Workaround for bubbles in LHM return flow

[white exec]

Sounds as if it just gets better and better. Perhaps residual air working its way out, and leaving everything working as it should do.

The same here, after Hydraurincage'ing, last sphere re-gassing, brake bleeding, and checking the reservoir connections. Ride staying consistently good for all journeys, and has now been like that for almost a year.

Struts can creak when setting to High, but not otherwise.

[CitroJim]

Simon, that's excellent news

[Zelandeth]

Interesting reading here, definitely gives me a few things to check on mine. I've got decent damping over large bumps, but still quite fidgety ride over smaller undulations and quite a lot of air in the LHM. Checking the intake hoses is obviously priority one.

Really wasn't obvious before as the spheres were shot, and it's far better now but can be better I reckon.

[chinkostu]

Interesting reading here, definitely gives me a few things to check on mine. I've got decent damping over large bumps, but still quite fidgety ride over smaller undulations and quite a lot of air in the LHM. Checking the intake hoses is obviously priority one.

Really wasn't obvious before as the spheres were shot, and it's far better now but can be better I reckon.

Thats the best word to describe it when theres air in there. Mines the same, sometimes it's absolutely wonderful, then you hit a hole and it jitters about horribly.

[Zelandeth]

Yep, you can go over speed bumps with complete impunity - then hit a tiny seam in the road and it causes an almighty thud that shudders through the whole car. Just seems that the suspension is really hard for about 1/2" then is as soft as it should be. Which given how the bypass valves etc work is exactly what I would expect if there was some air in there - that air has to be compressed before the valves will open and allow fluid to move which causes the brief overly hard suspension. Got hung up working on the garden today as the weather was so good, but hopefully get a look at it tomorrow.

[Mandrake]

Yep, you can go over speed bumps with complete impunity - then hit a tiny seam in the road and it causes an almighty thud that shudders through the whole car.

Yes, that is exactly the symptoms of air in the oil - large undulations are often handled OK but small, harsh bumps from broken surfaces such as potholes and road patches crash right through the body harshly to the point that it just about rattles your teeth...

A secondary symptom is that the car may feel like it's constantly reacting to small undulations on the road, never "settled" or "composed" feeling. When the air is eliminated (and assuming spheres are all in order and so on) the car becomes much more stable and resists moving in response to small undulations.

Just seems that the suspension is really hard for about 1/2" then is as soft as it should be. Which given how the bypass valves etc work is exactly what I would expect if there was some air in there - that air has to be compressed before the valves will open and allow fluid to move which causes the brief overly hard suspension. Got hung up working on the garden today as the weather was so good, but hopefully get a look at it tomorrow.

I've talked about this at length in other threads before but my understanding of what the air does is that it causes the column of oil between the piston in the strut/suspension cylinder and the sphere diaphragm to become slightly compressible. Normally pure oil would be a completely incompressible medium, however if some proportion of it is air, there must be some unwanted compressibility.

Since the oil medium between the piston and the damping valve in the sphere neck (and the damper valves in the Hydractive block on a Hydractive 2 model) should be completely incompressible, it follows that even the smallest suspension movement can only happen by forcing oil through the damper valve in the sphere - thus even the smallest suspension movements are damped and well controlled.

However air in the oil between the piston and the damper valve allows a small amount of suspension movement to occur without oil being forced through the damper valve (or to a lesser degree than normal) thus damping of small movements is reduced leading to an "unsettled" slightly under-damped feel to the ride. The more air, the larger the suspension movement that can occur without forcing oil through the damper valve so the less damped it feels.

On a standard model you have a relatively small amount of oil between the piston and damper valve in the sphere neck and then a little bit of oil in the 3.5mm piping back through a T joint to the height corrector, which is a "dead end" as far as suspension dynamics on the go are concerned. Any of this oil can have air mixed in it.

On a Hydractive 2 model there is far more oil in the circuit between the suspension pistons and the various damper valves. You have 10mm pipes from the struts instead of 3.5mm (about 8x the cross sectional area) and the pipes are also very long compared to a standard model - travelling around the wings at the front down to the bottom of the radiator. You then also have the 3.5mm pipe from there back to the height corrector. That's a lot of oil. The more oil volume in this critical section between pistons and sphere dampers, the more air volume there is for the same average air distribution around the system. (Eg, severity of air leak)

I believe this is one reason why Hydractive models seem to suffer more from this problem - they just have a lot more oil in this critical hydraulic path (Maybe 10-20x as much - haven't sat down to actually try to calculate it) and therefore have proportionally more air trapped there.

That explains the reduction in damping with air bubbles, but what causes the ride harshness ?

