Falling qualities of Citroën hydraulic ride

[Spaces]

The last time I was on the phone to Pleiades I mentioned my thoughts in relation to suspension behaviour, they hadn't considered this line of thought, so perhaps it hasn't been mentioned on this forum?

It deals with fluid dynamics. Increasingly disappointed by ride quality in successive Citroens and not convinced this was all down to cheap-to-produce suspension by Peugoet, I started looking for answers.

A GS or CX is going to be more comfortable and deliver more consistent road behaviour at high speeds and in poor conditions than PSA machines - you only have to study their chassis engineering, centres of mass and other such invisibles to see this - but even given the sticky and geometrically awful MacPherson strut, the self-steering Peugeot rear axle and other rubber nasties, electronics and bubble-filled electrovalves, rubber-mounted unruly anti-roll bars and a complex variable spring and damping suspension system I struggled to put my finger on a certain poor ride quality at lower speed and over harsh roads.

We all know (?) that Citroën's suspension comes into its own at higher speeds and in poor conditions, that low-speed ride over poor surfaces can be a little more accurate and harsh than a steel sprung car and that the reason Citroens were once light years ahead of other machines wasn't just because of the gas over oil springing and damping.

Hugely strong suspension and its location, metal bearings instead of cheap rubber bushes, rack and pinion steering, roadwheels at the extremities (taken up by many others sixty-odd years later), a very wide track, double wishbone front suspension with perfect steering geometries, the four piston brake calipers, slippery yet aerostable shapes, diravi steering, monocoque construction from 1934 (until it was realised the egg-shell construction idea fell apart when several large holes were cut into it,unless made very heavy) and by the seventies, ergonomics which made even a Saab look second-rate all added up to an other-wordly combination of qualities.

Given the wrong road, Citroëns increasingly seemed to patter and ride in a very wooden way - especially the PSA ones. Sometimes worn struts made it worse, as does general wear. Poor quality spheres or more complex problems could be blamed with some. Increasingly, though, I found Citroens which rode on fluid could be very poor on broken roads and washboard surfaces. Beyond 40 or 50 mph the problem often vanished.

I wondered if it was increasingly small amounts of tyre sidewall between road and rim - I tried a Xantia with much higher profile tyres - if anything, worse. Had times moved on and were some of the better Germans able to make metal springs and polyurethane buffers into smoother-riding, more supply suspended, even more capable rides than Citroen could? Surely not. French roads may be almost perfect today, but aren't today's graduates of les grandes écoles still versed in nids-de-poules and les planches à lavers of yesteryear? Or is Citroën, devoid of the multiply-talented genius minds of the 1930s-60s, simply convinced that steel suspension is better?

I happened to notice how the sphere design had changed - not the change from two halves into one or the green sphere to the flatter grey mushroom of the C5, but in the detailing of the damping orifice. This passes a lot of fluid to and from the sphere's 'spring' as the wheel goes up and down - especially when amplitudes are small and oscillating as on rippled, washboard surface. Initial movement which is slight enough not to displace the damping 'leaves' is all through this hole and turbuIence and resistance to flow is bound to affect the ride.

I fished out an old Citroën sphere. Around the hole was a ring of steel and the entrance to the hole was round-chamfered. My (limited) knowledge of fluid dynamics suggested this was similar to the carefully tuned bell mouth on a racing car's air intake pipe, or the mouth of a trombone - both designed for the minimum of lost energy with smooth air/fluid flow.

Later spheres lost the ring and instead just had a simple chamfered entry.

More recently, spheres have been made with a hole exiting directly from a flat surface, no chamfering. I reckoned that this would increase turbulence in fluid flow, especially when it was rapid to and fro movement - ie over a rippled road surface. Turbulence means resistance, so I used a drill bit to chamfer the entrance to the hole and was amazed by the difference in both ride and (more surprisingly) handling in a particularly low-mileage and pristine GS. The car rode much more smoothly and cornered with more grip and poise - more like they once had. I replicated the original design and was even more surprised, the ride and handling improved further.

I have altered the spheres on every hydraulic Citroën I have owned since this discovery, with benefits to the ride and often to the handling also.











Before some of you try to question this experiment, try it for yourselves first. I must admit I am sensitive to the way a car drives, but this is why I was amazed the first time I drove a hydraulic Citroën.

Those of you with keen eyes may notice other differences in the fluid paths in sphere entries, so suggest other causes for ride deterioration. You are probably right, but I thought this was more than enough to be going on with!

[Mandrake]

Very interesting post there, you might want to also look at the following thread if you haven't seen it where I ponder the same loss in ride quality with the Xantia versus the GS and CX:

http://www.frenchcarforum.co.uk/forum/v ... =3&t=40914" onclick="window.open(this.href);return false;

I'm very familiar with all three models, my first car was a GS (well, actually an 1129cc G Special, left hand drive no less!) I've driven and worked on many GS's, owned a 1978 CX2400 for a few years (although it was mostly driven by my Dad) and I'm now onto my second Xantia and have first hand experience of two other Xantia's.

There are a lot of compromises in the suspension design of the Xantia, many of which you mention, (McPherson struts is the biggest one, followed by rubber bushes in the geometry etc) but there is an underlying intermittentness to the slow speed ride quality of a Xantia, particularly the Hydractive 2 models which is yet to be fully explained, despite me putting a lot of effort into identifying it...(see the other thread) the frustrating thing about a Hydractive 2 Xantia is that the slow speed ride quality is sometimes very good, but often very harsh. The HA2 Xantia can and sometimes does produce an outstanding ride quality, but it is simply not consistent, perhaps due some design flaws...

By the way, I don't want us to over romanticise the ride quality of the GS, the GS has possibly the best suspension geometry and design of any car ever made, it's almost perfect, however one thing that it does suffer from is harshness in the front suspension over short sharp bumps. It's easy to forget this when you haven't driven one for years, but I have first hand experience with 4 GS's, and they all did it. It was even commented by many of the reviewers when the car first came out that whilst the ride is mostly excellent it does hit short sharp hump backed bridges quite harshly.

IMHO the reason for this is that there is no rubber isolation between the front subframe and the body - its directly bolted on, which is good for suspension geometry but bad for allowing direct transmission of jolts from the subframe into the body. I think this is compounded by the anti-dive geometry where the wishbone arm pivots are tilted forward - which tends to cause the pivots to hit sharp bumps at a right angle. Again, good for geometry, not so good for bump isolation.

The rear suspension subframe of a GS does have rubber bushes, albeit not self steering ones, and the extra isolation means the ride in the rear is far less harsh on sharp bumps.

If you exclude the DS, the CX definitely has the best ride of any Citroen, and in particular it has the best low speed ride over sharp harsh bumps such as broken surfaces, it just glides over them. Despite having some compromise in suspension geometry compared to a GS, (brake discs get in the way of opimal ball joint locations, and the increased unsprung weight) I think the major factor, apart from being a lot heavier in the front than a GS, is that the CX has an ingenious isolation system between both front and rear subframes and the body.

