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A six degrees-of-freedom ride dynamic model of a cab-over-engine supported on elastomer mounts is developed using lumped parameters. The lumped parameter model is analyzed for its free vibration response, while assuming the cab structure to be rigid. A finite element model of the suspended cab is developed and analyzed to establish the influence of flexibility of the cab structure on the ride dynamics of the tilt cab. The vibration modes of analytical lumped parameter and finite element models are compared to the dominant ride frequencies of the vehicle measured in the field. The lumped parameter model is then modified to achieve a comprehensive ride dynamic model for its further use in ride performance analyses.

Low frequency terrain induced vibrations transmitted to the off-road vehicle operators are quite severe and exceed I.S.O specified “fatigue decreased proficiency” limits. In this paper, the ride improvement of an agricultural tractor is sought through effective designs of passive seat suspensions. The dynamic analysis of existing bounce suspension seats is carried out to establish its ride performance behaviour. Optimal bounce seat suspension parameters are selected with an objective to maintain the ride vibration levels within 4 hours exposure “fatigue decreased proficiency” limits. The roll and pitch ride vibrations, perceived by the operators, can be attenuated through a gimbal arrangement mounted to the bounce suspension seat. The optimal parameters of the combined seat isolator are selected using parametric optimization techniques. Also a horizontal isolator, attachable to the bounce or the combined seat isolator, is configured.

The unique difficulties associated with low frequency and large amplitude ride vibrations of off-road tractor are summarized. Concept of a cab suspension system for improving the ride quality of off-road tractors in the bounce, longitudinal, lateral, pitch and roll modes is explored. Influence of suspension parameters on the ride performance is presented followed by selection of optimal suspension parameters. It is shown that a cab suspension would provide improved performance in the longitudinal and pitch modes alone. Ride analysis of the cab suspension with a sprung seat reveals satisfactory bounce ride. Roll and lateral ride of the off-road tractor can be improved significantly through alterations in the cab geometry. The ride performance of the optimal suspensions is assessed with reference to ISO (International Standards Organization) specified “fatigue decreased proficiency” (FDP) boundaries.