REAR-AXLE STEERING

Introduced for the 991-generation Turbo and GT cars, Total 911 explains how rear-wheel steering works and how it isn’t such a new technology

Porsche first experimented with passive steering of the rear wheels on the 928, developing the clever, toe-compensating ‘Weissach steering’ suspension setup. However, it wasn’t until the release of the 918 Spyder that a full active rear-wheel steering system made its way to a Zuffenhausen sports car. Since then the system has also been rolled out on all but the base 911 Carrera.

With its rear-engined layout the 911 has a tendency 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 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.

This is where the rear-axle steering system comes in. At speeds below 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 50mph the rear-axle steering turns the rear wheels up to 1.5 degrees in the same direction as the front. This effectively lengthens the wheelbase, making the car more stable in high-speed corners.

It also enables the rear wheels to load up faster, improving the 991

GT3 and Turbo’s ability to change direction. The whole system uses two electromechanical actuators bolted on to 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 to the car’s ECU 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.