Anti sink query

davetd · Unread post by **davetd** » 17 Apr 2006, 20:24

Can anti sink be fitted retrospectively to a pre anti sink Xantia?

My car is going in for it's bi-annual LHM change, and at the moment it sinks to the ground in 30 - 60 mins which is a tad quick. I'm going to question this at the garage but I was just curious to see if I could fit the anti sink unit without altering much

cheers

Davetd

AndersDK · Unread post by **AndersDK** » 18 Apr 2006, 00:07

Hi Dave -

Its not just fitting a valve into the existing pipework - front and rear. Its a complete rebuild of the system with new pipework and an extra sphere at rear.
To give you an idea : the antisink valve itself has 5 pipe connections - and you need a separate antisink valve front & rear. The antisink valve takes the feed for the suspension each axle - and then "communicate" with both the axle suspension cylinders and the Height controller. The "communication" is done by valves operating on pressure differentials in the system. There is no electronics involved.

No - I dont talk the sophisticated hydractive or Activa anti-roll system - it IS in fact the antisink system.

davetd · Unread post by **davetd** » 18 Apr 2006, 21:51

Thanks for the reply Anders. I'll leave it as it is

Why do pre anti sink cars sink at such different rates? Is there a non return valve to the pump or something? My car has an excellent service history and everything including the suspension has been serviced on time since new, so it's not poor maintenance, (which can lead to all sorts of horrors) that I've read about on this forum! I'm just curious

Regards

Davetd

Unread post by **citroenxm** » 18 Apr 2006, 22:42

Hi davetd

Its all to do with "Internal Leakage" between assosiated valves etc..

Think about all the LHM travelling around the pipe work etc, for what ever milage you have done, you will get wear - minimal though..

I guess, if you replaced ALL units - height correctors, pump, regulators, suspension struts etc, the car wouldn't sink so quickly..

Of course, OLD lhm can contribute to faster sinking too.

Regards
citroenxm

Mandrake · Unread post by **Mandrake** » 19 Apr 2006, 11:55

davetd wrote:Thanks for the reply Anders. I'll leave it as it is

Why do pre anti sink cars sink at such different rates? Is there a non return valve to the pump or something? My car has an excellent service history and everything including the suspension has been serviced on time since new, so it's not poor maintenance, (which can lead to all sorts of horrors) that I've read about on this forum! I'm just curious

Going down in an hour or so for a non anti-sink model is fairly normal.

The "leaky" components in the hydraulic system that primarily cause the car to sink are the height correctors (which affects sinking at both ends) and the brake control valve. (which affects sinking at the rear only)

Both height correctors and the brake control valve are necessarily somewhat "leaky" due to their design. (In fact the height correctors use no o-ring seals at all, and rely entirely on micrometre machined surfaces)

The anti-sink system adds an additional special low-leakage valve at each end of the car whose sole purpose is to isolate the height correctors from both the suspension and the high pressure supply, and also isolate the brake valve from the rear suspension.

The trigger to cause the valve to go into isolation mode is when the high pressure supply from the regulator drops below the pressure currently in the suspension. (Therefore front and rear anti-sink valves typically cut out at different times)

By doing this the only possible sources of leakage are the anti-sink valves themselves (which have very low leakage) and the suspension struts/cylinders, which also have very low leakage.

As for whether it could be retrofitted, the answer is yes, although it would not be a simple job.

It would require a front and rear anti-sink valve of the right type (hydractive and non-hydractive are different, and front and rear are different) an anti-sink sphere, a bracket to mount the anti-sink sphere on, a hydraulic circuit diagram for both anti-sink and non anti-sink models to compare the differences, and a means of making the necessary custom piping to re-pipe the height corrector etc..

Because most Xantia's already have anti-sink its probably not worth the effort on a Xantia IMHO, although I have considered fitting it to a CX if I ever get one

Regards,
Simon

jeremy · Unread post by **jeremy** » 19 Apr 2006, 12:52

I've never really understood this problem - so the car sinks at the rear after 30 minutes - so what - start the engine before fitting your seat belt etc - and by the time you're ready to move off its back at the correct height.

If the car drops - it shows that the suspension hasn't siezed! - also handy as it expells air from the system.

jeremy

davetd · Unread post by **davetd** » 20 Apr 2006, 21:28

Oh good so there's nothing wrong with the suspension, I remember a thread on the forum where people were saying their non anti sink xantias stayed up all day and I was just curious to see what made the difference.