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Why do our Citroens become so stiff on higher suspensio settings?
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Paulius   
Sun Sep 03 2017, 11:00pm
Joined: Aug 14 2014
Member No: #1821
Location: Vilnius
 This question has been bothering me for quite a while now.
Why does the ride become so extremely stiff on higher suspension height settings? On the highest setting there is virtually no wheel travel at all.

I would imagine one arguing that on higher suspension heights the overall suspension system pressure goes up so the spheres compress and, thus, become more stiff.

However, I personally imagine that system pressure depends on vehicle load, not it's height. I think that fluid volume (which you can increase with the hydraulic pump) translates into vehicle height, because it simply expands into the hydraulic cylinders. This process does not increase the mass of the vehicle, which, I think, is the only factor that actually affects the system pressure, which expands into the spheres. Physically looking at it, I would imagine spheres compressing briefly and inconsiderably whilst vehicle gains vertical momentum during the ascent process.

Where am I wrong? :-O
e3steve   
Mon Sep 04 2017, 06:40am
Joined: Jan 21 2013
Member No: #1163
Location: Warsash, Hants & Palma de Mallorca, Spain
 The volume of fluid fills the system completely until the pressure of the fluid on the road wheel side of the spheres' membranes exceeds the pressure of the nitrogen gas on the 'suspension' side of the membranes, therefore the gas can no longer react against the fluid...
travlician   
Mon Sep 04 2017, 03:16pm
Joined: Jan 22 2011
Member No: #350
Location: Paradera
 Not quite so e3steve, just checked the LDS pressure reading in Lexia when in normal, raised and fully raised positions and there is no significant change in pressure. Paulius theory seems correct. The way I see it there are two possible reasons, first, the programming of the suspension control may be such that the suspension is made stiffer (AMVAR), this makes sense as wheel travel in upper direction is limited (could not verify that).

Another reason might be that the way the momentum from the suspension is transferred to the suspension strut is non linear and can be needing more force to compress the strut (hence the nitrogen in the sphere) when in higher position. I don't really see it but need to draw it all out and recover my physics skills
gmerry   
Mon Sep 04 2017, 06:23pm
Joined: Dec 11 2009
Member No: #21
Location: Scotland
 Another way of restating what has already been said: ignoring temperature effects and the minimal stiffness of the diaphragm, the "spring" stiffness is just a function of the nitrogen pressure which is a function of volume. So we know the hydraulic pressure is the same, therefore the nitrogen pressure is the same, so nitrogen volume has not changed and therefore nitrogen spring stiffness is the same as when the car is at normal height.

However, look at the AMVAR damping curves (I posted these some years ago). The cross through zero (compression/rebound) at the zero displacement point of the suspension. Once the suspension is away from zero, they rapidly stiffen (as soon as there is any movement). On top of the damping change (function of velocity), there is also a static correction because otherwise the car would become unstable, especially at the very highest setting when say jacking up the car to change a wheel. In other words, at the highest setting the AMVAR damping is shut closed, when at the intermediate setting, the AMVAR units are early closed.

regards
Paulius   
Mon Sep 04 2017, 09:17pm
Joined: Aug 14 2014
Member No: #1821
Location: Vilnius
 Hmm, I didn't know AMVAR had a "shut closed" position. Are you sure? Also, wouldn't that show up on Lexia? AFAIK, you can monitor AMVAR position, has anyone noticed if suspension height setting actually affects AMVAR?
That to me seems like a plausible idea.

IMHO, the suspension could be a lot softer on at least the med-high position...
ul9601   
Mon Sep 04 2017, 10:35pm
Joined: Dec 14 2014
Member No: #1975
Location: Auckland
 I don't think it's all that complicated - all the hydro pneumatic Citroens I've had (CX, GSA, C5 & C6) regardless pre or post Hydractive go rock hard and lack suspension travel, so I'm inclined to go with Steve's explanation.
e3steve   
Tue Sep 05 2017, 07:04am
Joined: Jan 21 2013
Member No: #1163
Location: Warsash, Hants & Palma de Mallorca, Spain
ul9601 wrote ...

I don't think it's all that complicated - all the hydro pneumatic Citroens I've had (CX, GSA, C5 & C6) regardless pre or post Hydractive go rock hard and lack suspension travel, so I'm inclined to go with Steve's explanation.
That's my rationale, too. Through the years those clever French sods didn't keep re-inventing the wheel, so to speak.

All hydropneumatic Citroëns I've ever owned have behaved in the same way, whereby the volume of fluid within the entire suspension system overcomes the nitrogen's ability to react against it, thus leaving the tyrewalls to absorb the bump & rebound of the vehicle.

