Understanding permissible maximum combination mass

Overloaded goods vehicles do considerable damage to road infrastructure – but understanding how permissible maximum combination mass is determined will help truck operators to stay within the law. VIC OLIVIER explains

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

The information cannot be stamped on the data plate as it is determined by the composition of the combination and by a number of regulatory factors, including:

• The gross combination mass (GCM) of the truck tractor’s manufacturer;

• The sum of the axle mass loads;

• The power to mass ratio;

• The Bridge Formula;

• The traction ratio of five times the mass on the drive axles.

Truck manufacturers determine the vehicle’s GCM rating and stamp it onto the data plate, which is usually fixed to the truck tractor’s left-hand door. No vehicle should be operated if GCM limits have been breached.

The sum of 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 each
axle’s legal weight limits are not exceeded.

The road traffic regulation states that for every 240 kg of weight the truck must deliver a minimum of one kilowatt. To determine the maximum combination mass, multiply the truck tractor’s kilowatt rating by 240. The regulation is there to ensure that the vehicle has sufficient power to safely handle the load.

The Bridge Formula – a mathematical equation – is designed to protect the integrity of bridges and road surfaces 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 dimensions of such a group by 2 100 and adding 18 000.

To determine the maximum mass load on any axle group in the combination, measure the distance between each axle and calculate the permissible load allowance by multiplying the distance in metres by 2 100 and adding 18 000. The maximum permitted is 56 000 kg.

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

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 45 000 kg.