The fluctuations in the vertical forces applied to the road surface by the wheels of a vehicle have been measured for a two-axle semi-trailer fitted with a mono-leaf spring suspension. Measurements were made on a variety of pavements, at different static wheel loads and speeds, with and without axle dampers, and with different tyre types. The dynamic component of the load applied to the road surface was not small: the road loads had standard deviations of approximately 5-20 per cent of the static loads. This variation was mainly caused by the semi-trailer bouncing at about 3 Hz, with loads at all four wheels varying in phase. It was found that the standard deviation of the road load applied by each wheel increased almost linearly with the road profile roughness, that it increased with tyre stiffness and that it was unaffected by the addition of axle dampers. (A)

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