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Incompressible media in air bags

Discussion in 'The Hokey Ass Message Board' started by Ned Ludd, Jun 26, 2012.

  1. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,260

    Ned Ludd
    Member

    I'm quite ready for the bags-are-for-shopping crowd on the one hand, and for those, on the other, who hold that "doing it right" means doing it the way it is always done. This notwithstanding I can never leave an idea alone once it's got hold of me, and some of you might know how often I come back to suspension hydraulics, especially hydraulics used not as a mere means of altering ride height but as one to get suspension movements to a place where one can play them against one another in all kinds of cunning ways.

    In the course of that I've been looking at both Citroën and Moulton/BMC hydraulics. The former includes a lot of brilliant lateral thinking and the parts are available and surprisingly cheap, but the system is engine-powered, which means that the car gently settles down on its bump-stops when the engine is turned off. The latter is completely passive and avoids piston-sealing issues entirely, but it comprises nasty sealed assemblies which are moreover not very pretty, and which are becoming quite scarce. Neither is ideal, but it would be so cool to be able to combine the Citroën system's diy-friendliness and creative-configurability with the Moulton system's trouble-free, plastic-bag-with-water-in-it simplicity.

    It was therefore probably inevitable that the ubiquitous mental light-bulb should sooner or later illuminate the thought of taking an air-bag spring and putting an incompressible medium like hydraulic fluid in it. The thought following immediately is as inevitably that there must be a million reasons why it won't work. Imagine my delight, then, upon reading in a Firestone manual for industrial air springs that their use with a water/glycol mixture as medium is perfectly accepted practice! And before the point is made, there is no essential difference between industrial bags and automotive ones: in some cases they are exactly the same thing.

    It must be pointed out, to pre-empt yet another objection, that doing this causes the bag to stop being a spring and to become an hydraulic cylinder. The spring action in a bag doesn't come from tensile elasticity of the envelope but from the compressibility of the column of air inside it. Replace the air with the abovementioned mixture and the bag becomes a rigid strut. One would obviously put back the springiness by incorporating accumulators elsewhere in the system: the very point is that there are all kinds of ways to do this, depending on what else one wants to achieve.

    Now, bags are also not paragons of beauty; but I'd expect that one could safely use smaller bags in any given situation, because ride quality no longer depends on bag capacity. One just has to be safely within the structural capability of the bag.

    I'd got to this scratching my head around the Morris Minor's suspension. It was going in the direction of hydraulics somewhere in the system, but I was a bit wary of seals that start to leak in spite of all that expensive precision machining. Bags would be trouble-free and a lot cheaper, and used thus as cylinders the very smallest rollover-sleeve bags should be well up to the job.

    Thoughts?
     
    Last edited: Jun 26, 2012
  2. I don't know anything about old cars,but it seems like a mixed theme building a Morris Minor with air bag suspension.....
    They always seemed to be a Gasser or Drag theme car in my opinion.....
     
  3. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,260

    Ned Ludd
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    Well, the Moulton system would be Issigonesque enough, as it made it into the subsequent Mini and ADO16. It is extremely ugly, though, and too integrated for what I want to do with it.
     
  4. pastlane
    Joined: Oct 4, 2007
    Posts: 1,063

    pastlane
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    Could you use hydraulics to actuate a torsion bar system?

    One local builder set his up so the vehicle ( 50's chevy pickup) was at normal ride height & the hydraulics actually pulled the vehicle to the ground. His thought was in the event of a hydraulic failure the truck would return to normal ride height rather than try and become one with the ground
     

  5. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,260

    Ned Ludd
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    I'm sure one could. The obvious approach would be to have arms at the rear ends of the bars, with hydraulics acting on them.

    I've always thought hydraulics are best in series, e.g. wheel - spring - hydraulics - frame. I never could understand the set-ups I've seen where the hydraulics are parallel with the spring: it means the spring isn't doing anything and there is no springing except the tyres. The reversed hydraulic set-up you describe might be that way, in which case it only serves to lower the truck for looks when parked.

    Also, something of a principle of mine is that dampers should follow the motions of the springs they're damping as closely as possible. With hydraulics the ideal is therefore that the dampers should be parallel with the springs only, and not parallel with both the springs and the hydraulics.
     
