Xantia spheres

[Old-Guy]

I've noticed that the Green Lady's suspension is less than perfect:

1. She seems to have become a bit 'floaty' on the front end compared with the rear. Both ends bounce OK when unladen, but that's a pretty crude test.
2. Parked overnight on a level surface, the STOP light takes longer to go out than it used to and the back suspension always rises, over-shoots, and settles. The front just gives the classic Xantia groan, sinks slightly then rises back to where it was.

I know there's no sure-fire diagnostic for a dead/dying A/S sphere but 2. above leads me to suspect the A/S sphere needs replacing.

If the rear corner spheres are getting a bit flat, might that explain the apparent front-end floatyness?

The spheres were all last replaced about 8 years ago (Amtex from GSF) since when she's only done about 50,000 miles.

I haven't checked the tick rate on the regulator but that's easy enough, and I still haven't built a sphere tester even though I saved all the necessary bits from the VSX.

Gurus, any thoughts, please?

[xantos]

Quote from spheres-shop.com: "The spheres lose naturally in time 17% of pressure a year!"

Since it's 8 years since replacement, here is your answer I would say... Btw.: is your homepage still operational? I can't find it...

[Old-Guy]

I lost my home page (and everything else) when my then ISP went belly up. I didn't have the time then to do anything about it. Over the next few months, I'll be revising my guides and then posting them on the Wiki.

[MikeT]

I lost my home page (and everything else) when my then ISP went belly up. I didn't have the time then to do anything about it. Over the next few months, I'll be revising my guides and then posting them on the Wiki.

If it's any help, you may be able to retrieve copies from google cache https://support.google.com/websearch/an ... 7222?hl=en" onclick="window.open(this.href);return false;
or the wayback machine archives http://archive.org/web/" onclick="window.open(this.href);return false;

[Old-Guy]

Thanks for the suggestions Mike. I didn't make myself clear - I haven't lost the files (on my laptop, flash drive backups and also copied to my network server!), simply the web-space and domain name.

[Zelandeth]

Sounds like your spheres are on their last legs, the front of mine was very floaty when I got it, which progressed quickly to cartoon-car like bounciness. When I took them off I took the chance to put them on the tester and they were nigh on totally flat.

Ps. If you need a bit of web space I might be able to help out, drop me a message if you'd be interested.

[Peter.N.]

Normally 'floatiness' is linked to good spheres, when they start to loose pressure the suspension will become hard. The 'bounce' test is a pretty reliable one, the front should move 4-5" and the rear about 6" if the spheres are in good condition.

Peter

[Zelandeth]

What seems to happen is that the damping effect is reduced as the nitrogen pressure decays (the spring effectively becomes softer and with reduced travel), before going hard when there's nothing left as there is then no gas left behind the diaphragm in the sphere to compress.

Theres a difference between normal Citroen float and the "like driving a car with duff front shocks" float that I had prior to changing mine.

[Peter.N.]

Oh right, never experienced that.

Peter

[Old-Guy]

What seems to happen is that the damping effect is reduced as the nitrogen pressure decays (the spring effectively becomes softer and with reduced travel), before going hard when there's nothing left as there is then no gas left behind the diaphragm in the sphere to compress.

Theres a difference between normal Citroen float and the "like driving a car with duff front shocks" float that I had prior to changing mine.

My first reaction to this suggestion was 'Really?', then I thought about it and visualised what the effect of reduced gas volume would be: the springs will have a higher rate (less travel per unit load) meaning that the volume of LHM flowing through the (tired) damper valves will be significantly reduced and thus the damper rate is reduced.

The bounce test still works (to some extent) while there's still a decent volume of gas left.

The tick rate has gone up, the rear sinking then rising when the engine is started, suggests that the A/S sphere is (going) flat and next time there's good load in the back, I don't want a rear corner sphere to blow over the first 'calming' hump. Time for a new set of spheres methinks.

[Hell Razor5543]

I got my spheres last year from AEP Direct, for around £150 for a full set of 6 spheres. They had a production date on them, from May 2014. AEP did ask for a few details (vehicle reg) to make sure I got the right ones for my car.

