Acronyms regarding towing and trailer weights are being thrown around more and more today, and it can be very confusing when you first start looking into it.
We actually wrote a simple towing guide that covers everything you need to know when hooking a trailer up to your vehicle, including the 7 items that will keep you legal. You can read this here – Towing Capacity.
For the purpose of this post, we are looking at what ATM weight is, how to measure it, and why its important.
ATM stands for Aggregate Trailer Mass, and its a figure that the trailer manufacturer specifies as the maximum weight that the trailer can be. Most people just refer to it as ATM, or you are saying Aggregate Trailer Mass Weight, which is tautology!
ATM is the maximum total weight of the trailer, which includes the weight that is applied to the tow ball and wheels, together. This is important to get right, as some of the weight of your trailer is on the wheels, and a portion of it is applied to the tow vehicle as tow ball weight.
When you look at a trailer, you should be able to find a nameplate on it, which will list the tare weight (empty/unloaded weight) of the trailer, and the ATM.
The difference between the tare and ATM is the amount of weight you can legally add to the trailer, and you cannot exceed the ATM without having an illegal trailer.
This can void your insurance, increase the chances of things breaking and put you in a world of pain if something goes wrong and you injure or kill someone.
It is possible to get an ATM upgrade on a lot of different trailers and caravans, especially if the manufacturer is still around, and still able to be contacted.
In some cases, you can contact them and they will simply issue you with a new nameplate that has a higher ATM. This is the case if the components are already rated higher than the existing capacity.
Alternatively, there may be some physical work that needs to be done in order to increase the ATM; changes to the suspension, chassis bracing, wheel replacements and so on and so forth.
What is GTM?
There is another figure known as GTM, which refers to the maximum weight that can be applied to the trailer wheels, and has the tow ball weight removed from it. In other words, ATM = GTM + Tow Ball Weight.
Stay under the ATM
In summary, you need to ensure that the actual weight of your trailer is under the ATM that it is rated for. This should be easy if you’ve bought the right trailer for the job, but there are quite a few that have a surprisingly low amount of payload.
This issue is amplified if any aftermarket accessories have been installed, and the unit has been sold on. Think grey water tanks, toolboxes, air conditioners, battery and solar upgrades and so on and so forth.
I have seen instances where people have purchased a second hand van and its been almost at ATM with none of their gear in it!
ATM weight is an important consideration when towing, and if you haven’t measured yours, its well and truly worth while doing so.