Towing weights explained: Tare, kerb, payload and GVM weights

You’ve just bought a new vehicle with a 3000kg towing capacity, so this means you can go out and tow a three-tonne caravan, right?

Well, not necessarily. There are other factors that determine how much you can tow, such as the towbar strength and total weight of the tow vehicle. As well as understanding the different factors, you also need to master towing terminology. Let's simplify it 👇

JUMP AHEAD

• Trailer weights
• Vehicle weights
• Towing capacity
• Towball loading
• GCM and towing capacity
• Towbar rating

Trailer weights

Trailers (including caravans and horse floats) are usually assessed by their total weight, which means their unladen weight (also known as tare weight) plus the maximum payload they can carry. There are two different total weights to consider:

Aggregate trailer mass (ATM): This is the tare weight of the trailer plus its maximum payload when uncoupled from a vehicle. This is also referred to as gross trailer weight.

Gross trailer mass (GTM): This is the weight of the fully loaded trailer on its own axle, which will be less than the ATM as it excludes much of the trailer’s underpinnings. Not to be confused with gross trailer weight, which is another name for ATM.

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Vehicle weight

Kerb weight: This is the weight of a vehicle alone with a full tank of fuel. This figure doesn’t include payload including passengers, luggage and accessories such as bull bars and roof racks.

Tare weight: This is the weight of an empty vehicle with all of its fluids, but with only 10 litres of fuel in the tank.

Gross vehicle mass (GVM): The total weight of the tow vehicle – the kerb weight plus payload including passengers, luggage and accessories. Manufacturers quote a maximum GVM that the vehicle must legally remain below.

Gross combination mass (GCM): This is the maximum total weight allowed for both the vehicle and trailer. It is calculated by adding the vehicle’s GVM and the trailer’s ATM.

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Towing capacity

Most cars have an official towing capacity declared by the manufacturer.

This refers to the maximum trailer ATM a vehicle can legally pull. Towing capacity is based on things such as the vehicle’s power and torque, rear-axle strength, frame durability and even the grip of the standard tyres.

Braked towing capacity: This is the most commonly-quoted figure, and refers to the ATM of a trailer which has its own brakes.

Unbraked towing capacity: This figure refers to towing a trailer without brakes. In Australia, 750kg is the maximum unbraked towing capacity for all passenger and light commercial vehicles.

Towball loading

Also referred to as down weight, this refers to the maximum vertical weight that can be applied on the tow ball.

Often forgotten, this is an important figure, as excessive down weight on the towball affects a vehicle's ability to steer and brake.

Towball loading is a factor in determining towing capacity, but even a trailer that is lighter than the specified towing capacity can exceed towball loading if it is front-heavy, or riding at a different height to the tow vehicle.

The rule of thumb is that towball loading is around 10 per cent of the maximum towing capacity. For example, a properly weighted 3000kg caravan will exert 300kg of downward force on the towbar. Towball down weight is added to the vehicle's total GVM.

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GCM and towing capacity

Specified towing capacity can be misleading, as any weight added to the tow vehicle (including passengers, fuel and luggage) counts towards the vehicle's GCM.

For example, let’s say the maximum GCM allowable for your vehicle is 6000kg and it has a 3500kg braked towing capacity. This means your vehicle can weigh (GVM) up to 2500kg and still tow 3500kg.

That's fine if your vehicle weighs the same as it did when it rolled out of the factory, but adding accessories such as a tow bar (60kg), bullbar with winch (100kg), extra spare wheel (15kg), roof rack (40kg) and any extra camping gear (300kg) takes the GVM to 3015kg. That's before taking into account any passengers, or incorporating the towball down weight. This drops the vehicle's maximum towing capacity to 2985kg, in order to stay within the 6000kg GCM limit.

Remember that, although the towball down weight is calculated as part of the load's weight, it affects the GVM when attached to a vehicle. Therefore, when purchasing a vehicle with towing in mind it’s a good idea to have a buffer of roughly 10-20 per cent between the maximum towing capacity and your intended tow load.

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Towbar rating

A vehicle’s towing capacity is only as good as the towbar. The towbar’s maximum towing weight may be different to the one set for the vehicle.

For example, if your car has a towing capacity of 2800kg but the towbar is rated at 2500kg, you will only be allowed to tow up to 2500kg. Alternatively, if the towbar is rated at 3000kg and car’s towing capacity is 2800kg, you go with the lower number.