Rear Wheel Steering: How (and Why) Does It Work?
Rear axle steering. It’s one of those features that has popped up a few times over the years but was usually ahead of its time.
Or at least ahead of the complexity required and the engineering cost to make it reliable. But that hasn’t stopped automakers from trying it. Over and over and over.
Honda was one of the first to bring rear-steer to the market in the late ’80s. The four-wheel steering system was followed Nissan’s HICAS system but both didn’t stick around. GM had Quadrasteer, which was offered on full-size trucks and 2500 series Suburbans in the mid-2000s. Porsche’s 928 had a passive rear axle that could increase rear toe-in when braking, but that’s not exactly the same.
There are a few different effects from steering the rear wheels. Increased stability, improved maneuverability, a decreased turning radius. Oh, and more steering parts to wear out and then fail. Especially if the manufacturer decides that it’s not going to keep offering the feature.
After a big gap, the systems are back. Audi, Cadillac, Porsche, Lamborghini, Acura, BMW, and even Ferrari are offering it these days. So why do automakers keep trying it? And how does it work?
The reason they keep trying it is maneuverability. Cars keep getting wider and longer. That means that turning in tight city conditions can be a challenge. Look at cars with Volvo’s old transverse-mount inline six-cylinder engines for an example, where the amount you could turn the front wheels was limited by the long engine. Turning radius was best measured in miles. Even without that restriction, wide tires on sports cars can limit front wheel cut (the amount you can turn them) and long-wheelbase cars make wider turns because of the distance between the front wheels and the rear pivot point.
Turn the rear wheels the opposite way as the front wheels and the car pivots around the center axis. The rear wheels don’t turn much, less than 15 degrees in most cases and just 3-5 in others. It makes the wheelbase feel shorter. The Cadillac CT6, which is a very large car, trims its turning radius by a full three feet with active rear steering. That’s a significant difference and puts the rear-steer CT6 within inches of the turning circle of the much smaller ATS. Nearly tighter than a standard Porsche 911. On the Quadrasteer Suburban, the turn circle was slashed an amazing 12.2 feet, again within a foot of a new 911.
Turn those back wheels at speed, though, and things get more interesting. Ever try and crank the wheel in reverse when you’re moving quickly? So at speed, the rear wheels turn in the same direction as the front. That lets the vehicle almost crab walk over into the next lane. Instead of turning, you’re making a diagonal movement. The effect is like making the wheelbase longer. Like turning a bus instead of a sports car. It’s a smoother movement, so the car feels more stable as you make a lane change. If you’re towing, like in the pickups, the trailer gently follows. It actually reduces sway and makes it easier to control.
So how does it work? Basically, there is a small steering rack fitted to the rear axle. Since the wheels don’t turn very far, it can be much more compact than what you’d find on the front wheels. From that there are tie rods that link to the rear hubs, and those rear hubs can pivot. It’s like a tiny version of the front suspension layout, mounted in the back.
There’s no steering column leading to the back wheels either. The call to steering is done by computer. A sensor on the steering column measures what you’re trying to do. It tells the computer, which also knows how quickly you’re moving. From there, the computer decides to either turn the rear wheels with the fronts or the opposite. The speeds where the direction changes depend on the manufacturer and the type of vehicle. On a 911, for example, the switch happens somewhere around 50 mph. Cadillac’s system changes the speed depending on which dynamic mode the vehicles is in.
Audi’s system (which also shows up in the new Touareg) works a bit differently. Instead of the smaller rear steering rack, each rear wheel has a small electric actuator. These take advantage of Audi’s 48-volt system. A moving link from the actuator to the hub toes each wheel in or out to steer the rear of the car. It’s the same basic idea, though. It moves the rear wheels and either makes the vehicle turn tighter or change lanes more smoothly.
So that’s how big vehicles use rear-steer to feel smaller. Or sports cars use it to feel even more tossable still. And high tech electronics and computers make it work.
A version of this story originally appeared on GM Inside News.
Evan moved from engineering to automotive journalism 10 years ago (it turns out cars are more interesting than fibreglass pipes), but has been following the auto industry for his entire life. Evan is an award-winning automotive writer and photographer and is the current President of the Automobile Journalists Association of Canada. You'll find him behind his keyboard, behind the wheel, or complaining that tiny sports cars are too small for his XXXL frame.
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