The new C5 has a new suspension system, doing away with many solutions used on Citroëns for several decades, yet offering the same or even better comfort than before. Recent developments in electronics and computics made it possible to delegate many functions previously solved by mechanical-hydraulical components to electronic units.
This third generation suspension system retains the same basic functioning as the previous systems. It also comes in two flavors: a simpler Hydractive 3 reminiscent of the original hydropneumatic suspension of the DS–GS–BX–CX and a slightly more complicated Hydractive 3+, building upon the former Hydractive I and II (actually, Hydractive 3 is not hydractive in the sense we used this term before, its only special activity is to adjust the road clearance depending on speed and road condition).
Although the basic functioning is practically the same, the actual layout underwent significant changes. Most importantly, the previously mechanically operated height correctors became electronically controlled hydraulic units. And all hydraulic units except for the spheres—which were redesigned to give unlimited life expectancy—are now housed in a single unit, the Built-in Hydroelectronic Interface (BHI). This compact unit has three main parts:
- the high pressure for the new synthetic fluid (called LDS, orange in color) is generated by a five-piston hydraulic pump 1, driven by an electric motor (rotating at 2,300 rpm) operating independently of the engine, running only when necessary;
- the hydraulic units, including an accumulator 2 to even out the pressure pulsations of the pump, four electrovalves 3 and 4 and two hydraulic valves 8 serving the height regulation and anti-sink behavior, some in-line filters 5 and an overpressure valve 7 (taking the role of the pressure regulator of previous systems).
- the electronic computer 6, communicating with other computers across the multiplex network to read the inputs of various sensors and to control both the HP pump motor and the electrovalves.
In contrast to the height correctors of previous systems, operated mechanically via a linkage coupled to the anti-roll bars, the new system used electronic sensors to learn the actual height of the suspension and electric actuators to modify the ground clearance whenever needed. The main advantage of using them is that the ECU can implement very sophisticated algorithms to derive and apply height correction, what were impossible with the mechanically linked feedback with simple thresholds.
The computer 6 is connected to the CAN multiplex network, providing access to the messages sent by the BSI and its fellow computers controlling the engine and the ABS. The inputs the suspension ECU uses comprise of rear and front body height, brake pedal, vehicle speed and acceleration, open-closed status of the doors (including the tailgate), plus the steering steering wheel angle and rotating speed on the Hydractive 3+.
As usual with Citroëns, the driver can select from four height settings (although the selector is no longer mechanically coupled with the hydraulics, it is a simple electronic switch sending signals to the computer): high, track (plus 40 mm), normal and low. The selected setting is displayed on the multifunction screen in the dashboard. The computer also prevents unsuitable settings being selected. Neither high or low height is available when the car is traveling faster than 10 km/h and track height cannot be selected above 40 km/h.
In addition to the manual settings, the system adjusts the ground clearance automatically. Below 110 km/h on well surfaced roads the ride height remains standard but as soon as this speed is exceeded, the vehicle will be lowered by 15 mm at the front and 11 mm at the rear. This change lowers the center of gravity, improving stability, lowering fuel consumption (by reducing drag) and reducing the sensibility to crosswinds. The car resumes the standard ride height when its speed drops below 90 km/h.
On poorly surfaced roads (the computer learns about the road quality by monitoring data on vehicle speed, height and movement of the suspension) the ride height will be increased by around 13 mm but this setting is only used on very poor roads and with the vehicle traveling below 90 km/h.
The general height of the vehicle (filtering out rapid movements due to suspension travel) is checked, and if necessary, adjusted every 10 seconds and when any of the doors is opened or closed (even with the ignition switched off).
Just like its predecessor, this system also has two modes, firm and soft. A stiffness regulator—an additional sphere and a hydraulic control block per axle—isolates or connects the corner and center spheres. Its functioning is practically equivalent to the similar control block of the Hydractive II: the computer controlled electro-valve 4 opens the feed pressure onto the isolation piston 2 and by moving it, connects the center sphere 1 to the rest of the suspension, switching the suspension to soft mode. of the suspension. Closing the electro-valve 4 obstructs the hydraulic supply coming from the BHI; the residual pressure in the center sphere 1 moves the isolation piston 2 downwards into a position which closes off the center sphere completely: the suspension switches to hard mode.
The suspension has two settings the driver can choose from, Normal and Sport. The new stiffness regulators together with the center spheres are isolated in hard mode and re-activated in soft mode in response to the various inputs received and processed by the suspension ECU. The functioning of the computer is basically similar to the Hydractive II ECU: it uses tables and rules to set up thresholds on the value on many sensor inputs to determine when to switch to hard mode. Just like on its predecessor, the Sport setting does not mean constant hard mode, just lowered, more sensitive thresholds for the switching.
The computer observes the following input parameters: the height and sport settings specified by the driver (communicated by the BSI); the vehicle speed and the longitudinal-lateral acceleration of the body (communicated on the CAN), the angle and speed of rotation of the steering wheel (the type of the sensor depends on whether the car is equipped with ESP, in this case the sensor connects to the multiplex network instead of directly to the suspension ECU), the speed of suspension travel (using the values of the front and rear height sensors), the open-closed status of the doors (communicated by the BSI) and the movement of the accelerator pedal or butterfly.