If you have separate front and rear suspension subframes that mount to the main body with rubber bushes you get good shock isolation for low speed ride quality but now your overall geometry during cornering is compromised because the subframes will twist on their mountings causing sloppy handling. If you bolt the subframes directly on the handling is good but the ride quality will be poor due to a lack of isolation...

The CX solves this problem by having separate chassis rails under the body that directly link the front and rear subframes together, thus preserving suspension geometry under hard cornering etc. Front and rear subframes are bolted together through these chassis rails without any flexible medium between them, effectively forming one long suspension sub chassis.

At the same time the body is fully isolated from both the suspension subframes and the underbody chassis rails by rubber bushes, (from memory there's around 20 bushes) leading to exceptionally good low speed ride over broken/sharp surfaces.

The CX we had was 20 years old when we first got it, all we did to the suspension was fit a new set of spheres and the ride was outstanding, both high speed and low speed. Certainly the best of any Citroen I've driven. It did roll like a boat though Despite massive body roll it did get around corners rather fast.

Regarding the tapering of the bypass hole, I'm a little less convinced though, for one thing the GS and CX spheres that I remember (at least the replacement ones that were available in the late 80's and 90's) look more or less identical to Xantia spheres, with the centre bypass hole being in the middle of a large flat surface with just a very slight taper. I don't think the difference in the ride quality between GS/CX and Xantia can be attributed to small details in the sphere design.

I have a fair bit of experience with sphere tuning, including both increasing and decreasing the size of the centre bore to tweak the damping, so I have a pretty good idea of just how sensitive it is to changes in that hole and how much it affects the damping and low speed ride.

For example on the front of the CX the ride was a bit too floaty and under damped with the new spheres we fitted, by the time we got that CX in 1998 there were only one set of non turbo spheres available for the CX as replacements, and they were optimised for the Series 2 CX which has larger diameter hydraulic rams than the Series 1.

The end result is if you put Series 2 spheres on a Series 1 CX the ride becomes too soft and wallows rather sickeningly. From memory (which is getting a little hazy about exact figures) the supplied spheres had a 1.65mm centre hole, and we experimented with reducing the hole size between 1.25 and 1.5mm. Doesn't sound like a big difference but the difference in both ride and handling was enormous.

With 1.65mm the ride was very soft and floaty with uncontrollable wallowing over large undulations, quite scary to drive at speed in fact. (This is because the spheres are tuned for the larger rams of the Series 2...) Slow speed ride was obviously very good.

With 1.25mm the ride was rather firm and heavily damped - large bumps would be absorbed well but small bumps would cause the car to constantly "fidget", a bit like a HA2 Xantia in hard mode. At high speed the car was exceptionally stable and well controlled.

We eventually settled on 1.4mm, this was enough to get rid of the wallowing and overshoot in the damping characteristic but did not noticeably worsen the slow speed ride quality over broken surfaces. It was an excellent balance of ride and handling.

I think where you might be going a bit off track with the whole fluid dynamics and hole tapering theory is that you have to understand that turbulence of the oil flow through the small hole is where the damping comes from. The smaller the hole is the higher the velocity of the oil for the same displacement, the greater the turbulence, the more the friction, the more energy is absorbed and the more damping there is.

It's true that tapering the entry of the hole more as you have done will reduce the turbulence and make the ride softer, but that's because you are reducing the damping... you can achieve exactly the same result by simply drilling out the diameter of the hole slightly and leaving it square shouldered.

In other words (numbers made up here for illustration) a 1.2mm diameter hole with a large entry taper will perform exactly the same in terms of ride and handling as a 1.3mm hole with no taper and a square shoulder- its the total amount of friction caused by the turbulence that sets the damping, and the amount of damping sets the ride/handling balance. Don't forget also that you're only tapering one end of the hole - the inside end of the hole will still be square shouldered.

So, I don't think there is any fundamental difference between a tapered hole and a slightly larger square shoulder hole. (I have tried both) What you have observed is that the geometry of that bypass hole is exceptionally critical in controlling ride and damping, with very tiny differences whether it be diameter or tapering having large effects on the ride. Sometimes its possible to get a better result than the factory tuning with a slightly larger (or smaller) hole, and it does seem that as years have gone by Citroen have tended towards smaller bypass holes and more damping to reduce roll and firm up the handling a bit, which I'm sure is a conscious decision to "compete" with other marques.

When you think about it it is quite remarkable that the stored energy of a large heavy body weighing hundreds of kilograms trying to oscillate up and down can be dissipated by oil flowing through such a tiny hole. When you look at it that way you realise that a lot of turbulence is required to do the job, hence high pressure oil being forced through such a small hole to generate sufficient friction...

[Mandrake]

A small addendum to my post - I'd also forgotten that the original DS sphere design with the removable and re-shimable damper valves had a mounting bolt right through the middle of the damper where the bypass hole is on more modern spheres, with the bypass hole simply being a small hole drilled towards the outer part of the thick disc against which the washers were clamped.

Not only was the hole not central but it was also square shouldered on both sides, eg literally just a hole drilled through the flat disc at right angles with no attempt to streamline the flow of oil or avoid turbulence. Nobody would argue that a DS had a poor low speed ride.

As I said, the turbulence is actually wanted and necessary as that's where the damping comes from. Whether you adjust only the hole diameter or the shape or both to get the required amount of damping doesn't really matter, it's only an implementation detail.

[Spaces]

What you have to say is interesting too - down the years we have obviously being driving with very similar thoughts in our heads and engineering similar trials. I read (and had contributed) to your linked thread. Do you find the aeration problem much different in winter? Has anyone considered altering the viscosity of LHM slightly?

It looks like our thinking and experimentation does differ on the effect of a chamfered damper hole. I've tried it alongside a marginal hole diameter increase and found the results surprisingly different at all speeds on all roads, in a variety of model. Whereas boring out the hole made things a little smoother overall, it didn't have the effect that made you feel something rather clever and sublime had been altered, as I find chamfering the hole does. On the GS I mentioned I used a set of really old but un-used spheres (with the pronounced lip) regassed to the correct pressure, back to back with the newer spheres. The difference was very obvious with the hole the same size, then the newer spheres were drilled out so it could be seen if the difference was simply due to hole size. The car rode better with the larger holes but it was still wooden at times compared with the ride on the old design. Finally the new sphere were chamfered, which improved matters significantly but not to the point they were a smooth as the ride on the original Citroën spheres. Copying the old design with a ring of solder improved the ride again.

