Four-wheel steering
It can be a challenge trying to ensure cars handle more securely at high speeds without affecting their agility.
Chris Randall looks at one of the solutions.
Fitting a car with four-wheel steering isn’t a new idea. Daimler-benz designed just such a system for the German Forestry Service in the 1930s. Back then, the aim was to help vehicles negotiate corners on tight mountain roads, but it would be a few decades before other car-makers took advantage of the benefits for normal road use.
Japanese manufacturers were among the first to embrace the technology. Honda tested its own system in 1981, using the front ends of two Accords welded together. Launched to the public in 1987 on the Honda Prelude, the system was entirely mechanical at first (electric motor operation didn’t arrive until 1991) and used a shaft from the front steering rack to transmit steering angle to a planetary gearbox mounted at the rear axle; steering arms then moved the rear wheels by a few degrees.
Other manufacturers soon joined in. Nissan fitted some models with its HICAS and Super HICAS systems, which used computer-controlled hydraulic actuators to move the rear wheels.
Four-wheel steering then seemed to fall out of favour as car-makers concentrated on other technologies to improve the handling of their cars, but the last few years has seen something of a resurgence of interest and there are now a variety of models offered with such an arrangement. For some manufacturers it gives the opportunity to bring sharper, more dynamic handling to the mix. For example, some versions of the Porsche 911 employ electro-mechanical actuators to generate rear steering angles, while Renault’s Megane RS promises the ultimate in agility thanks to its 4CONTROL system. The latter can even be operated in the car’s Race mode, where the switch from opposite to same direction rear steering happens at 62mph rather than 37mph – perfect for setting fast times on a race track!
It’s not just high-performance vehicles that are benefiting. As modern cars get larger with each generation, one of the challenges facing carmakers is not only ensuring safe handling at high speeds – during emergency lane changes for example – but also increasing manoeuvrability in congested urban streets because four-wheel steering effectively shortens the wheelbase.
This is particularly relevant given the rise in popularity of large SUVS. One example is the 2018 VW Touareg. The first VW to be fitted with a steering rear axle, it claims to reduce the turning circle from 12.19 to 11.19 metres – a big improvement. Where modern systems differ from Honda’s 1980s coupé is that they are essentially governed by road speed rather than the steering angle applied to the front wheels. In the case of the Touareg, travelling at up to 37kph can see the rear wheels turn in the opposite direction to the fronts by as much as 5°, while above this speed they turn in the same direction.
Unsurprisingly, such systems have become increasing complex as they make use of the vast computing power that modern cars offer. Take Audi’s Allwheel Steering, for example. Fitted to models including the A8 luxury saloon and Q7 SUV, the system is managed by the Electronic Chassis Platform (ECP), which utilises a central control unit to analyse and record various parameters including road speed, roll and pitch movements, and yaw angles. Algorithms process information every 1 to 25 milliseconds, controlling fourwheel drive and adaptive suspension systems as well the rear-wheel steering that operates via an electric spindle drive and two track rods; by combining operation of front and rear steering, it can vary the overall steering ratio between 9.5:1 and 17:1.
The Dynamic All-wheel Steering fitted to the latest Audi A6 is even more impressive, combining rearwheel steering with what Audi refer to as ‘active super-position steering’ at the front wheels. This arrangement minimises the indirectness in the steering caused as the rear wheels turn in the same direction as the fronts, and also reduces the amount of lock needed to negotiate any given corner. It works by momentarily decoupling the steering column from the front wheels, allowing the controlling electronics to turn them independently at a sharper angle than that applied by the driver at the steering wheel. The result, Audi says, “combines direct, sporty steering response with unshakeable stability.”
Whatever the results, it’s certainly impressive technology and is another example of how modern car-makers are striving to make their vehicles safer and more entertaining to drive.