I believe what happens there is cavitation of the small air bubbles in the same path between the piston and damper valve. When small but sharp bumps are passed like broken surfaces, the damper valve will try to resist flow through the valve - this causes the bubbles to be compressed until they "collapse" and on the return stroke they then expand again, just like cavitation on a boat propeller. When cavitation is present, the relationship between oil flow and oil pressure becomes highly non-linear, which generates pressure pulses or shockwaves.

When you hit any kind of step change in the road (like coming out of a pothole) the wheel doesn't just move up and stop - its actually normal for the wheel to oscillate up and down on the tyre wall at about 10-15 times per second - the so call wheel hop frequency, for up to half a second or so. You can sometimes see this if you are following another vehicle and watching one of the rear wheels carefully as it passes over a pothole - it's particularly noticeable on buses due to the large unsprung weight.

One of the jobs of the suspension is not only to isolate you from this oscillation through compliance, but also to try to damp this oscillation via the damper valve. Normally this oscillation is absorbed and you don't really even realise that the wheel was bouncing up and down rapidly on the tyre, you just feel a gentle "thud" when you climb out of the pothole.

However when the air bubbles are there, every time the wheel oscillates up and down after hitting a step in the road the air bubbles are repeatedly collapsed and expanded causing a rapid series of cavitation shockwaves in the system which is passed into the body via the strut mounts. On a Hydractive 2 model I suspect there is an additional effect where these shockwaves transmitted through the 10mm pipe from the cavitation can actually passes through the pipe into the front wings via the attachment point where the hose is clamped.

Certainly it can sometimes sound like there is a rattle coming from the wing in these situations. Also on two of my Hydractive 2 Xantia's I've discovered that the steel pipe that passes under the battery mounting towards the hydractive block has come out of its plastic clips and was touching the body in places - any shockwaves being transmitted by these pipes would also rattle on the body if the pipe was touching the body metal to metal. It might not contribute much to the feel of the ride, but make a lot of noise and make the ride "sound" bad.

Anyway in short, air bubbles in the LHM = bad.

[white exec]

Simon, that's an excellent analysis, which seems borne out by experience. If spheres are in good condition, and fluid is air-free (which can take some persistence to achieve, in some cases), then the ride becomes exemplary.

I would say that brand new Hydractive 2 vehicles always give a design-perfect ride, and that the ride only becomes harsh/poorer after a few years of use, or higher mileages - always assuming sphere maintenance has been kept up. With age comes air ingress, probably aggravated by component and pipework disturbance.

Given decent spheres, the ride can be very acceptable, but the ingress of air is generally invisible, and often unobserved and overlooked - but enough to take the edge off the comfort as you describe: enough to cause irritation, but not always enough to alert to something fundamentally wrong.

I would agree about the apparent susceptibility of H2 systems to this problem, and the greater problem of eliminating it, compared to earlier and simpler systems. I do not remember a problem of harshness on BX when properly sphered, and CX is widely reported as delivering supreme comfort.

Chris

[CitroJim]

Simon, this is excellent. Well done

[Mandrake]

I would agree about the apparent susceptibility of H2 systems to this problem, and the greater problem of eliminating it, compared to earlier and simpler systems. I do not remember a problem of harshness on BX when properly sphered, and CX is widely reported as delivering supreme comfort.

My first car was a GS and I drove two others on a regular basis - none of these ever had this issue that I can remember. (although we're talking about 20 years ago now...maybe my memory is a little rose coloured)

Two of the GS's had exemplary ride, the third was a little bit harsh over the curb at the front but that was due to worn out roll bar clamp bushes that never got around to being done...the same problem also caused it to roll a lot more than the other two.

I also had a '78 Series 1 CX for a few years and the ride was amazing as well, probably the best of any Citroen I've driven. Despite having fairly worn rear suspension arm bearings (that sadly never got done before the cars demise) there was never any harshness in the ride and the way it could drop off the curb onto the road effortlessly always impressed me.

I'm now onto my third Hydractive 2 Xantia - the first a 1997 Petrol 2.0i, the second a 1998 V6 and now a 1997 V6. All three have suffered from this harsh ride / air bubble issue to varying degrees, with the first 2.0 litre one being both the "worst" of all and the best of all, due to being the most variable. I spent untold time doing everything imaginable to the suspension on that car (documented on the forum...) to try to solve its intermittent harsh ride, and never could. It was always variable, and reminds me a bit of Xantos's car...

On a good day (and the good days came a lot more often after all the work done) the ride was amazing and came close to rivalling the CX, amazingly smooth and stable even at high speeds with excellent cornering. On one of these good days I think it rode better than either of my V6's.

