Rebuild the Unrebuildable Sachs ZF Shock - GasGas & Other

Discussion in 'Trials' started by motobene, Aug 29, 2016.

  1. motobene

    motobene Motoing for 49 years

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    If you are going to use this thread to do your own work on the peculiar Sachs shock read all of the thread. I had a learning experience regarding assembly order with respect to the type of Schrader valve used to re gas the shock.

    This effort grew out of the following thread of putting a Reiger on an older GasGas. That freed the original shock up for playing with and servicing. Here's the other thread:

    http://advrider.com/index.php?threads/putting-a-reiger-on-an-older-gasgas-2005.1169872/

    The German-made Sachs ZF shock typical to many older GasGas and other bikes is a light, well made, simple shock that presents as un rebuildable, but is in fact, rebuildable by those skilled in such work. The Sachs is a very common design. Structurally it is similar to an Ohlins, and you can use some Ohlins parts in the shock, like shims, piston, separator piston, and even the seal head (may require some machining).

    The Sachs shock in this example is from a 2005 TXT GasGas. Here it is after I pulled it apart:



    After removing the spring, take a small punch or screwdriver and tap on the edge of the aluminum cover/dust seal to pop it out. You can then see the seal head and retaining ring. The retaining ring is impossible to remove without pushing the seal head into the shock body such that the ring isn't trapped by the step the seal head groove. Pushing the seal head in requires de pressurizing the shock.

    I depressurized my shock in the process of machining the body for a Schrader valve to later re charge the nitrogen. You can see in the photo above the counterbore and 1/4" reamed hole I made on an up-angle into the body chamber. Note the press-in Schrader valve I machined from a commonly available valve with 1/8" NPT male thread.

    Once the shock is degassed, the seal head can normally be pushed in and it will stay in or just slowly creep back out. If the shock has internally 'blown' from gas bypassing the separator piston, with the separator piston hard up against the bottom of the shock body bore, the shock will remain somewhat pressurized even after degassing the gas chamber. In that case a special fixture is required to push in the seal head to remove the retaining ring.

    In my case the shock was in excellent shape with no excess gas in the oil chamber. It was in such good shape with pretty clean green oil that I thought it no possible for it to be the original shock. However, 20 and 05 was milled into the cap over the seal head, so possibly it is.

    Regarding getting the seal head retaining ring out, Randy Lewis told me of a trick which I used. There is a tiny bleeder hole in the shock body. It is drilled on an angle, intersecting the retaining ring groove. The original hole is REALLY small at ~0.015". Per Randy I drilled the hole larger. He said 2mm (.078"). I chose 0.067 (#51 drill) to just clear a 1/16 steel welding rod to use as a push tool. The larger diameter the hole also becomes a better bleeder port when reassembling the shock:



    Photo error! That should be 0.067", not 0.670".

    When you drill out the hole, make sure to push the retaining ring around in the groove so the space between the two ends is where you will drill. You don't want to crash into the retaining ring when drilling!

    With the push tool and a tiny screwdriver, the retaining became easy to remove. I then worked the seal head up and out with up-pressure and a slow wobbling motion. The seal head popped right out, but the piston was then stopped by the second retaining ring. You can see here the two retaining ring grooves:



    The second, 'safety', ring is to keep the shock from flying COMPLETELY apart by stopping the piston. This scenario only happens after a blown shock has been repeatedly abused with hard top-outs such that the seal head retaining ring groove in the shock body fails fatiguewise. The probability of shock separation with a not-blown and not severely abused shock are just about ZERO, so that second ring just makes fully disassembly of the shock a royal pain!

    The second ring takes skill and patience to get out, as it is farther down in the bore and you have the shock shaft and seal head flopping around.

    The trick is to push a small, sharp tool behind and under one end, then push the end down deeper into the shock body to work it progressively out of the groove. It will eventually pivot in the bore, free of the groove.

