Air bubbles in the LHM gives harsh ride?

Mandrake · Unread post by **Mandrake** » 11 Dec 2006, 18:43

jeremy wrote:Liquids are non-compressible - but liquid/air mix must be compressible. Wouldn't this give a softer, rather uncontrolled ride?

Hi Jeremy,

Unfortunately the effect is nearly the opposite. The ride becomes harsh over irregular/broken surfaces, (potholes, manhole covers, curb etc) but as you say, at the same time there is an overall small loss in damping factor causing the body to tend to oscillate/pitch more over long distance undulations. (The effect is very paradoxical, but I have observed the appearance of the two symptoms at the same time on countless occasions....)

The loss in damping is easy to explain - especially on a Hydractive 2 model which has long large diameter feed pipes from the struts to the centre hydractive sphere - any air mixed in with this oil is slightly compressible, which means the suspension is able to move a slight amount without oil needing to flow through the damper valves (it just compresses the air bubbles a bit) therefore small movements are not properly damped.

When there is no air the fluid is completely incompressible between the suspension ram and the damper valves, and you get very good damping of small movements, preventing oscillations.

In this case the extra compressible medium (the air/nitrogen bubbles) is on the WRONG side of the damper valve....(the nitrogen in the spheres is always on the far side of a damper valve from the struts)

The other factor to consider is that just because air is compressible doesn't mean that it being in the fluid paths between the suspension rams and the dampers/spheres is equivalent to having that much extra nitrogen in the sphere - it's not.

What you have to analyze is the pressure vs displacement curve seen by the hydraulic ram in response to suspension movement - when there is only ONE compressible medium in the circuit - the nitrogen in the sphere, you get a smooth continuous curve.

If you now introduce a second seperate compressible medium - the air bubbles, which have a VASTLY smaller volume, the situation changes.

Without going into all the math suffice to say you now have a curve which has a discontinuity/spike in it at the working point - and it's this that gives the "harshness". (IMHO)

The practical effect of this is that it feels like suspension that has badly knackered balljoints or rear arm bearings.

The dead giveaway for me is that unlike poor Bernie, whos car is like this all the time, mine only does it occassionally, just enough to remind me that there is something still wrong... and when the ride is good (most of the time now, after doing various work, including replacing the seals in the pump) its outstanding, quiet, smooth, free from harshness, brilliant.

Then every now and then the ride goes completely to hell and it starts hammering and crashing over the potholes like you wouldn't believe - sometimes in the same day that it was previously riding brilliantly!

I can't think of ANY straight mechanical problem that could cause the ride to go from brilliant to awful and back to brilliant again in the space of a day...

Regards,
Simon

PS, it seems to me that a Hydractive 2 model would be MUCH more prone to air in the oil affecting the ride than a standard model - as you have long large diameter pipes in the operating circuit to the centre sphere for air to become trapped in - pipes which are harder to flush out, compared to a standard model.

On a standard model the air would have to be in the small distance between the ram and the sphere - only a few cm's.... and air in the small feed pipe back to the height corrector probably wouldn't have much effect on abrupt bumps as the small diameter pipe would limit the flow in that direction anyway.

Has anyone noticed whether this "harsh ride" symptom is occuring more commonly with Hydractive 2 models ?

bernie · Unread post by **bernie** » 11 Dec 2006, 18:51

Right, that's enough Simon

Swop your's for mine

and Yes I've bought another 124

A little piccy, isn't she lovely

bernie · Unread post by **bernie** » 11 Dec 2006, 19:00

This is for Steve, shows the led on the HA switch

steelcityuk · Unread post by **steelcityuk** » 11 Dec 2006, 20:23

Thanks Bernie, mines the same except that the rear window switch is nearest the steering column.

As Simon says (no pun intended!) it does seem as if it affects Hydractive cars more than other types. From reading various posts it seems that just replacing the spheres fixes their ride problems.

I'm going to check the ride height too this coming weekend. It will be interesting to see if curing the short regulator cycle has an effect on the ride quality. I think that my Xantia's had a used replacement pump at some stage so that's another unknown.

Steve.

citronut · Unread post by **citronut** » 11 Dec 2006, 22:05

steelcityuk wrote:I've just been for a look around the Xantia-L forum on Yahoo groups and it seems that I should have a dash light for the Hydractive system. I've never seen one on start up so that makes me think a bulb has gone seeing as the diagnostic LED says it's working as it should.

