MK 1 front sphere part number

[B-Hive]

Sorry if this has been previously covered, but can someone please point me to a resource that will tell me the correct part number for a front sphere for a Mk1 GT.. They are still the larger size but are stamped with 55 and not 70.

So from what I understand this makes these spheres have the same overall travel as the standard larger sphere (non GT-Mk1...stamped/pressurised with 70), but are firmer through out this range of travel

I want to get this right because I need to replace a sphere that was kindly offered to me at an 11th hour on a weekend...

Also, am I correct in guessing that a 55 can be charged up to 70 if required??

Thanks guys/gals

Ash

[B-Hive]

Bump!!!!

[Way2go]

I'm not sure why you want to charge to 70 but the following is a good authoritative source of info:

http://citroen.tramontana.co.hu/suspens ... nt-spheres" onclick="window.open(this.href);return false;

[MULLEY]

Contact Westroen, they may be able to advise on pressures etc...& may even have some in stock.

[Way2go]

Contact Westroen, they may be able to advise on pressures etc...& may even have some in stock.

Are you on commission from Westroen, Mulley?

There is still the question here as to why a non-standard pressure of 70 would be desired. I use the standard volume, pressure & damping types for the GTi which gives a nice firm ride. 70 seems way OTT to me.

[MULLEY]

If only, haha. I suppose an advantage of having a higher pressure is that the sphere will last longer? No idea what it would do to ride etc though....

[michaelr]

A higher sphere pressure results in a softer ride. It seems counter intuitive at first until you remember that a "flat" sphere, with little or no pressure, is rock hard. The "performance" BX had reduced pressure to firm up the suspension.

Damping on the other hand is a function of the valve installed in the neck of the sphere

[B-Hive]

Thanks for the info guys

AFAIK 70 is standard pressure for a standard MK1 (large sphere) especially as 70 stamped into the side.. This is a reason why the Mk1's ride better..

And as said the GT has same larger spheres but stamped 50 ,,so yes firmer but same wheel travel as normal MK1s..

[saintjamesy89]

I disagree, I did a lot of figuring and searching about spheres last night and the highest pressure BX sphere was 55 bar, see this table for spheres fitted to BX's. If you want a nice soft ride, then go for 500 ccm, 55 bar front spheres with a center bore of 1.8mm. For the rears go for 400ccm, 40 bar and 1.1mm bore - but apparently this drilled out to 1.2mm does improve the rear. I think you'll find that the larger cits were fitted with 70 bar and above, as these run higher pressure hydraulic systems and are heavier, so need the higher sphere pressure.

On one source (can't find it again now...), a chap pressured his spheres to 65 bar and this was getting close to the hydraulic operating pressure (65 bar is the sphere pressure when there is no LHM in the sphere, when car is running and on normal ride height, the nitrogen pressure roughly doubles as LHM is forced into the sphere to make the car rise to normal height) so when the chap did the bounce test on the front of the car, it dipped nice and softly, then rebounded and froze somewhere between intermediate and high height, as the sphere pressure was nearly equal to the LHM pressure, equilibrium was reached at the wrong height, so the front end didn't return to normal height.

Now 60 bar, this has been done and done successfully. But you do need to do it with 500ccm spheres apparently. This is what I plan to do with my fronts

To answer another of your questions, you can get a sphere re-gassed to above it's standard pressure, but I don't know if it'll keep this pressure for as long as the original - depends on the condition of the diaphragm. With the MK1 ride being more comfy, this I believe is due to roller bearings being used on the suspension rather than rubber bushes, more than this I can't tell you - because I don't know!

[B-Hive]

HHMMMmmm... the thot plickens

[Way2go]

A higher sphere pressure results in a softer ride. It seems counter intuitive at first until you remember that a "flat" sphere, with little or no pressure, is rock hard. The "performance" BX had reduced pressure to firm up the suspension.

This explanation makes no sense. You cannot use a failed state to justify a hypothesis.

You need to view it from when in equilibrium with the engine running and with the pump already maintaining LHM in the sphere. Say that now the diaphragm is approximately at the midway point in the sphere and you drive over a bump. The diaphragm will deflect a certain distance as the LHM compresses the Nitrogen. If the sphere pressure is higher then it becomes harder to compress the Nitrogen so the deflection will be less for that same bump (firmer suspension!).

Reducing the pressure will result in a softer ride but only to the point where Nitrogen still separates the diaphragm from the sphere wall when going over a bump. Beyond this point there is no longer a pneumatic "cushion" and "crashiness will result over the larger bumps and potholes.

[michaelr]

This explanation makes no sense. You cannot use a failed state to justify a hypothesis.

You are incorrect I am afraid, though I fully understand how you came to that conclusion. I was wrong though to mention a completely "flat" sphere. However, if what you suggest was true all our suspension spheres would be getting softer and softer as they lose gas.... and then suddenly become rock hard when it is all gone. As you know, that is not true.

The actual internal pressure of a sphere whilst supporting the car is a function of the weight it supports. The height corrector continues to supply fluid to the cylinder and sphere until it balances the load. The initial pressure of the gas in the sphere does not affect this, except is extreme cases.

Higher gas pressure however does mean that less fluid enters the system to achieve that balance and so the volume of gas is greater. A bigger volume of gas equates to softer "springs" as there is more of it to compress.

In the 16V BX Citroen specified lower gas pressures (45 bar as opposed to the standard 55 bar) which equate to a stiffer spring. They also fitted increasingly small damper orifices as the performance increased for "stiffer shocks".

Higher gas pressures (within reason) result in a softer ride.

[barry badger]

If a higher sphere pressure = softer ride and mk1 bxs were rated at 70bar, is this the reason for the fabled mk1 ride?

[saintjamesy89]

If a higher sphere pressure = softer ride and mk1 bxs were rated at 70bar, is this the reason for the fabled mk1 ride?

I have always thought the early MK1's were so nice to ride in because of a different bush/bearing set up - the mystic roller bearings rather than rubber bushes in the wishbones. In y research, the highest pressure sphere I have come across in a BX is 55 bar. Not that my lookings are exhaustive, but given the BX LHM operating pressure (150 bar I think, do correct me if i'm wrong), 70 bar spheres would be very very close to this when there is LHM in the sphere (about 140 bar) giving the system a very hard time to raise the car up from it's bump stops, and to function as it does generally.

[Way2go]

70 bar spheres would be very very close to this when there is LHM in the sphere (about 140 bar) giving the system a very hard time to raise the car up from it's bump stops, and to function as it does generally.

Actually, the system will raise the suspension up to normal height (or as set) whatever the pressure in the sphere or even with failed spheres as this is function of the height corrector, The height corrector gets it's information from the attitude of the anti-roll bars.
(however if you attempt to drive with failed spheres it will be jumpy as hell and you will have no springing or damping but to the eye the car will be up to it's ride height)
Likewise the sphere pressure will not alter the ride height, but only will change the ratio of LHM to Nitrogen either side of the diaphagm and the spring rate that this will cause in conjunction with the car weight.