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Analysis of commercial vehicles presents some special challenges: Many variations of the basic configuration exist. A complete model of a vehicle could easily have over 500,000 degrees of freedom. Response is desired for multiple road types, speeds, or other excitations. To reduce model complexity, Kenworth assembles Finite Element models of complete vehicle from a library of substructures and subassemblies. Substructures are reduced representations of components that retain the global mass and stiffness properties. To simulate multiple excitation cases quickly, a frequency domain approach is used. Frequency response functions generated by the FE model are combined with measured road auto- and cross-spectra. The response is weighted for human sensitivity per ISO 2631-1. Tuning of parameters such as stiffness or damping is performed quickly using system synthesis.