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In dynamic rollover tests many vehicles experience sustained body roll oscillations during a portion of road edge recovery maneuver, in which constant steering angle is maintained. In this paper, qualitative explanation of this phenomenon is given and it is analyzed using simplified models. It is found that the primary root cause of these oscillations is coupling occurring between the vehicle roll, heave and subsequently yaw modes resulting from suspension jacking forces. These forces cause vertical (heave) motions of vehicle body, which in turn affect tire normal and subsequently lateral forces, influencing yaw response of vehicle. As a result, sustained roll, heave and yaw oscillations occur during essentially a steady-state portion of maneuver. Analysis and simulations are used to assess the influence of several chassis characteristics on the self-excited oscillations. The results provide important insights, which may influence suspension design.

A new vehicle suspension control system that enhances vehicle stability and handling in fast evasive maneuvers performed close to the limit of adhesion is evaluated. The central idea is to use continuously variable magneto-rheological (MR) dampers to distribute the damping forces between front and rear axles in order to bring the vehicle yaw rate as close as possible to the desired yaw rate. This mitigates the vehicle oversteer or understeer tendencies during quick transient maneuvers. The basic principle of system operation is explained using known dynamic properties of MR dampers, vehicles and tires. The available control authority and the effect of MR damper settings on vehicle yaw response is then evaluated using computer simulations. The results of vehicle tests are presented. They demonstrate the benefits of the proposed control method in terms of improved vehicle response and reduced driver steering effort.