Falling qualities of Citroën hydraulic ride

[Mandrake]

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.

Leaky electrovalve ? I had a really leaky front electrovalve in my previous Xantia that I eventually had to replace. At its worst it was causing a regulator cycle time of only 8 seconds, low enough to seriously interfere with normal suspension operation. And yes, the leakage was much higher when the engine bay was hot...around 8 seconds, whilst more like 20 seconds when cold. As I said earlier in the thread, electrovalves really don't like heat.

Glad I'm not the only one who has experienced soft floaty ride that rapidly becomes harsh after as little as 15 minutes or so...I would dearly love to nail this fault down, you can be sure that the solution is quite simple if we could just figure out what the cause is...

Is your XM Hydractive 2 ? I assume you don't have the same problem, and if it is Hydractive 2 then it suggests differences between the individual cars rather than something that is fundamentally unavoidable.

[Spaces]

Does anyone have the figures for how much 'thinner' the LHM was made when its viscosity was changed for use with ABS - and had its name changed to 'LHM+' ? With hydraulic components which are more prone to wear and which have already worn slightly, the use of a marginally more viscous fluid (achieved by adding a calculated amount of a straight mineral oil) may have a beneficial effect in several different ways provided it wasn't heated up more. Even the Coanda effect as mentioned by Addo may increase, further improving the ride for those who have sphere damper holes which have faired in entries/exits?

[steelcityuk]

The previous 3 XMs have been HA2 whereas the current one is HA1. I think I prefer the HA1, it means I can run very soft suspension for general day to day but lock the car into a firmer mode when towing my trailer or such. The electrovalve doesn't give problems on the HA1 system either or so it seems. None of the XMs has had suspension as bad as the Xantia but the sphere layout seems better for maintenance on the Xantias. How about rigging up a cooling system for the LHM, maybe before the pump so that it isn't subject to such high pressure and it would help to keep the pump cool? Perhaps a transmission intercooler would work.

Steve

[Spaces]

I'm interested in your comments about HA1, Steve. I've often been annoyed with the programming of the HA2 ECU which obviously will work best on French roads. You never quite know where you are, as it were, not knowing how it will decide to deal with a section of approaching road. Bit like automatic windscreen wipers, you end up playing a mind game with the electronics rather than concentrating on just driving. To be fair, I have a series of spheres which I judge near-perfect and I have learnt the brain's thoughts so am generally pleased with its decisions, in both modes. Although coming home tonight, neither fast nor steadily, but quickly enough to use the sport setting for a smoother ride, it kept surprising me by switching into the softer setting - which in turn annoyed me as I wasn't in the mood for a super-absorbent ride or the paranoia of the computer not knowing which setting to use...

[steelcityuk]

Well the roads in Sheffield are notoriously bad so I've gone for soft suspension and tyres with a decent aspect ratio to help deal with the smaller sharper shocks. I think I'll experiment with chamfered sphere holes, it costs nothing so it's worth a try. Thanks for the tip.

Steve.

[Spaces]

I think the hole chamfering will be most obvious on cars where the original ride was intended to be optimum - ie if the body rides flat and the wheels follow the road that's how to achieve the best roadholding - but it's a few years since Citroën employed the minds which thought like that. Additionally with a PSA car, as Simon/Mandrake mentions, the fluid path between the piston and sphere is more convoluted so it's less likely the effect will be felt. What was most interesting was how, in the GS, it was more obvious on a smooth road than a really bumpy one, although washboard surfaces which are commonplace on the roads in the North York Moors were transformed, especially with the seventies spheres as in the first pic of this thread. I think a little experimenting with the LHM's viscosity could be worthwhile too, providing you don't stray too far from the original.

Higher profile tyres definitely the way to go for 21st Century English roads! Just wish I lived in rural Scotland or Wales, there you could begin to think about using near-standard springing and damping on a Hydractive car...

[addo]

I'm mid-experiment with the Xantia.

Have fitted Dee front spheres, riveted two-piece type with 40 Bar pressure (tested each). Also running at the low end of allowable tyre pressures.

After about five miles, the LHM has warmed up sufficiently to achive a good thinness, and the car does behave more like a Dee on undulating roads. The damping is marginally light-on and while there's no chance of a "Tacoma Narrows" type situation, a harmonic sequence doesn't die really quickly (this is a particular contrast with regular Xantia spheres).