A few points:
>>I'm not suggesting poor ride is all down to this - anything but - it's almost always a series of factors, adding up to the whole.
>>Why did Citroën alter their design to the one which which includes the circular lip then chamfered edges while they were still developing and improving their system, if there were no benefit?
>>I'm suggesting the machined surfaces smooth the initial flow of fluid into the sphere, which knocks off the 'ragged edges' of small road ripples when the fluid is moving back and forth many tens of times a second. Under certain conditions this may prevent the 'leaves' operating.
>>I reckon the threshold to entering the damper is marginally lower.

(Think of the fluid's molecules as a crowd of people who are being forced out of a small exit from an enclosed space. If there is a funnel then when the doors open/there is pressure to move, as soon as they open they can start passing through the corridor. They may even pass slower through this since there isn't a delay in finding the exit, when a 'bursting' pressure may begin to build before the route is used.)

>>It smoothes the ride when there is virtually no damping to do since there is less resistance to flow.
>>I compare with the carburettor bell-mouth used in racing/fast cars which are trying to minimise the gases' energy losses as they are forced through a small venturi.
>>For small and oscillating small wheel displacements, you're assuming maximum damping required, given the hole size. I'm not as sure as you are about this.
>>I can see that damping may be reduced slightly - not sure whether the shorter minimum width orifice is the sole reason of this as you seem to suggest, or whether there is a combination of this and the initial smoother flow.

Consideration of other points you make, not directly relating to the damper orifice:
>>The use of significant amounts of rubber in the system - sometimes I'm aware of this rubber setting up a shortish high-frequency oscillation when a sudden bump is encountered. This can lead to a crashy feeling ride of a different sort as different layers of rubber react differently to a shock load. Contrasts in the rubber's behaviour is sometimes obvious when temperature varies widely from one day to the next or day to night.
>>Different quality steel used through a car's life can adversely effect body stiffness and create differing responses to the different loads fed into it.
>>Tyre condition, size, age, make and wear affects how all bumps are dealt with - the suspension is set up to match the original tyres used on the car. Even the properties of these may alter down the years as manufacturers alter compounds.
>>Tyres meeting a sharp edge on the rise out of a pothole take so much of the shock. A CX riding on original 185-14 Michelins will deal with such a shock very differently from one with cheap make 195-70s or 185-70s in use. I used a GS for a while with 155 Michelins on the front: the car was better suited to English roads.

Like you, I admire the CX construction enormously. Citroën had been seeking a solution for how to construct a large car with the maximum strength for a given weight, and had reached this clever method after having tried the monocoque and 'base unit' ideas. The idea of structurally separating two different problems and making them work to their best free of compromise, as well as combining the two to work together where beneficial was as clever as so much other technology Citroen developed, but much less obvious unless you covered tens of thousands of miles.

I appreciate how the Australian firm Kinetic took hydraulic suspension beyond the bounds of Citroën and how the same man, who was originally inspired by Citroën, is developing high-speed marine suspension technology. This is surely more how the company would have advanced given its own design freedoms?

[Mandrake]

What you have to say is interesting too - down the years we have obviously being driving with very similar thoughts in our heads and engineering similar trials. I read (and had contributed) to your linked thread.

Opps, when I posted a link to that thread I hadn't noticed you'd already posted it in. I've been posting in too many threads lately, some of them with parallel or related discussions, so its easy to forget who said what and where.

Do you find the aeration problem much different in winter? Has anyone considered altering the viscosity of LHM slightly?

Funny you should mention that because until HDI's "Odd suspension issue" post came along I had been meaning to start a thread asking if people noticed changes in ride quality with temperature, because I certainly do.

In both this Xantia and my previous one (both HA2) I notice a big difference in ride quality with both ambient temperature and whether the car itself is warmed up or not. Invariably the ride quality is best in cold conditions and gets rapidly worse in hot conditions.

Even today I noticed this effect - we took some rubbish to the dump and the ride at first was extremely good, no real harshness to speak of, of course there was a little bit of extra weight in the back, but not much, maybe 50Kg. After that we went into the city (about 10 miles) and the ride was still good, but by the time we were heading back I noticed the ride was becoming quite harsh - a total driving time of about 30 minutes with a 15 minute pause before returning.

This is not an uncommon experience for me. Good ride at first, deteriorating in as little as 30 minutes as the car heats up. Now to be fair I haven't done the diode mod on the electrovalves on this Xantia yet, and its not unusual for dry joints on the internal diode to change with temperature - I had that problem on my previous Xantia where the back EMF diode in the front electrovalve measured and worked ok when it was cold but went high resistance when it heated up from engine bay heat (it is right in the radiator air-stream after all) causing the suspension to constantly cycle in and out of hard mode every few seconds. I caught it red handed with my diagnostic LED which visually indicated the feed to the elecrovalves was cycling on and off when it shouldn't be... Once I performed the diode mod I never observed that happen again in the months that I left the diagnostic LED on the dashboard.

However my previous Xantia still had a strong tendency towards riding well when cold and poorly when hot, so the diode didn't explain everything. Since that Xantia lived in New Zealand it was subject to a lot hotter weather than the UK ( sometimes hot enough to melt the tarmac on the road ) and quite mild winters so this could explain why I seemed to have this issue more than some of the UK forum members, as the car on average was subject to a lot higher temperatures than here in the UK.

As to why temperature seems to affect it, I don't really know. The only two things I can think of are the change in viscosity of the oil, and a change in aeration of the tank. Frothing of the oil is a significant issue in the hydraulic system, there are so many places where high pressure oil is forced through very narrow orifices, (including damper valves) so much so that LHM has anti-frothing agents in the oil which don't last indefinitely, hence one of the reasons for changing the oil. I'm assuming that when the oil thins with heat it has more tendency to froth under extreme pressure conditions than when its colder and thicker. (I've always felt that LHM is a wee bit on the thin side...)

The other possibility is that if hardened, failing rubber return lines are the source of air being sucked in and dumped in the tank in large quantities, its quite likely that the sealing of the worn rubber joints will change with temperature - heat may cause the rubber to expand and start to leak more letting more air in. From what I can remember I was able to correlate an increase in bubbles returning to the tank when the car and/or the weather was hotter, using my clear pipe section.

One other thought is that it may not be so much temperature but running time that causes the ride to get worse as you drive. If we assume that there is a steady stream of bubbles being sucked into the pump, (perhaps worsening with heat) then at first it wont affect the ride much as they will just be recirculating through the pressure regulator back to the tank, but there will be height corrections while driving - the rear suspension is constantly making small corrections due to acceleration, and the front suspension will make height corrections if you turn on a sharp lock for more than a few seconds, and also gradually with slow pressure loss which must be corrected. (About once every 10 minutes or so) Eventually this must lead to an accumulation of air in the suspension.

