Rear-wheel steering has some unique advantages.
Published on May 09, 2014 10:31:00 PM
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Rear-wheel steering schematic showing wheels steered in the same direction; when the vehicle is being driven at high speeds.
It isn’t everyday that you come across a driving experience that genuinely takes the game ahead. What’s even more thrilling is when it happens totally by surprise. But that’s exactly what happened when I first drove BMW’s then-new 750i back in 2009. Sure it was a BMW, but raw agility wasn’t what I was expecting from this massive barge of a car. What I was expecting, at best, was good stability and possibly a positive BMW-style turn in. What I got, however, was simply incredible high-speed agility and an ability to change direction at speed that bordered on spooky. The ability of this car to carry speed through corners without drama was simply baffling. The reason – a system very few customers opted for called Integral Active Steering; nothing more than four-wheel steering to you and me.
Now say the words rear-wheel steering to most people and their reaction isn’t very positive. On the contrary, most look like they’ve suddenly seen a hooded cobra pop up. The fear is understandable. We somehow imagine all things steered from the rear to be unstable. Much of this comes from the fact that cars don’t feel very stable when they are reversed at speed; the steering seems to want to flop from one side to the other. But this is only because the system has been designed to ‘drag’ the car around front the front. Think about it.
Actually steering a vehicle via the front wheels only is far from ideal. All four tyres carry the burden of taking the car onto a new path when the car is steered and the rear tyres and suspension are minutely side-slipped into place when you steer even a normal car. With four-wheel steering this de-stabilising transition phase is almost non-existent, and that allows you to accelerate and brake harder. So a properly designed four-wheel-steer system enhances agility.
That is probably why Porsche has gone hammer and tongs after this technology in recent years, a four-wheel-steer system appearing on both its top dogs; the new 911 GT3 and new 911 Turbo. The system comprises two electromechanical actuators that vary the steering angle of the rear wheels up to 1.5 degrees, depending on speed. Up to 50kph, the rear wheels are steered in the opposite direction to the front wheels. This shortens the wheelbase virtually by around 5.9 inches, making the 911 feel more agile. Above 50kph, the system steers the rear wheels parallel to the front wheels. Porsche says this archives a virtual 20-inch increase to the wheelbase, increasing high-speed stability.
Four-wheel steering will also feature on Porsche’s new hypercar, the 918, and I guess there’ll be others that follow suit as well.
Rear-wheel steering – it isn’t as whacked out as it initially sounds. Boats and aircraft, they are steered from the rear and the world’s fastest car, the Thrust SSC – the one that broke the sound barrier – that was steered from the rear too.
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