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Active Suspensions

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On: Thu, Sep 22, 2011 at 4:31PM | By: Peter C Sessler

Active Suspensions

Aren’t all suspensions “active”—that is, some part of the suspension is always doing something whenever a vehicle is being driven?  Like everything automotive, the suspension in your car is a compromise. A car could be made to corner really well—but that usually comes at the expense of a smooth ride. Or a car can be set up to ride softly, yet when it encounters a series of bumps the suspension gets flustered and the wheels lose traction. So, as long as the road conditions encountered are within the suspension's built-in parameters, and you’re aware of them, everything is fine.

What it comes down to is that a typical suspension is passive. It cannot compensate beyond its designed parameters and that is why you’ll have situations where you’ll be bottoming out or where your car will be screeching around turns, etc.

An active suspension system, on the other hand, has the ability to extend those built-in parameters because the system constantly adjusts itself to changing road conditions, sort of like your suspension system—your legs. On the other hand, it can’t overcome the laws of physics—if you go around a corner too fast, you’ll still end up in the bushes.

Several high-end manufacturers use active suspension systems; while these may differ from manufacturer to manufacturer, they basically work the same way, with adjustable springs and shocks, a series of sensors throughout the car and at each wheel, and an actuator or servo attached to each spring and shock. The heart of the system is the electronic brain (Electronic Control Unit or ECU).

The ECU is programmed to maintain a particular kind of ride character, so it is constantly adjusting shock stiffness and spring rates to maintain that particular ride character. Let’s say you’re in the process of making a right turn and you encounter a series of large potholes. A car with a conventional suspension can have its suspension system over-extended—meaning, as you hit these holes the car starts bouncing up and down with the possibility of losing control if you don’t slow down.

In active suspension, the sensors telegraph what’s happening to the ECU, which then analyzes the data and, within 10 milliseconds, responds accordingly. It signals the actuators at each wheel to stiffen up the springs and shocks on the wheels that are losing control so that the car just glides through the corner. An engine-driven oil pump operating at 3,000 psi does the actual adjustment at each wheel hydraulically. Once the car is through the potholes, the ECU instructs the wheels to resume their normal ride characteristics.

It’s quite an amazing feat, and the ride difference between a car with a conventional system and one with an active system is also amazing. The downside, of course, is increased complexity and cost. Pity the car owner whose warranty has expired when something goes wrong with an active suspension!

Currently, you’ll find active suspensions at Mercedes-Benz, BMW, Audi, Cadillac, and other makes as well. Still, like most things automotive, features that are found on the more expensive cars eventually find their way down to lower priced cars, too.


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