    I belt sanded a very small straight pick to push behind the ring. A tiny screw driver with a head lamp and magnification rounded out the two-handed+ job. Three times I got the ring started out of the groove, only to have it snap back home

    This infernal second ring will not be going back in my shock! With the addition of a Schrader and no second retaining ring, the Sachs will become a normal, easily rebuildable shock.

    With the rod and piston assembly out I could get the specs on the shim stacks. This is a simple shock having fewer shims than some other shocks. I also found, as Randy Lewis said I would, the piston assembled by Sachs upside down. He said in the 2000s the assembler(s) started flipping them over. When flipped over, two through-holes port to the compression stack:



    ...and four holes are ported to the rebound stack. That is contrary to convention, as these piston holes are like a main jet for high-speed compression, and usually you want more maximum flow capacity in the compression direction for the rapid big hits. The Sachs has really huge high-speed damping holes, however, so halving the flow area of the compression holes may not be that noticeable?

    The other effect of flow area is how much pressure surface area presents to the first shim of the stack for lifting force.

    Sachs shocks tends to feel more bound up in rebound, and the adjuster, which affects rebound speed more than compression speed, tends to like better closer toward minimum damping or even at minimum. Having the area of four holes acting on the rebound stack may possibly have tested out on a shock dynamo as a cheap improvement in overall damping?!

    The piston will go back in flipped 'right side up.' To deal with the bound-up feel of rebound damping I will add one or more 'rocker shims' between the larger rebound shims closer to the piston.

    For additional viewing pleasure, here is a comparison of shim stacks between this Sachs shocks and two shocks I got into previously. Unfortunately I did not take data on piston holes area.

    #1
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  2. the toe cutter

    the toe cutter Adventurer

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    This is great, thanks Motobene!
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  3. motobene

    motobene Motoing for 49 years

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    Spoke with Randy Lewis yesterday. He graciously offered his re valve specs and is sending me 10 shims. I'll end up with 1 more rebound shim and 4 more compression shims.

    We also discussed reassembling the Sachs shock as a gas-emulsion shock (like the KYB shocks on KLRs). This involves taking out and leaving out the gas separator piston, using gravity as the separator. Given this is a body-side-up shock, no separating piston works just fine, and it makes any further service and re valve work very fast and easy.

    There is no typical bleeding of air from the oil with a gas-emulsion shock. You simply fill the Sachs shock with about 90cc of oil (or set oil level x millimeters from the top of the body - I used 55mm on the GG45 Ohlins). Then you purge the gas bubble during the re gassing. With a Schrader valve purging is pretty easy. You charge and dump the charge several times taking the gas bubble from 78% nitrogen (atmosphere), to close to 100% nitrogen.
    #3
  4. motobene

    motobene Motoing for 49 years

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    Update: I found the Sachs shock bumper was REALLY destroyed inside. It looked fine on the outside but felt very soft when pushed on. It was basically just flakes and dust on the inside.

    Bumper failure can lead to the shock failing. The added compression excursion can have the shaft and nut of the shaft-piston assembly smack the gas separator piston at full compression, which can lead to gas building up on the oil side of the piston. A leaky seal or fitting can do the same thing, as the oil loss is taken up by the piston moving toward the piston assembly, and eventually the two will collide.

    The internally destroyed bumper accounts for the lack of rebound kick during full-compression loadings.

    I did not have a Sachs bumper, but did have a parts KYB shock off a Gen 1 KLR. Here you can see the Sachs destroyed bumper, a bumper off an Ohlins, and the KLR bumper before modification to fit the Sachs:



    The KLR bumper isn't as big in diameter, but it is quite firm. I cut it roughly in half and made a bumper for the Sachs:



    I also machined a spacer to reduce shock length to correct the tendency of the `05 225 to oversteer. Just backing off spring preload wasn't an option, because I'd be in running negative preload, resulting in a very sloppy spring. Shortening the shock was the only viable option. I wasn't going to attempt unscrewing the rod from the clevis, so using a spacer became the solution:



    I've received shims from Randy Lewis for the re valve. I have a quart of Ohlins oil, but I also found a quart of Motul Pro full synthetic very light shock oil. Good stuff, so I'll be using that instead of the Ohlins oil, which I think is a touch too thick and has a lower viscosity index.

