Sphere Volumes, possible misinfo

[John Malkovich]

Mr. Nastasic's book on XM left me in doubt: in his chapter on Hydractive II, he mentions that the introduction of middle spheres left engineers with more freedom and this was probably the reason to introduce the 450 cc spheres. I was left wondering if this is a mistake or are the larger spheres (previously built for CX) used on XM SII? Earlier I mentioned in this section that my technician fitted 450 cc rear spheres for a week test to my car and I am sure that I have smaller ones on my struts.

Thank you for your answers.

[kenhall1202]

My Mk2 XM originally had 450 cc spheres on the front suspension, ie 2 x 450cc/50bar (struts) and 1 x 450cc/70bar (centre). The other 5 spheres are all 400cc volume at various pressures.

I say 'had' because my last two sets have been Amtex Comfort spheres which I believe have similar volumes but different pressures and damper orifice sizes.

Ken

[Jan-hendrik]

The Berline/hatchback hydractive XM II has 400 cc spheres on the rear. The Break too, but the pressures are resp. 30 and 40 bar.
The front middle sphere is 500 or 450 cc, depending on the membrane, pressure 70 or 75 bar. Uhh, the 450 cc is fitted to the V6 and the 2.5 diesel. Confusing, eh .
But I think it's OK to experiment with different spheres

[techmanagain]

"But I think it's OK to experiment with different spheres"...............

I appreciate your right to experiment - after all it is your car - but I cannot for the life of me understand why owners want to change the suspension of a car that has been brought to its present comfortable and safe state by no doubt many hours and months of experimentation by the manufacturers. If an XM has the whole suspension system kept in the state intended by the manufacturer it is not possible - in my opinion - to improve on that without sacrificing something of its tried and tested design.
And I've no doubt that I've "put the cat among the pigeons" by that point of view!!!

[Jan-hendrik]

Hello Techmanagain.
I tend to agree with you. On my XM everything is according to specs, but I said what I said because slight deviations in setting up the suspension may satisfy some owners.
The manufacturer, when deciding on spheres, pressures, and god knows what else, also compromised. Were they tuning for the car being run fully loaded with 5 adults + a boot (trunk) full? Or only two people and a few shopping bags? Or just one driver and nothing else? Between the extremes there is a huge difference. That's why I said it was OK to experiment.
Once upon a time I used to drive a BX 19GTI. I mostly was the only driver/occupant. I found the rear sus too hard and bouncy and increased the damper orifices with 0.2 mm. Result: perfect damping to my liking Another solution could have been to put 1 or 2 bags of cement in the trunk as someone suggested, but that was not my choice

[DerekW]

At the risk of treading over old ground, if I were not satisfied with the softness of my suspension I would happily change the spheres - but not the damper orifices.

I think I would also limit my changes to the centre hydractive spheres.

The pressure in each sphere is always the same when the car is on its wheels, exactly enough to act on the suspension ram to support the car's weight. The secret is to increase the volume available for springing when the car is under load and this can be done by either increasing the charge presssure, increasing the static sphere volume, or both. The charge pressure has to be treated with caution, put it too high and that sphere will cease to have any springing action. My choice would therefore be to always fit the largest spheres available with the specified spessure and damper sizes.

Derek