Vehicle load terms
KERB WEIGHT – the vehicle’s weight when unladen. It often includes a token weight for a partially-filled fuel tank and a driver, but it’s essentially the stock weight of a 4X4.
GVM – Gross Vehicle Mass, or the most a 4X4 can weigh. It can vary through models and trim levels, but is always included on the VIN plate.
GCM – Gross Combined Mass, the maximum amount of weight of both vehicle and trailer. It should ideally be the GVM and braked tow rating added together.
ATM – Aggregate Trailer Mass. The weight of a trailer.
GTM – the Gross Trailer Mass is the ATM minus the amount of ball weight on the vehicle’s tow bar.
BALL WEIGHT – the force (in kilograms) exerted downwards on a vehicle’s tow ball by a trailer.
PAYLOAD – the difference between the GVM and the kerb weight of a vehicle -- how much weight it can carry. It includes fuel, passengers, bar work, operational equipment, winches and fridges.
AXLE LOAD – the weight the front and rear axles can handle safely. The sum of both axle ratings usually adds up to more than the GVM, giving a bit of flexibility as to where the weight is distributed on the vehicle.
BRAKED TOW RATING – the weight of a trailer (with trailer brakes fitted) the vehicle can tow.
Determining the payload
Payload and GVM are closely related. The weight the vehicle has on board when unladen – for example 10 litres of fuel, a front bar and a set of drawers and fridge in the back – the difference between that weight and the GVM is the payload. It is a measurement of how much weight the vehicle can carry.
A large SUV has a GVM from the factory of 3350kg and a curb weight of 2705kg, leaving a payload of 645kg. But bolt on a bullbar and winch (60kg), a rear bar and a second spare (70kg), a set of drawers (60kg), a 60L fridge on a slide (60kg), a rack (30kg), roof top equipment (70kg), a couple of passengers (150kg), a water tank (80kg) and a full long-range fuel tank (120kg) and that weight can be reached, and exceeded, very easily. At 700kg, or 55kg over GVM, the vehicle is illegal. And that is without a dual battery system, ball weight from a trailer, accessories or tools and recovery gear.
Being over GSM
It is easy to exceed GVM when the vehicle is loaded up for an extended trip through the outback.
Take a large SUV 4X4 with the 3.2 litre turbodiesel as a case study. It has a 3,100kg GVM, 605kg payload (with 80kg able to be carried on the roof) and a 3,000kg braked towing capacity.
Towing a loaded trailer, which comes with a 160kg ball weight, and add the factory bullbar, tow pack and a set of quality driving lights on – 50kg all up.
Now add a roof rack (20kg) to store the bulky gear up out of the way, a set of drawers (60kg) in the back to keep everything accessible and a long-range tank underneath (120kg when full).
With a driver and a couple of passengers on board (275kgs or so), the payload is at 685kg which is 80kg over the limit the vehicle is designed to carry. That is not factoring in any number of things like other operational equipment.
Weights being carted should be part of the packing regime. There are options for upgrading the GVM with aftermarket suspension, ranging from 400-600kg increases in load-carrying capacity, which will assist; however, it is absolutely imperative that the GVM is not exceeded especially if the vehicle has an upgrade. There are consequences.
Impact of towing
Many dual-cab utes are rated to tow 3500kg (braked).
Although this is an “official” rating it fails to consider GCM (the maximum weight of trailer and vehicle combined), a trailer’s Aggregate Trailer Mass (ATM) and its effect on the vehicle’s GVM. When towing, also consider the front and rear axle load ratings, or the amount of weight each axle can handle (which, when combined, usually adds up to more than the vehicle’s GVM).
Consider a ute with a GVM of 2,910kg and a GCM of 5,910kg, towing the maximum 3,500kg trailer. Subtracting the trailer weight drops the allowable GCM to 2,410kg (or the maximum the vehicle can now weigh).
Factoring in the kerb weight of 1,980kg gives 430kg remaining for occupants, bar work and equipment. When the ball weight of the trailer of around 350kg is taken off the GVM, it leaves only 139kg for the payload.
Towing when at GVM, the GCM of 5,910kg has to be reduced by the GVM of 2,910kg, leaving a 3,000kg towing ability. However the combined axle loads add up to 3,020kg (1,320kg front; 1,700kg rear), which is only 110kg higher than the GVM. As the vehicle is already at GVM, that leaves only 110kg of allowable ball weight (or a trailer weighing approximately 1,100kg). Because the tow ball (350kg) is located a metre or so behind the rear axle, the leverage changes the load on the axle (effectively making it greater) – so the reality is that even a 1,100kg trailer being towed at GVM by a ute with 3,500kg braked towing capacity is well over GCM. This adds stress to the axles, engine, brakes … and is illegal.
While these vehicles can tow that amount on paper, the driver will probably be in an illegal, dangerous vehicle, putting a huge strain on the mechanical components. These vehicles are not fit-for-purpose towing large loads regularly. Consider a vehicle with a higher GCM and GVM, and the highest torque numbers produced.