Why DS/SM have folded rear boots but later CX/MX have accordion boots?
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  1. #1
    Tadpole
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    Default Why DS/SM have folded rear boots but later CX/MX have accordion boots?

    The rear suspension folded boots used on DS/SM cars will develop a crack after 3 or 4 years while the front accordion style boots are much better shape. I noticed the later cars i.e. CX and MX have the accordion rear boot. Anyone know why Citroen didn't use the accordion boot on the rear for DS/SM cars?

    BTW, the CX boots have 2 return-pipes: one for breather and one for leakage oil return. Why DS/SM only has one per boot?

    Best,
    Brian

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  2. #2
    Fellow Frogger
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    Possibly due to the quite long travel at the rear of a DS. The change to two return lines might have been intended to precent the boot filling with LHM as happens with the DS. If a boot is full, you then have increased pressure on the boot and return line when the suspension moves.

    In my experience, it is the repro rear boots sold a few years ago for DS/SM that tend to split along the mould line after a couple of years. Perhaps, they've improved lately and more recent production will last better? Original factory rear boots, which you could order from Citroen until a couple of years ago, tend not to fail in the same way and certainly not as quickly.

    p.s. post #1. Welcome.

  3. #3
    Tadpole
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    David,

    Thank you. The DS/SM rear boot has the return pipe located at the highest location on the boot, thus the boot must filled up full before the leakage oil can return. I think it is an intentional design to leave any debris on the boot bottom.

    On the CX 2 return pipes, what prevent the oil from flowing into the breather pipe, and why the need for the breather? I don't see why we care if air trapped in the boot.

  4. #4
    1000+ Posts gerrypro's Avatar
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    Quote Originally Posted by sapote View Post
    David,

    Thank you. The DS/SM rear boot has the return pipe located at the highest location on the boot, thus the boot must filled up full before the leakage oil can return. I think it is an intentional design to leave any debris on the boot bottom.

    On the CX 2 return pipes, what prevent the oil from flowing into the breather pipe, and why the need for the breather? I don't see why we care if air trapped in the boot.
    The breather stop air building up pressure in the dust boot as the piston moves forward. The displaced air has to go somewhere and not inflate the dust boot! Hence the breather.
    Cheers Gerry

  5. #5
    Tadpole
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    If air or fluid collected in the boot builds up to the threshold pressure then air/fluid just simply flow back to the tank via the pipe. Even with no air, the boot still must handles the fluid built up pressure. I just can't see why the boot is subjected to higher pressure if there is air in the boot. Sorry I still don't see the reason for the breather pipe (with one end opens to ambient?).
    Btw, the front boots also have the breather pipe and not just the distance rear boots.

  6. #6
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    Quote Originally Posted by David S View Post
    In my experience, it is the repro rear boots sold a few years ago for DS/SM that tend to split along the mould line after a couple of years.
    In my case, both rear boots developed crack right on the folded line created with car at lowest position during long period storage. This makes me believe that an accordion boot is better for this because it has no folded line.
    Last edited by sapote; 30th January 2019 at 03:09 PM.

  7. #7
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    Quote Originally Posted by sapote View Post
    In my case, booth rear boots developed crack right on the folded line created with car at lowest position during long period storage. This makes me believe that an accordion boot is better for this because it has no folded line.
    I have often wondered about the wisdom of the inside out rear boot design with its inherent frailure point from creasing.. Here is another line of argument In theory there should not be a stress point IF the boot stayed concentric. The accordion boots stay reasonably concentric over their entire movement range. The rear boots do NOT stay concentric over their travel. That and the car's sinking after every use means the rear boots get stressed and because of the inside out double fold they get a fatigue point.

  8. #8
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    Quote Originally Posted by fritzelhund View Post
    I have often wondered about the wisdom of the inside out rear boot design with its inherent frailure point from creasing.. Here is another line of argument In theory there should not be a stress point IF the boot stayed concentric. The accordion boots stay reasonably concentric over their entire movement range. The rear boots do NOT stay concentric over their travel. That and the car's sinking after every use means the rear boots get stressed and because of the inside out double fold they get a fatigue point.
    I too have puzzled this design. The concertina looks so much more appropriate to the movement. Does anyone know of there is a concertina boot out there with an outlet for the return line I could experiment with.

  9. #9
    Fellow Frogger!
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    I have just been mucking about with the rear boots on my GS and I notice that the cylinder has an extra (compared to the D) groove not far from the seal which collects any leaks. The pipe attached to the boot is just a vent. The boot is just a dust seal, the odd crack wouldn,t matter!
    Woody

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    Best place to purchase DS boots.

  11. #11
    Tadpole
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    I think I know why the breather pipes on the later cars.

    So I was wrong thinking the breather pipe is the one closer to the cylinder (based on one of the photo thinking it is the breather). In fact the breather pipe is for lease the gas inside the boot, and the return pipe is for returning the leakage oil between the double-ring teflon seal and the O-ring seal within the cylinder.


    Citroen redesigned the system to return the oil that leaked pass the first seal (the double-ring teflon seal) before the oil passed the second seal (blain O-ring) and into the boot. This makes sense to return the oil before it got into the boot, and so with the breather on the boot, the boot no longer carries the full bladder of oil and it has less stress during it compressed/expanded cycle. This is the reason why the accordion type boots are used on the rear units on later cars with breather pipes


    To use the accordion boot on DS/SM, the return pipe needed to move to between the cylinder seals, and the boot just vented to ambient.

    Brian

  12. #12
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    For those with CX and other cars that have the suspension boots with the breather pipes, to avoid the breather dumping the oil causing a fail "MOT" due to the appearance of an oil leak, the boots should be drained periodically. The pressurized oil leaked pass the Teflon seal is returned to the tank, but a very small amount of low pressure oil might pass the second seal (O-ring seal) into the boot. In theory, the boot serves as a dust cover, and when you feel the boot heavy with oil then time to drain before it dumped out onto the chassis as the piston moving.

    Better yet, the boots should have a draining plug at the bottom to easily drain the boot oil. There is nothing wrong with the system if you find the oil soaked chassis with oil in the boots. Just drain it periodically.

    Brian
    Last edited by sapote; 1st February 2019 at 12:33 PM.

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