Brake-by-wire hits the road

A BMW minivan? Well, after a week of driving Mercedes' 2003 SL500, that's how my own BMW 528 felt when I got back behind the wheel. Now, the 528 is no slouch when it comes to tight handling, but the active suspension in the new coupe/roadster, coupled with the first use of electronically activated brakes, takes such control into the realm of dead-flat turns, no-dive stops, and level acceleration.

While the active suspension (initially introduced on the Mercedes 2000 CL Series coupe) is obvious in the reduced body roll, the brake wizardry is transparent to the driver-unless you're inadvertently at the edge of the envelope and the system brings you back to more positive control. Thus, while the laws of physics can't be changed, at the limits of control the car's systems are optimized to aid the driver.

Give me a brake. The pedal feels no different than with hydraulically controlled brakes. But between the driver and the wheels, the Bosch electronics are more than just a straight, lighter-weight replacement of a hydraulically controlled and powered system. Based on driver input (such as steering angle), the motion of the car (wheel speeds, yaw), the G forces acting on it, engine speed, and transmission gear selected, the controller drives modulator valves to produce the optimum brake pressure at each individual wheel. For example, if a driver enters a turn too fast and brakes, the electronics will apply most of the brake force to the two wheels on the outside of the turn to reduce the chance of skidding. The result is effectively a four-way split in braking functions as opposed to the twin, diagonally split systems, used on conventional cars with completely hydraulic brakes (see figure).

With no hydraulics to feed pressure back into the brake pedal, when the ABS function kicks in during a panic stop the pedal remains vibration free. And how's this for electronic legerdemain: In such an emergency stop, sensors detect the quick removal of the driver's foot from the gas pedal and signal the controller to prime the system with higher pressure and move the pads lightly against the disks (this is imperceptible to the driver). As soon as the foot presses the pedal, full brake force is applied using the high-pressure hydraulic accumulator. According to Mercedes Product Specialist Dave Larsen, this results in a roughly 3% reduction in stopping distance from highway speeds. At 40 mph, the reduction is about 6.5 ft.

Stop me. An electric pump pressurizes the system and an accumulator. This arrangement can deliver full braking pressure even with the engine off. And a "dry braking function" presses the pads lightly against the disks every 10-15 sec to keep them dry and ready for use when the windshield wipers are running.

If the controller fails or malfunctions, block values automatically slide open to connect the front brake disks to the brake pedal's hydraulic cylinder that now will act as a conventional master cylinder producing 80% of front braking power. Dividing pistons (check valves) in the front brake hydraulic circuits also isolate the backup system pressure from the rest of the circuit.

Other innovations include the previously mentioned ABC for active body (suspension) control, which was developed and produced by Mercedes. This system uses four hydraulic servos, one atop each steel coil spring at each wheel. The servo pistons apply force in response to signals from the ABC computer, which is updated with sensor readings every 10 msec, to drive the springs in relation to anticipated body movement. The active suspension eliminates the need for suspension stabilizer bars and handles low frequency(&5 Hz) motion. Conventional shocks and springs absorb higher frequency movement.

In its normal setting, ABC reduces body roll by 68%. In the Sport mode, 95% of roll is reduced, which produces enjoyable dead-flat turns and little body pitching motion. These level characteristics, combined with turning off the stability control to keep the engine revs up, allows exhilarating driving on twisty roads.

Mercedes engineers have ingeniously used aluminum, composites, and steel in the new SL500's structure to produce a car that is not only 80 lbs lighter than the previous model but has 20% greater body stiffness. And the aerodynamicists turned to a flat underbody, small spoilers ahead of each wheel, a rear-deck spoiler, and guided engine-compartment airflow to lower drag by 9% to a C_D of 0.29 (roof closed). Lift was also cut to improve directional stability. The ABC also contributes to cutting drag by lowering the body by 0.6 inches when the car reaches 60 mph.

With a base price of $85,990, the SL500 is not for most drivers. But we should look forward to its technology making its way into lower priced, more mainstream cars.

For more information about automotive electronics from Bosch, www.boschusa.com: Enter 543