Evolution: Xantia Hydractive Suspension Regulation - Info

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Evolution: Xantia Hydractive Suspension Regulation - Info

Post by GiveMeABreak » 27 Jun 2016, 10:54

I'm having a PC clearout and found this document all about the Xantia Hydractive system changes.
Before I zapped it, I thought I'd put a link to it here from my filestore for all the Xantia owners in case it is of any use to you - contains diagrams of the old and 'new' hydractive assemblies. You may have already seen this, but in case you're interested here it is: :wink:

Link to Xantia Evolution Hydractive Suspension Mod

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by Mandrake » 27 Jun 2016, 18:33

Interesting - I was not aware of the deletion of one of the ports on the later hydractive regulator, nor that it required a different anti-sink valve as well! (Although it looks like you could just blank off the unused port)

I have to admit that I can't quite see how the new unit can function with one of the small pipes deleted - on the older regulators one of the small pipes is from the output of the height corrector (via the anti-sink valve) which controls the suspension pressure and the second one is full system pressure to provide a feed for the electrovalve. I can't see how deleting either of these would result in a working system...

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by GiveMeABreak » 27 Jun 2016, 19:17

I feel a good old debate coming on! If nothing else it has at least raised one of your eyebrows Simon :-D

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by CitroJim » 28 Jun 2016, 11:50

Mandrake wrote:I have to admit that I can't quite see how the new unit can function with one of the small pipes deleted - on the older regulators one of the small pipes is from the output of the height corrector (via the anti-sink valve) which controls the suspension pressure and the second one is full system pressure to provide a feed for the electrovalve. I can't see how deleting either of these would result in a working system...
Same here Simon, I always believed that the electrovalve needed both full system pressure and an electrical signal to operate it...

Furthermore I always believed that low system pressure could cause the valve not to operate...

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by Mandrake » 28 Jun 2016, 14:47

CitroJim wrote:
Mandrake wrote:I have to admit that I can't quite see how the new unit can function with one of the small pipes deleted - on the older regulators one of the small pipes is from the output of the height corrector (via the anti-sink valve) which controls the suspension pressure and the second one is full system pressure to provide a feed for the electrovalve. I can't see how deleting either of these would result in a working system...
Same here Simon, I always believed that the electrovalve needed both full system pressure and an electrical signal to operate it...

Furthermore I always believed that low system pressure could cause the valve not to operate...
I just took a closer look at the diagrams and have worked out how the one pipe system works, and it's actually a rather ingenious design tweak. :)

For the original design Hydractive 2 control block with two small pipes you're right Jim.

The basis of operation is that there is a floating "shuttle valve" which via side ports opens and closes the flow both between left and right suspension sides, and also the flow from both sides to the sphere.

As well as one of the ports, one end of the shuttle valve is also connected to the pressure to the sphere, and the other end is connected to the output of the electrovalve, which can be switched to either full system pressure or overflow. Both ends of the shuttle valve are the same diameter so have the same hydraulic ratio. So the pressure provided by the electrovalve (either full pressure or no pressure) works against the pressure in the centre sphere - which is at a partial pressure - on an unloaded car about 80 bars at the rear and 120 bars at the front, but increasing proportionally with suspension load.

Using the front as an example, full pressure (145-170 bars) applied by the electrovalve to one end of the piston overcomes the 120 bars provided at the sphere and and thus pushes the shuttle valve into the "soft" position, (with the compliance of the sphere giving somewhere for that oil displaced by the shuttle valve movement somewhere to go) whilst zero pressure allows the pressure from the sphere to push it back the other way.

Notice some potential problems in the design already though - front suspension pressure is already 120 bars even without passenger load, add four heavy people and that might go up to 130 bars or more. Meanwhile the pressure regulator allows the pressure to drop as low as 145 bars before cutting in again - and that's assuming that the pressure regulator isn't old and tired with weak springs. As you've found they sometimes get sick and struggle to produce enough pressure either due to weakening reaction springs, or due to leakage.

If the available pressure is not reliably and significantly higher than the active suspension pressure, it will fail to lock the valve firmly into the soft position. Furthermore, if you hit a large bump, that sudden pressure spike will also be applied to the sphere end of the shuttle valve trying to push it back against the incoming supply pressure - if that pressure pulse exceeds a weak system supply pressure it could cause the valve to momentarily move into the hard position for the duration of the bump causing harshness. (Since you have abruptly gone into hard mode on the leading edge of a bump)

This is one of the theories of harsh ride I had years ago, and suggested the idea of introducing a one-way ball valve in the small feed pipe to the electrovalve to prevent any such back flow through the electrovalve when the suspension hits a bump, but I was never able to test the theory in practice.

Another issue that most owners will have observed is a propensity for the suspension to stick in the hard mode for a while after you have fully raised the suspension, left the engine off for a minute and then restarted the engine and lowered the suspension again - the reason for this is when the suspension is fully raised there is no longer a pressure differential between the electrovalve and sphere ends of the shuttle valve. When you start the engine again with the suspension still in high there is not enough pressure differential to push the shuttle valve back into the soft mode position, (towards the sphere) also because the gas in the sphere is heavily compressed it doesn't have much "give" to allow the displacement of the valve, as the oil displaced by the valve can only go into the sphere when the unit is in hard mode.

