongue Weight ; or Tow Ball Weight (caravan weight on tow ball)
TW is the amount of weight that is applied to
the car’s tow ball by your car. You can determine this using a scale built for
that purpose, or simply subtract your GTM from your ATM. The tongue weight
typically amounts to ten percent or more of your ATM. If your Tongue weight is
too low, that can cause a loss of control and lead to the caravan swaying while
you are in motion. Often times, this can be corrected by redistributing the
weight inside the caravan so that everything is not located at the rear of the
van.
In short…
You need to examine
your compliance plates – on both the towing vehicle and your caravan – to
ensure that all of your numbers align with the specifications issued by the
manufacturer. An overloaded caravan can be corrected by having an authorized
repair shop upgrade your ATM and GTM by providing larger electric brakes on the
axle group to bring everything into compliance. You cannot, however, increase
the GCM, GVM, or the tow rating – unless you tow rating is already below the
maximum, in which case another tow bar can be fitted that will bring you up to
the maximum.
Your first
concern should always be the rating and specs for your towing vehicle to ensure
that you have what you need to safely tow your caravan. That can help to
eliminate the risk of accidents of the type that even the most grizzled and
experienced drivers would be unable to manage.
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