    I now have the parts required to finish this project and see if one can attain a very good working shock from the lowly Sachs. Now for a few hours to put Humpty Bumpty back together again.
    #4
  5. motobene

    motobene Motoing for 49 years

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    Yesterday I blew out the separator piston and pressed the Schrader in using Araldite aluminum epoxy as a retainer-sealer. That will have cured by today.

    I knocked out the steel sleeves and lubed the spherical bearing of one shock eye (the Sachs uses a rubber bushing at the other eye). One foam square ring seal broke so I replaced it with an equivalent buna-N quadring.

    I built the valve stack almost to Randy Lewis's specs. I was was short a shim and changed another shim.

    Later today I will assemble and test the shock manually. I'll get to swapping the Reiger out as soon as I can for a real-world test of this reconditioned Sachs which I hope will match the Reiger's performance.

    If I don't like the results. the shock will be SO easy to revalve from now on given the Schrader gas port, no separator piston, and no redundant retaining ring.
    #5
  6. motobene

    motobene Motoing for 49 years

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    A great way to learn anything is to risk making mistakes, and then making a big one!

    I'm in the shop assembling the Sachs shock, all smugness about my careful work, then I suddenly realize (with a hot flash) that I cannot assemble the spring and spring nuts! I have stupidly blocked final assembly with a press-in Schrader valve!

    Ack!

    I like press-in Schrader valves for gas sealing. I've made them before for other shocks. The Sachs is a bit of an oddball in spring retainer ring, body, and spring nut thread, as it has a solid upper shock spring retainer with no slot to remove it. The body diameter is larger than the shock nuts thread at the seal head end, so you can't assemble the nuts from the rod end, even if the shock spring retainer was slotted and removable.

    Ack ack!

    And yeah, I forgot to thread the nuts on before pressing the Schrader in, which might have tempted me to slot the spring retaining ring (and weaken it).


    Given I don't want to slot the spring retainer, and removing a bonded-in Schrader requires heat and struggle, the simplest solution is to drill right through the bonded-in Schrader valve and tapping for the Schrader shown in the package.

    I've got a cherry 1/8" pipe tap, so I can machine a really clean threads for good gas sealing via Teflon tape.

    Gonna go get to it.
    #6
  7. motobene

    motobene Motoing for 49 years

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    Here's how I solved the assembly issue with respect to using a Schrader valve.

    I drilled through (R-size drill) the Schrader that was pressed in and bonded with Araldite aluminum epoxy. Then I tapped for 1/8" NPT. Here is the result:



    Note the photo blurs the threads. They are actually very sharp and clean.

    I assembled the piston-rod assembly to the body with a little shock oil on the seal head o-ring and piston band for lubrication. I flipped the shock over as shown below, then assembled the spring and nuts loosely:



    I filled the shock with Motul Factory Line VI400, 2.5W with a syringe to keep accurate track of the volume and measure the height of the gas chamber above the oil.

    Note this particular assembly method and use of protruding Schrader requires the gas-oil separator piston to be left out, for a so-called 'gas emulsion' type of nitrogen-charged shock. The KLR 250 and 650 have this type of shock. They are simpler than the 'DeCarbon' type shocks having separator pistons, and they work well.

    Gas-emulsion-type shocks must, however, operate rod-end down so the piston does not stroke through the nitrogen gas bubble. GasGas previous to '12 or '13 models had shocks oriented rod end down.

    Use of a needle-fill gas filling port as seen on some shocks, if that can be assembled such that the shock spring and shock springs nuts can pass over, would allow a separator piston and bleeding before pressurization.

