The concept of all 4 wheels steering has been around for many years. I first heard about it on an early model Honda Prelude, where the rear wheels would turn as well as the fronts.
At slow speeds, the rear wheels are turned in the opposite direction of the front wheels. This can lessen the turning radius by approximately 20%. At faster speeds on the highway, the rear wheels are turned in the same direction as the front wheels. This improves cornering and is particularly beneficial for high performance vehicles.
Daimler-Benz appears to be the first to develop a system primarily for forestry equipment and trucks that needed better steering through the trees. The first production car 4WS was included over 25 years ago on the 1988 Honda Prelude. This system was entirely self contained, automatic and mechanical: no electronics, sensors, computers or hydraulics. It was just two racks and a rod between them. Each rack had different ratios to allow the steering at the back to be narrower than the front.
A tube ran from a secondary pinion on the front steering rack, transferring 1:1 steering wheel motion to the rear rack. The rear rack transferred that rotational motion to a lateral motion. During the first 180 degrees of steering wheel rotation, the rear wheels are gradually steered in the same direction about 3 degrees, creating a slight under steer situation, improving straight line stability. During the second 180 degrees of steering wheel rotation, the rear wheels are gradually straightened back out. From 360 to full lock (450 degrees), the rear wheels are gradually steered in the opposite direction about 5 degrees, creating a significant overseer situation, improving parking lot manoeuvrability. The test car was actually two Accord front ends welded together! The tests started in 1981 after 3 years of design and theory.
Another version, called Quadrasteer and built by Delphi – part of GM, was primarily available on the bigger SUVs like the Yukon, Silverado and Suburban to help when towing vans or boats and for slow speed manoeuvring. Subaru use a compliance understeering mechanism on their all-wheel-drives to aid stability when cornering by adjusting the rear suspension loadings.
4-wheel steering has been used mostly by the Japanese manufacturers and Renault had Active Steering on its Laguna model that uses the G-Forces of the car to greater effect and thus reduces instability.
Sadly, this is a great idea that is not fully utilised by the industry – only a handful of models seem to have anything similar – with new computing technology coming to the fore, we might see 4WS become a standard for safety purposes. It certainly helps a vehicle corner better which improves efficiency.