Weighing-in-motion is generally defined as the process of measuring the dynamic tyre forces of a moving road vehicle (dynamic wheel loads) and estimating the gross vehicle weight (GVW) and the portion of that weight carried by each wheel, axle, and axle group of a corresponding static vehicle (the static wheel/axle loads).
There are a number of things that are important to realise from this definition:
- A WIM system will measure one physical quantity (dynamic wheel loads) and will calculate an estimate of something else (static wheel loads, axle loads and GVW) as output;
- The GVW is directly related to the total mass of the vehicle and static axle loads are related to the distribution of the mass over the axles. Like the total mass of the vehicle, the GVW and static axle loads are constant and can only be measured directly when the vehicle is standing still – and the brakes are released;
- When driving, the dynamic wheel/axle loads of a vehicle vary in time because of the movement of the complete vehicle (bouncing, rolling and pitching) and that of the individual axles (hopping). These movements are caused by the reaction of the vehicle to the pavement surface and external disturbances like braking, acceleration, steering and wind;
- The magnitude of the dynamic variation of a dynamic axle load depends on the quality of the road, especially the evenness of the pavement surface and the deflection of the road construction, the characteristics of the vehicle, and primarily the type and condition of the suspension system and the distribution of the load;
- The magnitude of the dynamic variation around the static loads is unknown to the WIM system. In general, the more disturbances, the higher the dynamic variations and the more difficulties the WIM system will have to calculate the static loads accurately.