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Why do our Citroens become so stiff on higher suspensio settings? |
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Paulius
 
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  Member No: #1821
Location: Vilnius |
ul9601 wrote ... I assume you've already read the Citroen technical literature concerning hydropneumatic suspension and how it automatically maintains constant ride height (and allows manual ride height adjustment)? I've read a good portion of what is available here. And I own a C6. I could of course be wrong, just trying to reason with reason. |
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ul9601
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  Member No: #1975
Location: Auckland |
You already know what I think causes the harsh ride and why. What causes a harsh ride on a steel sprung car? Stiffer springs (or riding on bump stops - essentially the same thing as a rubber bump stop is a spring in a way). To me, this is the obvious clue for us to look at the spring rate first (i.e. nitrogen). Does nitrogen gas spring have variable spring rate? Yes. To me harsh ride at higher ride setting is an unfortunate side effect of this variable spring rate. As long as you have completely sealed nitrogen chamber (as is the case here), there is no changing this. The only way you can lower the spring rate (and achieve soft ride) is to lower the hydraulic pressure but that will lower the ride height, so that's out of question. Of course I can't discount the other factor - as in your argument that this has to do with the suspension geometry but I just don't think it is likely. I'm happy to be proven wrong but unfortunately it'll take some serious experiment. Having said that, we are all speculating anyway... |
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Paulius
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Member No: #1821
Location: Vilnius |
Yes, we all are indeed speculating in a way, to the joy of our fellow friends here. And there is a thin line between pure speculation and reasoning. It's best to try to understand each other's theories when engaging in such discussions and, if theories contradict, pinpoint, where. I do understand how you can get different springing properties out of the system by simply changing the pressure. But you can't just go backwards and say that different springing properties indicate a change in the pressure. There can be and probably are other factors. From what I can understand, you can't just change the pressure to begin with. Pressure depends on car load. You also have dynamic pressure, which depends on road bumps and other dynamic factors (insert the neglible "car going up" momentum factor here). And that is LDS pressure. STATIC NITROGEN pressure of any given sphere doesn't even change when disconnecting or connecting spheres to the system alone. Connecting/disconnecting spheres changes the springing properties, as dynamic pressure (road bumps) works against variable amount of nitrogen volume. More spheres - more nitrogen on the compressing side of the system, hence you can absorb more LDS volume (as it doesn't compress, it means hydraulic cylinder travel) before nitrogen shows increased resistance (harder springing). Having said that, I tried to reason, why to me it seems that this has nothing to do with pressure, as there doesn't seem to be a logical explanation on why it should increase in any circumstance but the car load in the first place. And I didn't even refer to some of the replies by others, where they said that changing their ride height has no effect on the system pressure, as indicated by nothing else but LEXIA. |
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ul9601
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Member No: #1975
Location: Auckland |
My last post on this thread, as we clearly have very differing views on how significant (or not) contributing factor the nitrogen spring rate is in this argument. What hasn't been disproved though, is whether the ACTUAL nitrogen pressure changes or not depending on height setting. Sure it was ASSUMED by some here that it must remain the same regardless of the height as the LDS pressure readings at various height settings are not significantly different (how much is not significant?). |
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gmerry
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 Member No: #21
Location: Scotland |
Paulius and Robert, well reasoned thinking from you both. I think we all accept that behaviour at the low setting is mainly due to the bump stops, at the very highest setting the system is compressing against the end stroke of the struts. What happen at the intermediate setting is the bit we are mostly speculating on. One theory states that the suspension angle change is sufficient to work the strut such that the effective spring rate changes. One has to consider both compression and rebound to test that theory. Personally, I think the effective spring rate can be easily explained by a change in the throttling behaviour of the variable damper (away from the zero ride height datum). Spring rate = sum of kx and cxdot. Or in other words, changing c (orifice size) will change the spring rate. Regards G |
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Tjensen
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 Member No: #954
Location: Bergen |
This is not connected to variable damping/AMVAR, all hydraulic Citroens are harder on higher setting. It is something simple that is a function of the technology | ||
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gmerry
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Member No: #21
Location: Scotland |
Here is some additional explanation on how the damping force varies when the height is away from normal. Can be done completely in a mechanical fashion, which explains why DS etc have the same behaviour. For further explanation, read the book  |
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ul9601
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Member No: #1975
Location: Auckland |
The suspension behaviour in question has NOTHING to do with end-of-stroke damping described in the excerpt from this mystery "book". NONE. AT. ALL. It does NOT explain the lack of travel and the excessive stiffness at higher suspension setting AT ALL. These occur in both static and dynamic conditions - what does that tell us? It has to do with spring, not damping. Per the excerpt, "it is purely an additional damping force that decelerates the piston velocity" - except that the suspension has to move first before this "damping" kicks in at the end of stroke and "decelerate the piston velocity". But as anyone who has tried to move the suspension at higher setting can testify, it's nigh impossible to do so - how does it "decelerate the piston velocity" when it wasn't even in motion to begin with anyway? Therefore, you quoting the above excerpt to explain the behaviour is totally misleading and inappropriate. |
