Sinks lower and sooner now

Mandrake · Post by **Mandrake** » 30 Aug 2007, 00:47

MikeT wrote:Latest observations - STOP light comes on, after just a brief stop of 15 minutes or so, for several seconds while the car raises a little. This was a rare occurence before.

Hi Mike,

Theres nothing wrong with the stop light coming on for a few seconds after the car has been parked for 15 minutes - when you get into the car the height will drop, the height correctors will open trying to lift the car, this will draw on the pressure reserve in the accumulator and cause its pressure to drop below the point where the light comes on. (Approximately 80 - 100 bars)

It's also normal for the suspension to stay up longer, lift quicker, light go out sooner etc after spheres or LHM has been recently changed, and then gradually over a few weeks it deteriorates and then stabalizes. Nobody quite knows why, but they all do it.

If you're worried about the light taking a few seconds to go out after parking for a while you're chasing ghosts.

Now if it was taking 15 seconds you might have cause for concern.

Thought it might also be worth adding - I always thought the ride was a bit "boaty". There's a rippling (undulating) road nearby [troughs are in line with the drains at the kerb] that, while mildly accelerating from 20 to 40,I feel like I'm on a carousel ride! Altering acceleration does little to change that except amplify it.

Hmm, are you sure it has the correct spheres fitted ? The ride should not feel boaty....

I find conering at high speed (over 50mph) quite twitchy. It's easy to get the front to squeal on moderate to hard acceleration/cornering too.

I've driven cars at mad speeds before and you can get a certain confidence as you realise the limits but I get nervous at 70+ when approaching bends in the Xantia. Without the ABS currently working I just know that a heavy touch on the brakes when cornering will invoke a spin.

This just screams out poor tyres and/or wrong spheres to me...

The front on mine doesn't squeal at all even on hard cornering, and the rear NEVER feels like it's going to come loose and spin. (I have 205/60/R15 Michelin XM1's on mine)

If it feels like its tail happy you've definately got problems, Xantia's just aren't like that. (Or Citroen's in general) The only things I can think of that would do that is if the rear suspension was either very hard or extremely soft, or the rear tyres were crap. Perhaps if the rear passive steering bushes were very badly worn you might get a similar effect.

Even at lower speeds and heavy cornering I have to re-adjust (sometimes a more than once) the steering as the suspension levels back.

Don't understand what you mean there, can you elaborate ?

I get the impression the rear is all too ready to want to overtake the front given enough provocation but I have yet to prove that

Sounds bad. Needs looking into...

Start by locating the part numbers on the spheres if possible and posting them onto the forum and someone can look them up for you and tell you if they're the right spheres or not. It's not uncommon for people to fit the wrong ones, and depending on how wrong they are it can have disasterous results on the handling and stability of the car.

Common mistakes are fitting estate rear spheres to the hatchback (makes the rear too soft) or fitting normal strut spheres on a Hydractive 2 model. (also makes it too soft)

Also have a good look at your rear tyres... and perhaps post here what size and type they are...not all kinds of tyres suit the characteristics of Hydropneumatic suspension...

Regards,
Simon

jorgy · Post by **jorgy** » 30 Aug 2007, 03:20

...and to add to what's been told about the brakes: if u feel (and see) the tail of the car lifting even at the slightest braking effort, it means your brake circuit has air in it. You need to bleed the brakes -rears specially-. Your hydropneumatic Citroen should normally braking totally flat and be absolutely neutral in sudden braking in the middle of a corner.

regards
George

jeremy · Post by **jeremy** » 30 Aug 2007, 08:24

The geometry of the rear suspension will have some affect on the attitude of the car under braking. The rear suspension is done by training arms which are pivoted at the front end and as the car rises and falls the back wheels rotate (Stand besides the car while it rises and you can see this)

So apply the brakes to the back wheel and the effect is that the training arm tries to rotate and in fact seeks to lower the car. The height corrector has a built in delay - so the back of the car can drop a little.

Mandrake · Post by **Mandrake** » 30 Aug 2007, 11:04

jeremy wrote:The geometry of the rear suspension will have some affect on the attitude of the car under braking. The rear suspension is done by training arms which are pivoted at the front end and as the car rises and falls the back wheels rotate (Stand besides the car while it rises and you can see this)

So apply the brakes to the back wheel and the effect is that the training arm tries to rotate and in fact seeks to lower the car. The height corrector has a built in delay - so the back of the car can drop a little.

It can do yes, but its even a bit more complicated than that....

