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ENGINEERS LEFT CONVENTION ON THE SHELF: DESIGN USED BEST OF CARS, TRUCKS
By DALE JEWETT Engineering Editor
VANCE, Ala. - Just another leather-lined truck. When Mercedes-Benz announced in 1993 that it would build a sport-utility, many expected the company to follow conventional wisdom - start with a truck base, swathe the interior in leather and lots of amenities, and try to soften the ride as much as possible. The German automaker certainly wouldn't have had a technical problem doing that. With a sturdy frame, boxy styling and three locking differentials, the Gelaendewagen that Mercedes produced for military and public use in Europe already had an off-road pedigree. Although it has never been officially imported into the United States, the G-Wagen has a small but loyal following here. But Mercedes had a philosophical problem with that approach. The vehicle being built here in central Alabama is, instead, a testament to unconventional thinking. Mercedes passed up existing hardware and technology to enter the sport-utility market with an all-new design. Yes, the M class rides on a separate frame like most other truck-based sport-utilities. With eight inches of ground clearance, it provides that critical command-of-the-road view that many sport-utility buyers crave. But the M class rides on a four-wheel independent suspension. That suspension is prevalent on sports cars and luxury sedans but is a first in the compact sport-utility segment. And missing from the M class' drivetrain are the common locking differential and solid axle. When the M class needs traction, its antilock brake system helps send torque to the wheel or wheels that need it most. In short, the M class received the best of both worlds - trucklike utility, luxury-car ride. EXCEEDING EXPECTATIONS "The perception was that the M class would be a Range Rover or a Lexus. We wanted to combat that impression," says Stephen Cannon, regional marketing manager for Mercedes-Benz U.S. International Inc. in Tuscaloosa, Ala. Cannon, whose territory includes the United States, Canada, Mexico and Japan, says Mercedes wanted to offer more than an expensive truck with luxurious appointments. The technological mandate for Gerhard Fritz, M-class project engineering director, was to deliver the smooth ride of luxury cars along with off-road capability to claw over hills and run through streams without missing a beat. Fritz and the M-class team delivered. On the highway, a blindfolded passenger would be hard-pressed to tell the difference between the M class and a Mercedes sedan. On an off-road course constructed behind the Alabama assembly plant, the M class navigates terrain so tight and tough that instructors will not drive a Ford Explorer or Jeep Grand Cherokee over it. The M class' success in straddling the line between luxury car and off-road truck hinges on three elements: 1. Body on frame construction. 2. Four-wheel independent suspension. 3. Adaptation of electronic traction control to divert as much as 100 percent of torque to one wheel [This is incorrect. It should say: to one axle instead of to one wheel. - wolfgang]. To start its work, Fritz's team set two competitive goals. For on-road ride, Mercedes' own cars were the benchmark. For off-road performance, the goal was to become the competitive benchmark for all other makers. FRAME VS. UNIBODY As Mercedes engineers began laying down the first lines for the M class, the debate flared over whether the vehicle should be a body-on-frame or unibody. The "smooth-ride" camp backed a unibody. It made sense, since all of Mercedes' cars used unibody construction. And there was precedent. The Jeep Grand Cherokee, Nissan Pathfinder and Infiniti QX4 use a unibody construction, and those sport-utilities receive high marks for on-road ride quality. Other engineers argued that the unibody approach was not stiff enough to survive the rigors of real off-road driving. The debate was heated, and went on for several months. But the deciding issue came down to the ML320's off-road capabilities. "We had to deliver on the off-road credentials," says Cannon. "We were a luxury-car maker entering a new segment. For us to be taken seriously, we had to walk the walk." To meet the goal, the M class would ride on a stiff frame made up of steel, box-section side rails and three crossmembers welded together. A stiff, steel body sits on that frame, insulated by 10 rubber body mounts. Vibrations are further filtered out by front and rear subframes that serve as mounting points for the lower arms of the suspension, as well as for the rack-and-pinion steering gear. The front and rear of the frame are extended and closed off with bumperlike crossmembers to create "crash boxes." The steel squares deform in a minor accident to prevent the entire frame from being damaged. They are also easier to replace. THE INDEPENDENT ROUTE The heart of the M class' ride and handling is its four-wheel independent suspension - a first in the compact sport-utility category. In front, the upper control arms attach to the frame, while the lower control arms mount to the subframe, along with the steering gear. Torsion bar springs, anchored to the center crossmember, take up less space than coil springs, allowing the ML320 to show a lower hood line. At the rear, the upper control arm again mounts to the frame, and the lower arm to the subframe. The rear suspension uses progressive-rate coil springs, which react differently to different impacts. "It gives you a lot of flexibility to influence the ride of the vehicle, much more than a rigid axle," Fritz says of his suspension choice. Fritz's decision was influenced, in part, by the fact that high-performance off-road vehicles such as the AM General Hummer and entries in the Paris-Dakar road rally use fully independent suspensions. It was also a logical step forward in the evolution of sport-utilities, Fritz says. The first sport-utilities, such as the Ford Bronco and Jeep Wrangler, had solid axles front and rear. Over the last few years, most sport-utilities have switched to an independent front suspension, but kept the solid axle in the rear. Four-wheel independent suspension is being used on the smaller car-based sport-utilities, such as the Honda CR-V and Toyota RAV4. An