Understanding permissible maximum combination mass

Overloaded goods vehicles cause considerable damage to road infrastructure – but understanding how permissible maximum combination mass is determined will help truck operators to stay within the law. VIC OLIVER and NICK PORÉE explain.

Confusion and a lack of understanding regarding the permissible maximum combination mass rating of a truck tractor are common in the trucking industry since this information is not displayed on a vehicle’s data plate.

The information cannot be stamped on the data plate; it is determined by various calculations based on a number of regulatory factors, including:

• Maximum permissible mass on a single steering axle is
7,700kg

• Maximum permissible mass of a tandem (two-axle) bogie is 18,000kg

• Maximum permissible mass of a tridem (three-axle) bogie is 24,000kg

• Gross combination mass (GCM) of the truck tractor’s manufacturer

• The sum of the legal axle mass loads (e.g., 7,700 + 18,000 =
25,700kg)

• Power-to-mass ratio (240kg of Gross Vehicle Mass (GVM)/GCM per kW of engine power)

• The Bridge Formula: (2,100 x L) + 18,000kg

• Traction ratio: 20% of the GVM/GCM on the drive axles

• Maximum vehicle combination length is 22m

• Steering ratio (11% of GVM/GCM on the steering axle)

Truck manufacturers determine the vehicle’s GVM and GCM rating and stamp it onto the data plate, which is usually fixed to the truck tractor’s left-hand door. No vehicles or combinations should be operated if GCM limits are exceeded.

The sum of legal axle mass loads is determined by adding the mass load on each axle in the combination and recording the total. The regulation is designed to ensure that neither individual axle legal weight limits nor the gross combination limit are exceeded.

The regulation pertaining to the traction ratio is designed to ensure that the drive axle or axles have sufficient traction on the road surface to eliminate wheel slippage. The regulation limits the maximum permissible combination mass of a 4×2 truck tractor to 45,000kg.

The road traffic regulation states that the truck engine must deliver a minimum of one kilowatt for every 240kg of GVM/GCM. To determine the maximum combination mass, multiply the truck tractor’s kilowatt rating by 240. This regulation is there to ensure that the vehicle has sufficient power to maintain a road speed that does not impede other traffic.

The Bridge Formula – a mathematical equation – is designed to protect bridges and structures by ensuring that the axle spread of a vehicle or combination of vehicles is sufficient to eliminate point loading.

The regulation states that on a public road no person is permitted to operate a vehicle – or combination of vehicles – the wheels of which are fitted with pneumatic tyres, if the total mass load of any group of axles exceeds the mass (in kilograms) determined by multiplying the distance in metres between axles (or axle groups) by 2,100 and adding 18,000.

For example, a truck tractor coupled to a set of standard interlink trailers would have a distance of about 19m between its front axle and the rear axle of the back trailer. Therefore (19 x 2,100) + 18,000 = 57,900kg. The maximum permissible GCM is 56,000kg.