[chinkostu]

AEP are also pretty rapid when it comes to posting them out. next day if you're ordering early enough. they stock ifhs spheres and mine had stamps from only a few months prior. they're about £25 each IIRC.


the tick rate + sinking will be the accumulator, as the pressure rises the antisink valve opens but theres not enough pressure to keep the car up so it drops. mine used to do it until i replaced it. still rocking a rusty antisink here!

[Mandrake]

I think you're both actually a bit confused.

What seems to happen is that the damping effect is reduced as the nitrogen pressure decays (the spring effectively becomes softer and with reduced travel), before going hard when there's nothing left as there is then no gas left behind the diaphragm in the sphere to compress.

Lower gas pressure makes the spring stiffer, not softer. The ultimate extension of low gas pressure is no gas pressure - and that is infinitely stiff. (Only the springiness of the tyres and bushings remain)

Theres a difference between normal Citroen float and the "like driving a car with duff front shocks" float that I had prior to changing mine.

When the gas pressure (and hence springing) is normal, the suspension is able to absorb and damp the oscillations that would normally occur on the tyre wall itself. The wheel rim has a tendency to bounce up and down on the tyre, and part of the job of the damping of the the suspension is to minimise this oscillation. When the gas pressure gets really low and the springing of the suspension gets really stiff, the tyre sidewalls themselves become the dominant springing factor and because there is no real damping of the tyre springyness you get "bounciness". I would not describe this as floatiness however.

My first reaction to this suggestion was 'Really?', then I thought about it and visualised what the effect of reduced gas volume would be: the springs will have a higher rate (less travel per unit load) meaning that the volume of LHM flowing through the (tired) damper valves will be significantly reduced and thus the damper rate is reduced.

You're sort of on the right track but not quite. It's not so much that there is less flow of oil through the valve - because that's not necessarily the case depending on what you hit. For example if you hit a big step in the road the instantaneous travel of the suspension trying to absorb that will be more or less the same whether the suspension is really soft or only moderately soft. (Only if the suspension is really stiff will the travel be greatly reduced) What changes is how much force is imparted to the body, and how much energy is stored in the "spring".

The stiffer the spring the more energy is stored by compressing it the same amount. Thus when you hit a bump with soft suspension not only is the initial impact on the body less, less energy is stored in the spring because it doesn't take as much force to compress it the requisite amount for the step in the road the wheel just encountered. Thus there is less energy to absorb on the rebound meaning that you don't need as heavy damping as you otherwise would have to control the body oscillations. This is obviously beneficial for ride as excessive damping hurts the ride greatly.

The damper valves are tuned with this soft springing rate in mind, but when the spheres age and lose pressure the springing rate increases. The higher springing rate stores more energy on impacts as described above, and the car is now "under damped", because the rebound force from the spring is much stronger. A secondary effect is that because the body will have reacted more to the initial impact, there is also now more body movement to damp, but not enough damping to do it.

Basically if you were to lower the gas pressure on purpose to get a firmer ride, you would also need to re-tune the damper valves to be a lot stiffer than they are to control the rebound properly. Conventionally sprung cars with stiffer springing rates tend to have a lot firmer damping than Citroen's and that is just as much a factor in reduced ride quality as the springing rate itself.

[Old-Guy]

Simon, thank you for this brilliant dissertation on Xantia suspension behaviour. It explains exactly what we're experiencing. I think this ought to be part of the suspension Wiki.

A word of warning on spheres: I am comparing on-line prices for a complete set from AEP and GSF. Quoting the reg no to GSF produces a total of 8 different spheres with inadequate descriptions. There are two different 'Accumulator' spheres - if you look at 'More Information' the only difference is 70bar v. 62bar. I happen to have a 'sphere table' from long ago showing the old Amtex part numbers and specs of all Xantia spheres; that says ALL Xantias use the same 62bar accumulator. Heaven help the less knowledgeable!

[Zelandeth]

That table sounds like something worthy of a place in the Wiki too!