Technology explained: Rear-axle steering

Porsche first experimented with passive steering of the rear wheels on the 928, developing the clever toe-compensation ‘Weissach steering’ suspension setup.

However, it wasn’t until last year’s release of the 918 Spyder that a full active rear-wheel steering system made its way onto a Zuffenhausen sports car. Since then, the system has also been rolled out on the 991 GT3 and Turbo.

Thanks to its rear-engined layout, the 911 has tended to understeer due to a lack of weight over the front wheels. In order to rectify this on the 991 generation, the wheelbase has been lengthened (more so than the body) helping to effectively transfer more load to the front wheels compared to the 997.

This though has brought its own problem. The longer the wheelbase, the less responsive the car is when it comes to handling.

Under 31mph, the rear wheels are steering the opposite direction to effectively reduce the wheelbase.
Under 31mph, the rear wheels are steering the opposite direction to effectively reduce the wheelbase.

This is where the rear-axle steering system comes in. At speeds below 50kph (31mph), the rear wheels are steered up to 2.8 degrees in the opposite direction of the front wheels.

This effectively reduces the car’s wheelbase, making it more manoeuvrable at low speeds. As a comparison, 2.8 degrees of turn on the front wheels would equate to 45 degrees of steering lock.

When the car is travelling over 80kph (50mph) the rear-axle steering turns the rear wheels up to 1.5 degrees in the same direction as the fronts. This effectively lengthens the wheelbase, making the car more stable in high-speed corners.

However, it also enables the rear wheels to load up faster, improving the 991 GT3 and Turbo’s ability to change direction.

Above XXmph, the wheelbase is effectively lengthened as the rear wheels steer in the same direction as the fronts.
Above 50mph, the wheelbase is effectively lengthened as the rear wheels steer in the same direction as the fronts.

The whole system uses two electromechanical actuator bolted onto either side of the chassis just fore of the top wishbone. Steering arms connect to the top of the rear uprights.

The actuators are connected the car’s ECU (which measures road speed and steering angle) before sending a signal that causes electrical motors to either ‘push’ or ‘pull’ the steering arms to create the required angle and direction of rear-wheel steering.

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