Overview of Vehicle Weighing Methods

The weight of an object is defined as its volume multiplied by its Specific Gravity (S.G.). A way of illustrating specific gravity is to consider the difference between a tonne of feathers and a tonne of lead. Both weigh the same, but have vastly different volumes. This is because the specific gravity of a feather is far less than that of lead. The volume and specific gravity of an object such as a cube of wood is easy to calculate as it is a regular shape and is of the same material throughout. Unfortunately, a lorry cannot be thought of as having a regular shape, nor is it made of all the same substance, and therefore it is impossible to mathematically work out its exact weight.

We therefore have to weigh our lorry but comparing it to a known range of weights (i.e. grams, kilograms and tonnes) with the use of some form of weighing machine. On a set of scales, weights are added to one side of the scale until they are exactly the weight of the object on the other side. This is considered on of the most precise weighing methods, providing the weights used are accurate. However, to weigh a lorry, a more substantial weighing machine is obviously required, and there are several devices available for this. Some use the distension of a spring to move a pointer over a scale, others use strain gauges to give an electronic output, and yet others use the displacement of fluids to arrive at a result. The only drawback with all these methods is that their results are relative, comparing the object being weighed with known weights using an indirect means- be it a gauge, dial, scale or electronic output. Therefore it is important to recognise that no weighbridge can ever be totally accurate all of the time. (A simple test is to take a vehicle to several weighbridges under similar conditions of load and fuel and compare results).

On top of the problem of the actual accuracy of the weighing machine being used, there is also a problem when it comes to weighing something as large as a vehicle in the fact that it is not always possible nor desirable to weigh the entire vehicle in one operation. Sometimes operators wish to weigh vehicles axle to axle. 'Split' weighing, such as taking axle weights, is therefore the only solution. This in itself causes further problems, which will be covered in the next section.

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