Rear-Wheel Steering - Suspension Secrets

Written by: Matt Cowley

Rear-wheel steering systems can now be found on some of the elite handling machines of today such as the Mercedes AMG GT R, Porsche 991 RS models and Ferrari F12 TDF to name a few. In this feature, we are going to take a look at some of the technical aspects of the systems and find how active rear-wheel steering (which we will refer to as ARS for the remainder of the article) makes such a big difference to the handling and cornering potential of the car. The idea of rear-wheel steering is not a new concept. Looking back to the late ’80s and ’90s, Japanese engineers were breaking new ground for the road market by designing a system that allowed the rear wheels to be steered when the steering wheel was turned. The first car to feature this, a very unlikely suspect, was the Honda Prelude. Fast forward to the ‘90s and Nissan saw the appeal of using this system on their Godzilla range, so tested the idea out on the R34 Nissan Skyline GT-R. The system worked well; it allowed the car to generate more rear-end grip and also help such a big car (at the time) rotate and enter corners in a more smooth fashion and help to reduce the understeer. However, ARS slipped back out of sight due to the clunky and heavy mechanical systems required to make the idea work. Furthermore, the maintenance costs of the systems were climbing as their reliability over time came into question and moving parts began to wear, generating quite a sloppy feeling at the rear end.

Almost two decades after the R34 GT-R was launched, rear-wheel steering came back into the headlines on the likes of the Mercedes AMG GT R and then again on the handling supremacy of the Porsche 991 GT3 RS. As you would imagine, the systems are now much more refined, with the mechanical steering racks removed entirely, replaced by much more capable electronic actuators mounted in the same place the rear toe arm would usually sit. So, why has it now made a re-appearance? The main reason is quite simply that technology has finally caught up with the idea, and the components and electronics required to implement the system are now cost-effective for the manufacturers and much more reliable, not to mention that the high-powered actuators also have much faster reaction times which is key to making this system work flawlessly. The ARS works by altering the toe angle of the back wheels when the car changes direction. By altering the toe angle, the ARS makes the rear wheels turn in a similar way to the front wheels, however, they only turn by a very small amount unlike the front wheels. This tiny amount of steering at the back helps the car to take corners much faster and in a more stable manner than without the system. Sensors mounted all over the vehicle from roll sensors through to multiple engine sensors provide information to the suspension ECU allowing it to accurately compute how much toe change to make at the rear wheels and in which direction within milliseconds, constantly updating and recalculating as long as the car is moving. The ECU then sends a certain voltage to the actuators, making them move by a specified amount and in a certain direction to alter the handling characteristics of the car. So, how does this technology actually make the car handle better and go around a circuit faster? The ARS allows the car to generate a 'virtual wheelbase’. The wheelbase of a car is the distance between the centre of the front and rear wheels. This can vary from something like a Lotus Exige S3 with a 2.4-metre wheelbase through to a BMW F82 M4 with a 2.8-metre wheelbase. Using the benefits of short and longwheelbase cars has forever been a tuning factor for manufacturers. The Lancia Delta Integrale WRC car comes to mind where its short wheelbase for the time allowed the car to be much more nimble and turn more sharply on the rally course, helping it to victory. A long wheelbase, on the other hand, generates stability, allowing the car to travel at very high speed while staying safe and calm through long, fast corners. Although the wheelbase of a car is a very fixed physical feature, we can use toe geometry settings to alter the behaviour of the car in the same way that making a wheelbase change would do. If the rear wheels are set to toe in, creating a longer virtual wheelbase, this creates a more stable chassis at high speed and makes the car more planted and faster through high-speed corners. However, this holds the car back and can cause understeer through slower, tighter corners. If we toe the rear wheels out, creating a shorter virtual wheelbase, we generate the opposite effect. The car will be much more nimble and darty though slow, tight corners but would feel a little unsettled and twitchy through long, high-speed corners, forcing the driver to back off a little.

This is where the ARS system comes into its own. Having the ability to know when the car is cornering at low speed or high speed allows the system to create the ideal virtual wheelbase of the car for any given corner. The amount of toe in or toe out can also be varied for all of the different speeds and scenarios in between the ideal long fast corner and short tight twisty section. The ever-changing rear toe setting produces a car that can feel very stable and allow you to stay committed to the throttle through fast sections, and then when you enter the tighter sections, will completely alter the dynamics and allow the car to turn in and rotate towards the apex as if the wheelbase was much shorter than it is. This ability to constantly change the dynamics has created the ultimate, reduced-compromise all-rounder out on track. If you’re sat wondering if your handling will be forever compromised because your car doesn’t have ARS, there is still something you can do. Although your chosen toe settings will be fixed, our setups have the ability to fine-tune the fixed settings to create a neutral chassis by installing our own bespoke settings to the geometry. As standard, most supercars come out the factory with a relatively long wheelbase and toe settings that make the car feel even longer, therefore, we can tune the camber and toe settings on any supercar to generate a more nimble and dynamic chassis. @suspensionsecrets