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ZF supplies rearsteering system for Porsche 911 Turbo S Tom Murphy
<p><strong>ZF supplies rear-steering system for Porsche 911 Turbo S.</strong></p>

4-Wheel Steering Market on Rebound

Germany&rsquo;s ZF provides rear-steering as standard equipment on four Porsche models, and four other customers will use the supplier&rsquo;s technology within the next two years.

GREENVILLE, SC – The future for 4-wheel-steering systems appeared bleak a decade ago when Delphi’s innovative (but expensive) Quadrasteer system on fullsize General Motors pickups arrived and left the market within three years, partly because of a lackluster sales-and-marketing effort.

But today, modern systems capable of steering the rear wheels are finding new applications in performance cars such as the Porsche 911 Turbo, Turbo S, GT3 and 918 Spyder and in at least two luxury cars, the Acura RLX and new TLX.

At last week’s Los Angeles auto show media preview, Audi unveiled the Prologue high-end coupe concept with 4-wheel steering.

Since November 2013, German supplier ZF has delivered its Active Kinematics Control system as standard equipment in the high-powered Porsches as a way to improve handling at the track, where the rear-engine 911 has exhibited a tendency to oversteer.

At a media program here, ZF says it has three more customers (but won’t identify them) planning to launch AKC next year, and a fourth one year after that.

The Porsche system requires two electrically driven actuators – one for each rear wheel – but future customers will employ one actuator on the rear axle to control both wheels, the supplier says.

The ZF system can turn the rear wheels up to 5 degrees in either direction, and Porsche opted for 3 degrees for its applications. If the vehicle loses power, the rear wheels remain locked in the normal position.

At low speed, such as in parking lots, AKC turns the rear wheels in the opposite direction as the front wheels, which helps the back end come around more readily and results in a tighter turning circle.

In a standard vehicle without rear steering, the yaw axis around which the vehicle rotates is “quite a ways behind the driver,” says Mark Rotary, engineering manager of ZF’s Chassis Technology Division.

But with rear steering, that axis moves forward, just behind the front seats. “It feels like you’re turning on a top,” Rotary says.

On the highway, during a lane change, for instance, the system steers the wheels in the same direction as the front for improved stability. “You won’t have lag time between the steering input up front and the time the rear of the vehicle follows you,” Rotary says.

ZF demonstrated the technology on an abandoned airstrip here, allowing journalists to drive a 911 Turbo S equipped with AKC. On a slalom course, the car makes easy work of the maneuvers.

In the case of the 911, ZF’s Stewart Snead says the system is intelligent enough during aggressive maneuvers to detect oversteer and mitigate it by turning the rear wheels “in phase,” in the same direction as the front.

In general, the system requires less steering effort on the part of the driver, which is especially noticeable during an emergency lane-change to avoid a collision. “You don’t have to work the car as hard,” says Snead, global program manager for ZF Chassis. “It’s really evident on the Porsche.”

Keeping System Cost Down

ZF assembles the system at its plant in Lebring, Austria, and began shipping units to Porsche in May 2013. Those vehicles became available in the U.S. last November, Snead says.

In the past, rear-steering systems have been expensive, but ZF has addressed that issue by developing a modular tool kit, allowing use of as many common parts as possible.

“We will use common belts, pulleys, linear sensors and so on. We have a position sensor in the electric motor,” Rotary says. “There are a lot of common pieces that gives us the ability to build on two lines, and we get economies of scale.”

ZF may step up its development of rear-steering systems as it absorbs TRW, which specializes in steering, braking and safety systems. Some 79% of TRW shareholders have voted in favor of the acquisition.

ZF had a steering joint venture with Bosch but is surrendering its stake for antitrust purposes. Bosch has said it will buy out ZF’s share of the JV.

AKC was one of many technologies ZF featured at its 1-day TechAcademy, which covered the supplier’s two primary segments, chassis and driveline.

On the driveline side, ZF is one of the world’s largest independent transmission producers.

In 2013, 93.7% of the transmissions it sold worldwide were automatics, compared with 3.5% for manuals and 2.8% for dual-clutch units.

The company produced continuously variable transmissions in the U.S. 10 years ago for Ford, but that JV at a plant in Batavia, OH, fizzled after a few short years for a variety of reasons. The plant is closed now.

As fuel-economy requirements ramp up, automakers are showing greater interest in CVTs. Asked if ZF would consider re-entering the CVT market, James Potter, controls manager for ZF Powertrain Technology, says: “I would assume yes, if it made business sense for us, that we would get back into that business.”

Test vehicles with ZF 8-speed automatic transmissions offered for media evaluation at the event here included the BMW ActiveHybrid3, 5-Series, X4 and X5; Jaguar F-Type; Jeep Grand Cherokee; and Ram 1500 pickup. Also offered were the Chrysler 200 AWD, Jeep Cherokee and Land Rover Evoque, all with ZF 9-speed autos.

Vehicles with ZF 7-speed DCTs were the Porsche Boxster, 911 and 911 Turbo.

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