Sphere pressure

thing · Post by **thing** » 20 Dec 2002, 09:50

I drive a Xantia 1.9TD.
Anybody know what is the pressure (Nitrogen) inside the sphere? Any special tooling/equipment required to re-charge N2 to the sphere?

alan s · Post by **alan s** » 20 Dec 2002, 14:34

You can buy a sphere regassing kit from Andyspares at times which comprises amongst other things, valves similar to schraeder valves fitted to car tyres. This enables future regassing to be done without removing the spheres from the car.
There is a list of pressures on a list which I think is downloadable somewhere on this site that gives the pressures for various spheres on various models.
A point worthy of note is that to be successful, spheres need to be regassed when they are about 50% down due to potential damage that can be caused through a sphere being driven on when flat as well as the fact that a totally flat sphere often gets that way suddenly due to an internal rupturing of the membrane.
I've no doubt that Jon may be able to shed more light on this subject as regards the location of the list of pressures and price & availability of regassing equipment.
Alan S

Post by **Jon** » 20 Dec 2002, 14:40

Click here http://www.andyspares.com/downloads/ and click on the Sphere Application table. This tells you all the pressures, damper sizes etc.
Jon Wood
forum@andyspares.com

alexx · Post by **alexx** » 20 Dec 2002, 17:17

Mechanic at the local garage, where I get the refilled spheres, keeps telling me, that with lowering of the pressure in the spheres, suspension becomes softer first year or two, and than harder again. Did anyone notice such a behaviour? It sounds like nonsense to me.

tomsheppard · Post by **tomsheppard** » 20 Dec 2002, 17:26

very possibly. the high sphere pressure would be springy to start with(low compliance) and as the sphere's internal pressure decreased the spring rate would lower (High compliance) until the pressure got so low that you would feel the membrane (no compliance). Maybe he 's right.

alexx · Post by **alexx** » 20 Dec 2002, 17:58

Well. Before the car rises up, LHM is filled into hydraulic cillinders and the nytrogen in the spheres gets compressed to about 100 bar or so (depending on load), until there's enought pressure to lift the car, according to:
Force = pressure x area of the cylinder
If the pressure in the sphere was, say, 50 bar, nytrogen is compressed to 1/2 of sphere volume. If the pressure was 25 bar, nytrogen is compressed to 1/4 of sphere volume and the spring rate is 2x greater. I think it's that way, but not quite shure.
Did anyone really notice the softening of the suspension first two years?

Dave Burns · Post by **Dave Burns** » 21 Dec 2002, 00:32

It doesn't much matter what the pressure is inside the sphere, the weight of the car changes little exept for normal variations like fuel and passenger loadings, so its weight could roughly be considered as a constant.
That constant will allways compress the gas in the sphere to the same final pressure regardless of its initial pressure, meaning the higher pressure of the new sphere will allways give the softer ride, since the higher pressure means a bigger gas volume at the sphere's final pressure, where it is able to absorb suspension movements better, simply because a larger volume of gas at a given final pressure, will rise at a lower rate than a smaller volume of gas at the same given pressure and for the same amount of suspension movement.
Dave

alexx · Post by **alexx** » 21 Dec 2002, 14:05

Thanks, Dave!
So, you agree with me, the new spheres give the most soft suspension, and it becomes more hard as the spheres gradually loose their pressure.
In that case, there must be something wrong with my spheres. I did some measurments with meter and me as 'test weight'. On my BX, with newly refilled spheres, the spring rates are almost 2x higher than on father's old GS with spheres refilled in the other garage 2-3 years ago. The guy assures me it's normal, as my spheres are renewed, but I find no reason for that. Both cars weight about 960 kg, and have the same sphere dimension (400 cm3 front, 400 cm3 rear), which were refilled, so I was told, to the same pressure (55 bar front, 40 bar rear). I also did some math and it turned out that differences in diameter of hydraulic cillinders and different suspension layouts are not the reason for harder suspension on my car. Suspension height and other things concerning suspension are OK on both cars.
So, I think, the only explanation is that there's something wrong with the guy's pressure-meter and the pressure in my spheres is too low. What do you think?
Thanks for the answer and have a nice weekend.
Alex

thing · Post by **thing** » 21 Dec 2002, 15:15

Since the pressure in sphere is always being compressed to a final value, how do I know whether I need to regas the sphere or not? Or perhaps I need to jack up the car off the ground when I check the sphere pressure.