    I am more interested in robust simplicity than struggling with bleeding, so I went with a Schrader valve and the gas-emulsion format.

    I ended up with ~80cc of oil to achieve a gas chamber height of ~30mm with the shock fully compressed. With piston-rod extended fully the gas chamber height grows to ~36 to 40mm. Oil height remains well above the piston at maximum compression, so the valve stack does not stroke into open air.

    Lots of oil sloshing occurs as the oil jets out of the stack when the shock strokes. The action will emulsify some gas into the oil, but it does not make a shock feel like it is stroking through gas instead of oil. The action feels to my educated butt just like that of a shock with a separator piston.

    I was once concerned about oil shear in gas-emulsion-type shocks, then I disassembled an Ohlins I built that way after 3 years and found the oil to be near new in color and consistency.

    Sealing of the Schrader valve threads was by white Teflon tape. Three loops around starting after the first thread. Water and dishwashing liquid was sprayed on the fitting to check for gas leaks. No leaks were detected.

    The atmosphere is 78% nitrogen. I purged the other gasses simply by charging the shock and releasing the charge several times.

    I set the final nitrogen pressure to ~180psi. Anything between about 100 to 250 psi will work. Randy says he goes up as high as the mid 200s.

    The higher pressures will add a little to the extension force because the pressure is acting on the area of the shock rod, adding to the extension force of the shock spring. The area of a 14mm shock rod is 0.25 square inches. 100 psi yields 25 pounds, 180 psi yields 45 pounds, and 250 psi yields 62 pounds, minus friction of course. Seal clamping force goes up with pressure, reducing slightly the dynamic force due to added gas pressure.

    With the adjuster full open and 180 psi, the rod fully extends from full compression in about 1/2 a second.

    There is no check valve on the shim stack bypass circuit, the so called rebound adjuster, because it affects felt rebound response more than compression response. The bypass circuit bypasses some oil during both compression and rebound, so it does affect both.

    The end result, before preloading the spring:

    #7
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  8. motobene

    motobene Motoing for 49 years

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    The result of all the work and learning? Quite amazing really.

    I had three goals when I set out:
    - Learn the Sachs ZF shock in detail
    - Make the Sachs shock work like a more expensive shock
    - Get rid of squirrely handling and oversteer (butt end riding too high)

    All three were achieved!

    Here is the Sachs ZF assembled. Also shown is the Reiger Factory I took off the bike and against which my work was compared:


    The Sachs back on the bike:


    I rode the project bike with the re done Sachs shock 2-1/2 hours today, in the heat, but I didn't care. The shock was working SO good I was having a blast.

    I started as always with the adjuster full open. This lets me know adjustment range potential. A shock should be too fast with adjuster wide open and ideally right about a third of available clicks in (my opinion).

    In the ride test full open on the adjuster made the shock a little too fast... good! I achieved adjustment range. I settled on a few turns in. I can't say about clicks because this adjuster's clicks don't click. The setting was fine in hot weather and leaves room to back off in really cold weather when the oil becomes thicker.

    Performance was eye opening. Plush, controlled, deep feeling, yet active. I slammed walls and the shock soaked up the hits well and gave just enough kickback for forward rotation of the bike. I'd land on top level as if on the Stay Puffed marshmallow man. Oh yeah!

    Unloading response for small rear-end hops is correct and much like the the Reiger. Landing bigger rear-end hops is also right because the shock allows the rear end to plant and stay and not under or over react such that it affects balance negatively.

    Before the re valve, the action was both deader and less controlled.

    Very, very pleased, as the lowly shock is now hard to distinguish from an expensive aftermarket shock in a blind test. No more blown bumper sure helps, and the hard bumper made from a KLR 650 shock bumper works great. Flipping the piston back over the right way helped. Four holes supply the compression stack and two holes supply the rebound stack, which is correct. Sachs has assembled the pistons presumably wrong on thousands of shocks from about year 2000 to? A good spec oil that's fresh helps too of course. Having more shims and a better valve spec was also important. I'm not sharing that publically. It's Randy Lewis' specs plus a few of my tweaks like a rocker shim in the primary rebound stack.

