To make any real sense, cars need to be able to handle turns and corners.
Since its invention, every car has facilitated this by turning the front wheels to allow them to change direction. A more recent development, however, is the introduction of cars that can also rotate their rear wheels through a limited range of degrees.
These are commonly referred to as four-wheel drive (4WS) or rear-wheel drive systems. At a high level, automakers claim that today’s versions of this system allow the car to have shorter revs at low speeds, providing a practical benefit in navigating roundabouts. or tight parking.
However, when the vehicle is moving faster, the system increases stability, optimizing the vehicle’s dynamic performance. Let’s take a closer look at how these systems have evolved and their benefits.
History
While the basic concept of rear-wheel drive has been around for decades, the technology began to develop in the late 1970s, with models such as the Porsche 928 grand tourer featuring a Weissach axle.
This deflects the rear suspension bushings as the vehicle is slowing down to ‘hit’ the rear wheels (i.e. slightly inwards), thus further increasing stability and reducing overdrive.
The Japanese brands then took advantage of Porsche throughout the 1980s and 1990s, developing much more complex systems that could offer greater advantages.
Nissan first impressed with the Skyline generation R31, when it went on sale in 1985, offering a system called HICAS – High Power Active Controlled Suspension System.
While sharing the same goal as Porsche’s mechanical system, namely improving stability by reducing overshoot and undershoot, the Nissan system used computer-controlled hydraulic pistons to compress the hose. subframe lining.
This means that the vehicle can bring the rear wheel 0.5 degrees in or out to optimize stability at high speeds, regardless of whether the vehicle is slowing down or under load cornering, unlike like the Porsche system.
Honda released a competition system in 1987 on its Prelude coupe, which differentiated it by connecting the steering rack to the rear suspension tie rod, so that the number of degrees at which the rear wheel rotates is based on on the steering angle.
If the driver turns the steering wheel slightly (as expected when sweeping corners), the rear wheel rotates up 1.5 degrees in the same direction as the front wheel, improving stability.
At greater steering angles (as expected when going through roundabouts or tight corners), the rear wheel can turn up to 5.3 degrees in the opposite direction, which Honda claims reduces revs by about 10%.
Mazda, Mitsubishi and Toyota followed with similar electronically controlled systems on models such as the MX-6, sixth-generation Galant VR-4 and certain Celica models respectively. Honda and Nissan are also involved with second-generation, even more advanced computer-controlled versions of the 4WS concept.
The 1980s and early 90s were the height of Japan’s economic boom, and respectively the height of ambition for the Japanese auto industry, with a period of stagnation thereafter.
With that, customers are increasingly concerned that the potential cost, complexity, and reliability issues of these systems outweigh any tangible performance benefits. In the early 2000s, most Japanese OEMs phased out four-wheel steering from their product lines, in line with a more cautious focus on rugged, reliable, affordable products. affordable.
A contemporary revival: Cars available today with rear-wheel drive
The dynamic and performance benefits of rear-wheel drive have never been contested and most recently, several European brands, such as Mercedes-Benz and BMW (including Rolls-Royce), have led the way. beginning a modern revival of this technology.
The latest Mercedes-Benz S-Class and EQS models are said to feature the most advanced rear-wheel drive available today. The system can rotate the rear wheels 4.5 degrees, with the wheels moving in the opposite direction of the front axle under 60 km/h, and in the same direction above this speed to improve cornering stability.
Optionally (and in wheel sizes 20 inches or smaller), the system can rotate the rear wheels up to 10 degrees. This ensures the large S-Class has a revs of just 10.8m, similar to the A-Class, the smallest car in the Mercedes range, at lower speeds (in the form of a short wheelbase).
Other cars with rear-wheel drive today include the Mercedes-Benz C-Class, Renault Megane RS, Lamborghini Urus, Porsche 911, BMW 7-Series (and i7), as well as the Rolls-Royce Ghost.
Note, however, that due to ongoing semiconductor shortages, not all of the vehicles described above can currently be ordered with this feature. Eg, The newly launched Mercedes C-Class is temporarily unavailable with rear-wheel steering for new orders (some manual stocks may have this feature).
Related innovations
In the US, General Motors used rear-wheel drive to develop ‘Crabwalk’ technology for its new GMC Hummer EV. As the name suggests, this feature allows the vehicle to move diagonally, like a cornering corner, helping the vehicle to exit tight turns or overcome many off-road obstacles. This feature works by ensuring that the front and rear wheels rotate at the same angle, parallel.
Meanwhile, Rivian also demonstrated proof-of-concept “Tank Turn” on the R1T prototype pickup truck and R1S SUV. This feature allows the vehicle to rotate around a point, like a gyro.
Rivian claims that the feature takes advantage of the vehicle’s four-motor EV drivetrain to drive the front and rear wheels on one side of the vehicle forward, while those on the other side are driven forward. After that, allow the car to turn. However, numerous safety and reliability issues have delayed the availability of this feature on production versions of these models.
