Hydractive 2

The second incarnation of the hydractive suspension appeared at February 1, 1993 (ORGA 5929). It was designed to overcome the biggest problem of the previous system, the very uncomfortable hard mode.

Switching to Sport does not mean sticking to a hard, uncomfortable ride any more. On the Hydractive II, the relation between suspension modes and dashboard switch settings became more complicated: in both settings—Normal (the new name of Comfort) and Sport—the computer can switch to both hard and soft mode as it finds it necessary, however, when set to Sport, the suspension becomes more sensitive and will sooner and more often switch to the hard mode.

Many models were also fitted with an anti-sink system that locks the system when the car is not running, using yet another sphere. Its only purpose is to keep the car from sinking when not used, it does not influence the functioning of the suspension system in any way.

The center sphere circuits and supports were redesigned: they now house the electrovalves and the internal conduits serving the sphere were modified as well; the new control blocks connect, as previously, to the left and right corner spheres, the height corrector, and—depending on the control signal coming from the suspension computer—the center sphere. The elements are practically the same as on Hydractive I:

1 A sphere base;

2 A hydraulically controlled isolation valve;

3 A ball and piston valve;

4 Two damping elements;

5 An electrically controlled valve driven by the suspension computer.

The front and rear suspension circuits are identical but hydraulically independent. The electro-valves are driven simultaneously, in parallel.

Trapped among pistons

The electro-valve 5 is energized when the suspension is in its soft mode, hence, the default electrical position is hard. However, due to the indirect coupling between this valve and the isolation piston 2 inside the control block, the hydraulics can stay in either position for extended periods of time with the electric valve disconnected, depending on the pressure differences between the strut and the main circuits. If the main suspension circuit has nominal pressure, the system stays in hard mode with the electric valve off or disconnected.

The two modes are practically the same as on the previous Hydractive system: in soft mode the electro-valve 5 opens the feed pressure onto the isolation piston 2 and by moving it, connects the center sphere 1 to the rest of the suspension. In hard mode, the electro-valve 5 closes and lets the pressure inside the center sphere 1 move the control piston into a position which closes off the center sphere completely.

The center sphere 1 is now supplied directly from the height corrector in soft mode. This simplifies the ball valve arrangement with respect to Hydractive I.

Higher intelligence

The computer uses the same set of sensors as Hydractive I, the only difference is the vehicle speed sensor which is a Hall-effect sensor now. Its resolution have been doubled to 8 pulses generated per rotation, that is approximately 5 pulses per meter traveled (although this depends somewhat on tire size). It is located on the gearbox where the speedometer cable attaches, or in some versions, on the cable itself.

The internal algorithm of the computer became more sophisticated. While the Hydractive I had only one computer controlled mode (Sport switched the suspension to constant hard mode above 30 km/h of vehicle speed), the newer system has two such regimes of operation: in both Normal and Sport it dynamically activates the electro-valves of the suspension control blocks whenever it decides that the driving circumstances call for a firmer suspension. The difference is in the set of rules the computer uses to evaluate those circumstances: the rules are stricter for the Sport setting, with most of the thresholds reduced, thus the suspension will switch to hard mode much more readily.

The following table shows the thresholds of steering wheel angle. If the value observed by the sensor exceeds the threshold for the actual vehicle speed and the suspension setting, the suspension will switch to hard mode; it will revert to soft when the corresponding value drops below the threshold for at least 1.5 seconds:

There is a similar table for the thresholds of the steering wheel rotational speed as well. While the steering wheel is returning to the straight-ahead position, the thresholds are temporarily doubled.

The thresholds of the gas pedal sensor are:

The thresholds for body movement obey additional rules. Normally, when the compression (the body sinking) or the rebound (the body rising) exceeds the threshold specified in the Compression and Rebound values in the table, the suspension will be switched to hard mode with a timeout of 0.8 seconds (note that the thresholds are the same for both Normal and Sport suspension settings). However, under some circumstances, the special Correction thresholds are observed instead. There are two cases leading to these corrected thresholds:

• Flat tire correction: if the body movement speed exceeds 300 mm/s, the suspension switches to hard mode, and all thresholds are modified to 60 mm. The timeout of the correction will be 0.4 seconds.
• Excessive body movement correction: if the body movement exceeds 60 mm more than three times within three seconds, the suspension will switch to hard mode, and all thresholds are modified to 60 mm. The timeout of this correction will be 2 seconds.

These corrected thresholds will not be applied if the suspension is in Sport setting or the vehicle speed is over 159 km/h. The corrected threshold mode will be cancelled and the threshold returned to the original Compression and Rebound values in the table if the steering wheel angle exceeds the thresholds specified in the fourth column of the table.

With the improved resolution of the vehicle speed sensor, the rules formerly referencing to 30 km/h are changed to 24 km/h. Thus, the suspension switches to hard mode if the brake pressure sensor detects a pressure above 30 bar and a vehicle speed in excess of 24 km/h.

Similarly, the suspension will switch to soft mode if the ignition switch is turned on, for a maximum of 30 seconds, but reaching a vehicle speed of 24 km/h will cancel this mode prematurely. It will switch to soft also if any door or the tailgate is opened but the vehicle speed is below 24 km/h. The reason for this is to equalize the pressure between all three spheres of an axle. Without it, the center sphere would retain its former pressure and once the vehicle exceeds the speed of 24 km/h, opening it would make the car jump or drop, depending on the actual pressure.

It is important to note that the suspension will switch to soft mode even with the ignition switch turned off. Should the doors remain open with the ignition switch in the off position, the suspension soft mode will be subjected to a 10-minute timeout period to avoid draining the battery as the soft mode requires the electric valves to be energized.