So it ends up in a kind of "limbo" where the electrovalve is on (soft mode) but the shuttle valve is stuck in the hard mode position, unable to move into the soft position, but after a while the gradual pressure leakage through the valve allows it to move enough and suddenly it will break free - then the car pops up like a cork.

The new valve design solves all these problems. :) If you look closely at the valve diagrams you'll see on the new type there is an additional spring (item 11) on the shuttle valve that doesn't exist in the old design. I'm going to call this a "bias" spring. This is pushing the shuttle valve into the soft mode position. The electrovalve still controls the pressure on the opposite end of the shuttle valve but instead of getting a high pressure supply, it supplies pressure from the other end of the valve - eg the centre sphere.

So now when the electrovalve is on, it connects both ends of the shuttle valve together so that they are both at equal pressure - eg typically 120 bars at the front. Because of this there is no hydraulic force to push the valve one way or the other and it would otherwise flop loose, however the "bias" spring will firmly push the valve into the soft position. When the electrovalve goes off, it will connect its end of the valve to overflow and thus the suspension pressure will overcome the strength of the spring and push the valve back to the hard position.

The first problem of insufficient system supply pressure is completely eliminated, as supply pressure is no longer used. In soft mode the pressure across the piston is equal and its only up to the spring to keep the valve in the soft mode. If the supply pressure drops to 120 bars, no harm done - it will stay perfectly in soft mode. :)

The second problem of instantaneous pressure increase hitting a bump trying to push the piston back the other way is also completely eliminated - because in soft mode both ends of the piston are connected together - any pressure rise from a bump will be equal on both sides and not try to move the valve, with the spring maintaining control of the valve in the soft position. If you hit a bump in hard mode then the pressure increase would only serve to keep the valve even more firmly in hard mode.

The third problem of the suspension sticking in hard mode temporarily after being fully raised is also solved - because previously the problem was that you were trying to force more oil into an already nearly full sphere to let the slide valve move into the soft position, now when the electovalve turns on it connects both ends of the piston together which means it can move to the soft position by letting a small amount of oil out of the sphere instead of trying to force more in. :) Also, no matter what the suspension pressure is, there is no difference in the operating force applied to the shuttle valve, because in soft mode it is the force of the spring alone.

Yet another benefit of the new design is that in the old design, when the car was parked with the engine off, as the system supply pressure bled away from the main accumulator as soon as it dropped below suspension pressure the suspension would not be able to go back into soft mode when a door was opened as there was not sufficient pressure differential across the valve - again, solved by the new system. Now you could open a door hours after the engine had last been running and it would still switch to soft mode, because all it is doing is connecting both ends of the shuttle valve together, allowing a small amount of oil to flow out of the suspension spheres to push the valve back towards the soft position. :)

Brilliant! :) Not only does it solve these problems, it also means an entire pipe can be deleted from the design and a simpler anti-sink valve with one less port can be used. (I bet the bean counters loved that) In fact now that I've seen it I have to wonder why it was not originally designed this way, it seems almost like the original design was in error and has some significant shortcomings.

I'd almost be tempted to retrofit the later type unit onto the earlier car except for the fact that the unions on the large pipes have changed! #-o

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by CitroJim » 28 Jun 2016, 15:01

Simon, that is one very excellent explanation of it :D I'm most appreciative of it and it explains why S2 cars always seemed a bit less critical of system pressure than S1 cars...

Shame the retrofitting is not so easy...

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by Hell Razor5543 » 28 Jun 2016, 15:13

Well, now Simon has a new challenge to occupy him (assuming that Joshua will let him have enough time!).
Last edited by Hell Razor5543 on 28 Jun 2016, 15:20, edited 1 time in total.

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by Mandrake » 28 Jun 2016, 15:19

CitroJim wrote:Simon, that is one very excellent explanation of it :D I'm most appreciative of it and it explains why S2 cars always seemed a bit less critical of system pressure than S1 cars...
Only later Series 2 cars sadly - my 1998 S2 V6 (RPO 7905) had the earlier design unit on it, they didn't change to the new valve design until RPO 8155 according to that document. And between RPO 8053 and 8154 there was an intermediate version of the regulator that still had the old two pipe design, but had the new style large hydraulic unions! #-o
Shame the retrofitting is not so easy...
Retrofit from 2nd hand parts off a scrapped car is not possible (short of replacing the runs of large hydraulic pipe to the struts as well) as no car was shipped with the single pipe design with the old ISO fittings, however the document mentions:

"Vehicles manufactured up to RP N° 8052 :
a suspension regulator with internal pipework suited to the ISO hydraulic union is available from the Replacement Parts Division."

Which I interpret to mean that they made a special run of regulators for replacement parts stock only that used the old style ISO pipe fittings but with the new single pipe and spring design! No doubt NFP nowadays and laughably expensive when it was still available... :-D

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by GiveMeABreak » 28 Jun 2016, 16:42

Well I'm glad I posted the document now - seems to have encouraged a few 'eureka' moments! A very interesting read through your post Simon. :wink:

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by xantia_v6 » 02 Jul 2016, 06:02

Would you get most of the benefits of the later system by just fitting a bias spring to an early electro valve?