The torque from the rear brakes tries to compress the rear suspension as it tries to turn the arm as you describe, (it also tries to compress the front suspension slightly) while the torque from the front brakes is trying to LIFT the rear suspension. (Just try applying the handbrake hard while you're driving and watch the rear lift right up to the bump stops)

In addition to the torque effect which is simply torque about the wheel axes trying to tilt the body, you also have momentum which is trying to throw the centre of gravity forward, which pivots the car about a virtual axis which is somewhere between the front and rear wheels depending on the location of the centre of gravity, which is also trying to compress the front suspension and lift the rear suspension.

So whether the car drops, stays neutral, or lifts at the back depends entirely on the balance of braking efficiency between the front and the rear.

Good front brakes and poor rear brakes and the car will lift at the back under braking. Poor front brakes and good rear brakes and it will tend to drop at the back. Both brakes being good it will be relatively neutral.

It's also affected by the load in the rear as not only does this shift the centre of gravity further back, adding more load increases the rear suspension pressure and thus the maximum efficiency of the rear brakes.

When you take all those factors into consideration its amazing that the net effect at the rear suspension is relatively neutral, and just goes to show how cunning they were with the design.

Regards,
Simon

MikeT · Post by **MikeT** » 31 Aug 2007, 10:33

Looks like I'll be changing some spheres in the near future then. I'll get the current sphere numbers and tyre sizes and post them here later and see what you guys think.

Sorry I can't well express what's happening when cornering but it's as if there's some freeplay that needs constant reaction from me. The car rolls too easily for my liking and with the weight transfer makes cornering twitchy.

Under braking, the rear does not lift up.

MikeT · Post by **MikeT** » 31 Aug 2007, 22:03

Latest observations: Rear is sinking within a few minutes and for the first time, the rear stuck up in the air slightly when braking until I released the brake pedal.

Tyres are Uniroyal Rallye 540 185/65 R15's

Can't see sphere numbers except front one has a sticker with QBHC527 and 1/44/06

Mean anything?

AndersDK · Post by **AndersDK** » 01 Sep 2007, 17:27

Spheres have nothing to do whatsoever with height or sinking. Only on the drive softness.

The standing up on brakes is completely normal. You lock the rear wheel and hence also the rear swingarm. The rear can not sink then until you release the brakes.

Your LX trim Xantia would not have neither SMAC or HA2. Its "just" a basic hydropneumatic Citroen.

MikeT · Post by **MikeT** » 01 Sep 2007, 20:26

Hi AndersDK,

In the several thousand miles of driving this car (including some heavy braking to a stop) I've never known the rear to hold up in the air like it did the other day.

I had another Xantia last year for a few weeks and it was embarassing how easily and high the rear would stay up until the brakes were released. I would like to avoid that, if only for the sake of my passengers as I felt like I was being tipped out of my seat until brake release and it didn't just lower gently, it was quite a violent drop. I don't want this car to be like that.

AndersDK · Post by **AndersDK** » 01 Sep 2007, 20:38

If the rear tends to overdo the raising it may be high (!) time doing first Citaerobics, then bleed the brakes. Trapped air in the rear suspension may exactly cause rear shoot ups.

Also if the front is on the low side in suspension height setting, it will worsen matters with rear shooting up.

Mandrake · Post by **Mandrake** » 01 Sep 2007, 22:23

I think you two are talking at cross purposes here, there are two different things going on -

The rear lifting DURING braking is as Anders says, most likely air in the rear brakes - bleed the rear brakes and then see if there is still a problem with that. Until you bleed the brakes we're just speculating beyond that.

The rear staying up AFTER braking is due to the trailing arm geometry - if the rear suspension is too high (or too low) at the point where you finally come to a stop and you keep the brake applied hard, it prevents the arm from pivoting and the height normalizing.

The answer here is simply if braking hard ease off the brakes as you come to a full stop, and only hold the brakes on enough to remain stationary. (Or use the hand brake) Also if the cause of the rear lifting during braking in the first place is fixed this will happen a lot less.

Regards,
Simon

MikeT · Post by **MikeT** » 02 Sep 2007, 10:34

I frequently do run through the citroebics process (low for several minutes, then high for several minutes then repeat) but it doesn't seem to make any difference that I can tell.

How does air get trapped in the brakes in the first place?

DOH! I just remembered. I emptied my boot last week and there's less weight in the rear. I forget how these cars are sensitive to rear loading. Back to the drawing board - sorry guys.

jeremy · Post by **jeremy** » 02 Sep 2007, 10:50

There are 2 ways gas gets into the high pressure hydraulic system - the first is leakage from the spheres (through osmosis through the diaphragm) and the second is through leaks in the connection between the reservoir and the pump.

The gas/air should not get into the limbs running to the brakes themselves - however it will get into the rest of the rear brake pipework which of course is linked into the rear suspension.

On cars with anti-sink it can accumulate and cause problems as the LHM doesn't flow to the same extent as it did on earlier cars which dropped fully overnight.