Beyond All-Wheel Drive, All-Wheel Steering

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The notion of making vehicles turn better by steering all four of their wheels has inspired engineers for decades. The United States Army experimented with all-wheel-steering jeeps during World War II, and the benefits to maneuverability of rear-steering have long been known to firefighters assigned to hook-and-ladder trucks.

As demonstrated by its new flagship sedan, the Acura RLX, Honda Motor is taking its luxury division down the same well-traveled path. But the company is adding some new gloss to an old idea.

The 2014 RLX, on display in the Jacob K. Javits Convention Center when the New York auto show opened to the public on Friday, incorporates a feature that Acura calls Precision All-Wheel Steer. The system is intended to improve the handling of the front-drive RLX by adding steering capability to the rear wheels.

The cheeky acronym that appears on the car's trunk lid, P-AWS, seems well suited to a technology that the company claims will give the $50,000 RLX more catlike reflexes.

The image enhancement promised by the P-AWS system is something that Acura's top model could certainly use. Its predecessor, the 4-wheel-drive RL, was a chronic slow seller; to compete with the likes of the Audi A6 and A7, BMW 5 Series, Infiniti Q50, Lexus GS and Mercedes-Benz E-Class, the RLX will need the ability to take corners as well as these poised -- and predominantly rear-drive -- sedans.

Starting in the 1980s, Japanese automakers like Honda, Mazda, Mitsubishi and Nissan brought four-wheel-steering to performance models, and in 2002-5, General Motors marketed its Quadrasteer system on full-size pickups. But when sales dipped with the economy, the systems, which were heavy and costly, faded away.

With miniaturized electronics and the widespread adoption of electric power steering, rear-steering is now experiencing something of a resurgence. BMW offers it on the 5 Series, as will Porsche on the coming 911 GT3.

In the new RLX, the rear wheels are steered by electric actuators on each side of the rear suspension. As a result, the left and right rear wheels can steer independently of each other. The system, which was in development for five years, adds just 11 pounds to the car's weight, said Yousuke Sekino, chief engineer of the RLX, whose credentials include designing the suspensions for the 1986 Acura Legend and the 1990 NSX supercar.

Mr. Sekino likens P-AWS to the two skis of an Olympic slalom competitor. "By controlling both skis independently, the skier can control his turning and braking very precisely," he said.

Unlike the front wheels, which turn in a wide sweep, the RLX's rear wheels turn only a small amount, a maximum of two degrees left or two degrees right from center. They also react differently depending on the situation. Under braking, both turn inward slightly to increase the car's straight-ahead stability. As the driver turns into a right-hand corner, the rear wheels steer slightly left to make the car turn more quickly; in a left-hand corner, they do the opposite.

At parking lot speeds, this opposite-direction steering also makes the car more maneuverable in tight spots. At freeway speeds, the system automatically switches modes; the rear wheels then turn in the same direction as the fronts. That's to improve control in situations like diving across multiple lanes for an off-ramp.

The almost-undetectable labors of these systems are intended to make the RLX more agile and free it from understeer, or the tendency to plow forward even if the steering wheel is turned.

For those who have enough trouble steering just the front wheels, fear not; the actuators are not directly linked to the steering wheel. Rather, a computer determines from the driver's inputs into the steering wheel, brake and gas pedal how best to steer the rear wheels to improve handling. And, unlike Honda's previous four-wheel-steering system for the Prelude model of the 1990s, P-AWS shuts off when the car is in reverse. "Many drivers complained about that," Mr. Sekino said.

autonews

This article originally appeared in The New York Times.


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