Imagine a situation: you buy a modern Porsche Cayenne, Mercedes-Benz S-Class, or the latest BMW 7 Series. The car is lifted on a lift, and your gaze falls on the rear axle — and there you find steering rods. For an unprepared owner, this looks like something revolutionary. The dealer will gladly call the system an ultramodern innovation of the 21st century. However, the idea itself is not new at all: Japanese manufacturers installed similar solutions on production coupes back in the era of cassette players.
We are talking about the 4WS (Four-Wheel Steering) system — controlling all four wheels. The questions it raises are still relevant: how did engineers manage to implement the rotation of the rear axle without electronics? What physical principles allow the car to literally "shift sideways"? And why did the technology, which was developed in the 1980s, disappear in the early 2000s, only to return two decades later on expensive premium models?
If we turn to history, the very idea of turning all four wheels was discussed back in the early 20th century, although military equipment and special vehicles are not considered here. In the civilian passenger car segment, it was first actively introduced by Japanese automakers in the mid-1980s. It is generally accepted that the first mass mechanical system was 4WS on the Honda Prelude in 1987. Fans of the Nissan Skyline R31 may object: their HICAS hydraulic system appeared back in 1985. However, it was Honda that implemented a deep dependence of the rear wheel turning angle on the steering wheel position.
The design of Honda's 4WS was striking in its engineering purity. A cardan shaft ran from the front steering rack under the bottom to the rear rack, inside which there was a complex planetary mechanism. No computers or electric motors — only mechanics, capable of working with high precision and reliability.
The physics of the system depended on the speed and steering wheel angle. In fact, there were two modes.
At high speed, the rear wheels turned in the same direction as the front wheels, usually by 1.5–2 degrees. When changing lanes on the highway, the car did not "dive" its nose into the next lane, but shifted diagonally, maintaining parallelism with the markings. The body leaned less, and the stability felt almost rail-like.
During parking maneuvers, the situation changed: with a significant steering wheel angle, the rear wheels deviated in the opposite direction — up to 5 degrees. The turning radius of a large coupe was reduced by almost a meter. The car seemed to rotate around its own axis, which made maneuvering in tight spaces easier.
In the 1990s, the system became widespread among various Japanese brands — it was used on the Mitsubishi 3000GT, Nissan 300ZX, and Mazda 626. However, by the early 2000s, the technology had practically disappeared.
There were several reasons. First, weight and cost: mechanical drives or hydraulic pumps increased the weight of the car and made production more expensive. Second, the unusual behavior: when parking, the rear part could shift sharply, which became stressful for unprepared drivers and led to damage. Third, wear. Over time, play appeared in the rods and mechanisms, and the rear axle began to react less predictably, causing "wobbling" on bumps.
After about two decades, the system returned — now under the name Active Rear Steering. The reason is simple: modern cars have significantly increased in size and weight. If in the 1990s the BMW 7 Series weighed about 1.7 tons, then models such as the BMW X7 or Mercedes-Benz EQS reach 2.5–3 tons with a length of over five meters. Maneuvering such a car without auxiliary systems is difficult.
Modern solutions no longer use a mechanical shaft through the entire body. Compact electric motor actuators controlled by electronics are installed on the rear suspension. The on-board computer analyzes the speed, steering wheel angle, and data from the stabilization systems and determines the angle at which to turn the rear wheels. In the new Mercedes-Benz S-Class, the out-of-phase turning angle can reach 10 degrees, which allows a large sedan to turn almost like a golf-class car.
Today, 4WS primarily solves the problem of compensating for size and weight. The technology, once perceived as a sports attribute, has become a tool for increasing the maneuverability of heavy vehicles. At the same time, the complexity of the design and the cost of components remain significant: replacing the rear electric actuator on a modern Mercedes can cost 300–400 thousand rubles.
The evolution of four-wheel steering demonstrates the cyclical nature of automotive engineering. The idea, implemented in the 1980s using pure mechanics, disappeared due to weight, price, and wear, only to return in the digital age in a different technological embodiment. Now it helps to cope with the physics of large and heavy cars, confirming that even the most daring solutions can get a second life.