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by white exec » 02 Jul 2016, 14:37

That is an extrordinarily interesting service document.
And Simon, thanks for your forensic review of it.

I take it the "interim" service replacement regulator (ISO fitting, up to RP 8052) is no longer available? Part no.?

Could the all-important internal spring be retrofitted to earlier (including XM) units?
Plugging off redundant ports doesn't look a problem.
When I get a mo, will look at the possibility of creating the new 'internal pipework' by external means.
Also a possibility is the use of late units with thread-adaptors.

This basic mod looks capable of solving so many long-standing issues, as detailed by Simon.

Gosh. Well done you two. O:)

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by Mandrake » 02 Jul 2016, 15:57

xantia_v6 wrote:Would you get most of the benefits of the later system by just fitting a bias spring to an early electro valve?
Not really, no.

The spring might help a bit to prevent the shuttle valve moving into the hard position in response to a bump when the system supply pressure is lower than it should be, but it would not work as well as having equalised pressure on both ends of the valve - and remember that the whole idea of a bump being able to do this when system pressure is low is only theorising on my part - nobody has measured this or confirmed it in any way, other than anecdotally by observing that fixing a pressure regulator with low pressure output does seem to improve the ride. (I think Jim has found this ?)

The problem with the thought of retrofitting a spring is that they've almost certainly bored the shuttle valve hole a bit deeper to accommodate the spring, (it looks that way from the diagram although it may not be to scale) so it's not like you could just pull it apart and fit a spring, and if you could you're not getting the full benefit that changing the piping provides.
white exec wrote:That is an extrordinarily interesting service document.
I take it the "interim" service replacement regulator (ISO fitting, up to RP 8052) is no longer available? Part no.?
Part number ? Who knows... :) There were actually four versions of the regulator made according to that article:

1) Original regulator with two small pipes, old ISO fittings on the large pipes, used up until RPO 8051, (24 November 1998) so this will be by far the most common version since it was in use for 5 years.

2) "Interim" regulator from RPO 8053 (26 November 1998) to RPO 8154 (7 March 1999) which functions the same as the old regulator but has new "Citroen" fittings on the large pipe. This must be rare as hens teeth as it was only in use for 4 months...

3) "New" regulator described in this evolution. Fitted from RPO 8155 (8 March 1999) until the end of the Xantia's run some time around 2001, so there will be a fair number of these around but not nearly as many as the original type. It has both the new "Citroen" fittings on the large pipes like the interim regulator and the spring and modified internal piping.

4) "Replacement only" unit, presumably first made available in March 1999 with the old style Citroen fittings on the large pipes but the new internal piping and springing like the previous unit. Presumably they did a production run of these to have replacement parts available for cars prior to RPO 8053, but they would have never been fitted to a new car, and its likely they didn't make very many as the hydractive regulators very rarely if ever fail...

This replacement only unit is the one we'd want if we could get our hands on them at a fire sale... Fitting it would be a straight swap, you'd only have to blank off the no longer needed pipe from the anti-sink valve - in the service note they suggest to fit a bleed screw into the unused port in the anti-sink valve as a blanking plug.
Could the all-important internal spring be retrofitted to earlier (including XM) units?
As I mentioned earlier I doubt there is room for the spring, it looks like the hole has been drilled deeper to accommodate the spring in the new unit. (But we'd need a proper to scale cross section drawing to confirm this - which we don't have for the new unit)
Plugging off redundant ports doesn't look a problem.
When I get a mo, will look at the possibility of creating the new 'internal pipework' by external means.
Could be very difficult - on the old regulator there is no external connection that goes directly to the sphere, so you'd have to drill into the housing and tap a new port which you could then pipe across to the existing high pressure input, but depending on the geometry it might not be feasible, and if you can't fit a spring the whole exercise is moot anyway.
Also a possibility is the use of late units with thread-adaptors.
That seems like a much more practical approach. Of course if you had a late donor car with the new valves you could take the hydractive pipes to the strut tops as well and change those too - then no adaptor needed. Easy to swap on the rear but the front ones would mean removing the battery tray but otherwise it's not that difficult.

So I think if I was to attempt retrofitting I'd get the new valves and the hydractive strut pipes that go with them and swap the lot over.

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by Stickyfinger » 02 Jul 2016, 17:35

As I am not to sure of what to look for, but I have 2 s1 cars with a production date Aprox @ the change over date, and have S2 later parts "on stock" maybe Jim would like to check my parts stock for units/pipes etc next time he is down.....

Jim ?

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by Mandrake » 02 Jul 2016, 17:46

Not sure what you mean - the change over to the new valve design is well into the S2 run - March 1999. You won't find any S1 cars with the new valve design unless they had a faulty valve replaced after 1999.

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Re: Evolution: Xantia Hydractive Suspension Regulation - Info

Post by bobins » 02 Jul 2016, 19:24

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