On a bad day it would crash its way through potholes like nobodies business. Just really harsh over any brokenness on the road. The only correlation I could find really was that bad days tended to be hot weather (this was in New Zealand ) while good days tended to be cooler weather. I had a clear piece of pipe in the main return line to the tank for over a year and like Xantos there were persistent, large obvious bubbles in the return line that I could not eliminate. I even replaced the entire pipe from the tank to the pump with a custom made pipe - that did help somewhat but it was never completely cured.

Around the same time my parents also had Xantia's - but they were both non-Hydractive models. Dad's never came close to riding as well as mine did when mine was having a good day, and didn't feel nearly as safe around tight corners, (handling of the H2 model is far superior) but it also didn't have bad days... it's ride was middle of the road all the time - never brilliant (it never rivalled the GS or CX) but never awful either, and it never crashed harshly through potholes. Whereas mine went from brilliance to awfulness in the space of a day and back again.

Mums didn't ride as well as Dad's but it was never as well maintained and was a very high mileage diesel that I suspect had worn arm bushes, etc that never got done - but it too never crashed badly through potholes...

Since coming to the UK I've had two V6's, the first one right from the beginning (after I had changed all the spheres of course) had intermittent ride harshness that got worse over time. I did the "10 minute mod" on that one and it helped a lot, it went for about 3 months with no crashiness, but re-terminating the pipe on the tank may have helped seal a leak so the true cure is unknown. Later on the ride started getting harsh again and I discovered the pipe had split and I had to replace it as reported earlier in the thread and that restored crash free ride for a few months as well but it then started to get worse again.

This V6 initially rode well when I first got it, once the spheres were replaced, however a few months ago the hydraulic pump started leaking and I tried to do an in-situ repair of the top o-ring. In the process of doing that I tried to disconnect the hose at the tank and pump end and I simply could not get them to turn or let go in any fashion - they are moulded so hard onto the spigots that I genuinely though the rubber would tear if I tried any harder to get them off so I sheepishly ended up clamping them back on and doing the job without removing the pipe...(causing a significant oil spill in the process when I removed the pump piston)

Some time after that I noticed the ride was starting to get quite harsh and considered doing the 10 minute mod, however as a quick experiment I thought I'd try sealing the pipe at the tank with sealant and what do you know....

So at least in my experience, Hydractive 2 models seem to suffer ill effects from air bubbles much more severely, although that's not to say that standard models can't be affected as well.

[Zelandeth]

Had a brief poke around with this today in the evening. Doing a round of Citrobics usually resulted in a very frothy tank, especially as the rear dropped.

Had thought of sealing the joint on the tank with self amalgamating tape (that stuff has saved my bacon on many an occasion and should be in any toolbox), but there's nowhere to get at it so that idea went out the window. The original hose clamp.is definitely no longer doing its job as you can easily rotate the hose on the stub. I couldn't figure out how the original clips come off in the few minutes I had to poke around with it today, but could feel a good 3/4" of the stub past the original clamp so had an idea, grabbed a normal hose clip, split it and reassembled it around the hose next to the original one. This is an experiment after all.

Several rounds of Citrobics later and there is some slight clouding in the tank - but *vastly* less than before, when it looked like the head you would expect on a pint of beer - on the right track I reckon.

All of the return hoses look to have been weeping, so could do with getting new hose clips on there too...

[white exec]

Right track indeed. Fingers crossed.
If you do lay in some new worm clips, go for thin gauge ones (maybe stainless), which will be more flexibly circular than the usual thick and 'lumpy' standard item. Using in pairs no bad idea. Perhaps some joint-goo as well (eg Hylomar).

[CitroJim]

Right track indeed. Fingers crossed.
If you do lay in some new worm clips, go for thin gauge ones (maybe stainless), which will be more flexibly circular than the usual thick and 'lumpy' standard item. Using in pairs no bad idea. Perhaps some joint-goo as well (eg Hylomar).

This is a good argument for investing in the proper tool so the original clips can be re-used. They are circular...

Seen the problem with 'lumpy' Jubilee clips in the past... I never like them...

[white exec]

Have you got any details of it, Jim?

[Hell Razor5543]

I think this may be what CitroJim means;

http://www.ebay.co.uk/itm/FOR-CITROEN-C ... SwnLdWsIN3

If not, ignore me.

[xantia_v6]

I think this may be what CitroJim means;

http://www.ebay.co.uk/itm/FOR-CITROEN-C ... SwnLdWsIN3

If not, ignore me.

mine is more like: http://www.ebay.co.uk/itm/Sealey-Heavy- ... SwPc9W1PPj (although this one seems to have slightly different jaws to mine).

search for "clic pliers"