While four-wheel steering wasn't uncommon twenty-five years ago in the heyday of tech-heavy sports cars, we seem to have left some of those gizmos behind with the implied understanding that they were superfluous. Yet, with lengthening wheelbases in the current Porsche lineup and more and more weight, it's become a useful tool to make the latest fleet of Porsches rotate accurately, easily, and predictably.
If we're to trace the history of Porsche's rear-wheel steering back to its inception, we find the first system was used in the innovative 928. Its passive rear-wheel steering system, also known as the Weissach axle, was a means of mitigating lift-off oversteer—the tendency which made the 911 such a feared car in the sixties, seventies, and early eighties. While the semi-trailing arm rear suspension used with these early 911s was cost-effective and quiet in operation, it had a tendency to try and pull away from the Porsche's body during deceleration; the elasticity in the rubber bushings causing toe-out and the resulting instability.
With the Weissach axle, the front pivot bushing of the trailing arm was replaced by a short link, and the innermost mount was moved rearward. Additionally, a third, pivoted linkage sat between the front-most mount and the upright. This design caused the wheel to pull toward the car under deceleration; resulting in stable, reassuring toe-in. This passive system was the furthest Porsche took the rear-wheel steering idea until recently, though an electrically-controlled active system was tested in the 993 before it was shelved due to complexity.
Moving Forward Thirty Years
The current rear-wheel steering systems available on the Carrera S, GT3, and Turbo are much more sophisticated and adapt to conditions to provide different outcomes, but their operation principles are simple. At low speeds, where agility takes precedence, the rear wheels move in the opposite direction of the fronts. This shortens the curve the car must take and helps navigating parking lots or hairpins. That rotation also helps mitigate some of the potential for understeer; a trait that plagues 911s, especially with the longer wheelbase of the 991.
Up to 31 miles an hour, the rear steering effectively shortens the wheelbase by nearly 16 inches. The full 2.8 degrees of rear steering in this situation equates to something like 45 degrees of steering lock.
At higher speeds, stability is the aim. Above 50 miles an hour, the rears begin to steer in-line (as much as 1.5 degrees) with the fronts, effectively lengthening the wheelbase by up to 19 inches. Between 31 and 50 miles an hour, the computer determines the ideal course of action according to road speed, steering angle, and longitudinal and latitudinal acceleration. Additionally, the rears straighten in the event the driver is purposefully causing the car to oversteer. Clever stuff.
The steering arms are either pushed or pulled by actuators located just ahead of the top wishbone, the Porsche navigates the corner much more easily, and the driver familiar with the steering effort required in a 996 or a 997 is left flabbergasted at the sheer ease with which the modern breed can corner.
Watch as Chris Harris describes how little steering lock is needed to get the most out of a 991 GT3.