  6. The_Hermit
    Joined: Mar 31, 2009
    Posts: 128

    The_Hermit
    Member
    from France

    Hi

    good thinking on the Citroën system. Latterly, they actually managed to virtually eliminate the sinking when the engine isn't running. I think by adding 'extra' spheres that take up the slack and prevent the car from dropping too far.

    Aha, here we are : quick interweb search.
    - Tech guide on how it's done bt Citroën http://citroen.tramontana.co.hu/technical-guide

    - great set of pages on suspension tech. In the first page you clearly see the 'extra' spheres

    Hope that helps !
     
  7. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,260

    Ned Ludd
    Member

    Adding pneumatic capacity would certainly slow down the settling, but it wouldn't eliminate it. Citroën suspension struts are calibrated to "leak" at a rate between certain limits. The loss is made up from the central accumulator, which is charged by the engine, via a pressure regulator. With the engine off the accumulator can only provide so much fluid until it is unable to meet the regulator pressure. Using more or bigger accumulators one might be able to have enough pressure on hand to maintain ride height overnight or even for a few days, but eventually the car will settle.

    I know Citroën have also polluted their system with a lot of electronics. I don't know if they've now incorporated a dedicated anti-settle system. If so, it would have to bypass the strut "leak" with something solid.

    For all I know it is as simple as a very soft coil spring inside the strut, and not electronic at all. I don't know if Mercedes-Benz 450SEL 6.9s with the Citroën system settled or not, nor if not how.
     
  8. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,260

    Ned Ludd
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    I've taken a look at this: http://citroen.tramontana.co.hu/suspension/anti-sink-system

    The only way that I can understand it is that the bellows, into which the struts "leak" and from which fluid is constantly recovered back to the reservoir when the engine is running, are in the new system allowed to fill with fluid, and that the low-pressure return path is blocked with a special valve when the engine is turned off. It would appear that this valve is actuated by main accumulator system pressure and not by electronics (in which case I wholly applaud the innovation :)). It does mean that the bellows would now have to be capable of handling enough pressure to support the static weight of the car, rather than the barely more than atmospheric to which it was hitherto subject; i.e. it would have to become something like a fluid-filled air bag.

    Very interesting: thanks Hermit!
     
  9. All neat technology, but I'm not seeing the point. As you mentioned, at some point in the system, you need a springing medium. Unless you are going to connect bag to bag and place a coil spring (etc.) against the second bag; you will still need seals or a dividing diaphragm in there somewhere.

    I've seen both race shocks and hydraulic cylinders that were 20+ years old and never leaked a drop of oil, so it IS possible. I like the idea of the "remote" spring package a lot; I even designed it into the modified Mumford system at one point. Keep in mind that given the size of cylinders used, suspension fluid pressures will be relatively low by hydraulic standards.

    I'm interested in seeing what sort of solution you arrive at (as always!)...just not sure the technology is a necessity for leak-free operation.
     
  10. The_Hermit
    Joined: Mar 31, 2009
    Posts: 128

    The_Hermit
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    from France

  11. squirrel
    Joined: Sep 23, 2004
    Posts: 49,479

    squirrel
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    I'm trying to understand what you want to do. My guess is that one application would be to be able to adjust ride height, and use pneumatic springing rather than metal springs.

    The simple solution would be to have the bags partly filled with a liquid, which you can pump into or out of the bags to adjust ride height. The bags would also have air in them, and by adjusting the air pressure, you can adjust the spring rate.

    So you just need bags that have two fittings, one at the top and one at the bottom. The easy way to do this is to stack two shorter bags together. One is the air system, the other is the water/glycol system.

    And you're back to having a citroen, but with adjustable spring rate.
     
  12. An airbag filled with fluid will actually have a very small amount of give. Due to the elastic stretch of the rubber and lines. Yes it is very small, but it is more than a std hydraulic cylinder. Elastic modulus of steel is much higher than rubber, even reinforced rubber.

    I am not understanding what exactly you want to do with this hydraulic fluid inside the air bag? By taking away the compressible gas, you only only transferring the "spring" function to a separate piece. Which makes the system more complicated. Why not just keep it simple and have both spring and adjustability in the same airbag, rather than separate pieces?
     
  13. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,260

    Ned Ludd
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    I'm trying to avoid sliding seals. That's the advantage of the Moulton system: it relies entirely on flexible envelopes and has no sliding situations. Even the springing medium can be a common diaphragm accumulator: again no sliding seals; or as you say, a coil acting on a second bag.