Much as anything else, it's a great underbonnet look! Offside one barely clears some of the engine bay sheetmetal, though. I may have to get a set for myself (these are loaners) and experiment with pressure, as I feel that slightly more pressure will change the damping characteristics a little and also stop the occasional bottoming out.

[Mandrake]

I'm mid-experiment with the Xantia.

Have fitted Dee front spheres, riveted two-piece type with 40 Bar pressure (tested each). Also running at the low end of allowable tyre pressures.

DS spheres are 500cc compared to 400cc (standard) or 450cc (long life 3 dimple 3 layer spheres) typical on Xantia's, which means the springing rate is different for the same gas precharge pressure. (you'll notice that the specified pressure of a 400cc Xantia sphere is higher than a 450cc sphere for the same location on the car)

So 40 bars in a 500cc sphere is equivalent in springing rate to 45 bars in a 450cc sphere and 50 bars in a 400cc sphere, the difference being that it has a bit more linear displacement range. (eg copes well with a wider range of vehicle loads)

There's nothing wrong with putting DS spheres on another model as long as you can adjust the damper and gas pressure... we fitted some DS spheres with the removable dampers to the front of a GS once for lack of any alternative spheres, the pressure had to be set about 20% lower than normal but once the dampers were tuned properly it worked very well, in fact it worked better than the then (mid 90's) available replacement GS spheres.

I may have to get a set for myself (these are loaners) and experiment with pressure, as I feel that slightly more pressure will change the damping characteristics a little and also stop the occasional bottoming out.

You made this mistake in another thread too. More gas pressure means softer springing, which will mean more bottoming out if you leave the damping the same. If you want to stiffen the springing you need less gas pressure.

40 bars sounds fine though so what it really needs is a bit more damping. If that's not one of the removable valve types you'll have a hard time adjusting it though, the only thing you could adjust would be to reduce the bypass hole diameter, however if the leaf valves are too soft for a Xantia no amount of adjusting that hole will give the desired result. Worth a try though, you'd be amazed what a difference a very small (fractional millimetre) change in diameter makes to the damping. All I do is solder the hole closed and carefully drill it out again (to a smaller size) with a precision drill bit! Easy, repeatable, and undoable. (just use a minimum amount of solder and not too much heat so it doesn't flow right down into the bottom of the valve, you just want a nice thin smooth flow of solder across the end of the hole)

If you're trying to get a set of DS spheres for yourself, try to get the earlier ones with the bolted in damper valves - they can be removed and reshimmed with different numbers and thicknesses of washers in just a few minutes which gives full flexibility in tuning the damping to suit any car or any desired tuning on a given car.

Make sure they're not too early though, you don't want black LHS spheres as the diaphragm is incompatible with LHM, make sure they're green LHM types.

[addo]

More gas pressure (in the same volume) creates a greater "sink" for incoming forces. It's equivalent to stiffening a conventional spring. The reason it may seem softer is because it doesn't bottom out so readily.

[Mandrake]

More gas pressure (in the same volume) creates a greater "sink" for incoming forces. It's equivalent to stiffening a conventional spring. The reason it may seem softer is because it doesn't bottom out so readily.

Sorry addo but you're definitely wrong about this, you need to study how Citroen suspension works in a little bit more detail. I don't mean that to sound in any way critical, it's just that it's a point that a lot of people don't understand because it's not intuitive.

Gas precharge pressure is measured with no oil pressure applied, eg the diaphragm is sitting bottomed at the sphere neck end and the gas fills the entire sphere. The higher this pressure the more quantity of gas in the sphere. As an example lets pretend we charge a 500cc sphere to 50 bars.

When the sphere is in use the oil pressure required to lift the car is always significantly greater than the precharge pressure so the gas is compressed and the diaphragm starts to move. The front of a Xantia takes about 110 bars, but let's pretend it's 100 bars. This will compress the gas until it too reaches 100 bars. Boyles law says that because the initial gas pressure was 50 bars and it's now 100 the gas will compress to half it's volume - eg 250cc with the other half of the sphere full of oil.

If our initial precharge pressure was 25 bars instead of 50 bars the gas is still compressed to 100 bars but now the operating gas volume is only 125cc instead of 250cc and 3/4 of the sphere is full of oil.

So how does this relate to springing rate ? You have to go back to the definition of spring rate. A softer spring is a smaller change in force for a given suspension displacement, or alternatively a greater suspension displacement for a given change in force.