    And last, the handling quirk of extreme oversteer of this particular GasGas is now fixed. Steering in leaned turns us now neutral now and almost as planted as my Sherco. The wheel is no longer trying to turn into a lean and tuck under. This was accomished with three changes:

    - Forks lowered with TOP of the forks caps 2mm below the top surface of the top triple clamp (I could alternatively up the fork spring preload)
    - Shortened the Sachs shock to 10.864" (from 11.052") via an internal spacer
    - Run little shock spring preload - now only 1/8"

    The following is paper towel notes of the Reiger and the modified Sachs.
    I unfortunately did not take this data on the unmodified Sachs. The A dimension on that setup was notably higher than that of the Reiger, and at zero shock spring preload was still higher than the Reiger.



    There are few reads of this thread with one comment so far. If anyone Googles 'Sachs shock rebuild', however, this thread may come up and be very helpful.
    #8
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  9. jonnyc21

    jonnyc21 Trials Ninja

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    Your making me consider having something like this done to my wife's 2008 Gas Gas 125... Thanks for this great information!
    #9
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  10. motobene

    motobene Motoing for 49 years

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    I now know the what and how to achieve a great result. So, of course, does Randy Lewis and he's done many of them to my one.

    The ideal situation is a shock not needing parts, like a seal head or bumper. The Sachs is not a shock that has parts readily available, or so I was told. That means Ohlins parts or modified Ohlins parts to rebuild and old beater shock, so why not just buy a replacement shock?

    Clear to me, however, is the Sachs is a well made shock. A hard coated aluminum body is about as good as it gets, and the other parts are similar to other manufacturers.

    There is the lack of a gas port and the second, 'safety' clip that makes the shock a pain in the ass. Also the assembly order of the spring and nuts competing for space with means to re charge nitrogen.

    And last, the low number of shims (probably to save money). The Shoes on the 4RT has a bunch of shims, so you can shuffle them around to re valve. The Sachs requires purchase of about $20 in additional shims.

    And then there is the issue of tossing the separator piston, valid only in the former, shock-body-end-up applications. Some riders may believe performance will drop due to loss of separating piston. I have not found that to be the case.

    On the '16 Sherco, the frame is designed around a rod-end-up orientation of the Ohlins, so there is no choice, the separator piston must be retained. But the Ohlins has the spring and nuts assuming from the rod end, and a gas port, so revolving is just a matter of a conversational rebuild.
    #10
  11. motobene

    motobene Motoing for 49 years

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    Rode my `16 Factory Sherco yesterday for hours after doing all the test riding of the rebuilt Sachs the day before. I found the performance of the Ohlins to be quite good, but I was amazed to like the lowly modified Sachs better. The Ohlins is over damped, even with the adjuster at zero. The Sachs was plusher and gave better response to loading and unloading. A re valve is called for, and according to Randy, a shorter shock bumper so the travel feels deeper.

    I confirmed this by riding the 2005 GasGas again. Interesting!
    #11
  12. Zuber

    Zuber Zoob Supporter

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    Good job. Look at the Race Tech filler bolt for KTM shocks instead of the schrader valve. It only needs a 5mm threaded hole and the sealing surface doesn't have to be ultra flat. You fill it with a needle through its plastic center which is very common. The head only sticks out about 6mm, so your springs may clear.
    #12
  13. motobene

    motobene Motoing for 49 years

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    Thanks, you are right. I need to add this flexibility to shock work. It's a White Power filler screw on the Race Tech site:



    ...which of course requires purchasing the needle filler kit, which is something I've needed to do for a long time. Though an M6 hole is quite small, I could still syringe fill the oil through that small hole for the gas-emulsion version of the Sachs.

