Semi-Active Suspension

Active Roll Control

Tenneco Kinetic Suspension, e.g. McLaren PCC

Tenneco Kinetic suspension is brought to the world's attention by McLaren MP4-12C, on which the system is called PCC (Proactive Chassis Control). The latter is the technology's first application on production performance cars. It shows an amazing combination of smooth ride and excellent body control. Here we are going to see how it works.

The Kinetic suspension was invented by Australian engineer Chris Heyring. American component maker Tenneco acquired its patent in the late 1990s and developed it for production. Early adoptors include Citroen C4 rally car (before WRC banned the tech) and Lexus GX470 SUV. This suspension utilizes hydraulic and gas pressure to provide active anti-roll function, saving the need for conventional anti-roll bars thus improving ride comfort. It may also provide adaptive damping, therefore it is regarded as semi-active suspension.

The hardware of Kinetic suspension looks similar to Citroen's Hydractive suspensions, but it differs in details and principles. In place of conventional dampers are four hydraulic cylinders, each contains a compression chamber and an expansion chamber. They are interconnected in a way that the compression chambers on the right wheels link to the expansion chambers on the left wheels, vice versa. Besides, front and rear wheels are connected. The picture below should give you a clear idea:

There are two "roll accumulators", each locate at one of the hydraulic circuit. The accumulator contains oil (from the circuit) and pressurized gas, separating by a flexible membrane (like Citroen's suspensions). Electronic control unit can vary the gas pressure. If it pumps more gas into the accumulator, more oil will be pushed back to the circuit. If it releases gas pressure, more oil will be allowed to flow into the accumulator.

Now see my exclusive illustration below. In case the car steers quickly to one side, centrifugal force is built up at the opposite side, leading to body roll. As a result, the inside wheel dips and the outside wheel compresses. This leads to the orange circuit attains increased pressure from both cylinders, driving more oil into accumulator A. On the contrary, the blue circuit attains reduced pressure from both sides, therefore oil flows from accumulator B to both cylinders.

Now, if we increase the gas pressure in accumulator A, oil will be pushed back to the cylinders, pressing down the outside wheel and lifting up the inside wheel, thus eliminating body roll. The higher gas pressure injected, the stronger it resists body roll. As you can see, each roll accumulator is resonsible for regulating body roll to one direction.

What if the car is not cornering, but hitting a bump on the road ? Oil from the compressed wheel will flow towards other wheels, thus the suspension absorbs the bump comfortably. In contrast, conventional suspensions with anti-roll bars always return a stiff ride on road irregularities.

How does the Kinetic suspension achieve adaptive damping ? Turn back to our first picture, you can see the oil flows in/out the hydraulic chambers via a valve, whose degree of opening is adjustable. This ensure the ECU can control the speed of oil flow, hence the damping rate.

Copyright© 1998-2011 by Mark Wan
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