Consideration of Split Loads and their Effect

There are several methods of split weighing a vehicle, and the finer differences between machines available for weighing a vehicle are covered in a later section. For now, it suffices to say that they will all fall into one or two categories: -

1) Above the Ground machines- i.e. portable or surface mounted

The above ground methods of split weighing a vehicle is where the risk of transfer i.e. shift of the centre of gravity is greatest, and in this instance a portable weighing machine will illustrate the major pitfalls of split weighing.

Firstly one should remember that the closer the axles are to each other, the greater the chance of weight transfer and the greater the effects of this transfer of weight.

This is further illustrated if we take a 3-axle trailer bogey as another example. For the purposes of showing weight transfer, we will assume that the three axles are independently suspended and that the trailer chassis is rigid.

The only way to avoid this kind of weight transfer is to raise all the axles simultaneously to the same height above the ground.

2) In the ground machines-i.e. fixed weighbridge installation.

The in ground method of split weighing is less likely to give weight transfer so long as the ground on either side of the weighing platform does not significantly rise or fall under the remaining length of the vehicle. In this instance, an axle weighbridge will illustrate how weight transfer can occur on fixed systems.

As Illustrated above, where the roadway maintains a constant gradient, the weight transfer is zero to minimal. However weight transfer occurs where the changes in gradient as illustrated below.

To make matters worse, gradients are not the only consideration when thinking about axle weight. As shown below, both vehicles carry the same payload, both have a legal gross vehicle weight and yet one of them is illegal.

Thus we have seen that unless we have a machine that can weight the whole vehicle in one go, there are several important aspects that have to be taken into consideration when split weighing a vehicle in order to achieve an accurate result to avoid prosecution.

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