Whilst compensating systems are often fitted to vehicles with multi-axle bogies, in practice tests have shown that some types of suspension are unable to equalise axle loads within bogies effectively. Both static and dynamic tests have been carried out using different types of suspension system fitted to multi-axle semi-trailers. Results from roadside surveys have shown that air sprung multi-axle bogies fitted to semi-trailers compensate much better than their steel sprung counterparts. Static tilt tests have confirmed this result under controlled conditions. It has been shown that the compensation of some steel sprung systems can be improved by reducing the friction in the spring ends and equaliser pivot bush. A detailed analysis of the forces in the suspension components fitted to an instrumented steel sprung bogie when tilted during static tests demonstrated that under certain conditions loads were transmitted in the radius rods as well as the springs. This work showed that the geometry of the steel sprung suspension design is important in achieving equalisation within bogies. A different type of compensation mechanism fitted to a tractor unit which used pin-joints and shackles and did not have a radius rod to locate the axle showed much better equalisation of the axle loads within the 2-axle bogie fitted to this vehicle. Fully instrumented test vehicles have been used to monitor the dynamic behaviour of truck suspension systems running over different types of road surface. Results have shown that, as found using static tests, air suspended multi-axle bogies compensate much better than steel sprung systems.

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