independent suspension allows each wheel to move up and down separately from the wheel directly opposite. With a solid axle, hitting a pothole with one wheel also influences the opposite wheel on the same axle. "With a rigid axle, you use stiff springs to obtain good off-road capabilities," Fritz says, "but you get a poor ride on paved roads. If you make the springs soft for better on-road ride, you get a lot of self steering from road bumps, and that's unwanted. "For us it was clear that it would take an independent axle to combine the off-road abilities and the luxury-car ride," Fritz says. "We've compared the M class with the G-Wagen on the off-road test track. The M class runs faster." OTHER SAVINGS Fritz scoffs at the notion that an independent suspension is not tough enough to handle the rigors of serious off-road driving. "Conventional wisdom says a rigid axle is more stable off-road, but that is not the case," Fritz says. "If you hit a big bump, or boulder, you can damage the axle. Once the axle is bent, it's over. With an independent suspension, you might bend a control arm but it would keep going." Because an independent suspension contains several bushings between its elements, engineers have more options in tuning the suspension to block out vibration. Because the M class chose a heavier steel frame over a unibody, engineers had to find weight savings in other components. That made an independent suspension more appealing. The ML320 version weighs about one-third what a solid axle would have, thanks to extensive use of aluminum suspension components. The upper control arms of the front suspension and the upper and lower control arms of the rear suspension are made from aluminum, as are the differential covers. Reducing the amount of unsprung weight improved the M class' ride and handling. It also helped save money, Fritz notes. The aluminum components do not have to be reworked or painted after being cast. The independent suspension also let Mercedes lower the center of gravity about 2 inches, Fritz says, which lowered the step-in height and improved stability in turns. Even though about 75 percent of sport-utilities are equipped with four-wheel drive, the number of owners who regularly use it and go off-road is small, says Cannon. "Most customers don't know when to lock the differential to get the most out of their system," he says. Mercedes' response was to make its 4wd system as seamless to the driver as possible. The most revolutionary part of the M class' full-time 4wd drivetrain is the adaptation of Mercedes' electronic traction control system. The system will channel up to 100 percent of the torque to a single wheel. At the same time, the new traction system trimmed hundreds of pounds from the ML320. Fritz says it was a given that the M class would have three differentials to insure superior off-road capabilities. While many sport-utilities offer locking center and rear differentials, few offer a locking front differential - and then only at extra cost. The plan created a new challenge for the Mercedes team. Fritz and his engineers had to ensure the vehicle's ABS would remain functional at all times. On some sport-utilities, locking the differentials for better traction disables the ABS. That can be a problem, Fritz notes, when after a vehicle goes up a snowy hill, there is no ABS to help control the descent. At first the M-class team began testing torque-on-demand systems, which sense wheel slippage and automatically direct torque to the axle with the most traction. Mercedes has used such a system, dubbed 4Matic, on some of its cars. IT WON'T WORK While they were an improvement over the conventional 4wd systems, a torque-on-demand system would not achieve the team's goal, Fritz says. "There are disadvantages to the traditional system. In a 180-degree turn, it's not always smooth because the system continually engages and disengages," he says. Coincidentally, as the M-class team worked in 1993, Mercedes engineers were developing an electronic traction system for passenger cars. To see if it might work for the M class, Fritz had a four-wheel version of the system mounted on a G-Wagen. He did not expect it to work. 'We thought that the ETS would cause the brakes to heat up," Fritz says. But it didn't. "We discovered that when the system works, the vehicle is typically moving slowly," he adds. So not much heat is generated. "During testing, we had lights rigged inside to show when the brakes applied at each wheel. It was amazing to see how short a time a brake is activated - only about three to five seconds." The system had to overcome another technical problem. The Mercedes 4wd relies on three independent, or "open," differentials. Open differentials are simple since they lack components to coordinate their actions. Their weakness: They transfer torque to the wheel with less resistance. This was good for turning a corner, where the outside wheel turns faster than the inside. But in slippery conditions, a vehicle gets stuck when one wheel loses traction, and all torque goes to that wheel. Limited-slip differentials, by comparison, detect a spinning wheel and transfer torque to the other wheel. A locking differential mechanically locks the two wheels on an axle together. This ensures that traction goes to both wheels. But by being locked together, the outer wheel is unable to turn faster than the inner wheel during a turn, causing it to bind. That slows steering response and sends vibrations into the passenger compartment. The Mercedes traction system essentially "fools" open differentials. When a wheel's speed sensor detects that it is spinning faster than the other wheels, the brake is applied to slow the spinning wheel to the same speed as the other wheels. Slowing the wheel channels the torque through the open differentials to the wheels with traction. The breakthrough enabled Mercedes to use the simpler open differentials. That allowed the vehicle to save about 15.5 pounds, compared to a torque-on-demand system, Fritz says. It also was less expensive. The only limit to the M class 4wd system is the friction of the road, Fritz says. "Earlier this year we had 20 days of rain in Alabama," he says, "but the M class was doing 180-degree turns with no problems."
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