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Certain sections of Motor Vehicle Safety Standard 121 require that air braked commercial vehicles have the capacity to produce an average deceleration of more than 17 feet/sec2 from 60 mph to stop without prolonged wheel lockup on a surface characterized by an ASTM skid number of 75. Since commercial vehicles commonly include a wide variety of geometric and load configurations, careful steps must be taken by the manufacturer to assure conformance of each vehicle produced. Ford Motor Co. has found it useful to approach this problem with a program combining vehicle testing and computer simulation. A simple computer model is employed to select critical vehicles for subsequent testing. These vehicles serve a twofold purpose; to demonstrate empirically conformance to the stopping distance requirements of MVSS 121, and to permit correlation of a more sophisticated simulation. This more comprehensive model is then utilized to calculate the longitudinal braking performance of other vehicles.