Car and Trailer Weights – The Words (and Letters) We Use
GVM and ATM
2a. Gross Vehicle Mass GVM
In my previous article, we explored what Tare Mass was and how you may never see it (for your car or trailer) again after it leaves the factory – if not before.
Gross Vehicle Mass (GVM) is that Tare Mass PLUS whatever things you put in your car.
It’s important to note that GVM relates to the car – not the trailer.
So, the minute you put more than 10 litres of fuel in the tank; when you get in the car yourself; when you put seat covers on; or later when you put passengers in the car; that box of tissues and the shopping you have an Actual GVM.
The Actual GVM of your car
The Actual GVM of your car will change in some way with each trip you take. But, its the holiday travel that really weighs you down (pun intended).
I’m sure that there is a three-year study grant in this somewhere… It would be great to plot the travel loads of a family as they go through their life – there’d have to be a spike somewhere where the kids are in their early teens when we all pack a couple of bikes, a skateboard, some surfboards, clothes, towels, chairs, the dog, food, plus a fridge etc.
How the maximum GVM comes into play
This is where that all pervasive Maximum GVM comes into play. This is the magical number that all manufacturers have to apply to their cars as determined by the tyres, suspension, brakes, axles, differential, gear box, engine, body and chassis.
Do you know what your car’s Maximum GVM is? It’ll be in your manual or technical data on the internet. If your in a position to load up your car, you really ought to know this number for your car.
What is your car’s Actual GVM?
Do you know your car’s Actual GVM? This is the weight that your car weighs right now in the driveway. Do you know how much you push that weight out to when you take a trip away? This number can be found by going to your council tip, recycling yard, public weighbridge or by a mobile weigh station in your area.
Maximum GVM
Remember that the Maximum GVM is just that – the maximum. The Police, transport officials and your insurance company will be interested to know these numbers in the even that something unfortunate may happen.
There’s some homework for you.
2b. Aggregate Trailer Mass
Just as your car has a Gross Vehicle Mass, your trailer will have an Aggregate Trailer Mass (ATM).
Interestingly, in common use ‘aggregate’ simply means the combined amount of things.
For some reason, the nomenclature in the trailer industry and associated legislation has come to define the ‘Aggregate’ Trailer Mass as the “… maximum load recommended by the manufacturer” (emphasis added) (source:)
But we need a name for that weight of the trailer – that weight where once you put into, or on, it that cutlery set you bought to christen the new van, the solar panels, the second battery, that roll-out awning and the steel fire pit you bought for a spot of free-camping. Everything…!
So, I’ll call that our ‘Actual’ ATM.
Think of the whole trailer suspended from a set of fish scales – that’s your Actual ATM. Part of your trailer’s ATM (the Tow Ball Mass) will be included in the car’s GVM when it is hitched (refer to my next article on Tow Ball Mass).”
Actual ATM can change with each trip
Again, this Actual ATM can change with each trip and most likely gets higher on the return leg as you bring all those shiny rocks home to edge your garden beds. Here’s a reminder to sit down and watch Lucile Ball in The Long, Long Trailer (1953).
Trailer manufacturers are required by the Road Rules here in Australia to stamp or etch the Maximum ATM onto the trailer’s compliance plate (located on the drawbar, in one of the cupboards or near the doorway). This figure has been derived by engineers looking again at the tyres, suspension, brakes, axles, chassis etc and ought not be exceeded.
What do we need to question?
The same questions still apply… Do you know your trailer’s Maximum ATM? Do you know your trailer’s Actual ATM when you hit the roads? You need to.
More homework
It’s somewhat funny to note that trailer manufactures / resellers often advertise the Maximum ATM of their caravan and espouse how it that will suit, or not suit, your tow vehicle. Then, many of us go off and fill up the cupboards, spaces and tanks with all our travel ‘stuff’ (perhaps) never thinking how the Actual ATM is comparing to the Maximum ATM.
Perhaps this is seemingly an unintended legacy of simply referring to the maximum as the ATM.
Perhaps too this would be a great place to start an awareness program on loads. Then the discussion around the campfire may go something like this…
“Hi Jamie. Nice rig you got there. 19 foot hey…? What’s your Actual ATM?”.
Another great watch is George Carlin’s standup routine on ‘Stuff’ –
When it all becomes too hard….take a look at our very own app LoadGood
