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Suspension Of Disbelief

Magnetorheological (MR) shock absorbers use tiny particles of iron suspended in a viscous fluid as the damping medium.

Magnetorheological (MR) shock absorbers use tiny particles of iron suspended in a viscous fluid as the damping medium. A 12-volt electromagnet stationed on the outer surface of the flow channel acts as the valving mechanism; it constricts the flow of the fluid by magnetizing the iron particulates. The amount of constriction depends upon the energy fed into the magnet (usually a maximum of 5 amps). The fluid can go from free-flowing to rock-solid faster than you can read this sentence. Control algorithms take readings from sensors around the vehicle (yaw rate, steering angle, wheel speed, etc.) to determine the exact damping rate needed at any moment, and adjusts the magnetic field accordingly. This real-time damping ability alters the relationship between ride and handling by allowing the use of softer springs and smaller anti-roll bars without adversely affecting body motion and wheel control. All of which makes MR technology something of a silver bullet, though one that has yet to get the use its ability suggests is achievable.

“Some OEMs still see MR as a risky technology, in part due to its exclusive use on GM vehicles [the Corvette, various Cadillac models and Buick’s Lucerne], and partly because they are unfamiliar with its in-use history,” says David Hoptry, marketing manager, Chassis Systems at Delphi (Dayton, OH; www.delphi.com). “Yet, if anything, our analysis of high-mileage parts we’ve collected from around the country show that our testing procedures in the lab are too stringent, and far exceed anything we’ve seen out in the field.” Concerns about leaking seals, clumping of the iron particles, and wear of moving parts is misplaced, says Kristopher Burson, marketing manager, Materials Business Unit, Lord Corp. (Cary, NC; www.lord.com). “We are the only company out there with a commercially available MR fluid in large part because we have spent the past 20 years developing the additive packages and granular shapes and sizes necessary to make this technology work everyday, no matter the weather.”

The beauty of MR fluid, says Hoptry is that it has “damping authority and lots of it” that can be altered near-instantaneously in an analog fashion. “It’s more than on-off,” he says. And that ability to infinitely vary the damping force within the jounce and rebound parameters of a shock absorber not only fundamentally changes the ride-handling compromise by increasing the suspension’s effective bandwidth, it offers automakers the opportunity to standardize a platform’s specification while customizing each model’s feel. “Imagine the overall savings available as manufacturers specify exactly the same dampers for each vehicle built off a common platform, and download a specific ride-handling algorithm at the end of the assembly line,” says Hoptry. “Especially when you realize the MR units have 60% fewer parts than controllable dampers built using conventional technology, and are no bigger than non-controllable units.” Unfortunately, as Hoptry will admit, OEM purchasing departments often view components on a piece-price and not a systems-cost basis, which lessens their enthusiasm.

Nevertheless, Delphi and Lord continue to refine the fluid and control software, and revise its capabilities to work with other vehicle technologies or within other systems. These include using the damper’s capabilities to expand the dynamic envelope of an electronic stability control system, delay or soften its initiation through greater body and wheel control, and adapting the technology to other parts of the vehicle. “We have seen increasing interest in MR engine mounts, fan clutches, and differential clutch packs for all-wheel-drive systems,” says Burson, who admits the technology also has been used in sprung seats on over-the-road trucks and to provide force feedback on a steer-by-wire forklift technology demonstrator.

Despite the reticence on the part of some companies to adopt MR damping units, Hoptry says the next five years will likely see it move from luxury vehicles to sports cars to SUVs and into leading edge mid-size family sedans. “It may take 10 years to move below that level,” he admits, “but the six applications we now have with GM will expand significantly by 2007.” Expect 11 applications worldwide by 2007, including two from non-GM European automakers.—CAS