Rear Axle Layout for the Speede Research Vehicle

The foundation of a dynamic as well as safe driving behaviour is, even in combination with modern vehicle control systems, based on the passive set-up of the suspension. Considering the constraints of a powerful rear wheel drive with torque vectoring functionality and centre of gravity position towards the rear of the car, special attention has to be paid to the rear axle layout. Regarding the kinematics the toe and camber curves are defined after choosing the position of roll and pitch centres. A linear increase of toe-in during bump motion reduces the general tendency towards oversteering of the vehicle. At the same time, toe angle changes during wheel travel must be limited to ensure sufficient directional stability on uneven tracks. To maximise the forces transmitted by the tyre in longitudinal and lateral directions, the tyre contact patch area must be as large as possible at all times. Thus a progressive increase of negative camber angle in bump motion is desirable to partly compensate the reduction of the contact patch area induced by the roll motion of the vehicle body during cornering. Here, as well, a compromise needs to be found regarding directional stability requirements. Besides the wheel trajectories the elasto-kinematics, that means the change of wheel alignment due to longitudinal and lateral forces, play an important role while designing the vehicle dynamic characteristics. A tendency towards toe-in at the outer rear wheel in a turn will reduce the oversteering tendency of the vehicle. Under longitudinal braking and propulsion forces only small changes of toe angle are desirable.