Design of hydropneumatic suspension
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  1. #1
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    Default Design of hydropneumatic suspension

    I am looking for someone to give me guidance on the subject of hydropneumatic suspension. Currently I am looking into retrofitting citroen suspension onto a Suzuki Sierra. My proposed objective is to achieve the usual citroen armchair ride with the ability to raise and lower the car with highway, rough road and extra clearance modes.

    I would like to achieve a large range of movement, if I could get a total of 300mm I would be wrapt. I am proposing to have a rocker arrangement so that 100 mm movement at the suspension movement results in 200 mm at the axle/wheel. With this 1:2 ratio I think that I would need to go for as large a suspension unit as possible although I also need it to be fairly compact. How much do the suspension units vary in size? Which model has the most compact design?

    Sphere wise I think that the DS spheres would be that way to go. I suppose the main performance/weight limitation would be the ability of the suspension system to handle the pressures involved. What is the function of the accumulator sphere in the from of most systems/

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    Has anyone ever re-plumbed citroen suspension? What type of joints are used? Are they similar to the usual flared joint used in conventional braking systems?

    Any information or suggestions would be gratefully accepted. As well as supply of suitable second hand parts.

  2. #2
    Real cars have hydraulics DoubleChevron's Avatar
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    This has been done before. I've seen on ebay a landrover that had a 3litre Jap V6 fitted, along with the suspension from an SM fitted, they also fitted the SM daravi steering with 2turns lock 2 lock from memory.

    The task would be MASSIVE. I think those bloody great DS front suspenion arms would probably break the little suzuki. Possibly you need to have a look at a GS. Find a rusty one and pull it apart and figure out how it works. Citroen uses 'bubble flares'. ie: you need a flaring tool that will put a small bubble in the hydraulic line.

    seeya,
    Shane L.
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  3. #3
    Fellow Frogger! Paul Smith's Avatar
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    As Shane says - a huge job - you would have to get rid of the live rear axle as well I assume - not much point in using that with a lot of travel in a small car.

    The accumulator sphere is a reserve so that the pump is not working all the time - otherwise it is likely to fail in a short time - you have to have a regulator to control the pressure anyway, so adding an accumulator sphere is simple then.

    The GS would be a good start to look at - the rally ones (and some export cars) had cast lower suspension arms, which are much strronger than the steel ones. How you handle the rear I am not sure - look at a BX or Pug Mi16 4WD setup?

    Might also be worth looking at something like the 'Kinetic' system - they used a modified Citroen setup at one stage (and their design is so good it has been banned from WRC!)

    http://www.kinetic.au.com/

    Here is the Landy that Shane was talking about - but it had a custom chassis - I suspect thay needed something to do in those long northern winters!



    http://www.citroen-ds-id.com/ds/Land...uspension.html


    Paul
    Paul Smith

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  4. #4
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    Icon7 more info

    I didnt see a note as to what sort of driving you want the hydro-kinetic suspension for, but as a bit of a 'heads up', I'll tell you something else about that rover - its not used off road a heck of a lot. I grew up around Calgary: and know the Rovers there, and the local 4WD areas. That thing may 'accelerate out of corners like crazy', and go great on gravel roads with dust and washboard, but doesnt see anything except light trail work off road around the Calgary area. Tires, ride height, stance, and lots of little bits are 'wrong' for HD off road use here. The usual trail challenges are deep, deep ruts, thixotropic clay beds (aka - quick clay), and deep water crossings on mountain tracks. oh, and for what its worth, snow isnt particularly bad driving with the right rig - Calgary is noted for its very, very low humidity, and for its winds. you might find snow drifts 2' deep, but certainly not prolonged running in 2 feet of snow. its also generally a dry, powdery type of snow that you can either bowl through, or thats gone crusty, and you can drive on top - but I'll forgive you folks down under for not knowing that

    the best active suspensions I've heard of for off-road, and mixed on-off road driving are made in Aus: they're called 'fielder' suspensions but they're just straight pneumatic, not hydro-pneumatic. I've seen videos of a few range-rovers fitted with fielder suspensions doing trials - absolutely incredible. not sure how to post a link on here, but here's a URL that'll get you a look-see. http://www.4wdonline.com/Suspension/Fielder.html

    chris aka wanderer4x4

  5. #5
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    I was only lanning on using the hemi-shock body units. These would ideally be mounted to the chassis and the centre of a rocker arm whose inner end is also attached to the chassis and the outer end has a link which is connected to the live axle. All 4 corners would have the same setup. This would reduced the unsprung weight of the live axle while still retaining its robustness.

