Hydractive for Dummies

[tim leech]

I've had over 50 hydraulic Citroens over the past 20 years but my Xantia Athena Diesel has a funny button in front of the height adjuster which I imagine Is hydractive.

The car rides well but isn't as good as my Desire which is running comfort spheres and is non hydractive.

As the instruction manual is missing (thankfully the service book is present) can anyone give me a idiots guide to what it does!.

Cheers

[RichardW]

It has 3 spheres per axle rather than 2. The centre sphere is undamped and softer than the outer 2, and can be isolated from the other 2 via an elctrovalve. Thus you can have 2 suspension modes 'hard' when only the outer spheres are in the circuit and 'soft' when the centre one is connected - and due to the way it is plumbed in, hard mode also serves to reduce body roll (but not eliminate - for that you need an Activa). A suspension ECU decides when you should have hard or soft mode - hard is selected under hard cornering, acceleration or braking. The switch merely selects 'normal' or 'sport' mode - which doesn't turn off the hydractive, but in sport mode it selects hard mode with less provocation.

There are lots of threads about poor ride on hydractives. First is to ensure the electrovalves are working properly, you should hear them hum for about 30s after you open the door, and there should be a noticeable difference in suspension stiffness between when they are humming (soft) and when they are not (hard). If they do not hum, then they possibly need additional diodes at the ECU (Search for e-crofting diode mod); or they may be knackered. If they hum but there is little or no difference, then the centre spheres are probably shot. They also seem to suffer from air ingress to the system (signified by a frothy LHM tank) - there are a couple of (very!) long threads about this somewhere. A mod to the return pipe in the tank can help to reduce this, and take some of the harshness out of the ride.

Some prefer the normal ride, as hydractive can be a bit crashy at times.

If your's is an Athena, then it's probably reg RXXX ?KV? - these were run out Mk1s registered by Citroen en-mass in 1997. I had R140 OKV and several others have surfaced on here over the years.

[tim leech]

Hi Richard

Yes its R305OKV! a very high spec with roof/air, leather steering wheel and oil temp gauge (looks like a VSX with cloth seats)

The previous owner bought it from his Citroen local garage when it was a few months old with very low mileage on and the V5 had Citroen UK as the first owner.

I had no idea it had hydractive, the ride seems fine to me, but I may fit some new comfort spheres (if this is possible) for ultimate waftyness. It does seem to roll less than the Desire.

I aim to bring the Desire to the Chevrons/CMX Rally in Sept so will ask Sir Jim of Eastment to cast his eye over it.

Thanks

[xantia_v6]

One slight correction to Richard's description: the centre sphere is not "undamped", the damper elements are mounted in the valve block (one per side) not on the sphere.

Don't fit normal comfort spheres, as the pressure and damping are wrong when the hydractive spheres get switched in, and the car wallows too much.

Some people like to fit normal non-hydractive spheres to the corners of a hydractive car, as this gives a similar effect to fitting comfort spheres to a non-hydractive car. I didn't like the effect, but I don't like comfort spheres either.

Some vendors of comfort spheres do sell versions for hydractive cars (and they might suit you) but note that because the centre damping elements are not part of a sphere, comfort spheres won't have optimal spring rate/damping balance. On a hydractive car with original spec spheres, the natural frequency of the suspension is similar in soft and hard modes, with softer corner spheres fitted, the natural frequency tends to be higher in soft mode.

[tim leech]

Thanks for that, the spheres seem ok, but I have no record of when they were last changed.

[Mandrake]

One slight correction to Richard's description: the centre sphere is not "undamped", the damper elements are mounted in the valve block (one per side) not on the sphere.

Don't fit normal comfort spheres, as the pressure and damping are wrong when the hydractive spheres get switched in, and the car wallows too much.

Some people like to fit normal non-hydractive spheres to the corners of a hydractive car, as this gives a similar effect to fitting comfort spheres to a non-hydractive car. I didn't like the effect, but I don't like comfort spheres either.

Some vendors of comfort spheres do sell versions for hydractive cars (and they might suit you) but note that because the centre damping elements are not part of a sphere, comfort spheres won't have optimal spring rate/damping balance.

Fully agree with all your comments above, but...

On a hydractive car with original spec spheres, the natural frequency of the suspension is similar in soft and hard modes, with softer corner spheres fitted, the natural frequency tends to be higher in soft mode.

Is not correct. The natural frequency of soft and hard modes is quite different, with soft mode having a lower natural resonance frequency.

For example see here:
http://www.citroenkerho.fi/xantia/pdf/o ... tive_2.pdf" onclick="window.open(this.href);return false;

" The suspension has a soft setting with low resonance frequency, and a hard suspension setting with a higher resonance frequency. "

That article doesn't quote the frequencies but from another document (which I can't find an online link to and don't have with me at the moment) the soft mode is 0.7Hz and from memory hard mode is either 1.0Hz or 1.2Hz. There is no way that the resonance frequency of a soft and hard suspension could be the same as the mass of the car is staying the same but the springing rate is changing. Stiffer springing rate will always increase the resonance frequency.

Damping also changes - soft mode is slightly "under damped" (a small amount of overshoot in the rebound recovery, albeit slow and well controlled) while hard mode is significantly "over damped", eg there is no over shoot at all during rebound recovery. When I say over damped I don't mean that it is too heavily damped, simply that it is more than critically damped, eg a resonance Q of less than 0.5. At a "guess" I would say that the damping in soft mode is a Q of about 0.6 and in hard mode a Q of about 0.3, but it doesn't directly translate to the Q of say an electrical circuit as the damping itself is very non linear with stroke and velocity unlike a damping resistor which is completely linear.