alexx · Post by **alexx** » 21 Dec 2002, 17:03

I did the 'testing' this way. I sat on the rear bumper of GS, and it dived 9-10 cm (had to raise the car to the 2nd position to measure it, because in 1st the rear end sat on the rubber bounce stops). Repeated the procedure on the front, which dived about 4.5 cm. My weight is 71 kg. From that, I calculated the sphere pressure about 40 bar front, and 30 bar rear (hope may calculation is ok). It seems still fine, as the car has 2-3 year old spheres.
On the other hand, similar BX with newly refilled spheres dived 6 cm at the rear and 3 cm on the front. I think it's not enough, and the ride comfort is far worse than in 22 year old GS. But the mechanic ensures me it's ok.
Of course, Xantia, XM and C5 are heavyer, so the spring rates a expected to be proportionally higher, and the car will dive less than GS/BX under the same weight (depending also on the sphere type).
Alex
Edited by - alexx on 21 Dec 2002 20:24:41

Dave Burns · Post by **Dave Burns** » 23 Dec 2002, 00:26

The quality of the ride from the sphere's is also greatly affected by the damper size in them as well as the fill pressure of the sphere.
Don't concern your self with the pressure of gas in the sphere when its on load, the condition of the sphere as far as its fill pressure goes is checked while it is at rest, if the correct pressure is present while the sphere is at rest then all should be ok.
Dave

alexx · Post by **alexx** » 23 Dec 2002, 03:22

Maybe I wasn't clear (sorry for my non-so-perfect english, as I am from the southern part of Europe). I measured 'spring rates' of my BX and father's GS (with meter and me as 'test weight' as I said) and from that calculated the pressure in spheres at rest (I wrote a little program for that). I am a mechanical engineer and the calculation is not very complicated. Maybee I overlooked something, but I checked everything several times and found no error. I tested the program on data available for C5 (from Julian Marsh site) and got almost perfect match. Results on father's GS also seems to be OK, having in mind that spheres were refilled 2-3 years ago. Zeljko Nastasic's program also gives very similar results, despite of the little math simplification in his code. So, my BX seems to be the only exception here, and the pressure-meter in the garage is the only suspect left.
Of course, the ride quality is affected by the damper valves. Central bores on spheres for both cars are equal (1.8 mm front, 1.1 mm rear), but the number marks near the bores are different. Each damper has 8 more holes under the small metal disc around central bore, which are probably slightly different, or the disc thickness is different. But I don't think it's a big differece, like ride confort, which is way batter in GS than in my BX.
So, I'll have to check the spheres for correct pressure is some other place.
Edited by - alexx on 23 Dec 2002 12:16:16

allmond · Post by **allmond** » 23 Dec 2002, 17:41

<BLOCKQUOTE id=quote><font size=1 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>
I did the 'testing' this way. I sat on the rear bumper of GS, and it dived 9-10 cm (had to raise the car to the 2nd position to measure it, because in 1st the rear end sat on the rubber bounce stops). <hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>
Hi
Maybe I'm missing something here, but if one person sits on the back of the car and it goes to the bump stops, something's wrong isn't it?
I'm not familiar with the GS specifically, but I've owned a BX and two Xantias, and if you load them, they pump back up to normal ride height. I wouldn't expect them to hit the bump stop with just one person, even if he's on the back bumper!!
Happy Christmas or Joyeux Noel anyway.
Jamie

alexx · Post by **alexx** » 23 Dec 2002, 18:10

Of course, you must be quick, because after a few seconds, the corrector will open HP supply to the hydraulic cyllinders and the car will raise back to the normal height. It's a little tricky to do the 'measurment' anyway.
Edited by - alexx on 23 Dec 2002 13:21:19

JohnW · Post by **JohnW** » 24 Dec 2002, 02:34

Dear Alexx,
Is it possible to get a copy of your calculation programme? Thanks for an interesting discussion thread.
Regards
JohnW