  6. #6
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    Default N.American input?

    I dont know how well you know N.American gear, or if you follow N.American offroad technology at all, but there's been a number of rigs made using air over hydraulic suspension based on convensional hydraulic rams. Specifically, there was a rig from here in kelowna than won the 4x4 of the year trial run by off-road magazine a few years ago. Th eowenr was / is Gebby Wager, and he's the stepfather of a good friend of mine ( my friend was actually gebbys 'co-pilot' at the trial)

    anyway - point is THAT rig used custom built, equal length tubular A-arms, independent diffs, and double acting hydraulic rams for the suspension on all 4 wheels. (also had 4 wheel steering !) Anyway, by putting a bleed off valve, to a reservoir from one side of each ram, then putting a measured 'shot' of oil, and an air chuck cap onto the other end, Gebby was able to turn conventional hydraulic utility rams into fully adjustable air over hydraulic suspension. By vary the pressure head on the remote reservior he could change not only the overall ride height, but the rie height ( and thus % of weight) on ech individual wheel. I havent talke dot my friend a whole lot about the rig, but I believe there was even a joystick setup in the cab to control them for off-road use.

    Not sure what you're regulations & inspections are like down there, but for on-road, INSPECTED use, you can find similar compponents under the following vehicles. ( I'm a stickler for making LEGAL rides, BTW - ALL of my modifieds are inspected and meet, or exceed Canadian regs)

    a) Mercedes sedans with Boge/hydromat suspensions - nitrogen charged hydro-pneumatic rams, transverse mounted on swing axles

    b) Audi 5000 CD turbo - Hydraulic ( nitrogen) accumulator spheres, and a 2 stage belt driven ZF pump ( low pressure for the power steering, high pressure for hydraulis, all fron one pump)

    c) equal length A-arm suspension, and IFS diffs with gearing options ( including limited slips, manual and auto lockers, and spools) toyota mini-trucks and 4Runners ( Surfs?) 1985-1995 ( pre-Tacoma/T100 years)

    if you combine parts form those 3 source vehicle types, you should be able to find all the bits to build a hydraulic over nitrogen charged suspension that can do varyign ride height/ride stiffness.

    ***Do note - all of this is just 'blue sky' project stuff thats running around in the back of my head. I have no idea about the practicallity of finding, or buying ( $$$) the assorted parts in Australia.

    regards
    chris aka wanderer4x4

  7. #7
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    Hi Chris,

    Thanks for your input. I find it satisfying to get a response to my queries. It sort of keeps me on track. I have looked into airbag systems for 4WD but I was quoted 10K AUD from a local supplier to install a system into the suzuki. I also have doubts about air bags actually achieving my objectives. On road I dont want the car wallowing which I think would happen with the airbags in 'low' mode with little air in them. Alternatively offroad you want plently of movement with height which again you I feel you are unlikely to acheive with the airbags pumped up hard in 'high' mode.

    With a hydro-pneumatic setup I was hoping to have 4 settings, as with the DS, GS, CX & BX, 1) bumpstops, 2) onroad, 3) offroad and 4) wheel changing or big obstacles. What I am looking for is someone to bounce my ideas off and keep me in the right direction. I dont expect someone to have a design for me. What I really need is to know how a standard, ie pre active, system works. Hopefully from there I can work the rest out by 'playing'.

    What I will do next is post a list of questions that I would like answered. Hopefully that will provide me with the information I think I need to design the system.

    I will drop the mob with the DS system in the Rover a line to find out what they did with the system and how it turned out.

