- ATM (aggregate trailer mass)
The total mass of a trailer carrying the maximum load its manufacturer recommends, including the part of its weight carried on the towball. It's shown on the trailer's compliance plate. Weigh a loaded trailer on its own and you have its actual ATM.
- Braked towing capacity
The heaviest trailer with working brakes a vehicle's manufacturer allows it to tow.
- Breakaway system
A device that applies a trailer's brakes automatically if it separates from the tow vehicle. Required over 2,000 kg GTM, and the brakes must stay on for at least 15 minutes.
- Compliance plate
Also called VIN plate.
The permanent plate on a vehicle or trailer that shows its identification and mass ratings. For a trailer it must show the ATM, GTM and tare mass.
- Electric brakes
Trailer brakes applied by an electrical signal from a brake controller in the tow vehicle. They are the usual way to meet the rule that trailers over 2,000 kg GTM have brakes controlled from the vehicle.
- GCM (gross combination mass)
Also called GCW.
The most the loaded tow vehicle and loaded trailer may weigh together, set by the vehicle's manufacturer. Often less than GVM plus towing capacity.
- GTM (gross trailer mass)
The mass a hitched, fully loaded trailer puts on the ground through its own tyres. It leaves out the towball download, so GTM equals ATM minus towball download. Brake rules are based on GTM.
- GVM (gross vehicle mass)
Also called GVW.
The most a loaded vehicle may weigh on its own wheels, including accessories, people, cargo and, when towing, the towball download.
- Kerb weight
Also called kerb mass.
A vehicle's empty weight with all fluids and a full fuel tank, but no accessories, people or cargo. Most manufacturers publish this figure.
- Over-run brakes
Also called override brakes.
Mechanical trailer brakes applied when the trailer pushes against the coupling as the vehicle slows. Allowed only up to 2,000 kg GTM.
- Payload
How much a vehicle or trailer can carry. For a vehicle it's GVM minus kerb weight. For a caravan it's ATM minus tare.
- Safety chains
Rated chains connecting the trailer to the tow vehicle in case the coupling fails. One is required up to 2,500 kg ATM and two above that.
- Tare mass
The empty mass of a vehicle or trailer, ready for service. For a vehicle in Australia it includes only 10 litres of fuel. For a trailer it doesn't include accessories fitted after it was plated.
- Towball download
Also called towball mass, TBM, ball weight, nose weight, tongue weight.
The downward weight a trailer puts on the towball. It counts against the tow vehicle's GVM and must not exceed the vehicle's or towbar's maximum.
- Towbar rating
The maximum trailer weight and towball download a towbar is built for, stamped on the towbar. It can be lower than the vehicle's towing capacity.
- Unbraked towing capacity
The heaviest trailer without brakes a vehicle may tow. Often 750 kg, and never useful above 750 kg GTM because heavier trailers need brakes anyway.
- VSB 1
Vehicle Standards Bulletin 1, the national standard for building and modifying trailers up to 4.5 tonnes ATM, published by the federal Department of Infrastructure. Revision 6 was published in August 2026.
- Weighbridge
A large scale for weighing vehicles. Public weighbridges let you weigh your loaded rig to compare it with its limits.
Towing terms glossary
Every towing term used on this site, in plain English. Australian usage throughout.