Porsche - Active, Not Passive

Active, Not Passive

In the new Porsche Panamera, the rear axle steers too.

The boss gets straight to the point. He dictates where things are going. Everything else falls in line. In the chassis of a car, the boss is the steering system on the front axle. The boss of the vehicle, of course, remains the driver: his or her commands are executed by the complex steering system. In the new Gran Turismo from Porsche, that execution is now better than ever before. Porsche engineers in Weissach have made the steering system 10 percent more direct as compared to a Panamera without rear-axle steering. That uptick in agility can be felt in every driving situation.

In most cases, such a steering setup comes with a catch. A steering system that reacts sensitively to fine-grained nuances tends to be erratic at higher speeds. Not so in the Panamera, because the boss on the front axle has an assistant to which to delegate: the optional rear-axle steering system. At speeds over 70 km/h, it ensures that the rear wheels also steer—in the same direction in which the front wheels are pointing. The only difference is the lower maximum steering angle: 1.5 degrees. This minor adjustment has a major impact. It essentially extends the wheelbase of the sporty sedan. The result, in turn, is that the car is significantly more stable on the road, even during fast lane changes. Indeed, the rear-axle steering yields benefits even when the car is simply driving straight ahead. The engineers call this a “reduced yaw tendency.”

Yet this task is not the assistant’s only job. After all, the powerful Panamera also moves at slower speeds on occasion. At such times—while parking, for instance—maneuverability is key. The rear-axle steering now steers up to 2.8 degrees in the opposite direction of the front axle. This results in a virtual shortening of the wheelbase and a 60-centimeter decrease in the turning radius, which in practice permits more elegant parking maneuvers. In general, the car’s agility increases at lower speeds.

Rear-axle steering is not entirely new to Porsche. It has already proven its mettle in the 911, but the system was improved for the Panamera. Unlike in the 911, where two single levers are at work, here there is a central actuator that converts electrical impulses into mechanical motion. It is located directly in front of the rear axle and above the differential and executes the steering motion with the aid ofa steering spindle. It is connected to the wheel mounts on both ends with a tie rod.

At least as important as the mechanical connection is the electronic connection to the vehicle. A controller in the central actuator continuously exchanges information, including driving speed, steering wheel angle, and longitudinal and lateral acceleration, with other control devices. From the incoming signals, the controller calculates the optimal steering angle for the rear wheels in fractions of a second. The system even recognizes when a driver is intentionally drifting and politely cedes control. Electronics notwithstanding, the real boss in any Porsche is the person sitting behind the wheel. And if that person wants to swing the rear out safely, then that is exactly what will happen.

By Johannes Winterhagen
Illustration ROCKET & WINK