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Tare
Mass? GVM? Kerb Mass?
GCM? These terms and
acronyms can be found on
your vehicle's
compliance plates, in
your owner's manual and
many articles and
discussions about
weights, but what do
they actually mean?
They all refer to how
much of a load your
vehicle is designed to
carry or tow, which is
crucial to its safe and
efficient operation.
Therefore, it's im-
portant stuff to know.
Two terms you'll often
see in these
descriptions are 'gross'
and 'mass' but if you're
unfamiliar with them in
this context, have no
fear. Gross simply means
the entire amount of
something, in this case
weight. Mass is
different to weight in
strict scientific terms
but for ease of
description here, it
means the same thing.
All of these weights are
expressed in ei- ther kg
or tonnes.
The easiest way to
measure these important
weights is by using your
nearest public
weighbridge for a
moderate fee. They're
easy to find with a
quick search online or
through local business
directories. The design
of public weighbridges
can vary, from the
traditional single deck
with an op- erator
on-site to those
featuring multiple decks
and self-serve 24/7
kiosks with automated
credit card payment. So,
let's start at the
lightest weight and work
our way up.
Tare Mass or Weight
This is the weight of an
empty standard vehicle
with all of its fluids
(oils, coolants) but
with only 10 litres of
fuel in the tank. We
assume 10 litres was
chosen as an industry
standard to allow
otherwise empty vehicles
to be driven to and from
a weighbridge.
Kerb Mass or Weight
This is the same as Tare
Mass, but with a full
tank of fuel and without
any accessories fitted
(bull bars, tow bars,
roof racks etc). Think
of it as your standard
vehicle literally parked
at the kerb and ready
for you to get in and
drive away.
Gross Vehicle Mass (GVM)
or Weight (GVW)
This is the maximum your
vehicle can weigh when
fully loaded as speci-
fied by the
manufacturer. You will
usually find this GVM
figure on the ve-
hicle's weight placard
(generally found in the
driver's door opening)
or in the owner's
manual. So GVM is the
Kerb Mass plus all
accessories (bull bars,
roof racks, winches etc)
and payload (see below).
And if you're towing
something, GVM includes
the Tow Ball Download.
Payload
This is simply the
maximum load your
vehicle can carry as
specified by the
manufacturer. Just
deduct your vehicle's
Kerb Mass from its Gross
Vehicle Mass (GVM) and
what's left is the
amount of stuff you can
load into it. Don't
forget this includes all
passengers and their
luggage, which can
seriously bite into your
payload. For example, if
your vehicle has a 1000
kg (1.0 tonne) payload,
five big blokes will use
up about half of that
before you even start
throwing in their
luggage and a couple of
cold slabs!
Gross Vehicle Axle
Mass or Weight
It's important to know
that your vehicle's GVM
has been distributed
even- ly
This is the maximum load
that your vehicle's
front and rear axles can
carry as specified by
the manufacturer. You
will usually find these
figures in the owner's
manual. The combined
gross axle weights
usually exceed the GVM,
to provide a safety
margin. Even so, it's
important to know that
your vehicle's GVM has
been distributed evenly
for safe and efficient
op- eration.
Tare Trailer Mass or
Weight (TARE)
This is the weight of an
empty trailer. The term
'trailer' covers
everything you can tow
or 'trail' behind a
vehicle, from a
single-axle box trailer
or camper trailer to
motorcycle and jet-ski
trailers right on up to
heavy duty multi-axle
boat trailers and
caravans. If it's a
camper trailer or cara-
van, its Tare Mass
unlike a motor vehicle
does not include fluids
like wa- ter tanks, LPG
tanks, toilet systems.
Also known as Dry Weight
for obvi- ous reasons.
Gross Trailer Mass (GTM)
or Weight (GTW)
This is the maximum axle
load that your trailer
is designed to carry as
specified by its
manufacturer. It is the
combined weight of your
trailer and its payload
but does not including
the Tow Bar Download
(see separate heading).
The GTM is usually
displayed on the trailer
or in the owner's
manual.
Aggregate Trailer
Mass (ATM) or Weight (ATW)
This is the Gross
Trailer Mass (GTM) plus
the Tow Bar Download
(see separate heading).
In other words, the ATM
is the maximum towing
weight of the
trailer/caravan as
specified by the
manufacturer.
Gross Combination
Mass (GCM) or Weight (GCW)
This is the maximum
weight allowed for your
vehicle and trailer com-
bined, as specified by
the tow vehicle's
manufacturer. This is
where you have to pay
close attention to your
vehicle's GVM and your
trailer's ATM, because
those two figures
determine the GCM and
one directly affects the
other.
For example, say your
vehicle has a Kerb Mass
of 2500 kg, a GVM of
3500 kg and a GCM of
5000 kg.
At its Kerb Mass of 2500
kg the manufacturer says
it can legally tow an-
other 2500 kg, but that
towing weight decreases
in direct proportion to
how much the tow
vehicle's weight
increases. So if you
loaded up the towing
vehicle to its GVM of
3500 kg (or a payload of
1000 kg), that would
only leave a towing
capacity of 1500 kg to
meet the GCM of 5000 kg.
If the tow vehicle's GVM
dropped to 3000 kg (or a
payload of 500 kg), its
towing capacity would
increase to 2000 kg and
so on.
Tow Bar Download
(TBD)
The amount of weight on
your tow bar is crucial
to safe and efficient
tow- ing and needs to be
mentioned here. Any
quality tow bar will
have a placard or
similar showing the
maximum tow bar capacity
(kg) and maxi- mum tow
bar download (kg). Make
sure the tow bar you
choose is de- signed
specifically to suit
your vehicle and your
towing capacity
requirements.
Typically the TBD should
also be around 10-15
percent of the Gross
Trailer Mass (GTM),
which for peace of mind
can also be calculated
using the GTM and TBD
figures as shown here:
TBD divided by GTM x 100
= % of GTM.
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