75The damping constant of a polyurethane seat is known to vary with displacement, velocity, preload, delay between the stabilization of the deformation under static compression and the onset of cycling, and with the number of cycles preceding the one used to compute damping [35, 40]. Owing to the extreme complexity of PUT behaviour, the detailed modeling of which would constitute a major project in itself, a single equivalent damping constant was selected for inclusion in the model, which was computed on the basis of energy dissipated during a loading/unloading cycle [41]. For this purpose, the force-velocity characteristics were measured under a sinusoidal excitation, and the area bounded by the hysteresis loop was computed to represent the dissipated energy E. The equivalent viscous damping C constant is then computed from:C A2 go(3.1)where A is the amplitude of cushion deflection and a) is the angular frequency of excitation.The analysis of the data acquired by BRP under a 63.2 mm static compression, arising from a 576 N load corresponding to 70 percent of the body weight of a 83.9 kg driver, and excitations at different frequencies and amplitudes, revealed that the damping constant varied between 900 and 1000 Ns/m. The lower bound of the damping constants is used in the vehicle model.3.2.2 Track ModelIn the model development, the track mass and pitch inertia are added directly to the rail and the frame rigid bodies by associating the upper half of the track to the frame