A road simulator system has been developed for heavy duty vehicles. The system utilizes vertical inputs to the tires and corrects for heavy duty vehicle simulation problems such as solid axle cross-coupling, different tire contact patches for front and rear tires, tire nonlinearity, and variation in vehicle speed. The system operates in two modes which allow reproduction of response measured on a particular road, or simulation of vehicle response to a class of roads described by a PSD Matrix.The road profile generation techniques for each mode of operation are described. These techniques determine effective road inputs from measured vehicle response through a multiple input-multiple output systems analysis of the vehicle. Laboratory test results for both modes of simulator operation are presented.