My D Safari, CX, GS & BXs had no AMVAR or clever electronics. Just a low-pressure switch and a STOP light...
travlician   
Wed Sep 06 2017, 01:25am
Joined: Jan 22 2011
Member No: #350
Location: Paradera
 Time for some physics classes.... The CX and BX (no experience with D/GS) in highest and lowest positions were tight against the end stops so logically no suspension, the C6 is something different! The description of going rock solid is when the nitrogen is depleted from the sphere, which eventually also happens with the C6. But that is not height position related.
ul9601   
Wed Sep 06 2017, 05:08am
Joined: Dec 14 2014
Member No: #1975
Location: Auckland
travlician wrote ...

Time for some physics classes.... The CX and BX (no experience with D/GS) in highest and lowest positions were tight against the end stops so logically no suspension, the C6 is something different! The description of going rock solid is when the nitrogen is depleted from the sphere, which eventually also happens with the C6. But that is not height position related.

Can you confirm that you saying that suspension spring rate remains the same for all four height settings as long as nitrogen pressure in the spheres are not completely depleted - all else being equal?
e3steve   
Wed Sep 06 2017, 06:45am
Joined: Jan 21 2013
Member No: #1163
Location: Warsash, Hants & Palma de Mallorca, Spain
travlician wrote ...

Time for some physics classes.... The CX and BX (no experience with D/GS) in highest and lowest positions were tight against the end stops so logically no suspension, the C6 is something different! The description of going rock solid is when the nitrogen is depleted from the sphere, which eventually also happens with the C6. But that is not height position related.

Always happy to listen & learn. But...

With the exception of the intermediate-high setting, the C6 is 'tight against the end stops' when at its highest and lowest positions. At its lowest, it's on the bump stops; at its highest, the rams -- at the front, certainly -- are fully extended, ergo no incompressible fluid transfer movement against the spheres' membranes is possible...

In both extremities of suspension travel, the tyrewalls are left to absorb bump & rebound.
arconell3   
Wed Sep 06 2017, 11:17am
Joined: Jun 28 2012
Member No: #922
Location: Kalkar
travlician wrote ...

Not quite so e3steve, just checked the LDS pressure reading in Lexia when in normal, raised and fully raised positions and there is no significant change in pressure. Paulius theory seems correct. The way I see it there are two possible reasons, first, the programming of the suspension control may be such that the suspension is made stiffer (AMVAR), this makes sense as wheel travel in upper direction is limited (could not verify that).

Another reason might be that the way the momentum from the suspension is transferred to the suspension strut is non linear and can be needing more force to compress the strut (hence the nitrogen in the sphere) when in higher position. I don't really see it but need to draw it all out and recover my physics skills


That is indeed the correct explanation. Think of it as a simple vector analysis. When in high position, the lower wishbone points downwards. Any force on the road wheel will then be partly deflected and split up in a force on the lower wishbone, compressing it against its inner mounting bushes (stiff) and another component force pushing against the suspension strut (soft).

At normal ride height the wishbone is practically horizontal, the force from the road wheel is (almost) completely absorbed by the suspension strut.

The same holds true for the earlier hydropneumatic systems with McPherson suspension. I am not familiar with the DS suspension setup, I think that was with trailing arms?

Robert
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Tjensen   
Wed Sep 06 2017, 11:51am
Joined: Jul 17 2012
Member No: #954
Location: Bergen
 Even the trailing arms of the DS and GS and CX became totally stiff in the high position. Driving with big Cittroens since 1980 and never though about this mystery !
ul9601   
Wed Sep 06 2017, 06:34pm
Joined: Dec 14 2014
Member No: #1975
Location: Auckland
 Nah - I don't buy that. Regardless of the suspension arm design differences, the spring action of the sphere via the strut remains the same. Besides, the angle of the wishbone has to be very acute to make the ride that stiff, i.e. the vector parallel to the wishbone must far exceed that normal to the wishbone.
Paulius   
Wed Sep 06 2017, 07:09pm
Joined: Aug 14 2014
Member No: #1821
Location: Vilnius
 (popcorn)
gmerry   
Wed Sep 06 2017, 07:16pm
Joined: Dec 11 2009
Member No: #21
Location: Scotland
 I think we have established that the spring stiffness (aka nitrogen volume/pressure)) remains the same for low, normal, intermediate high and extra high positions. For the totally low position, (depressurised) the spring stiffness will be lower because all the weight will be on the suspension stops.

So what is happening at the intermediate high position. It must be something to do with a restriction in the fluid flow. So either at the damper (AMVAR) or back at the electro-hydraulic valve block.

Any other possibilities?
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