Talk:Hydropneumatic suspension

Too broad a term?
"Hydropneumatic" is too broad a term for what this page describes. I was going to move it to "hydropneumatic suspension", but the Citroën uses hydropneumatics for brakes and clutch as well.

The main point, though, is that "hydropneumatic" is a very general term. Mackerm 13:15, May 26, 2004 (UTC)


 * Good point, it should probably be a disambiguation page at some point! --Pete Richardson 09:43, 28 May 2004 (UTC)

Most of the alleged advantages are complete gibberish
Whoever wrote the article has no idea why cars have damping, including those with the Citroen system, in which damping is provided by flow restrictors in the spheres. The idea that a non-linear spring cannot resonate is nonsense, as anyone who has ever watched a pendulum swinging through more than a small angle knows. We are told that increasing the loading leaves the spring rate constant in one paragraph, then in the next that the springing has stiffened up. All in all it reads like the ill-informed ravings of an enthusiast. About half this article needs thrown out as rubbish and the rest heavily rewritten. 87.113.37.249 (talk) 20:58, 11 December 2012 (UTC)

“Its nitrogen springing medium is approximately six times more flexible than conventional steel"
This phrase means essentially nothing. Conventional steels vary in stiffness (Young’s modulus) by a good factor of six themselves, and nitrogen, as a gas, has no characteristic stiffness; gasses can expand infinitely and (within the range of pressures relevant here) can be compressed infinitely as well. The “stiffness” of a gas cylinder depends on the pressure within it; it’s meaningless to compare that to the ”flexibility” of a solid material, whatever that might be.

The whole second paragraph needs replacement, but I'm not sure what main point to highlight. —Preceding unsigned comment added by 65.38.32.131 (talk) 10:43, 11 February 2009 (UTC) I agree. The allusion is to the behavior of a cantilevered leaf spring. The internal friction in the leaf spring acts to damp its motion just like a shock absorber does. The natural vibration of a cantilevered leaf spring is a function of its density, internal friction and stiffness. But the spring's mass and internal friction are negligible compared to the mass it supports and the shock absorber's friction. The nitrogen in the spheres has a very low density (compared to steel) and low internal friction. Its spring rate, however, is a dynamic function of the load it carries. This nonlinear spring rate must be part of the reason a Citroën doesn't have a natural bouncing frequency. I'd welcome discussion on this. CharlieAndy (talk) —Preceding undated comment added 20:08, 27 September 2010 (UTC).

"large loads make no real changes to the handling"
That would require, at least, central tire inflation, like a military Humvee, but load dependent. Some cars, like the Renault R4, have wheels that don't change angles with load, but the load on the tires affects car handling. That is the main reason the DS had wider tires in front than in the rear and Porsches have wider tires in the rear than in front. David R. Ingham 03:39, 13 January 2006 (UTC)

Small car

 * Shouldn't the first small car with hydropneumatic suspension be the Citroën M35? Hektor 20:34, 11 April 2006 (UTC)


 * The M35 was really just a prototype that a few customers actually tested (sort of like the 1964 Chrysler Turbine car) plus was the M35 really older than the 1970 GS?

Diagram

 * Needs a key/index. 87.254.67.236 04:16, 9 January 2007 (UTC)

Air ride suspension link invalid
I removed this link as it doesn't have any information about hydropneumatic suspension. Having produced a photobucket album containing jpeg images of a Citroen manual, a link was attached but removed. I will attempt to attach it again as I believe that much more can be learnt from this information. —The preceding unsigned comment was added by 202.43.226.11 (talk) 12:14, 9 May 2007 (UTC).

list of Citroen cars
Hi. As regards the list of car with hydropneumatic suspension: C4, Picasso and C8 are not equipped with hydropneumatic suspension but with classical. Rgds. François

Trivia
Too much trivia for a technical subject and it needs cites;

Alastairward (talk) 12:42, 16 September 2008 (UTC)
 * The accumulator sphere typically has enough pressure left for multiple hard brakings (up to 50), even if the engine (and high pressure pump) stops working.
 * Some hydropneumatic Citroëns can drive on 3 wheels. One can remove a rear wheel, then raise the suspension to 'high' position. The suspension, being pressed firmly against its rebound stops, has no flexibility: the rigidity of the chassis raises the corner with the missing wheel.
 * Maseratis in the 1970s borrowed some parts of the system for braking and power steering, but the only Maserati that did use a hydropneumatic suspension was the Quattroporte II. Citroën owned Maserati at this time and produced the hydropneumatically suspended SM.
 * Rolls-Royce cars used the system (for height leveling) in combination with a standard suspension.
 * In the early production of hydropneumatic cars different synthetic fluids were used (CH12, SP19, LHS liquide hydraulique synthétique).   Significant problems were inherent with these fluids.  They were hygroscopic, absorbing atmospheric water which pitted hydraulic valves and pistons/cylinders. They lacked lubricative quality, limiting the lifespan of hydraulic components, and they would decompose at moderate temperatures, causing acidity in the fluid.  In 1959, an attempt was made to solve the problem of temperature by including ventilators in the upper surface of the front wings of the DS, but the better solution was found in chemistry. LHS2, which resolved the thermal instability but not the problem of hygroscopy, nor the lack of lubrication.  In September 1966, une liquide hydraulique minérale, LHM, replaced LHS2, necessitating the redesign of all the neoprene rubber seals/diaphragms/covers/boots with a material suitable for mineral hydraulic fluid.  The problem of hygroscopy disappeared, and the lack of lubrication became a situation of perfect lubrication.  With correct maintenance, the Citroën hydraulic system could remain in reliable service for hundreds of thousands of miles
 * In early production, slide valves/valve bodies and pistons/cylinders were matched to tolerances of within 10 micrometres. Such was the tolerance necessary to guarantee hydraulic sealing at a pressure of up to 2200 psi, and air-conditioning was essential in the factories producing such parts.  Such tolerance in machining did not exist elsewhere in auto manufacture.  The parts could not be machined to 10 micrometres, but could be measured to that accuracy.  The valve barrels and pistons were machined as accurately as possible, and then matched up in pairs.
 * The "poor man's" alternative to hydropneumatic is British Leyland's Hydragas system, which uses pre-pressurised spheres which are interconnected between axles, and various air suspension arrangements.
 * From 1974 to 1981, the USA outlawed cars that lower as the hydraulic system depressurises.
 * Over 9 million vehicles have been produced with this type of suspension.

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