Soft suspension system keeps car stable, level

The problem with conventional suspension systems is that theycan't change with the road surface.

The typical Chicago driver might want a soft suspension for ridecomfort over snow packed roads and urban streets. At high speeds,the same suspension will cause the car to roll excessively incorners, and dive during hard braking. While antilock brakes,traction and stability control prevent the driver from losing controlin these situations, the better alternative is a suspension whichkeeps the car stable and level.

Active Body Control, Mercedes-Benz's new active suspension system,does just that, by making instantaneous adjustments based on roadsurface, vehicle speed and other inputs to an on-boardmicroprocessor. The system, introduced on the 2000 CL coupe, isavailable for 2001 on all CL models and S Class sedans. The systemreduces body role by 68 percent in "comfort" mode, or up to 95percent when the driver switches the switches the suspension to"sport" mode.

Thirteen sensors monitor the car's level, vertical and transversemovement, providing the on-board computer with new information everyten milliseconds. In addition, the computer interfaces with over 20other on-board computers, for information on driver behavior, engineload, ambient temperature, road surface conditions etc. Opticalbuses transmit the data between microprocessors, reducing the amountof wiring in the car, and making it easier for technicians todiagnose any electronic problems.

Based on this information, hydraulics adjust the suspensionutilizing servos on top of each steel coil spring. An engine drivenoil pump maintains 2,840 pounds per square inch at two pressurereservoirs (one at either end of the car). When the computercalculates a need for stiffer springs, hydraulic fluid moves into theservo reservoir within milliseconds. The system rechecks itselfevery 10 milliseconds, and an oil cooler maintains correct operatingtemperatures in the hydraulic system.

"ABC is a performance enthusiast's dream," said AMG ProductManager, Rob Allen. "In the past, car enthusiasts have installedstiffer shocks, springs and stabilizer bars to optimize high speedhandling. The problem is that these suspensions made the car hop onuneven road surfaces, and the cars weren't comfortable to ride in. The electronic technology gives the driver the best of both worlds,something a traditional mechanical system simply can't do."

Because active suspension handles low body frequency movements(five Hertz or less), stabilizer bars are no longer necessary. Thatbrings the additional advantages of less body weight and fewermechanical parts to wear out during the life of the car.

Mercedes-Benz engineers began testing prototypes for activesuspension in 1987. Two years later, a version was developed for theMercedes Group C race car. The passenger car version of the systemdebuted at the 1996 Hanover Show on a Mercedes-Benz high decker bus.According to the manufacturer, ABC is the first active suspensionsystem in a production car.

Nina Padgett-Russin is a certified mechanic and Phoenix-based autowriter. Write to her c; o Chicago Sun-Times, Fourth Floor, 401 N.Wabash, Chicago 60611. Please enclose a stamped, self-addressedenvelope.