Active Axle Kinematics A rear axle that thinks
As even your supermarket trolley would agree, it’s easier to drive round a tight bend when all four wheels can move. Even so, putting this principle into practice in a passenger car is very far from straightforward.
For decades, it’s been high on auto developers’ agendas: the ability to adjust rear-axle tracking. The concept has advantages in almost every driving situation. Driving slowly through narrow streets, it would give you a tighter turning circle. Driving faster along a winding road, it would improve your vehicle’s handling. At really high speeds, it would enhance your car’s stability. But until recently, the technical challenges were insurmountable. Add an extra steering system to the rear axle? A system that’s mechanically controlled by the steering wheel, just like the front wheels? No, it’s simply not feasible: too costly, and there’s not enough space.
The “Rear Axle Kinematics” approach
Which is why ZF engineers decided on an alternative approach – now christened AKC (standing for “Active Kinematics Control”) and ready for series production. The idea is relatively simple: to develop a suspension system capable of adjusting its characteristics as the car is driving along. The main focus of the AKC System is the actuator unit integrated into the rear axle that electromechanically adjusts the axle kinematics depending upon the driving situation. As a result, the toe-in angle varies by 3 degrees and more. Although this doesn’t sound like much, when combined with the steering angle on the front wheels, it can have a significant influence on the vehicle’s driving behavior.
Thus ZF’s AKC system with its actuator unit only springs into action if the electronic control system (ECS) issues the appropriate command, following the “power-on-demand” principle. Under the intelligent control of the ECS, the shifts in track angle can be applied and adjusted so they precisely match the driving conditions. At low speeds, the active rear-axle kinematics turn the rear wheels in the opposite direction to the steering angle of the front wheels, reducing the vehicle’s turning circle and increasing maneuverability. Then, as the vehicle picks up speed, the rear wheels start to turn in the same direction as the front wheels, making the moving car safer and more stable.
Gallery: AKC – Steering on Demand
In fact, the system is in series production in a sportscar with two actuators (dualactuator principle). But generally speaking, the AKC system works perfectly well with a single actuator positioned in the center of the rear axle (center-actuator principle).