Citroen XM Forum

Hydractive shouldn't switch to hard mode at all below 18mph.

Jorgy and Dieselman have both de-railed me bit over the behaviour of Hydractive over speed-humps, so I've done a bit of checking.

The speed bumps here which can be a bit unpleasant are effectively elongated ones (often carrying a full-sized pedestrian crossing), and are typically 3-4m in length (in direction of travel), and up to 15cm high. They are flat-ish topped (as a rule), but have either straight-and-gentle or sometimes bull-nosed approaches/departures. The latter can be quite nasty.

As the car crosses the hump, the front wheels rise, and, if the vehicle speed is more than about 10km/h, the suspension switches to firm (a dash LED extinguishes) before the back wheels have mounted the hump. The suspension stays firm, and the back wheels come off the hump with a bit of a thud (but no bounce). If the road speed is kept below about 10km/h, the suspension stays soft, and the hump is taken comfortably.

It's been suggested that this isn't correct, but the Consultant Technician training manual (B7, June 2001, on Hydractive II), states, on page 59:

SWITCHING TO FIRM BY REACTION

Body movement

The vehicle speed must be greater than 10km/h and the movement [of the body, as detected by the front ARB/sensor] (attack or release) must be greater than a limit as a function of vehicle speed.

The suspension will return to soft state . . . when the amplitude of movement is less . . . and after a 0.8s timer.

The graph on the same page shows the limits of body movement against speed: (Normal Position, Attack)

Below 10km/h: No shift to firm
10-40km/h: 84mm+ of movement shifts to firm
40-90km/h: 54mm+ of movement shifts to firm
etc

So, my reading of this is that, if a speed hump raises the front wheels by more than 84mm at a road speed of just 10km/h+, then things go firm. Hence the back-end bump, particularly as the car suddenly exits the hump, before the suspension has had a chance to revert to soft.

Or am I wrong on this?