Abstract
Keywords - Driving Dynamics, Chassis Development, Steering System, Rear Wheel Steering, Parameter Control
The new steering system based on the successfully introduced and awarded BMW Active Steering supplemented by an all new Integral Rear Axle Steering system. Only the combination of both active steering systems enables totally new degrees of freedom for a perfect chassis application.
The Active-Steering consists of a hydraulic assisted rack-and-pinion steering system with an integrated planetary gear. The Active-Steering varies the steering ratio on the front wheels depending on the vehicle velocity. In this way the steering effort will be significantly reduced and applicated to a perfect driving performance. Additionally the system stabilizes the car in critical situations for example during oversteer or braking on different friction levels.
The new Integral-Rear-Axle-Steering is based on newly developed mechatronic actuator which operates directly on the wheel carrier and therefore on the rear wheels. By using a jackscrew actuator as well as a concentric electric motor, steering angles up top 3 degrees on the rear wheels are possible. In contrast to the Active-Steering, the Integral-Rear-Axle-Steering not only improves the steering effort but also has a positive impact on the driving characteristics. These effects are measurable by yaw rates, lateral accelerations and attitude angle. The functional logic is implemented within a superordinated function unit, the BMW Integrated Chassis Management. The target requirements are transferred as steering angles via Flexray to the actuator ECU´s.
Customer Benefits: Within the lower velocities, the Integral-Rear-axle-steering steers the rear wheels in the opposite direction to the front wheels. This leads to a significant enhancement of agility and dynamic.During the higher velocities the rear wheels steer in the same direction to the front wheels. The result is a phase-identical transverse force and lead to a considerable enhancement of driving dynamics, particularly for stability; driving behaviour by steering input and reduced roll angles. In summary, the customer gets a more sovereign driving performance.
The principle behind rear axle steering systems in combination with conventional steering is that they have the disadvantage of an understeer tendency. Furthermore, the steering effort at a higher velocity increases with the result of an inert driving performance resulting in reduced curve willingness. Only the combination of the Integral-Rear-Axle-Steering and the Active-Steering enables the compensation of the above mentioned disadvantages.
The Integral-Active-Steering is a meaningful extension of the Active-Steering by the Integral-Rear-Axle-Steering. In combination with the Integrated-Chassis-Management, the new system has lead to a precise steering behaviour without any compromise in every driving situation.