FAQ #176
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Hydractive Suspension
Citroen's hydractive suspension was first introduced on the XM and is a refinement of Citroen's justly famous oleopneumatic suspension first seen on the Traction Avant as long ago as 1952. It uses a central ECU to control the firmness of the suspension, cutting down roll and sharpening the reactions to the steering, while keeping the ride quality good.

At the heart of the system is a third sphere at each end of the car, in addition to the traditional wheel spheres. This is switched in and out of circuit by the computer as required - Citroen claimed in 0.05 of a second. The effect of switching the extra spheres out of circuit is to firm up the suspension considerably, as the springing is handled by only 2/3 of the gas volume. This gives the system two different modes: normal (soft) and sport (firm). The basic suspension set up can therefore be made quite soft and comfortable for normal driving, with the computer automatically switching to firm mode and back when conditions require, giving owners the best of both worlds: a wonderful ride and superb handling.

The microprocessor based control system has inputs from five different sensors:

1. gives information on the angle and rotational speed of the steering wheel

2. monitors the speed of movement of the accelerator pedal

3. assesses braking effort

4. measures body movement

5. measures vehicle speed

These help the computer to determine the overall sensitivity of the system.

In addition to the height lever present on all hydraulically suspended Citroens, Hydractive cars posess a mode switch. This enables the driver to select the normal mode, or a firmer program - 'Sport'.
Here is a Video of how it all actually works:

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FAQ Posted by C6Dave
Info Created: 07 October 2009
Last Updated: 06 February 2015