Tech: Towing capacities explained
Get a handle on the weights and balances of your rig and tow vehicle.
Among the many different ratings, there are two that are most important when shopping for a new tow vehicle or rig: the rated towing capacity of the tow vehicle, and the ATM – or Aggregate Trailer Mass – for what you’re towing. If the ATM is the same or less than the towing capacity for the vehicle, you are on track.
The ATM can often be found in a brochure or spec sheet, but the most reliable guide is the compliance plate, where the official ATM is quoted.
Vehicle manufacturers commonly quote more than one figure for towing capacity, but it need not be confusing. Just look for the one figure that applies to you particular situation, and then match it up with the ATM.
Where a manufacturer quotes figures for braked and unbraked trailers, the unbraked figure is much lower. Unless you are planning to tow something very light that is not fitted with brakes, it usually won’t be the figure you need.
Some vehicles offer various towing packs with different capacities – a car could offer a choice of 1200, 1600 or 2400kg. Choose the one that suits your needs, and that is the figure you will need to match up with the ATM.
As for the various other ratings and capacities for safe towing, here are some explanations for the main terms found on compliance plates and spec sheets.
GLOSSARY OF TERMS
ATM (Aggregate Trailer Mass): The mass of the laden caravan when carrying the maximum load recommended by the manufacturer – including water, luggage, the lot. This includes both the mass resting on the wheels of the van, and the mass resting on the towbar of the tow vehicle. This figure must not exceed the rated towing capacity of the tow vehicle.
GTM (Gross Trailer Mass): Almost the same as the ATM, but always a lower figure. This is what rests on the rig's tyres, but does not include what rests on the towbar when coupled to a tow vehicle. Like the ATM, it includes the maximum carrying load recommended by the manufacturer. This figure must not exceed the rating for the axle group (wheels, tyres, suspension and axle) specified by the chassis manufacturer.
Tare: Effectively the "empty" weight, this is the mass of the van with all OEM equipment fitted, but with no luggage or personal effects on board, and with empty water tanks. This is what a van weighs when it leaves the production line. Like the ATM, but unlike the GTM, it combines the mass that rests on the tyres and the mass that rests on the towball.
Ball weight: The ball weight (or ball load) is the load (or force) exerted on the towbar of the tow vehicle. The ball load should always be eight to 15 per cent of the ATM. Vehicle specifications include a ball load figure, although it is not generally as easy to find as the rated towing capacity. A van should always be loaded with any heavy items near the axle, and not at the extreme ends where they will alter the ball load.
Load-carrying capacity: The total permitted load that may be
legally carried in the van – your water, luggage, food and the like.
Subtract the Tare from the ATM and you have the load-carrying capacity. Any
options or accessories not included in the Tare will obviously reduce the
load-carrying capacity available for luggage and personal effects.
For more advice, visit the tech and towing archive