Four-wheel steering is employed by manufacturers when they want to improve the stability of a car at higher speeds, or its manoeuvrability at lower speeds. As the name suggests, the concept involves steering the wheels of both axles. There are two types of four-wheel steering in the automotive market. The first is active four-wheel steering, which involves electronically or mechanically steering the rear wheels to achieve the desired effect.
The other, often called 'passive rear-wheel steering', uses special bushings and suspension components to slightly alter the direction of the back wheels in certain conditions. This less costly and complicated solution can deliver several benefits but, compared to active set-ups, is far more limited in terms of what it can achieve.
Active four-wheel steering
Active four-wheel steering system features a mechanical, hydraulic or electronic system that can steer the rear wheels; in the case of the more advanced electronic or hydraulic systems, a controlling computer will use a plethora of sensors to decide the best steering angle. There are typically low- and high-speed steering profiles for the rear axle in these set-ups. When the car is driving slowly, the rear wheels will often be steered in the opposite direction to that of the front wheels. This makes the car far more nimble and can greatly reduce its turning radius, which is useful when parking or negotiating tight corners.
When the car is travelling at higher speeds, however, the rear wheels will turn in the same direction as the front wheels. This improves stability, as the car doesn't yaw as much, which makes manoeuvres such as high-speed lane changes more predictable and easy to control. In most cases, the rear wheels are steered by fewer than eight degrees - an amount that is usually just visible.
There are some potential downsides to active four-wheel steering systems. They are complicated and can potentially induce odd handling characteristics when the system fails, while their added complexity drives up the overall cost of the car.
What about passive rear-wheel steering?
These configurations features specifically designed suspension components, such as precisely engineered bushings, that cause the rear wheels to toe in or out in a controlled fashion as the suspension loadings change.
For example, when Porsche was developing the 928, it was discovered that the rear suspension caused the outer rear wheels - in particular the one on the outside of the corner - to toe out when decelerating. If the driver lifted off in the middle of a bend, this trait could cause unexpected oversteer.
A new rear axle was promptly developed that would instead cause the rear wheels to toe in when decelerating; this passive rear-wheel steering reduced the tendency to oversteer and consequently improved stability.
It doesn't take much of a change in toe to cause the desired rear-wheel steering effect, either, with the deflection of the rear wheels sometimes being measured in just fractions of a degree. Unlike active systems, however, passive set-ups offer far less adjustability and no low-speed benefits.
A brief history of four-wheel steering
Porsche first introduced passive rear-wheel steering in the 928 in 1977, using what it dubbed a 'Weissach Axle' at the rear of the car. Other manufactures later adopted passive rear-wheel steering, including Ford, Peugeot and Saab, and the set-up is now commonplace.
Nissan then released the first production active 4WS system in 1985, fitting HICAS - 'High Capacity Actively Controlled Steering' - to its R31 Skyline; Honda followed shortly after, launching a Prelude with 4WS in 1987. Many other manufacturers have since made use of active four-wheel steering systems, including Porsche, Mitsubishi, Renault, Ferrari and BMW.