Optimization and Potentiality Study of a Variable Kinematics Suspension

The main active suspension devices (Citroe¨n Hydractive, Mercedes ABC, BMW Dynamic Drive, CDC systems in general, etc.) produced in the last years in the automotive industry try to manage the stiffness at the ground and finally the handling, controlling the spring and/or the antiroll bar stiffness. From a theoretical point of view such a solution is not the only one to control the tire vertical force. If we consider the vehicle dynamics equations obtained with the lagrangian approach, the tire vertical force is the product of the physic element characteristic (spring, damper and antiroll bar) times a purely kinematics term, called jacobian. Authors imagined to have an innovative device, constituted of a variable kinematics suspension, in which the spring and the damper are passive and the geometry is active. In this way it’s possible to control the suspension effects at the ground modifying the jacobian and not directly the stiffness and/or the damping values. While in a passive suspension the vertical stiffness at the ground is a function mainly of the hardpoints position and the vehicle height is a function of the spring preload, in a variable kinematics suspension both the stiffness at the ground and the vehicle height become function of the kinematics actuator too. This further dependence allows to control the tire vertical force, the car static height and finally the overall handling. A stiffness variation of ± 50% and a height variation of ± 20 mm become a reachable target so that an antiroll bar is not needed anymore, reducing the vehicle mass. Besides, the height control could be useful for all those car that have a well designed and sophisticated aerodynamics (i.e. with a flat undertray to increase the ground effect). Because of the jacobian dependence on the geometry, the suspension hardpoints location has been optimized to match the all targets as better as possible. Authors developed a multi-body model in Matlab/SimMechanics and they linked it to a genetic algorithm in order to find the best kinematical configuration. After this development step, a deep analysis of the potentialities of this kind of suspension has been performed comparing the handling behavior in ISO maneuvers to the one of the same vehicle equipped with a standard passive suspension.