Rear Steer

| December 12, 2008

European system improves maneuverability, conserves tires.

The roads outside the small German village of Kulmbach are typically European: narrow, windy and mountainous. But on a warm July day, the cramped farm roads are no match for a bright blue Class 8 tractor. It cuts through the Bavarian countryside like a sedan and winds through alleyways and congested village streets like a small van.

Even European trucks, which tend to have more wheel cut than their U.S. counterparts, would have trouble with the turns made by this German-made MAN bobtail. The MAN, one of Europe’s most popular brands, features an electronically controlled rear-axle steering system that dramatically reduces turning radius and saves tire wear in tight turning situations.

The system, the Servocom RAS-EC, is made by German manufacturer ZF, known in the United States for its two-pedal automatic ZF Meritor Freedomline transmission. Like other rear-axle steering systems found in some city buses and off-road equipment in the United States, the Servocom RAS-EC is applied to one of two rear axles on a standard vehicle, in this case a cabover. Although rear steering systems can be applied to drive axles, the ZF system was mounted on a non-drive axle.

The system tested in Kulmbach adds 15 degrees of wheel cut to the rear of a truck with an already robust 52-degree front axle cut. Combine that with the better turning radius of a cabover and the truck begins to turn like a car – a small car.

Rear steering systems can also cut down on wear. Normally in a tight turn, tires on the rear axles tend to drag rather than roll. The same turns at higher speeds can make a truck tippy. But with a rear steering axle working in tandem with a truck’s front axle, the tires don’t drag, and the truck’s body doesn’t pitch and yaw as it does on a standard truck.

Docking maneuvers become a cinch because the system aids in backing just as it does in navigating turns. The area needed to back a truck into a tight spot decreases dramatically.

Truck makers selling to the European market, faced with ancient city roads built for horses and wagons, have long designed vehicles with high maneuverability. Japanese automakers offered four-wheel steering in cars to U.S. consumers 20 years ago. Many specialized off-road construction vehicles and some emergency equipment also feature four-wheel steering. City buses have adapted the technology, too.

But those vehicles, as well as sport-utilities and pickups that have rear steering, are a far cry from the average over-the-road tractor-trailer, and with good reason, says ZF’s Peter Walker. “For a highway truck, a system like this would be a waste,” he says. Plus, Walker says, the wider U.S. roads can accommodate trucks with less maneuverability. ZF has no plans to export its system here.

That’s not surprising to Steve Slesinski, product manager for Dana’s drive axle division. Even in the application that makes most sense – city delivery – the going would be tough in the States. Urban delivery vehicles tend to be low-cost; if rear steering adds several thousand dollars to the vehicle’s price, as well as more maintenance costs, owners might balk. Consequently, he says, “It’s something we probably won’t see anytime soon.”

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