    Mike
    Citroen BX 16TRS
    Citroen BX 19 ESTATE
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  8. #8
    Fellow Frogger! chris's Avatar
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    hmmm,

    The suspension units may not be the problem, they just replace a conventional spring/shock. If you want more travel, why not just move the attachment point closer to the pivot and use sphere off a heavier car? The front of a CX would, I imagine, be much heavier than the front of a Suzuki.

    The problem that might cause you to scratch your head is the attachment of the ride-height corrector. In case you don't know, these regulate the volume of fluid in the suspension cylinders to keep the car at the set height. In Citroens they're worked by a linkage off the middle of the anti-roll bar (pulling from the other side lets you set different heights). Getting that bit right would be crucial, I suspect. Does your live rear axle even have an anti-roll bar?

    Straight pneumatic systems are not famous for longevity. Air contains water, dust, etc. Hydraulic systems are basically sealed and last forever.



    Chris
    GS 1220 break. Beige cars go faster

  9. #9
    Tadpole
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    Default presurised suspension

    chris - I happen to agree about air suspension, but hydro-pneumatics as a general term, encompasses a lot more than just 'air' charged systems. all of the suspension components I listed are NITROGEN charged. the compressors & pumps vary the hydraulic fluid volume. Hydraulics work on the principle of liquid incompressability, and with those suspension pieces adding fluid decreases the space inside the component, which increases the gas pressure ( same mass of gas compressed into a smaller volume = higher pressure) additionally, I beleive the Boge/hydromat components use a gas / liquid bladder as a seperator, so even if the hydraulics leak, or are bled out for servicing, the system will remain nitrogen charged. **** the importnant note here is that the nitrogen pre-charge is what provides the upper & lower pressure limits of the system. if you're moving from one vehicle to another, its important to find out the lbs per inch upper, and lower limit, and locate the mounts accordingly.

    the bonus of the 2 part gas/liquid system is that the gas-over-liquid format provides a certain amount of damping. passive hydraulic systems dont respond well to very fast, repeated pressure changes, as would experienced by repeated suspension cycling. without the gas charged buffer, sooner or later that liquid incompressability crops up just as the suspension hits a bump, and 'wham' it feels like someone hit your rear with a barnboard, and you're 3 feet of the ground. ( for those who've never seen hydraulic suspension, unlike shock absorbers, there ISNT a pressure bypass, and there is only 1 chamber in each wheel unit. on a shock absorber, fluid motion is restricted for damping, but it does flow between upper, and lower chambers in the shock. the % of restriction is what causes the damping effect)

    regards
    chris aka wanderer4x4

  10. #10
    Tadpole
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    Default presurised suspension

    chris - I happen to agree about air suspension, but hydro-pneumatics as a general term, encompasses a lot more than just 'air' charged systems. all of the suspension components I listed are NITROGEN charged. the compressors & pumps vary the hydraulic fluid volume. Hydraulics work on the principle of liquid incompressability, and with those suspension pieces adding fluid decreases the space inside the component, which increases the gas pressure ( same mass of gas compressed into a smaller volume = higher pressure) additionally, I beleive the Boge/hydromat components use a gas / liquid bladder as a seperator, so even if the hydraulics leak, or are bled out for servicing, the system will remain nitrogen charged. **** the importnant note here is that the nitrogen pre-charge is what provides the upper & lower pressure limits of the system. if you're moving from one vehicle to another, its important to find out the lbs per inch upper, and lower limit, and locate the mounts accordingly.

    the bonus of the 2 part gas/liquid system is that the gas-over-liquid format provides a certain amount of damping. passive hydraulic systems dont respond well to very fast, repeated pressure changes, as would experienced by repeated suspension cycling. without the gas charged buffer, sooner or later that liquid incompressability crops up just as the suspension hits a bump, and 'wham' it feels like someone hit your rear with a barnboard, and you're 3 feet of the ground. ( for those who've never seen hydraulic suspension, unlike shock absorbers, there ISNT a pressure bypass, and there is only 1 chamber in each wheel unit. on a shock absorber, fluid motion is restricted for damping, but it does flow between upper, and lower chambers in the shock. the % of restriction is what causes the damping effect)

    regards
    chris aka wanderer4x4

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