Weights, towball mass and towing limits

By the end of this topic you can

• You can explain tare, ATM, GTM, GVM and GCM in plain words.
• You can find the towing capacity of a vehicle and the rating of a towbar.
• You can work out a sensible towball mass for a loaded trailer.
• You can check that a combination stays within all of its limits.
• You can describe the main width, length and projection limits for a trailer load.

What you need to know and understand

• Tare is the empty mass of the trailer or caravan.
• ATM (aggregate trailer mass) is the total mass of the loaded trailer when it is not hitched, including what would rest on the towball.
• GTM (gross trailer mass) is the mass carried by the trailer's own axles when hitched, so it does not include the towball mass.
• GVM is the most the towing vehicle can weigh fully loaded, and GCM is the most the vehicle and trailer can weigh together.
• The loaded trailer must not exceed the lowest of the towbar rating, the vehicle's towing capacity, the trailer's own rating and the tyre ratings.
• In NSW, if the vehicle maker gives no towing limit, the trailer may weigh up to 1.5 times the towing vehicle's unladen mass if the trailer has working brakes, or up to the vehicle's unladen mass if no brakes are needed.
• NSW guidance is to carry about 5 to 10 percent of the loaded trailer's mass on the towball.
• In NSW, a towing combination can be no more than 19 metres long including the load, and a trailer load can be no more than 2.5 metres wide.

Reading the plates and labels

Look in three places. The vehicle's owner's manual or compliance information gives its GVM, GCM, maximum towing capacity and maximum towball mass. The towbar has a label showing its rating, and the lower of the towbar and vehicle limits applies. The trailer or caravan has a plate showing its tare and ATM.

Each limit is separate. You can be under the vehicle's towing capacity but still over the GCM if the car is loaded with people, water and camping gear.

Getting the towball mass right

Too little weight on the towball makes a trailer prone to sway. Too much overloads the rear of the car and lightens the front wheels, which affects steering and braking. Load the trailer so roughly 60 percent of the weight sits in front of the axle centre, keep heavy items low, and aim for a drawbar that is level or slightly nose down when hitched.

A simple weight check

1. Write down the vehicle's GVM, GCM, towing capacity and towball limit, and the towbar rating.
2. Note the trailer's ATM from its plate.
3. Load up as you would for the trip, including water tanks and gas bottles.
4. Visit a public weighbridge and weigh the car, the trailer and the combination.
5. Compare each figure with its limit and move or remove gear if anything is over.

Size and load limits

• In NSW, a load may stick out no more than 150 mm beyond the side of the trailer, and the total width must not exceed 2.5 metres.
• In NSW, a load that sticks out more than 1.2 metres behind the trailer must carry a red flag at least 300 mm square.
• At night or in poor light, a projecting load needs a red light or red reflectors on its end.
• Other states have similar limits, but check your state handbook for exact figures.

Common mistakes

• Using the trailer's tare instead of its loaded weight. Always plan around the real loaded mass.
• Forgetting that passengers and gear in the car count towards GVM and GCM.
• Loading heavy gear at the very back of the caravan, which reduces towball mass and increases sway.
• Trusting the advertised towing capacity without checking the towbar rating.
• Filling water tanks after weighing. Weigh the combination as you will actually drive it.

Test yourself: Weights, towball mass and towing limits

Answer each question, then read the explanation. Questions come in a random order each time.