Power spectral density of road profiles

The road profile is usually considered to be a random process x(d), where x is the road height and d is the distance along the road. As the vehicle tr avels along the road with velocity v, the random process x(d) is converted to a random process x(t) which is input to the vehicle suspension via the t yre. The random process x(d) is usually described in terms of its power spe ctral density as a function of frequency in either radians or cycles per un it distance. However, there are several different ways of defining power sp ectral density, and this makes it difficult to compare published data witho ut knowing how the power spectral density has been defined. The proper calc ulation of RMS values of vehicle response for an assumed road power spectra l density is explained by an example.