    A big factor in all this is the cost of bags compared to cylinders of the sort of quality that would give twenty years of leak-free service.

    All this is still fermenting. I'll post if something crystallizes.
     
  14. yetiskustoms
    Joined: May 22, 2009
    Posts: 1,930

    yetiskustoms
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    This is a new and interesting approach to bads. I like all the ideas. Somone need to build one or put their thoughts into a blue print to follow and adjust.i bet the ride would be alot better for sure with the mix fluid.
     
  15. mrconcdid
    Joined: Aug 31, 2010
    Posts: 1,157

    mrconcdid
    Member
    from Florida

    Since were brain storming here ( not spending any money ) why not use an airbag as your lift and support system and add a shock that has that trick magnetic oil in it that mercedes uses that can adjust firmness of the ride with voltage, the airbag will lift and support the vehical and the magnetic oil shock will control firmness.

    build a set of struts that are ( firmness adjustable with the magnetic oil ) and put them thur an airbag. Totally expensive but compact.

    Godspeed
    MrC.
     

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  16. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,260

    Ned Ludd
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    Squirrel, you've almost described the Moulton Hydragas system! You've added the facility to alter both fluid and air, so you can go softer with more air and less fluid and vice versa, without having the ride height linked to that. That is certainly one application.

    As always I'm looking for ways to do interconnection: that's what got me to the idea. What got me to posting it here is that it's another tool in our technological toolkit, for which one may or may not ever run into a use :) The way I see it, the more options we have the better.

    Depending on the application, the elasticity of the bag may be useful or problematic. And I'd expect it'd depend a lot on the pressures one is running: relatively big bags on low pressure might not give enough to be noticeable. In fact the idea that led to this involves pressure only slightly over atmospheric most of the time (though it isn't really quite formed yet).

    As I said, I'm after interconnection, which led me to the idea of using a bag as an hydraulic cylinder. But as I also said, this isn't really about my intended application, which is admittedly as yet half-baked, but about the possibility of using "hydraulic" bags for whatever use you can find for them.

    Thanks, but I'm trying to have less in the way of electronics, not more!
     
  17. Dyce
    Joined: Sep 12, 2006
    Posts: 1,917

    Dyce
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    What about using nytrogen springs? Haul trucks use a nytrogen spring and shock cylinder all in one unit. If you have ever driven a large haul truck with this suspension they ride very smooth. Not sure how you would displace the nytrogen in the cylinder to raise and lower the height, but you could use a high psi cylinder and have it refilled and exhaust it when you lower the car....
     
  18. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,260

    Ned Ludd
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    A real limitation I've come across in considering all this is that the bags' ability to collapse makes them unsuitable for pressures below atmospheric. While one is just looking for something to support the weight of the vehicle this won't be a problem, but once one gets into the sort of simple analogue hydraulic logic circuitry in which I find myself too often mired it is all too easy to need to deal with sub-atmospheric pressures. One has then to take care that pressure stays above atmospheric in every possible permutation of conditions.

    Analogue hydraulic systems are all about cylinders rather than valves, and their logical terms revolve around "in so far as" rather than "if". For example, "in so far as a suspension movement has a roll component" something happens and/or something else doesn't.
     
    Last edited: Jun 28, 2012
  19. nofin
    Joined: Jan 7, 2010
    Posts: 321

    nofin
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    from australia

    They do settle, but it takes a fair time (can be over a month) depending on how leaky the system is. I don't know about Citroens but the 6.9 has a "lock" valve in the hydraulics for servicing so when the car is lifted the wheels don't drop; it may be possible to incorporate something like this to prevent sag when the engine is off.
     
    Last edited: Jun 28, 2012
  20. The_Hermit
    Joined: Mar 31, 2009
    Posts: 128

    The_Hermit
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    from France

    Ouch, you're getting technical, and I don't understand theory very well (i'm the hands-on kind). Can you illustrate this with a diagram or in simple words with no more than 3 syllables ? Pretty please ?
    I do understand mathematical automatons and turin devices if that helps. I also have a basic understanding of hydraulics (diameter*pressure etc.). But why the need to have more than atmospheric pressure at all times ?