It looks like our thinking and experimentation does differ on the effect of a chamfered damper hole. I've tried it alongside a marginal hole diameter increase and found the results surprisingly different at all speeds on all roads, in a variety of model. Whereas boring out the hole made things a little smoother overall, it didn't have the effect that made you feel something rather clever and sublime had been altered, as I find chamfering the hole does. This was particularly obvious on the GS I mentioned. Using a set of really old spheres (with the pronounced lip) which had seen little use and regassed to the correct pressure gave an even more sublime ride. The damper hole was altered so it was the same on all, after slightly smaller holes with chamfering had been driven back-to-back with those without any chamfer and a slightly larger hole.

Interesting, maybe you do have something there then. One of the other problems with the Xantia versus CX/GS is that on the older models there is a straight flow from the piston to the sphere damper valve at the same diameter of the piston, so no turbulence at all. Look at the strut top of a Xantia by comparison and not only do you have a greater than right angle joint, there is an offset hole of only about 10mm connecting the sphere with the piston inside the strut.

Furthermore the oil must flow through two small holes (about 5mm diameter) at right angles into the top of the strut shaft and then after a sharp 90 degree bend all the way down inside the shaft to reach the actual piston. So there is a rather small diameter long path with a number of bends and right angles between the sphere and the actual piston surface that is going to cause unnecessary turbulence. Then in HA2 you have the 10mm pipes that are a good 1.5 metres or so long down to the hydractive regulator, so more tortuous path to flow. I've often wonder whether this byzantine path the oil has to flow through isn't part of the problem with the ride on a Xantia... I've noticed that if the steel pipes are not mounted correctly they will bang against the wing with the jolt caused by the sudden oil flow from a bump - like water hammer in an unsupported water pipe...

A few points:
>>I'm not suggesting poor ride is all down to this - anything but - it's almost always a series of factors, adding up to the whole.
>>Why did Citroën alter their design to the one which which includes the circular lip then chamfered edges while they were still developing and improving their system, if there were no benefit?

If I had to guess I would say cost savings, pure and simple, and by the time the later spheres came out the original designers that invented the hydropneumatic system were probably retired if not no longer with us. When you look at some of the design of the Xantia and later Citroens you definitely get the feeling that there is a new generation of engineers who whilst coming up with some good ideas (Hydractive 2 on the whole I think is a good idea with some implementation errors) are lumbered with this "legacy" system that they don't fully understand. Some of the small details and intricacies of why certain things were done certain ways are lost on them, and so some mistakes have been made that the original designers with a more thorough understanding of the system would not have made.

>>I'm suggesting the machined surfaces smooth the initial flow of fluid into the sphere, which knocks off the 'ragged edges' of small road ripples when the fluid is moving back and forth many tens of times a second. Under certain conditions this may prevent the 'leaves' operating.
>>I reckon the threshold to entering the damper is marginally lower.

So do you think the threshold at which the leaves open is where the harshness occurs ? So that a bump amplitude that just opens and closes the leaves repetitively results in harshness ?

(Think of the fluid's molecules as a crowd of people who are being forced out of a small exit from an enclosed space. If there is a funnel then when the doors open/there is pressure to move, as soon as they open they can start passing through the corridor. They may even pass slower through this since there isn't a delay in finding the exit, when a 'bursting' pressure may begin to build before the route is used.)

>>It smoothes the ride when there is virtually no damping to do since there is less resistance to flow.
>>I compare with the carburettor bell-mouth used in racing/fast cars which are trying to minimise the gases' energy losses as they are forced through a small venturi.

I don't know enough about fluid dynamics to agree or disagree with this, but I suspect it doesn't work quite like this. At the end of the day you have a volume displacement versus pressure curve - if you apply a certain amount of pressure on the hydraulic ram, how rapidly is the piston allowed to move. This relationship between volume displacement and instantaneous pressure is what defines the damping characteristic, and with any sort of threshold damper like this its a non-linear line with a kink in the curve where the leaves open.

The way I would go about tuning the suspension would be this - once spring constant is chosen (and thus oscillating frequency) I would start with no (or very stiff) leaf valves and adjust the size of the bypass hole until the rebound damping is correct - just enough damping (hole just small enough) that the rebound doesn't overshoot. Then I would make the leaf valves softer until the rebound starts to overshoot and then back it off a bit so that they can't quite open on rebound alone.

This way the leaf valves don't open in response to rebound, (the slow rebound plays out through the bypass hole) but a bump that is moderately more sudden than the rebound rate will open the leaf valve and thus be absorbed easily. I believe this is the way most Citroen's are tuned, or at least used to be, although it gets a lot more complex with the HA2 system as there are two sets of dampers with different tunings connected in parallel while in soft mode...

>>For small and oscillating small wheel displacements, you're assuming maximum damping required, given the hole size. I'm not as sure as you are about this.
>>I can see that damping may be reduced slightly - not sure whether the shorter minimum width orifice is the sole reason of this as you seem to suggest, or whether there is a combination of this and the initial smoother flow.

Don't quite follow what you're saying there. Although the damping just below the leaf opening threshold is heavier than after the leaves open,the damping still does reduce below that as damping is proportional to the oil velocity...so damping drops to near zero at near zero velocities.

Consideration of other points you make, not directly relating to the damper orifice:
>>The use of significant amounts of rubber in the system - sometimes I'm aware of this rubber setting up a shortish high-frequency oscillation when a sudden bump is encountered. This can lead to a crashy feeling ride of a different sort as different layers of rubber react differently to a shock load. Contrasts in the rubber's behaviour is sometimes obvious when temperature varies widely from one day to the next or day to night.

Yes I notice this as well, although the rubber does isolate rough surfaces quite well in some ways it does have its own oscillatory frequency at quite a high frequency, probably around 10-20Hz. Bear in mind though that a tyre does this as well - the unsprung mass of the wheel and some of the suspension mass works together with the compliance of the tyre itself to oscillate at the wheel hop frequency - also around 10-20Hz, which also varies with tyre pressure and hence temperature. If you watch a car wheel hit a step in the road it oscillates at this 10-20Hz frequency for at least 5 cycles or so. If this sets up a sympathetic vibration in the suspension joints themselves (eg worn rubber joints like the lower arm bushes) then it can cause quite a harsh jolt that feels more like a rapid vibration.

When the HA2 system is malfunctioning (for as yet unexplained reasons) I believe its this rapid wheel/rubber joint oscillation when hitting steps in the road that leads to the harshness - for example image that the soft/hard mode valve opened and closed with every cycle of this 15Hz oscillation when hitting a step in the road - you'd feel a number of harsh jolts in under a second.

>>Different quality steel used through a car's life can adversely effect body stiffness and create differing responses to the different loads fed into it.
>>Tyre condition, size, age, make and wear affects how all bumps are dealt with - the suspension is set up to match the original tyres used on the car. Even the properties of these may alter down the years as manufacturers alter compounds.

None of the tyres originally specced for older Citroens are still available - even the original Xantia types are long gone, so who knows how the newer typically harder tyres will interact with the resonances in the suspension, thats a good point.

