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Keeping it fluid

From air to fluids that respond to magnetic fields, suspension systems have come a long way from struts

india Updated: Aug 28, 2009 11:27 IST
Grease Monkey

We revisit suspension one final time. Last week, you read that McPherson struts for the front is the usual choice of manufacturers today. We also learned that they’re not the choice for performance cars, because, as the wheel travels up and down, it also describes a slight arc. This means that it isn’t always dead vertical, and the bit of the tyre that is in contact with the road — its ‘footprint’, so to speak — becomes smaller due to that angle. This will usually happen on a turn, when you need maximum grip at your disposal. Therefore, luxury cars and the sporting ones, of course, have got complex systems, and they’ll bandy about phrases like ‘double wishbone’ and ‘multi-link’ to convince you that their system is all that and then some. The ‘wishbone’ is named after the chicken bone, because the two look similar. These make sure the tyre is dead vertical at all times, for maximum grip and safety.

The right suspension
Air suspension is the logical step forward for manufacturers, since it allows the car to ride softly on a bumpy, uneven road, and when the driver wants to make like Karthikeyan, he can firm up the suspension. This can be achieved in one of two ways: one is to change the size of the opening of the valve, and the other is to change the viscosity of the fluid in the dampers. You can’t do the latter very effectively with air, so manufacturers have come up with an elegant solution.

Which is...
The valve adjustment is generally available on adjustable suspension units like aftermarket ones, or for racing applications. Changing the viscosity of the fluid is a complex problem, though. Let’s take a step back, though: ‘viscosity’ is, in a way, the opposite of fluidity. Honey is viscous, as is maple syrup and oil paint. Air and water are fluid. Do not mix the property of their fluidity with their being a fluid — water is a fluid, but air is fluid.A fluid whose viscosity can be changed will serve the purpose ideally, as it can be made fluid for a soft ride, and viscous if better handling is desired. This will also eliminate the need for a system to adjust the valves on the fly. Cars with air-based damping systems usually have a ‘Comfort’ and a ‘Sport’ mode to satisfy the driver’s requirements, but other cars go a step further.

The Audi R8 and Ferrari 599 GTB, for example, have the option of a suspension whose dampers have a ‘smart’ fluid that responds to magnetic fields due to tiny particles in it. These tiny particles increase or decrease the viscosity of the fluid depending on how strong the magnetic field is. It is then a matter of supplying an electric current to an electromagnet situated around or at the damper to stiffen or soften it. For the record, both cars mentioned have had their respective semi-active suspensions developed by Delphi, and the fluid is called a ‘magnetorheological’ fluid. A few followers of the old school, however, will have nothing to do with these new technologies, and still swear by a well-tuned normal suspension. Sometimes you do see the sense in that, like the time I came across a premium SUV with air-filled dampers that had squatted on all fours — and all because a switch got stuck!