Unpacked: How to calculate payload on a flat deck link

A FOCUS reader recently posed an interesting question: “What is the maximum legal load on a flat deck link pulled by a 6×2 single drive axle truck tractor?” While this might seem like a straightforward query, the answer is far more complex than it appears. KEVIN VAN DER MERWE, executive manager of certification and operations at the Road Freight Association (RFA), provides a detailed explanation.

To accurately determine the maximum payload, we must refer to the National Road Traffic Act 93 of 1996. The question pertains specifically to a single drive axle 6×2 truck tractor combined with a link trailer, focusing on the legal limits of the load that can be carried on this combination.

Understanding payload calculation

Calculating payload involves knowing two key masses: the combination’s permissible maximum mass and its unladen mass. While this may sound simple, several technical factors must be considered when determining these values.

Let’s start by understanding the unladen mass of the combination. Often, operators make the mistake of simply adding the known tare masses of the vehicles in the combination (truck tractor plus trailers). However, tare mass only represents the vehicle’s weight as it comes off the production line (this excludes the weight of fuel, the driver, additional staff, tools, spare wheels, personal belongings, ropes, chains, and other non-standard equipment). The actual unladen mass can therefore differ by several tonnes from the tare mass. To get an accurate unladen mass, all these additional items, including fuel and staff, must be accounted for before the vehicle departs the depot.

Determining the permissible maximum mass

Next, we determine the permissible maximum mass of the vehicle combination, referred to as “V.” This involves considering six factors, with the smallest value among them being the final permissible mass:

1. Sum of Permissible Maximum Axle/Unit Mass Loads: Each axle or axle unit in the combination must be evaluated based on three factors:

• The axle manufacturer’s rating on the manufacturer’s plate (GA/GAU).
• The tyre manufacturer’s rating for each tyre, expressed in kilograms, pounds, or load index.
• The legal road carrying capacity for each axle/unit configuration.

After considering these factors, the smallest value for each axle/unit is taken as the Permissible Maximum Axle Mass (A). The sum of these values across all axles/units gives the maximum axle mass load for the entire combination.

2. Gross Combination Mass (GCM): This is the maximum mass allowed for the combination, including the payload, as specified on the truck tractor’s manufacturer’s plate.
3. Engine Power (P/D): The engine power, expressed in kilowatts (kW) on the truck tractor’s plate, is multiplied by 240 to determine the maximum mass in kilograms that the truck tractor can move, including its load.
4. National Road Traffic Regulations Limit: South African regulations cap the maximum combination mass (including load) at 56,000kg on public roads.
5. Bridge Formula: This formula is applied to the largest axle group, measuring from the centre of the first axle to the centre of the last axle of the combination in metres to the 10^th of a metre, rounding up to the next 10^th of a metre (for instance, 19.13m gets rounded up to 19.2m – not the nearest 10^th of a metre as per normal mathematical practice). This distance (M) is applied to the formula to get the maximum mass for the combination as per the Bridge Formula: (M x 2,100) + 18,000kg = max mass.
6. Drive Axle Mass x 5: There must always be sufficient mass on the driven/powered axle (driven axles are usually identified by the differential or “diff” incorporated in the axle) to ensure traction, otherwise there is insufficient grip on the road surface to move the combination forward. This can lead to slipping on inclines and the combination potentially dangerously sliding backwards instead of driving forward. A very important consideration in this instance is the actual weighed mass of the drive axle. This is multiplied by five to ensure there is sufficient mass on the powered axle for traction. For example, if the drive axle’s weighed mass is 4,500kg, multiplying this by five gives a maximum combination mass of 22,500kg.

After determining these six factors, the smallest value obtained is the permissible maximum vehicle (combination) mass (V).

Applying the calculations

To find the maximum payload for the 6×2 link flat deck combination, subtract the unladen mass from the permissible maximum vehicle mass. For instance, if the unladen mass of the combination is 20,000kg, and the smallest permissible maximum vehicle mass is 45,000kg, the maximum payload would be 25,000kg (45,000kg – 20,000kg).

It is important to remember that the 56,000kg limit is only one of the six factors considered, and the actual permissible mass could be lower depending on other factors. For example, if the drive axle’s mass is limited to 9,000kg, the total permissible mass might be capped at 45,000kg. Therefore, the maximum payload could vary depending on the specific circumstances of the vehicle and load.

What seems like a simple question is actually quite complex, illustrating the intricacies involved in determining legal load limits for road freight vehicles.