It sounds as if these air bubbles are a common thing. I know they are extremely small - you can't see them without a torch - but if they are making up 10-20% volume of the LHM then that's gonna be a problem isn't it? I also suppose that once air bubbles are in the liquid it would take quite a while for them to dissipate. Maybe they are being introduced by a leaking pump. I'm thinking this way because parts of the pump must be at low pressure and/or vacuum so a faulty seal here would suck in air and so on. I think a trip to the breakers is needed. Could this also be the reason for rapid regulator cycling?

Steve.

the regulator cyclling frequenly is almost certainly down to acumalator sphere going or gone flat,in wich case it will dump its gas into the system,i belive people are becoming very paranoid over the air in the tank
regards malcolm

steelcityuk · Unread post by **steelcityuk** » 12 Dec 2006, 09:28

I hope the sphere isn't the fault it was changed only about 3 months ago. Even when just fitted the cycle time was still very short. I see your point but what's the tie in that makes a Hydractive car behave for a short after having it's fluid replaced?

I'm sure we'd all like to get to the bottom of this issue.

Steve.

jeremy · Unread post by **jeremy** » 12 Dec 2006, 09:47

Its not just the hydractive ones that do that. They all feel different with new LHM.

When I got my BX the warning light would come on at high speed after a while (lovely!!!) - and eventually after a new pump and doseur and othe bits I was getting desperate. One of the problems was that every time I tinkered it would seem OK for a few days until the new LHM I'd added to top up what was lost got mashed up again and the thing returned to normal.

My solution was to fill it up high, and when tinkering to carefully clamp the pump supply hose to minimise loss - and try not to top it up!

Nothing will work properly with a flat accumulator - and its very dangerous as the accumulator provides that power for the ABS system!

steelcityuk · Unread post by **steelcityuk** » 12 Dec 2006, 12:30

I still have no reason to suspect that the new accumulator sphere is faulty. The point I'm trying to make is that Hydractive cars are magically transformed by LHM changes, is that the same with non HA Xantia's - does the ride become so much smoother?

Steve.

citronut · Unread post by **citronut** » 12 Dec 2006, 13:18

if i remember rightly citroen used to say fluid change every 60,000 miles,maybe after you have changed your fluid you will bleed the system of air,this might be why it seems to ride ok for a while
regards malcolm

FrenchLeave · Unread post by **FrenchLeave** » 12 Dec 2006, 18:21

Bernie,

Isn't that a two door Lada?

Derek

bernie · Unread post by **bernie** » 12 Dec 2006, 18:42

FrenchLeave wrote:Bernie,

Isn't that a two door Lada?

Derek

Bloody hell, Derek, which way to Specsavers

FrenchLeave · Unread post by **FrenchLeave** » 12 Dec 2006, 18:55

You'll find them next to the Fiat factory that sold the design of the 124 to the Russians in (I think) 1972.

And yes, I know it was the 124 saloon they sold; and yes, I know they didn't include the coupe. Pity you didn't appreciate my subtle attempt at humour.

Derek

steelcityuk · Unread post by **steelcityuk** » 14 Dec 2006, 12:50

I tried the suggestion of raising the suspension to max but the regulator ticking didn't slow down. Also the standing on the towbar didn't make the car raise after switching off the engine, could this be because of the Hydractive system or anti sink isolating the height correctors?

Yesterday the good then bad all the way home from work. <sigh>

Any ideas how I track down the low cycle time problem?

Steve.

citronut · Unread post by **citronut** » 14 Dec 2006, 13:37

no you dont raise it up to do this test,you leave it at normal hight
regards malcolm

jeremy · Unread post by **jeremy** » 14 Dec 2006, 14:22

Low cycle time? - do you mean too short an interval between cycles?

Usual cause is flat accumulator - cure - change - they are about £20 or so new from GSF and similar outlets. This should be done first.

If its still there then its probably the non-return valve in the regulator which needs re-seating. If the regulator is conveniently placed you can do the job in the car and it costs nothing. (remove accumulator, remove bolt from accumulator mount face, retrieve ball bearing. Stick it back in its hole with some grease, tap it smartly ONCE using a brass punch and re-assemble. You MUST use a brass or similar punch - otherwise you may put a flat on the ball bearing)

Re-seating must NOT be carried out asa substitute for replacing the accumulator. A sound accumulator is essential for the proper functioning of the system.

The car doesn't rise when you do the boot test. What makes you think your accumulator is any good?