Rear axle becomes steerable… by wire

February 06, 2015 // By Christoph Hammerschmidt
With an innovative approach to automotive kinematics, ZF Friedrichshafen emphasises its commitment to electronic technologies in automotive segments hitherto defined exclusively by mechanic parameters. The company that recently took over mechatronic systems supplier TRW Automotive now introduces a steerable rear axle design in which decisive elements are controlled by software.

Until now, steerable rear axles were too expensive, too complicated or too fuel-inefficient to implement. Such systems which assist the steering of the front axle for better driving behaviour have been on the agenda of OEMS throughout the automotive industry - and due to their high price and complexity they were restricted to a few high-end vehicles. Now ZF says it has a system ready for volume production. ZF's AKC system, which facilitates steering movements of the rear axle by modifying the toe angle, demonstrates its advantages as a standard active rear axle steering in Porsche's 911 Turbo and 911 GT3 models.

Track adjustment is an important task when developing the suspension. The aim is to promote optimum vehicle handling because the toe angle on the chassis is responsible, among other things, for directional stability when braking and for the steering precision perceived by the driver. Suspension components such as toe links and control arms ensure that the track setting defined during chassis development on a volume production vehicle is maintained precisely.

As soon as the track for a vehicle is defined, it cannot be varied anymore – and right at this point ZF steps in with its latest development. The company's developers developed a length-adjustable toe link, which lies at the center of the active system: Electromechanical actuators can vary the toe angle while the vehicle is moving; control software integrated into the vehicle electronics issues the commands. One advantage: A steering movement is produced by modifying the track angle. This is actually small (about three degrees) compared with the front axle, but steering intervention at the rear axle has a greater impact. Interacting with the steering angle of the front wheels, the result is a distinctly noticeable and positive impact on vehicle handling. Depending on requirements and available installation space, the AKC system, which operates energy-efficiently according to the power-on-demand principle, is available in two variants: with an actuator in the centre of the rear axle, also called a "central actuator" system, or with one actuator per rear wheel as is the case in Porsche's 911 Turbo and 911 GT models.