>>Tyres meeting a sharp edge on the rise out of a pothole take so much of the shock. A CX riding on original 185-14 Michelins will deal with such a shock very differently from one with cheap make 195-70s or 185-70s in use. I used a GS for a while with 155 Michelins on the front: the car was better suited to English roads.

Funny you say that, my Dad's GS had 155 Michelins all round and it definitely rode and handled better than the original 145's.

Like you, I admire the CX construction enormously. Citroën had been seeking a solution for how to construct a large car with the maximum strength for a given weight, and had reached this clever method after having tried the monocoque and 'base unit' ideas. The idea of structurally separating two different problems and making them work to their best free of compromise, as well as combining the two to work together where beneficial was as clever as so much other technology Citroen developed, but much less appreciated.

There was certainly a lot to admire in the CX. If you compare the front suspension of any modern car including a Xantia it looks positively flimsy by comparison, with the top half of the suspension only connected through the body. The front suspension of the CX (and GS for that matter) is massively strong, how they managed to shoe-horn a large transverse engine and gearbox in there and still keep the one piece suspension chassis with over the top box section I don't know. To then connect the front and rear chassis with under body chassis rails would be positively overkill in many peoples eyes, and yet they did it, and it worked.

I have first hand experience of the solidity of the GS suspension as I had a 30mph head on collision in mine (not my fault I might add, even though I was only 18 at the time ) which completely crushed the front bumper and under-tray, literally wrapping it around the engine, along with the left hand wing which was wrapped around the tyre. After bending the wing away from the tyre not only did the car start first time and drive home, but there was no structural damage to the suspension or its geometry - apart from the destroyed bodywork all that happened was that the whole suspension subchassis moved back on its mounts on the body on the left hand side by about 10mm. The suspension chassis itself retained its integrity, no arms were bent, even the wheel alignment was fine, we just had to slacken the mounting bolts and move its position slightly on the body as the mounting holes on the body had been stretched.

I appreciate how the Australian firm Kinetic took hydraulic suspension beyond the bounds of Citroën and how the same man, who was originally inspired by Citroën, is developing high-speed marine suspension technology. This is surely more how the company would have advanced given its own design freedoms?

Wasn't it Kinetic that were responsible for the cross linked anti-roll system in the Citroen Xsara WRC car of a few years back ? Now that's something I'd like to see on a road Citroen, and something that could be implemented on an Activa like car...

[addo]

I wouldn't underrate the Coanda effect in terms of what may pass through that orifice.

Note to Xac: Android dictionary has "Coanda"... Bet iPhone doesn't.

[Spaces]

I wouldn't underrate the Coanda effect in terms of what may pass through that orifice.

In which case the movement of fluid out of the sphere would be significantly aided by the chamfering or smoothed circular ring.

Opps, when I posted a link to that thread I hadn't noticed you'd already posted it in. I've been posting in too many threads lately, some of them with parallel or related discussions, so its easy to forget who said what and where.

-Just to let you know I had thought about this too.

(I've always felt that LHM is a wee bit on the thin side...)
If I had to guess I would say cost savings, pure and simple, and by the time the later spheres came out the original designers that invented the hydropneumatic system were probably retired if not no longer with us. When you look at some of the design of the Xantia and later Citroëns you definitely get the feeling that there is a new generation of engineers who whilst coming up with some good ideas (Hydractive 2 on the whole I think is a good idea with some implementation errors) are lumbered with this "legacy" system that they don't fully understand. Some of the small details and intricacies of why certain things were done certain ways are lost on them, and so some mistakes have been made that the original designers with a more thorough understanding of the system would not have made.

With you on that. Except I'm not sure you interpreted my wording correctly - are you suggesting that chamfering and the circular ring would have cost less than without?

Funny you say that, my Dad's GS had 155 Michelins all round and it definitely rode and handled better than the original 145's.

I always thought the sidewall depth on a 145 Michelin looked a little mean for the GS. Larger tyres at the front, original at the back helped reduced understeer through roundabouts yet didn't create any lift-off neutrality at speed...
Equally, when running a typically late-ninties-ratty Series 2 CX TD I stuck a pair of 195-14s on the front. The engine was the quickest of its type I'd driven so easily managed the slight gearing-up. The slightly shakey car (poor steel, some corrosion) was much sweeter on the bigger tyres. Gone was the crash and twitter every time a certain sharp bump was encountered.

I don't know enough about fluid dynamics to agree or disagree with this, but I suspect it doesn't work quite like this. At the end of the day you have a volume displacement versus pressure curve - if you apply a certain amount of pressure on the hydraulic ram, how rapidly is the piston allowed to move. This relationship between volume displacement and instantaneous pressure is what defines the damping characteristic, and with any sort of threshold damper like this its a non-linear line with a kink in the curve where the leaves open.

The way I would go about tuning the suspension would be this - once spring constant is chosen (and thus oscillating frequency) I would start with no (or very stiff) leaf valves and adjust the size of the bypass hole until the rebound damping is correct - just enough damping (hole just small enough) that the rebound doesn't overshoot. Then I would make the leaf valves softer until the rebound starts to overshoot and then back it off a bit so that they can't quite open on rebound alone.

This way the leaf valves don't open in response to rebound, (the slow rebound plays out through the bypass hole) but a bump that is moderately more sudden than the rebound rate will open the leaf valve and thus be absorbed easily. I believe this is the way most Citroen's are tuned, or at least used to be.

I thought this explanation of setting up a sphere was very neatly described! It was pure (half-educated) trial and error with my Hydractive Xantia, a 2.1 with 195 tyres so quietly rapid - I would use 450cc spheres on the front in summer and 500s in winter, damping holes to my own size. (Centre spheres from XM, one end from the estate I think, rears I can't remember but they're well matched.) A few years down the line and the 500s (think it was 55bar) have lost just enough pressure for it to be perfect in all conditions (on poor roads at speed you used to have to engage sport more to prevent too much suspension displacement, this is a rare occasion today - suggesting it's somewhere near right?), it's quite happy on the old-fashioned French road at 90(miles)+, no sign of excess movement, big and small bumps swallowed with a grace which Citroëns were once known for. Cornering is very decent, it turns in without hesitation, deviation (or repitition one hopes!) on the smooth as well as bumpy - you can feel the ball-bearing snuggling into its hole! You just have to wary of wet corners, as with many PSA cars. I think the original setup was designed to corner on smooth roads at the maximum speed possible. Not well-suited for the real world, in my book.

I'm pleased you can understand a little of what I am getting at. The difference was too great, in my mind, for anything else with the GS - which is going to be particularly sensitive to any improvements with its relative lack of weight and pure and simple suspension.
The steady simplification of design being in cost interests and nobody understanding the enormous subtlies of the hydraulic components. Maybe they thought that with struts, what was the point? I wonder where else in the hydraulics other apparently tiny alterations have been made? Interesting to hear about the implementation errors with Hydractive 2, what are they and can something be done?

