The Hydractive I suspension system appeared with the XM. Unlike the simpler hydropneumatic suspension used on the DS, GS/GSA, CX, BX and some XMs, this one has two modes of operation, soft and hard. The suspension functions in soft mode but it will be switched to the hard mode when the computer deems this necessary for the sake of roadholding and safety.
To achieve this, the first hydractive system adds two spheres (one for each axle) and an electric valve to the struts and spheres of the standard hydropneumatic setup.
During normal driving, the computer keeps the suspension in soft mode most of the time but—based on the input provided by many sensors (steering wheel, accelerator pedal, body movement, road speed and brake), including the Sport/Comfort switch on the dashboard—the suspension ECU decides when to switch to hard mode; in other words, when to deactivate the additional spheres for extra roadholding and safety.
When the driver selects the Sport setting, the suspension is switched to hard mode constantly. This setting is not what any Citroën driver would call comfortable… The successor system, Hydractive II overcomes this limitation.