    Been riding the 225 GasGas with the Sachs ZF shock in gas-emulsion form. I'm still very pleased with how it is working. Now the forks could use some improvement to come up to where the shock is performing :-) Endless projects for a bike that isn't worth much! Just because, I recon....
    #13
  14. broma345

    broma345 n00b

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    Man! Great thread! I have a small Sachs project going myself. I am putting a slightly later Sachs ZF from an '06-'09 Aprilia Tuono with a replacement spring from Racetech or Ohlins on an NC700X. I would love to know more about actual re-valving options for the Sachs since it seems like it has great potential. Did you get further into into adding/adjusting shims? Any tips in what you have found? Thanks!!
    #14
  15. motobene

    motobene Motoing for 49 years

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    The Sachs is generally over damped so the adjuster needs to be run close to or at minimum. I'll typically add some 'rocket shims' to the rebound stack. That is, one to three 0.20mm shims that are much smaller in diameter than the larger shims that are earlier in the stack. The smaller diameter shims allow the larger shims the 'rocker' into the spaces they create to reduce rebound damping.
    #15
  16. PMK

    PMK Been here awhile

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    Not sure how this will compare to my upcoming rebuild of the Scorpa SY 250R rear damper, but thank you for a look inside.

    After reading through all the posts, wanted to add a couple things.

    1) While the Race Tech bolt does work, they have a proven record off leaking nitrogen when used on KTM WP dampers. In an emulsion setup, the oil may help act as a seal.

    2) You mentioned the second snap ring is a safety clip. While true it can accomplish that task, there is a primary function that improves damping performance. In a non remote rezzy design with no compression adjuster AND a circlip retained sealhead, the second circlip stabilizes the sealhead. During a HS compression event, it is very common to float the sealhead. In that situation, the shaft velocity is rapid enough to “ plunger effect” the main piston, and with this, simply hydraulically compress against the IFP and gas pressure while at the same time drawing the sealhead assembly into the shock body. The ability of the sealhead not to move is dependent upon either almost zero compression damping in a non rezzy setup, excessively high gas pressure, or the second circlip.

    The better setup, is having a threaded sealhead that is stationary, but more expensive to manufacture.

    Omitting the second circlip can cause wear to the damperbody bore, the sealhead, and sealhead O ring.

    Often, simpler less expensive dual sport machines will lose gas pressure over time but are continued to be ridden. In a similar fashion, they too float the sealhead. Also, some of the early mtb rear shocks had similar issues.
    #16
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  17. motobene

    motobene Motoing for 49 years

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    I could see that second clip stopping the seal head from pushing in, but I think this subtle detail may be based on some assumptions.

    Sachs is a DeCarbon shock, not a gas-emulsion-type shock. DeCarbon means a gas-oil separating piston, name after the perhaps Dutch guy who invented it.

    These second retaining rings in Sachs are simply not present on the vast majority of shocks having modern seal heads, DeCarbon or otherwise. Also whether or not a shock is gas emulsion or DeCarbon, very low or no gas pressure will have the same effect of wanting to collapse a shock into the low-pressure bubble.

    I think it is legacy issue on the Sachs, that second ring, as I recently acquired a 2004 Sachs with the old-style brass seal head compressed against an elastomer... that second ring serving as a compression stop.

    In 2005 Sachs changed to a modern seal head like Ohlins, but left the second ring in. That makes it irritating for disassembly, but they probably figured, why not keep it in as a line if defense if gas pressure is lost? A stupid decision, me thinks, along with assembling all their shocks for many years with the pistons in upside down, with the smaller high-speed compression holes plumbed to the rebound stack and the larger high-speed rebound holes plumbed to the compression stack.