[Mandrake]

If I had to guess I would say cost savings, pure and simple, and by the time the later spheres came out the original designers that invented the hydropneumatic system were probably retired if not no longer with us. When you look at some of the design of the Xantia and later Citroëns you definitely get the feeling that there is a new generation of engineers who whilst coming up with some good ideas (Hydractive 2 on the whole I think is a good idea with some implementation errors) are lumbered with this "legacy" system that they don't fully understand. Some of the small details and intricacies of why certain things were done certain ways are lost on them, and so some mistakes have been made that the original designers with a more thorough understanding of the system would not have made.
With you on that. Except I'm not sure you interpreted my wording correctly - are you suggesting that chamfering and the circular ring would have cost less than without?

What I mean is if you look at the three spheres you pictured each later type would most likely have been cheaper and easier to make. I'm most familiar with the middle one that you show, thats the type of fitting that all of our GS/CX spheres (obtained late 80's onwards) had... I can't actually remember what my last Xantia had, but I think it was the same too - I don't recall seeing any like the 3rd one you post, possibly thats a non OEM brand ?

Funny you say that, my Dad's GS had 155 Michelins all round and it definitely rode and handled better than the original 145's.
I always thought the sidewall depth on a 145 Michelin looked a little mean for the GS. Larger tyres at the front, original at the back helped reduced understeer through roundabouts yet didn't create any lift-off neutrality at speed...
Equally, when running a typically late-ninties-ratty Series 2 CX TD I stuck a pair of 195-14s on the front. The engine was the quickest of its type I'd driven so easily managed the slight gearing-up. The slightly shakey car (poor steel, some corrosion) was much sweeter on the bigger tyres. Gone was the crash and twitter every time a certain sharp bump was encountered.

Yep we had 195's on the front of the CX as well and 185 on the rear. Helped with the grip and the handling balance a lot. CX's are said to understeer but I found the CX on 195's had so much more front end grip than a GS that out of the two the GS was more understeery than the CX.

I'm pleased you can understand a little of what I am getting at. The difference was too great, in my mind, for anything else with the GS - which is going to be particularly sensitive to any improvements with its relative lack of weight and pure and simple suspension.
The steady simplification of design being in cost interests and nobody understanding the enormous subtlies of the hydraulic components. Maybe they thought that with struts, what was the point?

The McPherson struts came about mostly as a way to use the same chassis as a steel sprung Peugeot...plus some cost savings no doubt. Lots of compromises in geometry, ride quality etc. Even things you don't think about like hard braking loading up the bushes in the strut preventing it from moving smoothly with low friction reducing grip on bumpy surfaces while braking...

There is a small glimmer of hope though - both the C6 and the Mark 2 C5 have double wishbones again, and from what little I've seen of it it looks like they've actually done a good job of the design, at least the geometry. I'd love to get some in depth technical info on them so I could study it a bit more.

I wonder where else in the hydraulics other apparently tiny alterations have been made? Interesting to hear about the implementation errors with Hydractive 2, what are they and can something be done?

Heres a few off the top of my head:

1) When you accelerate from stationary the suspension goes into hard mode for about 3 seconds to minimise rear squat, but both rear and front switch to hard. Why ? Why not just switch the rear into hard mode and leave the front in soft ? When the front switches to hard it is HARD. Intersections often have broken surfaces so this is a large source of harsh ride.

Under all circumstances the rear and front switch to hard at the same time, and I know its done for reasons of safety as switching only one end to hard while the other is soft is a potentially dangerous handling change at high speeds. (This is mentioned in the training manual) But surely all they needed to do is say as long as we're below 15mph and going straight ahead its safe for us to leave the front in soft when driving away from the lights to avoid unnecessary harsh ride.

2) The front electrovalve should never have been mounted just behind the radiator. Not only does this lead to the internal diode soldered joints failing, it also leads to the rubber seals failing. Front electrovalves seem to fail much more frequently than rear ones, and I believe the difference is temperature.

3) There should be a one way ball valve on the high pressure inlet to the electrovalve. One source of problems is that the only thing that keeps the shuttle valve in the hydractive block locked in soft mode is the supply pressure being greater than the suspension pressure. (Each is applied to one end of the shuttle valve) If the car is heavily loaded the suspension pressure isn't much lower than the supply pressure, meaning that if the supply pressure is a bit lower than it should be (faulty or tired pressure regulator with lazy springs) then hitting a large bump is enough to cause some pressure back flow through the inlet of the electrovalve and cause the shuttle valve to move abruptly switching to hard mode mid-bump. As described earlier this could happen 15 times per second in response to a suspension oscillation.

A simple one way valve at the entrance of the electrovalve would prevent such back flow, solidly locking the shuttle valve in soft mode even if the inlet pressure from the regulator was only marginally higher than the nominal suspension pressure.

4) The ECU monitors the front body movement sensor on the roll-bar for abrupt large movements to trigger a switch into hard mode, whilst this is generally an excellent idea, and in fact almost a necessity for sudden compressing movements that would otherwise bottom the suspension dangerously, (such as going down hill and meeting a bridge suddenly) it also does it for the reverse where the height is suddenly increased - for example suddenly coming over the top of a rise where the ground drops away from you, and the sudden switch to hard is very unnerving and causes a loss of grip as the wheels are unable to drop quickly to stay in contact with the road. They really should have been less sensitive to movements in this direction. Another minor flaw with the body movement sensor is that its clamped onto the roll bar quite far to the right, meaning it doesn't respond to left and right wheel movements equally.

5) The hydractive regulators are just too far away from the struts, especially the front one. Yeah I know there isn't really anywhere else to put them, but it really needed to be above the back of the engine between the strut tops. The long pipes to the front mean that you've got a long section of large diameter piping with a lot of oil in it, which has a lot of movement mass - every bump you hit is causing this large volume of oil to flow abruptly which can lead to hydraulic hammering of the pipe unless it is well clamped and secured. Consider the long, large volume of oil in these pipes to be the hydraulic equivalent of high unsprung weight.

It also leads to problems with bleeding air from the front suspension, which is naturally going to flow up the pipes and accumulate near the top of the strut tops. Trying to purge air bubbles down hill to the hydractive regulator is a real challenge, a LOT of oil has to flow quickly to flush any bubbles all that way through and out past the height corrector. Basically only a full up/down cycle will do it, it won't self purge.

6) The steering wheel sensor is simply a slotted LED chopper system which has no mechanical straight ahead reference, it then relies on a very complex and somewhat fallible heuristic algorithm to constantly try to figure out where straight ahead is. Every time you start the car it figures this out again from scratch. I noticed when I had my monitor LED in place that it was quite common for the suspension to be in hard mode a lot more than it should be because the ECU was still trying to figure out where straight ahead was, thinking I was cornering when I was not. When you consider that at high speeds (60mph+) angles as small as 7 degrees can trigger hard mode its important for the straight ahead position to be accurately known. On very long sweeping curves the heuristics are fooled and the straight ahead position is erroneously reset.

