Suspension III: Active Suspension Systems


In our past two installments in this series, we looked at suspension theory and componentry. We also examined various spring and shock arrangements, and the advantages and disadvantages of each.

In this month's Tech Center column, we'll take a look at active suspension systems. Then we'll close by glancing at several current car models that offer this technology.

What is active suspension, and how does it differ from traditional suspension?

When we refer to a traditional or a conventional suspension system, we mean a system that comes "as is." In other words, a conventional system is a passive system. Once it's been installed in the car, its character changes very little.

This has certain advantages and disadvantages. On the plus side, the system is very predictable. Over time, you will develop a familiarity with your car's suspension. You will understand its capabilities and its limitations. On the down side, once the system has reached these limits, it has no way of compensating for situations beyond its design parameters. Thus shock absorbers bottom out, struts overextend, springs respond sluggishly, torsion bars get tweaked.

An active suspension system, on the other hand, has the capability to adjust itself continuously to changing road conditions. It "artificially" extends the design parameters of the system by constantly monitoring and adjusting itself, thereby changing its character on an ongoing basis. It's schizophrenic, if you will, but with a purpose. With advanced sensors and microprocessors feeding it information all the time, its identity remains fluid, contextual, amorphous. By changing its character to respond to varying road conditions, active suspension offers superior handling, road feel, responsiveness and safety.

Before we dive into active suspension systems, though, a word about why you should care.

True, this type of technology typically appears on very expensive cars. But, as with any new technology, a "trickle-down" effect takes place. Rapid advances in microprocessor science will soon bring these features to a whole new range of vehicles, including family sedans, minivans, trucks, SUVs, even compact cars. So consider this a primer about what's coming down the pike toward you. Hey, Velcro was developed in the space program, and look where it is today.

Active suspension systems (also known as Computerized Ride Control) consist of the following components: a computer or two (sometimes called an electronic control unit, or ECU, for short), adjustable shocks and springs, a series of sensors at each wheel and throughout the car, and an actuator or servo atop each shock and spring. The components may vary slightly from manufacturer to manufacturer, but these are the basic parts that make up an active suspension system.

As mentioned above, active suspension works by constantly sensing changes in the road surface and feeding that information, via the ECU, to the outlying components. These components then act upon the system to modify its character, adjusting shock stiffness, spring rate and the like, to improve ride performance, drivability, responsiveness, etc.

In a sense, active suspension mimics the functions of the human body. Consider:

  • The sensors are nerve ends — seeing, feeling, hearing, even tasting the road surface (ugh!) and delivering that data back to the ECU.
  • The Electronic Control Unit represents your mind, defective though it may be (residual damage from the sixties) — collecting, classifying, interpreting and analyzing sensory input. Once it has interpreted this data, it makes decisions and sends "marching orders" to the outposts in the hinterlands.
  • The wires connecting the whole thing are the central nervous system, stout deliverers of commands and directives. They dispense decrees, elucidate edicts, issue instructions. Hey, don't kill the messenger.
  • Lastly, the servos and actuators resemble the muso-skelatel portion of our show. They're the enforcers, the brutes. They carry out the commands. Don't get in their way, though; they don't think much. Think of them as Teamsters.

Okay, following that brief primer in Physiology 101, let's put the system to the test.

You're cruising down the road in a car with a fully active suspension system. Rounding your first turn, you hit a series of potholes, each one larger than the next.

Now, in a car with conventional suspension, these potholes might present a serious challenge to the suspension system. Their ever-increasing size could even max out the system, setting up an oscillation loop — a situation wherein the car begins to bob up and down higher and higher and gets a little out of control. But you're not worried, because your car has active suspension.

The sensors on the right front of the car (you're turning left, by the way) begin to monitor the situation. They pick up yaw and transverse body motion, and send this information back to the ECU. They also sense excessive vertical travel, particularly in the right-front region of the car. This data is likewise forwarded to the ECU. Rotary-position wheel sensors and a steering angle sensor confirm the data coming off the other "nerve endings" on the car.

The ECU collects, analyzes and interprets the data in approximately 10 milliseconds. It sends an urgent message to the servo atop the right-front coil spring to "stiffen up." To accomplish this, an engine-driven oil pump operating at nearly 3000 pounds per square inch sends additional fluid to the servo, which increases spring tension, thereby reducing body roll, yaw, and spring oscillation. A similar message, but of a slightly less intense nature, is sent to the servo atop the right-rear coil spring, with similar results.

At the same time, another set of actuators kicks in to temporarily increase the rigidity of the suspension dampers (a.k.a., shock absorbers) on the right-front and -rear corners of the car.

The vehicle glides through the turn, not even breathing hard. And neither are you.

The ride potential for such a system is truly spectacular. As we mentioned at the outset, several state-of-the-art production passenger vehicles offer fully active suspension systems as standard or optional equipment. Here's a list of all 2007 vehicles with this feature.

Acura - MDX

Audi - A8, Q7, R8, RS4, S8, TT

Bentley - Continental Flying Spur, GT, GTC

BMW - 5, 6, 7, Alpina B7, M5, M6, X5

Buick - Lucerne

Cadillac - DTS, Escalade, ESV, and EXT, SRX, STS, XLR, XLR-V

Chevrolet - Corvette, Suburban, Tahoe

Ferrari - 599 GTB Fiorano

GMC - Yukon and XL

Jaguar - S-Type, XJ, XK

Land Rover - Range Rover, Range Rover Sport

Lexus - GS430, GS450h, GX470, LS 460, LX470

Maserati - Quattroporte

Maybach - 57, 62

Mercedes-Benz - CL, CLS, E, GL, M, R, S, SL

Porsche - 911, Boxster, Cayenne, Cayman

Rolls-Royce - Phantom

Toyota - Land Cruiser

Volkswagen - Touareg 2

Volvo - S60, V70

Suspension Basics I: Shake, Rattle and Roll
Suspension II: Still Rocking After All These Years
Suspension III: Active Suspension Systems
Suspension IV: Improving Your Suspension

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