RV FEATURE
How much do you really know about Gross Trailer
Mass? Of the four weights – ball weight, Tare,
and Aggregate Trailer Mass being the other three
– GTM is arguably the least understood.
According to VSB1 – Technical Requirements,
GTM “is the mass transmitted to the ground
by the tyres of the trailer when coupled to a
towing vehicle and carrying the maximum load
recommended by the manufacturer or importer,
approximately uniformly distributed over the load
bearing area”.
Put simply, in the case of a pig trailer such as
a caravan, it is the maximum amount of weight
that can be supported by the trailer's axle or axle
group. It is an often overlooked rating and one
that, if exceeded, can have serious repercussions.
DYNAMIC WEIGHT
It is important to realise that the weight resting
over the trailer’s axles is not a static weight. It
will vary depending on the attitude of the trailer
when it is connected to the tow vehicle.
If the trailer is sitting dead level when unhitched,
the centre of gravity will be in a position above the
floor of the trailer. Because a caravan is a very tall
trailer, with a considerable amount of weight up
top, the centre of gravity will be quite high.
When hitched to the tow vehicle, if the trailer
sits nose down, the centre of gravity will shift
forward, placing more weight on the towball and
less weight over the axles. If the trailer sits nose
high, the centre of gravity shifts towards the
rear of the trailer, decreasing towball weight and
increasing weight over the axles.
Ideally, for the vast majority of caravans
manufactured in Australia, you want to have
between eight and 10 per cent of the trailer's
total weight resting on the towball. If you have
an adjustable-height tow hitch, the trailer should
sit very slightly nose down. When this is set
correctly, you can move your cargo around to
ensure correct weight distribution.
EFFECTS OF A WDH
A Weight Distribution Hitch will also have an
effect on the weight over the trailer's axles. As
the name suggests, these are essentially springs
that counter the downwards movement of the
rear of the tow vehicle when the trailer is hitched
up. By doing so, they redistribute some of the
weight back towards the rear of the trailer and
over its axles, as well as over the front wheels of
the tow vehicle.
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