A given suspension displacement injects or removes a given cc of oil to/from the sphere based on suspension leverage and the area of the hydraulic ram, and the compression or expansion of the gas causes a change in pressure, therefore a change in force with displacement.

I'm not going to do the math here but the end result is that a large operating gas volume (250cc in example 1) results in less change in pressure for a given suspension displacement since the change in gas volume with displacement is a small percentage of the total gas volume. Less change in force for a given displacement is soft springing.

In example 2 where the operating gas volume is only 125cc, the same suspension displacement thus oil displacement changes the gas volume twice as much by percentage than the 250cc example, therefore the change in pressure is twice as much, therefore the change in force is twice as much, which corresponds to a springing rate that is exactly twice as stiff.

In essence the spring softness is directly proportional to the operating volume of gas. The greater the operating volume, the softer the springing. Operating volume (for a constant weight loading the suspension) is dictated by both gas precharge pressure and sphere volume.

The higher the precharge pressure the greater the operating gas volume when compressed to operating pressure, and the larger the sphere volume the greater the operating gas volume for the same precharge pressure, since you started off with a larger volume of gas and compressed it by the same ratio.

In simple terms - higher precharge pressure is softer, larger sphere is softer.

Hope that makes sense and explains what may seem counter intuitive. I can absolutely categorically state that higher gas precharge pressure provides a softer springing rate, there is no doubt whatsoever about this.

[addo]

Jeez, I just composed a reply and the internet dropped its bundle.

My misassumption was based on stroke volume of the cylinder, which I'd assumed to be close to - or greater than - sphere volume.

For the table below I nominated a half-stroke displacement of 38cc.

[Mandrake]

My misassumption was based on stroke volume of the cylinder, which I'd assumed to be close to - or greater than - sphere volume.

For the table below I nominated a half-stroke displacement of 38cc.

Yes exactly, the total suspension stroke is only quite a small percentage of the sphere volume. It's even less than your estimate in fact.

For example rear rams on most hatchbacks are 35mm diameter and due to the trailing arm leverage (approx 6 to 1) the piston stroke for full suspension travel is only approximately 30mm, which works out to a volume displacement of ~29cc for a complete stroke from bump stop to bump stop, compared to the sphere volume of 400cc, so well under 10%.

For a given suspension load the full suspension travel is quite a small movement of the diaphragm in the sphere, so even though the pressure vs displacement of the sphere is actually a curve with an asymptote at the fully compressed end, its relatively linear over such a small portion of its range, as long as you aren't too close to the fully compressed end. Load the suspension with more weight and the equilibrium point moves and now you have a new point around which the diaphragm moves. This is in contrast with normally sprung suspension where the suspension travel is a large percentage (if not all) of the available spring operation range...

Nice spreadsheet, you can see clearly from that that increased pre-charge pressure results in less change in force with a given displacement, eg softer springing. It's not precisely double stiffness for half the gas volume because that assumes that the displacement around the equilibrium point is infinitesimally small - because its not infinitesimally small and the pressure vs displacement is a curve instead of a straight line there is a small error, (the slope between the two extremes is different to the tangent of the half way point) but its still approximately proportional to operating gas volume until you get down to the point where the gas is compressed into quite a small percentage of the sphere, then it starts to become highly non-linear. (This is why spheres with very low gas pressure give a non linear suspension that gets harder very rapidly as you press it down, while normally gassed spheres give very linear springing...)

There is a limit to the softest possible springing though, the gas pre-charge can't be any higher than the operating pressure or the diaphragm will be bottomed instead of within its working range...you have to allow a displacement for the suspension to extend as well so in practice if your equilibrium pressure with the car unladden was 110 bars - as it is at the front of a Xantia, then the maximum possible pre-charge for the front spheres would be approximately 90 bars. (Highest used in the front HA2 regulator sphere is 75/80 depending on sphere size)

The only way to increase the softness further would be to use a larger capacity sphere - since you can get the same working gas volume from a lower pre-charge pressure. A larger sphere always works better and provides a greater minimum load to maximum load working range (as well as better linearity at heavy loads) but the drawback apart from increased size and cost is that more oil has to be pumped into the sphere when pressurising thus a longer delay before the suspension lifts. (Not really an issue with anti-sink to be fair, since it doesn't fully depressurise)

[CitroJim]

Gosh I've only just read this as before I was unable to absorb it properly but by golly what an excellent thread. I have learned so much.

Thanks! Made me realise just how much I don't know about hydraulic suspension... My knowledge is now greater...