    Another issue is what is the minimum pressure to prevent reservoir collapse? It may indeed be whatever the impulse pressure under fast compression is, and I don't know what that is. I do know it doesn't take much pressure at all for the seal head to be almost impossible to push in. Case in point those internal-reservoir DeCarbon shocks that have lost just enough oil for the piston rod to smack the DeCarbon piston at max compression. The bit of nitrogen to spritz by the piston and goes into solution in the oil makes shock disassembly without drilling a relief hole through the seal head just about impossible, as often the DeCarbon piston is backed hard up against the shock bore bottom.
    #17
  18. PMK

    PMK Been here awhile

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    Yes, agree his name is DeCarbon.

    I understand all tnat you say. If the damper is capable of a moderate amount of compression damping, and the shaft velocity is adequate, the sealhead will float in a monotube design with an IFP. Less likely in an emulsion style as the fluid is in theory uniform throughout the body, and is less compressible than IFP N2.

    Most, but not all, single circlip dampers have a compression adjuster that tends to stabilize the sealhead via the orifice effect of flowing the fluid.

    If the compression event is sufficient to cavitate the oil between the sealhead and piston, the gas pressure in tnat region is in theory within a vacuum setting, and with high moves to low, the sealhead may float.

    The benefit of floating the sealhead is often a smoother ride for a slower woods rider. In trials, the compression events seem to be more low speed, so floating the sealhead is less likely, plus the tire does take the majority of HS compression events.
    #18
  19. motobene

    motobene Motoing for 49 years

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    Is IFP internal fluid pressure?

    I wondered how one might know if a compression hit ever moved a seal head inward from a cavitation event on a big compression hit. The collapse has a certain 'bloop-doyg' feel one can directly experience compressing a shock by hand at near-zero gas pressure. In 45 years I've never felt that on a shock except once, and that shock was blown.

    I did feel it several times on a computer-controlled prosthetic foot I was developing, but that was when gas pressure had declined to below 15psi.

    I'm thinking major cavitation event is darn near impossible if gas pressure is - I'm guessing - say, 80+psi? Most shocks are charged 150 to 250psi, so quite a bit of oil (or gas) would have to leak out before the feel gets wonky.

    But again, I'm speculating based on my experience, and that is never extensive enough.

    What's interesting about this conversation is that it's likely only a few can understand what the heck we are talking about :-)
    #19
  20. PMK

    PMK Been here awhile

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    Agree, some will understand, while others may not and possibly some may learn from it. I learned bits I need to know regarding the Sachs damper since it is one I have never needed to work on, but do now.

    Interesting comment regarding how does a floated sealhead feel to the rider. Years ago, myself and other suspension tuners discussed this.

    A gas charged damper does not need to be low on gas pressure to cavitate. Cavitation can occurr, for a multitude of reasons. Gas pressure, aids in the reabsorbtion of the cavitated fluid into a solid column of fluid.

    Gas pressure, has a drawback as it resists damper rod movement, and even though not damped, in some cases, for some riders creates compression spikes. In some cases, lowering gas pressure or increasing gas pressure can improve wheel control.

    IFP = Internal Floating Piston, you make see reference in discussions with longtime suspension tuners of the term ISP. ISP = Internal Seperating Piston, both are identical.

    The internet has a video, of a clear bodied damper assembly, run on the shock dyno. Even properly assembled, serviced and with proper gas pressure, the cavitation is clearly seen.

    In simple terms, the compression force exceeds the IFP force. The smaller the shock body diameter, the less IFP present, (F=PA). A loss of gas pressure makes the situation worse.

    FWIW, Fox, for various reasons, and to overcome a poor design has now increased IFP pressure to 500 psi. This is on a small body diameter monotube MTB shock assembly. The one benefit of the Fox design, is the sealhead is not snapring retained, but rather the threaded style.

    Converting to emulsion in many cases is considered a downgrade. However, with no IFP to contend with, emulsified can provide a firmer column of resistance to compression and since the fluid already is cavitated being an emulsion style, floating a sealhead is less likely, or less severe.

    In returning to the need for the second snapring, while it may act as a safety device, the true purpose is to provide a better performing damper assembly.
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