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jamescarruthers
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 Member No: #1548
Location: Cambridge |
Ouch! | ||
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e3steve
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 Member No: #1163
Location: Warsash, Hants & Palma de Mallorca, Spain |
gmerry wrote ... Here is some additional explanation on how the damping force varies when the height is away from normal. Can be done completely in a mechanical fashion, which explains why DS etc have the same behaviour. Interesting read but not necessarily relevant to Citroën’s sphere-type suspension, Gerry; I’m sure you’ll agree. Although the damping part is, of course, relevant. Carrying on the ‘stiffness’ debate, chaps: Although our C6s do have somewhat conventional shock absorbers which, I’d say, contribute greatly — if not entirely — to the limiting of the supspension’s elevation at its fullest height, thus the gas in the spheres is at its most incompressible. At its lowest height a similar harshness is introduced by the fact that the car’s weight is being entirely absorbed by the bump stops and tyrewalls. Nothing to do with AMVAR(?)... |
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arconell3
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 Member No: #922
Location: Kalkar |
e3steve wrote ... ...... Carrying on the ‘stiffness’ debate, chaps: .....At its lowest height a similar harshness is introduced by the fact that the car’s weight is being entirely absorbed by the bump stops and tyrewalls. Nothing to do with AMVAR(?)... I'm afraid that that is a very common misunderstanding. The C6 in its lowest position is theoretically still driveable, although Citroen advises against it. To get the C6 on its bump stops, the (hydraulic) system needs to be depressurized, which can only be done using the Diagbox diagnostic tool. I know that many people, including garages who should know better, think that in its lowest position the system is already depressurized, but that is definitely not the case, as the relevant document in the technical section here also shows (C6 Suspension how to top up the LDS Fluid.pdf). Robert |
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gmerry
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Member No: #21
Location: Scotland |
Hi all, some more stuff for those who are curious and have some engineering knowledge:- Das Book (hint as to original language, by the way 5 minutes with google will find it) points out that suspension stiffness depends on inherent elasticity in the spring medium, Coloumb friction as a simplification and hydraulic resistance. Note that not all these forces are necessary are of equal force in both directions (and as a function of displacement). It is NOT solely a function of the stiffness of the spring medium. For hydraulic Citroens, all the way back to the TractionS (rear only) and forwards to the C6, the elastic medium is compressed nitrogen. Now the behaviour is different depending on ambient temperature and also on excitation frequency. Low frequencies, the spring response is isothermal, high frequencies, the spring response tends to adiabatic. Generally but not always, the Coloumb friction is low and can be ignored in any simplistic analysis. Getting to the hydraulic resistance: it is true that this is modelled as displacement first derivative term, however, the constant can be very high (or the velocities can be very low). As previously explained, a simple test of this resistance is available to any owner who has Lexia or Diagbox at his disposal. Go through the damper test routine, and one can have American 1960s wallow, German stiffness or near locked suspension, all at hand pushed low frequency excitation. (Now consider that force = mass x acceleration and compare the force from a hand push versus self mass excitation from our 2Tonne cars, at even modest vertical "g" forces) So, we have established that suspension stiffness depends enormously on hydraulic resistance. Now consider that the French Engineers who conceived the original hydro-pneumatic suspensions and all the designers of subsequent refinements, were not stupid people. They knew that jacking a car up high and allowing for the suspension to remain soft and wallowy would be OK for a circus ring, but inappropriate and dangerous for a road going car. So they designed their systems so that this would not happen! Cheers |
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ul9601
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Member No: #1975
Location: Auckland |
gmerry wrote ... ...Now consider that the French Engineers who conceived the original hydro-pneumatic suspensions and all the designers of subsequent refinements, were not stupid people. They knew that jacking a car up high and allowing for the suspension to remain soft and wallowy would be OK for a circus ring, but inappropriate and dangerous for a road going car. So they designed their systems so that this would not happen! Again, this is no more than a pure speculation. If you look at pre-Hydractive Citroen suspension dampers, it does not incorporate any additional elaborate end of stroke damping circuit to render suspension rock hard at high setting. In fact, it is the simplest hydraulic cylinder damper design you'll ever see. So how is it that pre-Hydractive Citroens are able to behave the same way as C6 (which by way of computer control, hardens the ride to prevent drivers from driving dangerously at high suspension setting, according to you). I haven't tested out C6 suspension damping with Lexia so I'll take your word for it - but the outcome of the test does not necessarily prove your hypothesis for C6 or pre-Hydractive Citroens. In my opinion, high setting is exactly where you want longer suspension travel and softer ride - it is pretty pointless having an increased ground clearance if your suspension can't cope. |
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ul9601
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Member No: #1975
Location: Auckland |
gmerry wrote ... Hi all, some more stuff for those who are curious and have some engineering knowledge:- Das Book (hint as to original language, by the way 5 minutes with google will find it)... And anyone with some engineering knowledge knows to reference a material when quoting it as opposed to suggesting to "google it"... I don't really understand why you don't just name the book and we'll be all the wiser... |
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ul9601
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Member No: #1975
Location: Auckland |
Question to gmerry: Since C6 dampers are electronically controlled with 16 laws (or something like that), 16 different degrees of damping, if one can override the damping dependency on height settings, is it possible, in theory, to be able to drive with soft and long suspension travel at high suspension setting? |
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