The three most significant challenges in getting it there were making: 1) its rheological properties consistent over time, 2) its CIP not abrasive, and 3) its CIP not settle out of the fluid when the car was at rest. How Lord solved these problems, particularly the first, is a bit unclear. When asked specifics, Lord Corporation continued the company's policy of not commenting, then referred SUPER CHEVY to some public domain information.
The oil refining industry uses additives to make abrasive substances suspended in liquids nonabrasive. We believe similar additives are introduced into MRF which either coat or lubricate, or both, such that CIP becomes nonabrasive. The settling problem is also addressed with additives. Though we're not sure whether they form a buoyant coating on the CIP or whether the additive is a "thixotropic agent," which thickens the fluid while it's at rest so that it resists settling, but liquifies as soon as the fluid moves.
Changes in the fluid's rheology and abrasion are virtually nonexistent. MRF is resistant, but not totally immune, to the CIP settling. If a MR shock is at rest for six months or longer, there can be some settling, however, it takes only a few strokes of the shock for it to work as intended.
MR Shock HardwareThere are no valves in a Delphi MagneRide shock absorber. All are replaced by an electromagnet inside the piston assembly resulting in a 40 percent reduction in each shock's part count.
The rest of MR shock architecture is similar to that of a traditional, gas-charged, mono-tube unit. The shock tube bolts to the suspension. The piston rod bolts to the chassis. The rod ends in a piston, which moves in the tube. A piston seal prevents MRF from bypassing the piston and a piston rod seal keeps fluid from leaking out. At the bottom of the tube, a divider piston and seal separate the magnetorheological fluid from a gas chamber, which pressurizes the fluid to prevent foaming.
Each piston/electromagnet assembly has four annular channels. As the piston moves in the shock tube, magnetorheological fluid flows through these channels. When the magnetic field is weak, it flows freely. As the magnetic field gets stronger the yield stress of the fluid is altered.
This change in yield stress is localized inside the piston channels. As MRF flows into the channels, the CIP align in matrices and as the fluid exits, the matrices dissipate. The altered rheology fluid inside the channels restricts flow and dissipates the kinetic energy of the piston's motion thus absorbing the "shock" of suspension movement.
Control and the "Sky Hook" AlgorithmIt took scores of people, hundreds of millions of dollars and a decade of work to perfect Magnetic Selective Ride Control. The lion's share of that went into the last word: "control."
The heart of MR is a powerful, dual-processor controller capable of making 1,000 adjustments per second in each of the four shocks. At highway speeds that's a damping change about every inch the car moves.
MR's position sensors measure wheel movement. With that data, the controller determines three types of body motion: pitch, roll and heave. Pitch (front end moves up or down) and roll (the car leans), are terms we've heard before, but heave might be a new one. When the car heaves, both ends move the same way. If you drive over a rise, the body heaves up, then down. MR's controller also uses brake, throttle, steering angle, lateral acceleration, vehicle speed, and air temperature data in making its decisions
The controller processes sensor data with various software algorithms. When SUPER CHEVY talked with Mike Neal, Corvette's lead ride-and-handling engineer, we heard about "sky hook," an algorithm focused on isolation and body control. It gets its name from the idea that, if the car could be hooked to the sky, it would ride nice regardless of how nasty the road gets. Sky hook is not a literal term. As good as MR is, it cannot isolate the car completely.