Riding the (Magnetic) Wave

COLORADO SPRINGS - If you believe some of General Motors Corp.'s most impressive recent technical achievements have come from its work on advanced suspensions, you're about to have more supporting evidence.

The automaker's '02 Cadillac Seville STS will be the first vehicle in the world to offer MagneRide, a valveless suspension system - developed in conjunction with Delphi Automotive Systems - that can adjust "on the fly" from tight damping for sharp cornering and high-speed touring to supple-yet-comfortable ride quality for decayed urban streets.

MagneRide joins Mercedes-Benz's Active Body Control (see WAW - June, '99, p. 44) in employing newly available technology to deliver performance approaching the definition of "active" suspension. Delphi, though, is careful to describe MagneRide merely as "semi-active" - specifically referring to the system as continuously variable real-time damping (CVRTD).

The continuously variable damping system employs a unique "Magneto-Rheological" (MR) fluid in its monotube dampers. The fluid contains magnetically "soft" iron particles in suspension; a special additive keeps the particles from settling to the bottom. Under normal soft-driving conditions, the fluid in the dampers flows unimpeded by any sort of valving, giving a soft, compliant ride.

To offset the input of large or high-frequency bumps or heavy cornering forces - or even acceleration "squat" or braking "dive" - an electromagnetic coil in each damper activates and the iron particles immediately respond, aligning themselves in fiber-like strands that suddenly make the MR fluid's rheology nearly plastic. The almost instantaneous effect is a firmer damping force, the firmness calculated and controlled via the input from several sensors.

The beautiful delicacy is that this happens in about one millisecond - and that the effect is infinitely variable, based solely on electrical input to each strut's electromagnetic coil.

Cadillac's MR-based system will be 10 times faster than current mechanical variable-valve suspensions systems, GM says; response time, in context, is 25 times faster than an eye blink.

It appears esoteric, but the MagneRide system's MR fluid works on a well-understood principle - and importantly, does not require the elaborate sensors and actuators essential for other automakers' active suspension control systems, GM says. MagneRide uses a combination of existing sensors to observe damper speed and displacement from the vehicle's four corners and also correlates steering wheel and braking input from existing ABS/stability control sensors.

Equally important, there's precious little energy input required. Previous systems often sucked extravagant amounts of engine power to run pumps that energized mechanically motivated dampers. A Delphi spokesman says the MR system's full-power draw is roughly equivalent to juicing a 75-Watt light bulb, and that full power is seldom required for more than brief moments.

With enhanced control over chassis motions via computer algorithms instead of the mechanical capacities, the MR system also is easily tweaked; response and tuning for future vehicle architectures can be achieved without a major tear-up of the vehicle's existing chassis hardware.

Delphi will make MagneRide at its Kettering, OH, facility.

Even before the MR system is introduced, Cadillac can boast of outperforming its rivals in several controlled handling tests. With journalists competing against one another, the current '01 STS - equipped with the enhanced StabiliTrak stability control system - punishes the rear-drive Toyota Motor Corp. Lexus GS400 and Mercedes E430 in a timed demonstration on loose dirt, wet pavement and a slalom course. Only BMW AG's excellent 540i keeps up with the Caddy.

And Delphi recently treated us to an on-road demonstration of the MR system piped into a current Cadillac Catera. On a familiar stretch of two-lane road characterized by treacherous camel-hump features - "familiar" only because we've been seriously airborne and scraped oil pans on this road more than we'd like press-fleet administrators to know - the MR-equipped Catera is almost magical, handling speeds that quite possibly would damage or even cause serious loss of control in a conventionally sprung vehicle.

Based on our early experiences, the Cadillac MR suspension already appears formidable. But Delphi development engineer Sinisa Mandich says there's room for improvement before Cadillac puts MR into production.