Steerable rear axles supporting the steering angle of the front axle stand have long been on the agenda of the automotive industry because can improve both driving safety
05 Feb 2015 | 9717 Views | By Autocar Professional Bureau
Steerable rear axles supporting the steering angle of the front axle stand have long been on the agenda of the automotive industry because can improve both driving safety as well as driving dynamics by this technique. So far, however, their implementation was too expensive, too complicated or too intense usage.
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Steerable rear axles supporting the steering angle of the front axle stand have long been on the agenda of the automotive industry because can improve both driving safety as well as driving dynamics by this technique. So far, however, their implementation was too expensive, too complicated or too intense usage.
Now ZF has launched a system for series production, which makes possible a toe angle change steering movements of the rear axle. The advantages the AKC system displays for the active rear is now standard in the Porsche models 911 Turbo and 911 GT3.
The tuning of the track is an important task in chassis development. This is done in the interest of an optimal driving behavior, because the toe angle in the chassis is responsible, among other things, for the straight-line braking and for perceived by the driver's steering precision. Suspension components such as track and control arms provide for a series vehicle that defined in chassis development adjust the tracking maintains accurate.
When setting the trace is defined once for a vehicle, it is no longer variable. ZF chassis engineers tested what effect a dynamic adjustment of the track has while driving on the driving behaviour. They developed an adjustable-length track link which is at the centre of the active system: an electromechanical actuator can vary the toe angle while driving, integrated into the Aktuatorelektronik control software gives the orders. An advantage is that due to the toe angle change results in a steering movement. Although this is about three degrees small compared to the front axle, a steering intervention on the rear axle has higher effects. In conjunction with the steering angle of the front wheels, it makes a much more tangible positive influence on driving behavior. This is the principle of Active Kinematics Control (AKC).
Depending on the requirements and available space there is energy efficient for the ‘Power-on-demand’ principle working the AKC system in two ways: either with a controller in the centre of the rear axle, also known as ‘central plate’ system designated or Porsche 911 Turbo and 911 GT3 with one actuator per wheel.
"The technique brings benefits in almost every situation," said Dr Peter Holdman of the IF Division suspension technology for the development and the business unit Responsible Axle Systems. "When it comes slowly through narrow streets, they deflected in opposite direction to the steering angle of the front wheels and produced a higher yaw rate of the vehicle." In order up to ten percent, then reduced the turning circle, cars are more easily maneuverable. "At higher speeds, ie from about 60kph, the system steers the rear wheels in the same direction as the front wheels and thus improves directional stability and driving dynamics," says Holdman.
The steering assistance is performed by AKC by electromechanical actuator, which has no mechanical connection to the steering wheel. It is therefore a pure ‘by-wire’ system. This has the advantage that the AKC can be integrated into the active control network of the respective cars. Then it supports the function of other active systems – for example, in combination with ESP. Linked to AKC and improving the anti-lock braking system, stabilizing interventions of brake and rear axles, the handling of the vehicle during braking. Thus, the system increases safety and driving dynamics at the same time. When braking on different gripping surfaces the braking distance is shortened.