    Cheers
     
  21. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,260

    Ned Ludd
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    In my experience it's usually not the big words that cause the trouble. People coin them when they're looking for accuracy, and consequently they end up meaning one thing and one thing only for quite a long time. It's the little words you have to watch out for: they can have so wide a range of senses, and such subtle differences between senses, that it is extremely easy to get it wrong. People speak metaphorically when they want to explain an unfamiliar idea in familiar terms, i.e. little words. When a metaphor works really well other people start using it to express the same thing, and eventually it gets accepted as another established sense of the word. Philologists always have a tricky moment when it's unclear if an historical author was speaking metaphorically or using a word in an established sense, or in a sense that has since fallen into disuse. They have to look at how others used the word around the same time lest they get the wrong end of the stick entirely.

    Interesting thing, language: but be that as it may.

    What I mean is, in a system where one has two bags connected by hydraulic lines, when one pushes on one bag one would get a pretty-much-proportional reaction at the other bag - within the limits of the envelope tensile elasticity that 38Chevy454 referred to. However, if instead of pushing one pulls on the first bag, the reaction at the second bag would not be nearly proportional, because both bags would start to cave in. The transition between the two conditions is when there is pressure equilibrium between the inside and the outside of the bags, i.e. at atmospheric pressure.

    Whether or not this is a problem depends on what you want the system to do. Either way it is a factor to keep in mind.
     
  22. The_Hermit
    Joined: Mar 31, 2009
    Posts: 128

    The_Hermit
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    from France

    right. So if the pressure inside the system is equal to or lower that the external pressure, if you try to 'depress' that system you will not get the same reaction all over.
    ... If the internal pressure is higher than atmospheric, then the internal vacuum will have more effect than the external pressure, and bag 2 will compress when you pull on bag 1. Understood.
     
  23. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,260

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    In simple terms, one tries to avoid pulling at all, and tries to push less instead. That is, at the maximum "pull" there is still more "push" than in a completely uncompressed system.
     
  24. stealthcruiser
    Joined: Dec 24, 2002
    Posts: 3,748

    stealthcruiser
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    This is getting pretty heavy here!
    If you are planning on using bags in this scheme,(with either a hydraulic or compressible medium), what will you do about response times and "medium metering", due to the limiting sizes of the inlet,(and common outlet!), on the bags?

    Thinking along the lines of a 3/16 or so diameter that might be common size on the inlet of the bags.

    I'll be watching!..........I owned a 77 Citroen CX for 4 years whilst living in Europe, and a LOVED the suspension on that whale!
     
  25. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,260

    Ned Ludd
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    I've noticed that a lot of bags have much bigger holes in their end plates. I've also noticed that a lot of bag-kit manufacturers hack the end plates about with merry abandon. Ideally I'd need to be able to connect to ½" hydraulic lines.
     
  26. mrconcdid
    Joined: Aug 31, 2010
    Posts: 1,157

    mrconcdid
    Member
    from Florida

    The largest bags inlet holes, I have seen on the market is dual 1/2 inch.

    Im running single 1/2 with true 1/2 line and valves, its fast, way fast with 120psi, at 165 or greater and were talking airborn.

    Godspeed
    MrC.
     
  27. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,601

    Rusty O'Toole
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    The Moulton system is brilliant. I don't know why more car makers didn't use it. Years ago I saw a mechanic repair an Austin 1100 system (I think it had a rusty line) and refill the system with windshield washer fluid and pressurise it with an air hose.

    In case you don't know... here in Canada where we often get freezing weather we fill our windshield washers with a mix of water and alcohol with a dash of detergent and some blue or green dye. It costs $2.79 for a 4 liter plastic jug. The mixture is such that it will not freeze down to 30 (blue) or 40 (green) below 0 fahrenheit.

    I don't know why you couldn't make your own system using air bags and washer fluid or in your case, if you don't need to worry about freezing weather, water and something to prevent mold and algae.
     
  28. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,260

    Ned Ludd
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    One somehow doesn't associate Austin 1100s with Canada, but during the few years of my teens that we lived in Ottawa one of my best friends' dad had an 1100 called Fred. As I remember much of the bodyshell started with steel somewhere deep inside and gradually transformed into grp towards the surface. The tops of the front wings were a good inch higher than stock, so thick was the accumulation!

    The Morris Minor system is starting to take mental shape. We're moving this weekend, so I'll be offline until I'm back at the office on Monday. The home data/phone line is only due to be reconnected next week. Not that my wife would take kindly to my being on the HAMB while she's dealing with cardboard boxes! I'll let the Minor ferment in the background while finding new places for mortars and pestles and little baskets that had been gifts from someone ...
     
    Last edited: Jun 29, 2012

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