This report examines the ways in which Heavy Goods Vehicle suspensions apply unexpectedly large loads to road pavements. Roadside surveys show that for many multi-axle bogies the axle loads are not equal, and tests with an instrumented semi-trailer show that these inequalities persist with the vehicle in motion. Air suspensions show good equalisation. Goods vehicles in motion bounce continuously on the tyres and suspension; this can increase the loads applied to the road. Tests have shown that the pattern of loads along a length of the road during repeated runs at the same speed is rather consistent for a particular vehicle. Instruments in the road have shown that at a given speed the strains in the road structure are approximately proportional to the instantaneous load applied to the road by an axle. Tests of a semi-trailer on alternate air and steel suspensions running over the same road profile have shown that on some parts of the road the air suspension produced smaller dynamic loads, but on other parts of the road the air suspension produced higher dynamic loads. The TRRL suspension research programme is described. It is seeking to identify what characteristics of goods vehicle suspensions minimise road loading.

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