I'm not sure how you would mechanically identify where straight ahead is accurately when you have misadjusted track rod ends to contend with, but there must be some better way of doing it...

There are plenty of other issues with the suspension in the Xantia, but they're not specific to Hydractive 2.

[Peter.N.]

Fit an accumulator sphere to each corner - beautiful ride

Peter

[Spaces]

There are plenty of other issues with the suspension in the Xantia, but they're not specific to Hydractive 2.

I'm sure others would be interested to hear about them. For me, beyond the obvious compromises of reverse-engineering hydraulic suspension to a very ordinary layout designed for metal springs, it's the inconsistency with which the rear axle steers - according to the load carried, temperature, sidewind-induced lateral forces and mid-corner undulations. On a dry road the cars can corner blisteringly fast. However, I find that in the wet and in slippery conditions the back end needs to be treated with care - similarly if the grip changes half-way through a corner taken quickly - which has distorted the rubber bushes - then the back end may lose grip more rapidly than the front as the axle begins to steer straight again.

I became aware of the nature of PSA's rear axle many years ago when driving a BX on a winding, frosty road. My girlfriend was following in a GS and when we stopped asked why I had driven so steadily, for once. I'd been aware she had been pushing me on, but in the BX I was constantly avoiding spinning off the road at 45mph, so I slowed to 40. I reached our destination quite worn-out. It's rear wheels were vertical and everything about the suspension was in good condition, plus the tyres were Michelin. In contrast the GS was on an unknown, cheap brand which were over half worn. I tried the GS back over some of the road and was amazed at its grip, or at least consistency and poise. It was good for at least another 10mph and was totally relaxed even as it did begin to slide on all four wheels.

Other cars with passive-steer rear axles don't seem to suffer this huge contrast in grip between grippy and slippery surfaces, does the PSA setup generate significantly more steer or are other geometries at work I wonder?

[steelcityuk]

I still think that a 'cold' hydractive system rides better because there's less leakage and more consistant pressure.

Mandrake - I fixed my Xantias hydraulic problems by geting rid of it and buying a XM....

Alright, who shouted coward?

Steve.

[Mandrake]

There are plenty of other issues with the suspension in the Xantia, but they're not specific to Hydractive 2.

I'm sure others would be interested to hear about them.

Well you've already covered most of them.

1) McPherson struts - very compromised geometry compared to double wishbones, just try turning the hub with the track rod disconnected at normal height and you'll see how much strain there is on the strut top rubbers just from the simple act of turning the steering. A major problem with hydraulic McPherson struts is that the dynamic loads due to cornering, braking, accelerating etc apply additional side thrust to the bushes which means higher static friction and worse ride especially small bump ride. If you're braking or cornering hard the struts can't move smoothly with small movements which means fidgety ride and loss of grip. Not as bad when they're nice and new but when they get worn the extra friction under side thrust is considerable.

Wear at the normal ride heigh is an issue - the bit of strut that contacts the bushes at normal height becomes more worn than the rest of the strut and causes a "sticky" section of the strut at the normal ride height (See my pic from the other thread) which leads to unusual ride and handling variations depending on small height errors.

Lack of lubrication - the way the strut is designed the very important top strut bush (bronze bush ?) runs dry by design. Lubricating this top bush with LHM or lithium based grease often makes a huge improvement in the slow speed/small bump ride, (almost to the point of old Citroen ride quality) but its usually only temporary as the lubrication eventually disperses. I thought about various ways of permanently lubricating and wetting this bush but never came up with a working solution.

One idea was to glue a thick felt washer to the top of the strut cylinder that wraps around the shaft that is then soaked in LHM, providing a small LHM reservoir that constantly wicks LHM onto the shaft with movement and collects and stores any excess LHM. Keeping that top bush wet with oil should make a big improvement to the ride.

2) Strut tops - related to the McPherson strut geometry compromise is the fact you have a huge rubber bush that has to support the weight of the car and yet has to twist a large amount with steering movement and suspension travel. Anyone who has watched a strut top as the steering has turned from lock to lock will marvel at just how much that thing can flex. Earlier strut tops were badly designed - in salted road environments (eg the UK) the metal frame rusted and lead to catastrophic failure, meanwhile in hot humid climates (NZ, Australia, Malaysia etc) the metal frame didn't rust but the rubber perished and lead to similar catastrophic failure as the top and bottom metal pieces did not overlap - meaning that if the rubber tore or split the joint came apart.

This is such an issue that the first question on a second hand Xantia buyers lips is not "when was the timing belt last done" but "have the strut tops been replaced".

3) Connecting the roll bar to the strut top instead of the lower arm like the BX. This leads to the annoying Xantia habit of the ride height increasing when the car is on full lock. If the struts are a bit sticky the extra friction of the struts under side load will also lead to the height then oscillating. Both things that double wishbone Citroens did not do.

4) Anti-sink - great idea overall, but I think it does cause the air ingress in the suspension problem to become a lot more problematic, as it can't self purge each night.

5) Height correctors connected to the roll bar with an overly complex linkage system consisting of springs, pivots, and sliding surfaces...works great in a climate like NZ where car underbellies don't rust but is a complete nightmare in the UK where everything rusts. This is a good example of over engineering something when it isn't necessary, and leads to inaccurate height correction (overshoot) due to the sliding surfaces dragging due to rust.

The CX/GS approach of a simple torsion bar clamped to the roll bar with a teflon (?) bush acting as a single pivot at the other end and a lever arm pressing on the height corrector joint is simple and elegant and almost never goes wrong. There are no metal pivots, no sliding surfaces, no springs, no nothing. It just works, and its almost impervious to rust. Even the BX had a system similar to the GS/CX, I'm baffled as to why they made it more complicated and less reliable for the Xantia.

6) Rubber bushes on the lower arm...(say no more)

For me, beyond the obvious compromises of reverse-engineering hydraulic suspension to a very ordinary layout designed for metal springs, it's the inconsistency with which the rear axle steers - according to the load carried, temperature, sidewind-induced lateral forces and mid-corner undulations. On a dry road the cars can corner blisteringly fast. However, I find that in the wet and in slippery conditions the back end needs to be treated with care - similarly if the grip changes half-way through a corner taken quickly - which has distorted the rubber bushes - then the back end may lose grip more rapidly than the front as the axle begins to steer straight again.

