Technology In Depth: Magnetorheological Damping

By David L. Miller

The Perfect Ride

Imagine that you are driving down the street when—wham—the impact of a pothole bottoms your suspension and nearly chips your teeth. You probably remember it from a couple days ago if you live in Chicago, New York, or any of a number of especially pot-holed cities. Even here in usually sunny southern California, when the rains come, so do the potholes.

Now imagine the same scene, but instead of the wham-bam of pothole percussion, your ride gently glides over that gaping crater, barely disturbed. Sure it will, if you’ve got some kind of Star Wars Anti-gravity Glider, but not in this world. Right?

Not so fast, Motoring Skeptic. There is nothing in physics saying that tire has to fall all the way into the pothole, hit the trailing lip, then bounce up until the suspension hits the stops. It does so because your shock absorbers are not up to the task—yet.

The perfect shock would anticipate that pothole and keep the tire from falling into it. The tire would glide right over it. Actually, we’re talking about an entire active suspension at this point, one that can move the tire to follow the contours of the road. That is still largely a concept except perhaps in some DARPA-sponsored lab, but part of that concept, electronic damping, is here right now. With electronic damping, your shock can be stiff when it needs to be, such as when the wheel is falling into that pothole. With stiff enough damping, it won’t fall all the way in. It can be soft a few milliseconds later when the wheel first strikes the pothole’s lip, lessening the initial jolt. Then it can quickly stiffen up again as the wheel travels upward, preventing the suspension from reaching its upper stop. Sweet!

Electronic Damping

Several methods have been implemented to electronically control shock absorbers’ damping. Some have worked better than others. One key to a system that functions well is speed of response.

Imagine a speed bump a foot across. You are approaching it at 60 mph. I know, not likely, but bear with me. At this speed, your tires will travel across that bump in about 11 milliseconds. A shock absorber which can handle that speed bump will need to change its damping characteristics over a wide range, from soft to stiff to soft again, very quickly. And it will need to know how much to change the damping during that 11 milliseconds. Enter the microprocessor, electronic sensors, and a magnetorheological liquid.

Magnetorheology and Computers—An Excellent Combination

A magnetorheological (MR) material changes its viscosity in response to a magnetic field. Viscosity is a measure of the “runniness” of a fluid. Apply a magnetic field from a coil and the MR fluid becomes viscous, thick, almost solid, like cold honey. Remove the field and it becomes thin and runny, like water. And it does all this within a millisecond! If we replace the oil normally found in our shocks with this fluid and add a microprocessor-controlled magnetic coil, voila, a millisecond-response variable damping shock absorber! We call this type of electronic damping system the MR shock.

We could, with some trial and error, tune a conventional shock so that it could respond optimally to one type of impulse, or bump, at one speed of travel. But with the MR shock, we can have optimum response at every instant of the suspension’s motion. This is not quite our ideal Anti-gravity Glider, because we can’t yet put energy into the suspension to drive the wheel up over an obstacle just as the tire reaches it, but we are getting close.

MR shocks are a major advance over conventional passive shocks if engineered correctly for the car in which they are installed. An accurate physical model of the suspension must be entered into the microprocessor, the appropriate acceleration and position sensors must be in place, and the control software must be correctly written. We’re talking almost rocket science. Sorry, you aftermarket mavens, this type of electronic damping is OEM territory.

Where Can I Buy MR Shocks?

Several companies include MR shocks in their cars. This concept was pioneered by GM, who included Delphi’s MagneRide system in the 2003 50th Anniversary Corvette. It is now used by GM in Cadillac (CTS-V), Chevrolet (Corvette Z06/ZR1, Camaro ZL1), as well as some high performance HSV-tweaked Holdens in Australia. Audi uses MR on its R8 and TT, Acura on its MDX and ZDX, BMW on its 3-, 5-, and 7-series, and Ferrari on its 599 and the new 458 Italia. The military has it on its Humvees. However, it is not clear from most manufacturers’ literature how quickly and often the electronics varies the damping rate, whether in response to immediate suspension position and velocity (millisecond scale) or to body pitch and roll and average road conditions (seconds to minutes scale).

Incidentally, Delphi is now owned by Beijing West Industries Group (BWI). Could MR suspension be coming to you soon in a BYD or Geely automobile? Time will tell.