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Technology: How To Turn on a Dime

6 minute read
Philip Elmer-Dewitt

“Let me explain it to you,” says Jackie Mason in his television commercials for the new Honda Prelude with four-wheel steering. Jabbing his elbows this way and that, the Borscht Belt funnyman proceeds to confuse a subject that is already complicated: “The car is going like this, the wheel is going like that, you’re going like this because you can’t figure out where did the back go?”

The pitch may be skewed, but the viewer gets the message: something new and strange is happening to the way cars are steered. Like so many technological advances these days, this one was made in Japan. Honda and Mazda began showing 1988 models with four-wheel steering in U.S. showrooms in September, and san and Mitsubishi are expected to follow quickly. Detroit’s carmakers say they are still studying what is turning out to be the most talked about automotive innovation of the year.

What is four-wheel steering? The concept is simple. Rather than controlling a car solely by the angle at which the front tires meet the road — the method used by wheeled vehicles since the horse-drawn carriage — four-wheel steering turns the wheels simultaneously at both ends of the car. The idea is intuitively appealing to any city driver who has ever pulled up to a too-short parking space and wished he could point all four tires toward the curb and crab right in.

Not so easy. For starters, the rear wheels of a four-wheel-steer car do not always turn in tandem with the front wheels. Depending on the speed of the car, the rear wheels may turn in the same direction (same-side steering) as the front wheels, or in the opposite direction (countersteering). Most of the new four-wheel-steer autos are capable of both countersteering and same-side steering. In sharp, slow-speed turns, countersteering can shave a full yard off a standard sedan’s turning radius. At high speeds, however, countersteering can make a car dangerously unstable, while same-side steering actually improves the ride.

The difference comes from the dynamics of high-speed motoring. When a driver traveling at highway speeds turns the wheel of a conventional, two-wheel steering car, the front tires immediately begin to pivot and the car’s forward momentum generates a powerful sideways or cornering force at the front axle. The rear tires, however, have to wait until the car has actually started its turn before they begin to generate a corresponding force at the rear axle. That is why a car with two-wheel steering fishtails during lane changes, the back end is trying to catch up to the front. In extreme cases, or under slippery conditions, the rear of the car may fishtail out of control.

In a four-wheel-steer car, this high-speed sway can be damped or even eliminated through the use of same-side steering. When the rear wheels are turned at the same time and in the same direction as the front wheels, the back end turns with the front, and the cornering forces occur at both axles simultaneously. The car slides smoothly to the side without sway or fishtail.

But how can the rear wheels turn one way at low speeds and another direction at high? That is the central engineering problem in four-wheel steering, and the competing Japanese automakers have come up with an astonishing variety of technological solutions. Mazda’s 626 four-door sedan solves the problem electronically. Sensors monitor the car’s speed and its front-wheel angle and pass the information to an onboard computer, which determines in what direction the back wheels should turn. At speeds less than 22 m.p.h., the rear wheels countersteer; at more than 22 m.p.h., they turn in the same direction as the front wheels. Mitsubishi’s version is simpler but more limited. On its four-wheel-steer Galant models, now available in Japan, the rear wheels are incapable of countersteering, but they can turn with the front wheels at high speeds. The action is controlled hydraulically: as speed increases, so does the oil pressure in the car’s rear differential, the gear assembly that transfers the engine’s driving force to the rear axles. The higher the oil pressure, the more sharply the rear wheels turn.

Honda’s system is strictly mechanical, taking clever advantage of the fact that high-speed turns require relatively small movements of the steering wheel, while sharp turns at low speeds require large movements. When the steering wheel is turned through its first 140 degrees, a system of gears and shafts turns the rear tires the same way as the front. Between 140 degrees and , 246 degrees, the rear tires return to the straight-ahead position. When the wheel is turned beyond 246 degrees, the rear tires begin to pivot the opposite direction from the front tires.

Ingenious? No doubt. But how do these cars handle on the road? The news from test drivers, though generally positive, is mixed. All agree that shoehorn parking is dramatically easier — so much so that first-time users sometimes pivot the car too sharply and clip their fenders. At high speeds, an even more surprising effect sets in. As predicted, the cars glide effortlessly from lane to lane. But they also exhibit a slight sluggishness, as if drivers had to rotate the steering wheel further to get the same results.

It turns out that the sway the Japanese engineers worked so diligently to eliminate actually helps cars get around corners more quickly — a fact that race car drivers use to their advantage when they skid around sharp turns. Undaunted, Nissan has begun showing off a computer-controlled system that counteracts the perceived lack of crispness by reintroducing a touch of fishtail. During a high-speed turn, the system puts the rear wheels through a split-second of countersteer before turning them in the same direction as the front wheels. Result: the enhanced safety of four-wheel steering is retained, but the apparent sluggishness of the turns disappears.

Experts differ on four-wheel steering’s potential. Jerry Rivard, vice president of Bendix Electronics, a major auto supplier, calls it a “dramatic jump in technology” and predicts that it will be standard equipment on cars of the future. Ron Glantz, an auto analyst at Montgomery Securities, feels otherwise. “Other than parking,” he says, “the only benefit is on gravel roads at speeds over 70 m.p.h.” In Japan, where the technology was first marketed more than two years ago, car buyers seem favorably impressed. Nissan reports that 40% of the Japanese who pick the flashy Skyline model ask for four-wheel steering. Some 75% of those buying new Honda Prelude in Japan have purchased the high-tech option.

At the Big Three U.S. automakers, spokesmen say the companies are still assessing market demand. Donald Runkle, a director of advanced vehicle engineering at General Motors, acknowledges that four-wheel steering can improve handling and maneuverability. The question, he says, is whether Americans will be willing to pay the premium of some $1,000 the Japanese are charging. “We have come down on the side of it not being worth what it costs ! right now,” says Runkle. “But we could be wrong. Honda could come in here and clean our clock with four-wheel steering.”

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