I became aware of the nature of PSA's rear axle many years ago when driving a BX on a winding, frosty road. My girlfriend was following in a GS and when we stopped asked why I had driven so steadily, for once. I'd been aware she had been pushing me on, but in the BX I was constantly avoiding spinning off the road at 45mph, so I slowed to 40. I reached our destination quite worn-out. It's rear wheels were vertical and everything about the suspension was in good condition, plus the tyres were Michelin. In contrast the GS was on an unknown, cheap brand which were over half worn. I tried the GS back over some of the road and was amazed at its grip, or at least consistency and poise. It was good for at least another 10mph and was totally relaxed even as it did begin to slide on all four wheels.

Other cars with passive-steer rear axles don't seem to suffer this huge contrast in grip between grippy and slippery surfaces, does the PSA setup generate significantly more steer or are other geometries at work I wonder?

I'll be honest and say that I have never noticed a tail happy characteristic to the HA2 Xantia as you describe, in fact I found the handling generally excellent.

My previous Xantia was a 2 litre 8 valve petrol Auto with Hydractive 2, the original tyre size was 185/65/15 on alloys, but I upped it to 205/60/15, (Michelin XM1) the same size that comes on the V6 I now have, and I found despite my battles with intermittent slow speed ride harshness, the handling on that Xantia was always excellent, ever since I put those tyres on it.

I pushed that poor car very hard around the corners and never once had the back slide out, even in the wet. In fact I recall commenting on the forum a few years ago just how neutral the handling of that car was for a front heavy front wheel drive car, pushed too hard in the wet it would drift sideways almost neutrally and very controllably. There is a tiny bit of lift off oversteer under some circumstances, but my GS did that as well.

On the other hand I did find my Dad's non-Hydractive Xantia a bit more "lively" at the rear end, it was ok up to a point but if you pushed it really hard it did get a bit twitchy at the back, and didn't feel as safe and controllable on the limit as the HA2 model. I never felt comfortable pushing that car to the limits. Whether thats a difference in the tuning of HA2 versus non-HA2, or whether the car had other issues like worn bushes, (it was 3 years older and higher mileage) or whether it was the inferior and mixed front/rear tyres that he had, I don't know.

I haven't really had a chance to push my V6 to its handling limits (ironically) but its handling feels very similar to the other HA2 car despite an extra 200Kg at the front. My feeling is that what you describe doesn't happen in a properly working HA2 Xantia.

My suggestion would be that the most likely cause of what you experienced with a twitchy tail happy rear end is worn rear arm bearings - not only will the negative camber introduced cause a large loss of grip especially with wide tyres, the stiffness of the arm pivots under sideways thrust will prevent the arm following the undulations of the road and cause loss of grip.

I did the rear arm bearings on my previous Xantia, and to be honest they weren't that bad before I did them either, and the ride and handling at the rear of that car was excellent.

The GS doesn't suffer from these problems because the roll bar itself is the pivot onto which the arms are mounted - so even if the rear arm bearings become worn it will not cause the arm to tilt and introduce negative camber. (The CX on the other hand suffers from the same rear arm bearing problems as the Xantia, perhaps more so)

Another possibility is the rear steering bushes being badly worn or replacements fitted incorrectly - if their orientation is wrong when replacing them the amount of rear steer will be incorrect.

One final though if the car was HA2 - handling stability can be compromised if one end switches to soft while the other is in hard. In particular if the front is soft but the rear is hard this will induce oversteer especially on rough surfaces. This could happen if the car should be in soft but the rear electrovalve is sticky, leaving the rear hard while the front is soft, or if the rear hydractive regulator sphere is flat. (Leaving the rear always hard)

Another combination that can happen is if the rear strut spheres are very flat but the rear hydractive regulator sphere is ok. In this situation the soft mode will be mostly ok but when you corner hard and the ECU switches to hard mode the rear will suddenly become almost rigid mid corner, this could lead to the rear end sliding out since the handling bias will abruptly shift from neutral towards oversteer.

It's important with HA2 for the spheres to all be in good order and for the HA2 system to be working properly to avoid sudden changes in under/oversteer handling bias as it switches modes between soft and hard.

[Mandrake]

I still think that a 'cold' hydractive system rides better because there's less leakage and more consistant pressure.

Can you elaborate ? Less leakage where ? More consistent main system pressure ?

Mandrake - I fixed my Xantias hydraulic problems by geting rid of it and buying a XM....

Alright, who shouted coward?

Ahem.

[steelcityuk]

Well with the Xantia the hydractive only felt floaty for the first few minutes of the day, the cycle time of the pressure regulator was longer first thing too. So I took it that the LHM being cold would be thicker which would mean less leakage past the various valves that had worn in the years since manufacture. Sure you can drop off a leak back hose and check that leak back isn't excessive but we have no real data when it comes to tolerances and what is acceptable. The pressure regulator could be upped to try make up for this 'leakage' but surely that would be self defeating, more pressure, greater fluid temperatures, thinner fluid and more leak by/leak back. The way 'sinkers' drop to their knees very quickly as the years go by perhaps indicates the amount of wear the hydraulics are subject to.

Small cramped things those Xantias.

Steve.

[Spaces]

Maybe I exaggerate my perception of the tail-happiness a little, partly because I associate Citroens with accurate, linear handling which could always be trusted at all speeds in the worst conditions. A small amount of rear arm bearing wear would never cause a CX to lose grip at the rear. Having experienced the edginess of the BX's handling on the slippery road, in complete contrast to the serene GS, I took it to a level and smooth large open space with a couple of inches of virgin snow. I started to accelerate slowly into a large circle, and continued to increase the speed gently until the car lost grip. With the BX there was little warning - the tail suddenly shot out and the car spun quickly with no chance of catching it. I tried the same test in a friend's Golf, it began to slide progressively, front first, until the circle could not be held, reaching a couple of miles and hour less than the BX.

I have carried out this little test with almost all other cars since, it's interesting and informative. (I spent a winter in the Alps, driving a variety of motor cars.) A non-Hydractive Xantia was the car which seemed to grip as well as any modern Citroën, with the Hydractives behaving quite poorly, but all with the distinctive sudden PSA tail slide. Of all the cars I drove in this slippery circle, the Saab 900 (Saab version) was the most grippy and most predictable, closely followed by two cars from the same stable - Citroën's GS and CX. Which pretty much bears out what I have found on the road.

The Saab had no anti-roll bars, allowing a more even cross-axle weight distribution. On the road, the lack of these undamped torsion springs (which render an independent suspension system anything but) has a more pronounced effect when doing anything than travelling steadily in a straight line on a perfectly smooth road. Those on the GS and CX were minimal, the standard hydraulic setup allowing for some balancing of axle load from one side to the other. The hugely strong bars fitted to many more 'modern' cars have a hugely corrupting influence on vehicle handling, it seems to me they're a sop to the school-run Mums who are frightened of a little heel around the roundabouts. It's interesting to note the latest McLaren sports car has none and several of the better luxury 4x4s are using alternatives and/or decoupling them as much as possible.