Saloon spheres on an estate

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jono
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Saloon spheres on an estate

Post by jono »

Greetings people,[:0]
I have just discovered that the rear spheres on our Xantia 1999 hdi estate are the ones for a saloon[:(!]. The volume is the same at 400 but the pressure is 30 instead of, I think, 40. Any predictions as to what will happen to the ride quality. My independant mechanic better have a good reason for not fitting the correct units[:(!].
Thanks
Jon[:D][:D]
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Mandrake
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Post by Mandrake »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by jono</i>

Greetings people,[:0]
I have just discovered that the rear spheres on our Xantia 1999 hdi estate are the ones for a saloon[:(!]. The volume is the same at 400 but the pressure is 30 instead of, I think, 40. Any predictions as to what will happen to the ride quality. My independant mechanic better have a good reason for not fitting the correct units[:(!].
Thanks
Jon[:D][:D]
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Hi Jon,
We've just been through the exercise of 40 bar spheres vs 30 bars on a saloon.
Basically the lower pressure will make the suspension a bit stiffer, and on the saloon at least, 40 bars was a bit too soft and lively, and we prefered 30 bars for the saloon.
But for the estate, 40 may be optimum due to various factors. (Heaver unladden weight, wider expected range of load weight, possibly different suspension components, I'm not sure if there are mechanical differences in the saloon/estate suspension for the Xantia)
The main problem with a pressure thats too low is that apart from riding a bit firmer, with the car fully ladden the diaphram in the sphere may be close to running out of travel, causing limited suspension travel.
Try loading the car up with as much stuff as you're ever likely to carry, and see how it rides, and try the bounce test. If you find the suspension travel seems to be severely limited with the full load, the gas pressure is probably too low...
Regards,
Simon
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Post by Peter.N. »

Did I read that right, The LOWER pressure will make the suspension stiffer?
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Mandrake
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Post by Mandrake »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Peter.N.</i>

Did I read that right, The LOWER pressure will make the suspension stiffer?
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Yes that's correct.
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fastandfurryous
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Post by fastandfurryous »

It also means that as the spheres slowly loose their charge, they will need changing sooner. 40bar ones would (of course) have to leak all the way down to 30bar to get the ride quality you are now experiencing, which could well be many years of motoring.
That said, if you wanted a stiffer, sportier ride, then you have it...
There is still the question as to why mr. mechanic fitted the wrong component to your car. Makes you wonder what else he's done which is wrong.
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Post by Mandrake »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Mandrake</i>

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Peter.N.</i>

Did I read that right, The LOWER pressure will make the suspension stiffer?
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Yes that's correct.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
I guess I should elaborate on that, as its something that seems counter-intuitive to a lot of people.
You have to go right back to the basic definition of springing stiffness - which is measured in something like Newtons of force per millimetre, or other similar units. The more Newtons of force it takes to move one millimetre, the stiffer the springing.
The quoted gas pressure of a sphere is when the sphere is not under compression from the oil - eg, the system is depressurized or the sphere is sitting in a box on a shelf.
When the suspension is presurized the operating gas pressure in the sphere is ALWAYS equal to the oil pressure in the cylinder, and the oil pressure needed is directly proportional to the weight the suspension is carrying, based on the suspension parameters such as arm lengths, piston diameter etc, which are a constant on a given model of car.
Therefore the "gas pressure" of the sphere is just a convienient measure of how MUCH gas is in the sphere, not an indication of the gas pressure while the suspension is operating.
Compared to a higher pressure sphere, a sphere with a lower starting gas pressure will have the gas compressed to a smaller working volume at the operating pressure, (diaphram closer to the top) which means that for a given volume displacement of oil, there is a larger percentage change in gas volume, which causes a greater variation in gas pressure, therefore a greater change in oil pressure, therefore a greater change in force applied to the suspension, all of which adds up to stiffer springing.
When the diaphram gets too near to the top the springing also starts to become very non-linear - just like conventional springs, because the volume displacement of oil for the full suspension travel is now a comparable amount to the size of the remaining gas volume, instead of being only a small percentage of it.
Thats why on an estate, as the rear spheres go flat you'll notice the effects first with maximum load where you'll find that you don't actually have much travel at all compared to an empty load...
Regards,
Simon
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Post by fastandfurryous »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Peter.N.</i>

Did I read that right, The LOWER pressure will make the suspension stiffer?
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
As the spheres loose their charge, the suspension gets harder and harder.... I'm sure every citroen owner has felt that. As soon as you put new (fully charged) spheres back on, what happens? the suspension goes floaty light and soft again.
jono
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Post by jono »

Well I contacted my mechanic and he said that knowing how I appreciate a cars handling I would find this rear setup more rewarding! We have an understanding now that we chat for a short while before any 'mods'. Thanks for the info Simon
Regards
Jon
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