All Wheel Steering

All-wheel steering, abbreviated AWS and effectively synonymous with four-wheel steering, is a system that allows the rear wheels to be turned in addition to the front wheels. In a conventional car only the front wheels steer, while the rears remain fixed in line with the body. By giving the rear axle a small but controlled steering capability, AWS addresses two opposing demands that a fixed rear axle cannot satisfy at once: nimbleness in tight, slow situations and stability at speed.

The behaviour is speed-dependent and this is the key to how it works. At low speeds — manoeuvring in a car park, performing a three-point turn or negotiating a tight city junction — the rear wheels steer in the opposite direction to the front wheels. This counter-phase steering effectively shortens the car's turning circle, pivoting it more sharply around a tighter arc, so a long saloon or large SUV can turn almost like a much smaller vehicle. Above a certain threshold, typically somewhere around 40 to 60 km/h, the controller reverses the logic.

At higher speeds the rear wheels steer in the same direction as the front wheels, known as in-phase steering. The angles involved are very small, often only a few degrees, but the effect is significant: the car changes lane and responds to steering inputs without the rear stepping out of line, producing a more planted, stable feel during motorway lane changes and fast sweeping bends. In essence, the system makes the car behave as though it has a shorter wheelbase when agility is wanted and a longer one when stability is wanted.

Mechanically, the rear steering is delivered either by a central electric actuator that pushes a tie-rod assembly to turn both rear wheels together, or by individual actuators at each rear wheel. The front steering remains a conventional rack-and-pinion arrangement, usually with electric power assistance, and an electronic control unit coordinates the rear angle against vehicle speed, steering input and yaw. Because the rear actuators are electrically driven and software-controlled, manufacturers can tune the response precisely and even integrate it with stability control to help correct a skid.

The principal benefits are improved low-speed manoeuvrability for large, long-wheelbase vehicles and enhanced high-speed composure, along with reduced steering effort and better trailer handling in some applications. The drawbacks are added cost, weight and complexity, plus extra components in the rear axle that can require maintenance or repair. Different manufacturers market the technology under their own names, but whether described as all-wheel steering, four-wheel steering or rear-wheel steering, the underlying principle is the same.