What does ATM weight mean?

Believe it or not, there are multiple ways to measure the weight of a caravan or trailer. And each one will give you a different answer. But it's important to know exactly what you're measuring, because there are some serious legal and insurance ramifications to getting this wrong and trying to tow a too-heavy or poorly arranged load.

So let's tackle the ATM abbreviation, or Aggregate Trailer Mass. And what is ATM weight? It's the maximum ATM weight allowable for your fully loaded, as-towed caravan or camper or trailer or boat, unhitched and sitting on its wheels (and jockey wheel).

The ATM can be found on the manufacturer's plate (often on or near the hitch or in the manufacturer's handbook for the van). Go beyond this mass and you're in trouble. Even aside from the legal; problems, a caravan that is over its ATM may behave strangely and be dangerous to tow.

So how do you work out the actual ATM mass of your van, so you can compare it with the maximum allowable mass? Basically, you need a weighbridge (many councils have a weighbridge at their local tip) and the method involves weighing just the caravan (unhitched from the towing vehicle) with the van resting on its wheels and the jockey wheel. The van or camper-trailer also needs to be fully loaded to measure ATM weight meaning that the water tanks, gas tanks and all of the gear normally carried in the van are on board at the time. Then you take that mass and compare it with the maximum allowable caravan or trailer ATM for that particular make and model.

If your weighbridge ticket is lower than the maximum ATM specified by the manufacturer, you're good to go. If the caravan or camper trailer weight is greater than the maximum allowed, you need to lose some of the gear you were going to carry in it to stay safe and legal.

The ATM of a towed load is distinct from the GCM which is the total mass of the trailer and the car towing it, and the tare mass of the trailer which is the weight of the empty caravan or other trailer. That's distinct from kerb mass which is the same trailer, but will all manufacturer-fitted water and gas tanks full.

Neither should ATM be confused with GTM which is the Gross Trailer Mass and is the weight of the trailer once it's hitched up to the tow-vehicle. Payload is another term you'll hear and this equates to the mass of all the gear you carry in the trailer that isn't part of the trailer itself.

ATM vs GTM can be confusing, but they're really quite different measurements. Tare vs GTM is another source of confusion, but if you remember that tare weight is the mass of the empty trailer, unhitched, then you're on the right track.

The GTM is also related to Tow Ball Download (or tow ball weight) which is the amount of downward force your trailer places on the car's tow-bar. Again, there are strict manufacturer's limits for this.

If you get online, you'll find sites that offer topics such as boat weights and caravan weights explained, as well as subjects like towing weights explained. But be aware that some are better than others and they all have their basis back at the fundamentals we've just discussed. And trust us, this is not a subject on which you can be vague in your calculations.