Truck drivers can walk a fine line between carrying a legal load and being overweight on one or more axles. With freight taking up a significant proportion of some companies’ costs, optimising the delivery weight while reducing the risk of a fine can significantly improve profitability.
Trucks have weight limits to help reduce damage to the road’s surface. When a truck drives on the road, it causes the surface to flex. This opens up minute cracks. Water gets into the cracks and works its way into the substrate. This can cause localise depressions that then hold more water when it rains. If it freezes, the water in the cracks expands, breaking up the road. More traffic gradually dislodges pieces of the road and a pothole forms. The heavier the truck, the more the deflection in the road, so roads are engineered to take a maximum deflection. This also applies to the road’s ability to protect underground services such as electrical and data cables, and water, stormwater and gas pipes from being damaged.
The maximum weights per axle also help with tyre wear and preservation. Loading too much weight onto a tyre will make it more susceptible to punctures, and it could overheat and delaminate more readily.
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Understanding the characteristics of your truck
Any company that owns one or more trucks should first understand how to calculate axle weight limits – this is often done via some kind of training program. The first part of this is the tare weight (empty weight) of the truck and how that weight is distributed across the axles. This can be done at a weighbridge. Take the truck when the fuel level is full, and with any auxiliary equipment that you might usually carry such as a pallet jack. The weighbridge will give a readout showing the weight for each axle.
The second step is to understand your local maximum weights for each of the axles. This is usually dependent on the axle type, the tyre size and the number of tyres on each axle. For example, the front steering axle on a semitrailer will not be permitted to support as much weight as the rear axle of an 18-wheel semitrailer that has duals (i.e. four tyres on the axle). This is because the load’s weight is spread over a greater area when there are duals.
If there are axle sets (e.g. two or three axles in a group), they will have a combined weight that is not the sum of the individual weights. For example, if a single axle with two tyres can support 7000kg, a tri-axle set of duals (12 tyres in total) will not be permitted to carry 21,000kg, but probably more like 18,000kg, depending on where you are in the world.
The third step is to understand what permits you own, or what charges you pay for using the road. This might be a registration fee, road user charges, high-productivity motor vehicle permit, over-weight permit, etc. This is the legal amount of weight you can carry if your truck and tyre combinations support it.
Finally, you need to understand the limitations of your truck. The truck’s data plate, certificate of loading or registration documents will say how much load capacity it has.
From all those figures, you take the lowest. For example, you might have at truck and trailer capable of weighing 60,000kg, but the maximum legal weight on the road is 38,000kg, and you are only paying for 35,000kg, in which case, the maximum gross vehicle mass (GVM) can only be 35,000kg, otherwise you would get a fine.
Trialling configurations
Now you understand the maximum weight your truck can weigh, you can deduct the tare weight and that is the maximum load you can carry. For example, if your truck’s maximum load weight is 25,000kg and the tare weight was 14,500kg, then the maximum load you can carry is 10,500kg.
However, this has to be spread across the axles in a way that doesn’t overload any one particular axle. The front of the truck usually will have the engine weight over the front axles, so you can’t load heavy loads near the front, but also you don’t want to load everything at the rear because that could overload the rear axles.
Calculate the weight of the load you are carrying and then use your best guess to load your truck so it is as balanced as possible. Now take it back to the weighbridge and see whether you are within the legal limits, or whether you need to rearrange the load to make the distribution better.
A few visits like this will give you an excellent idea of how different loads can sit on your truck. It also means that you are optimising your load in terms of delivery capability, you are decreasing your tyre wear, and you are reducing the risk that your driver will be pulled over and given a fine for the truck being overweight.
