Magnetorheological dampers are adaptive dampers that are filled with a specialised magnetorheological fluid. The characteristics of this fluid can be altered by magnetic fields, generated by electromagnetic coils, allowing for the damping characteristics of the damper to be changed on the fly.
This means, at the press of a button or from a command issued by a control unit, the behaviour of the damper can be altered to better suit the road or driving style at hand. Adaptive dampers are not uncommon but they often rely on solenoids and valves that control apertures through which oil flows in the damper. A smaller aperture restricts flow, which firms up the damper, and vice versa.
In a magnetorheological damper, there are no moving parts - aside from the rod and piston assembly - because all the adjustment is carried out by applying current to the integrated electromagnet. This more straightforward design results in improved reliability. As magnetorheological dampers cannot put energy into the suspension, they are classed as a type of 'semi-active' or 'adaptive' suspension. Unlike the more complicated and expensive active systems, though, magnetorheological dampers consume far less power.
How do magnetorheological dampers work?
A conventional passive damper effectively consists of a piston in a cylinder filled with oil. The piston has tiny holes in it and, as it moves up and down as the wheels encounter bumps, oil is forced at pressure through these holes. The force required to push the oil through the openings in the piston, as the damper compresses and extends, slows down the motion of the suspension - helping damp out bumps and control the motion of the vehicle.
During the design of the damper, the diameter of these passageways - and the weight of the oil - will be adjusted to tune its damping forces to best suit the vehicle at hand. The response of these dampers cannot be changed once they on the car, however, so most manufacturers have to opt for a compromised calibration that offers a balance between appropriate handling and ride comfort - although there are some workarounds, such as ZF's 'Sensitive Damping Control'.
Using an adaptive damper, such as magnetorheological units, allows for the damping force of the suspension to be adjusted - so it can be automatically adapted to best suit the conditions, or switched between distinct modes. Unlike a conventional damper, a magnetorheological damper is filled with a magnetorheological fluid.
This 'MR' fluid consists of magnetic particles suspended in oil. Left to their own devices, these magnetic particles are distributed randomly in the oil. It consequently flows through the orifices in the damper's piston with ease.
When a magnetic field is generated by electromagnetic coils integrated into the damper's piston, the particles travelling through the piston align themselves with the magnetic field - effectively increasing the viscosity of the fluid. This restricts the flow through the orifices in the piston, increasing the damping force.
The dampers themselves are regulated by a control unit. Motion sensors at each wheel deliver data on the surface of the road, while driver input - and mode selection - is also monitored. Data from the sensors is processed, the appropriate damper force calculated and the correct amount of current applied to the coils within the dampers. This process is carried out, according to the manufacturer, up to 1000 times a second. The continual adjustment of the damping force consequently allows a car with such a suspension system to deliver improved handling, control and comfort.
A brief history of magnetorheological dampers
Magnetorheological dampers were first introduced in January 2002 as an option for the Cadillac Seville STS. The system was developed and supplied by the GM-owned Delphi Corporation, which dubbed it 'Magneride', while the magnetorheological fluid itself came from the LORD Corporation.
The system then made its way into the 2003 C5 Corvette, in which it was called 'Magnetic Selective Ride Control'. GM also used in it several other Cadillacs and continues to use it in many of its cars today, including the Camaro ZL1 and Corvette C7.
Magneride dampers were also used in the 2006 Ferrari 599 GTB Fiorano and the Audi TT Coupe of the same year; they have since been installed in a plethora of other cars - including the Ford Mustang, Audi S3 and R8, Land Rover Discovery Sport, Acura & Honda NSX, Ferrari 458 Italia and LaFerrari, GMC Yukon and the Lamborghini Aventador.
Today, Magneride systems are supplied by a Chinese group called Beijing West Industries. Delphi was spun off by GM in 1999 but begin to struggle and, in 2005, the company filed for bankruptcy protection. During its restructuring, the brake and suspension division was sold to BWI in 2009.
BWI continues to develop and produce Magneride systems and its customers including Ford, BMW, Audi, Ferrari, GM and Jaguar Land Rover. BWI has also developed magnetorheological engine mounts, which are used by companies such as Mercedes-AMG and Porsche.
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