Modifying Another Sachs ZF

Discussion in 'Trials' started by motobene, Aug 1, 2018.

  1. motobene

    motobene Motoing for 46 years

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    Yup, I got suckered by a friend into modifying another Sachs ZF (2005 GasGas 300), and I'll shortly be cursing that blasted second retaining ring getting the bloody thing apart!

    Degassed and the Schrader thread put in.... IMG_20180801_152606.jpg
    #1
  2. motobene

    motobene Motoing for 46 years

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    Only 15 minutes to get that accused clip to roll out!

    And here it is again, yet another Sachs ZF piston assembled upside down, with two holes facing the compression shim stack and four holes facing the rebound stack. They are at least consistent!
    IMG_20180801_163716.jpg

    Whew! I found four pages of notes on the mods in my suspension files, so I can duplicate the developed hacking and whacking and shim shuffling with only 4 or 5 brain cells.
    #2
  3. Kiharaikido

    Kiharaikido Been here awhile

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    Fancy shop. Is that your home setup?


    Sent from my iPhone using Tapatalk
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  4. motobene

    motobene Motoing for 46 years

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    Yup, the ranch shop. It's a nice space. Not big, but well equiped.
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  5. motobene

    motobene Motoing for 46 years

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    Lucky me. Had enough shims left to re valve another. IMG_20180801_172437.jpg
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  6. Sting32

    Sting32 Trials Evangelist

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    Did I read that post right, that Sach's shocks had been assembled incorrectly? or are you saying you are holding the shock upside down or something?
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  7. wheelieman14

    wheelieman14 Adventurer

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    VintagePro says the same thing about the Sachs shocks installed in Beta Evo bikes. I understand that compression stroke should be quicker than rebound, so I'm also baffled that Sachs assemblers have them built with only 2 orifice holes for compression and 4 holes for rebound.

    I'm amazed at how well only a few people can modify trials shock with such noticeable improvements (such as long-travel) and the factories keep building bikes that don't seem to tap the potential of suspension enhancements.
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  8. alpineboard

    alpineboard Been here awhile

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    Is this reversed as compared to other shocks..? Just a guess, but is it possible that the amount of holes does not effect dampening, but is for heat dissipation, to keep heat from building up in the first place, by having more holes for flow area for rebound/ increased surface area..? If there are 6 holes, why not 3 and 3?
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  9. motobene

    motobene Motoing for 46 years

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    Yes, the piston inside the shock is assembled in the less optimal of the two choices.

    I could explain the fluid dynamics in more detail in about 15 minutes face to face. Not so easy in text here. In very short:

    It's not about number of holes but rather the flow area for high-speed damping and the presentation of the holes to their respective first shims, a sort of blow-off area, atfecting and lower-speed damping.

    Converting mechanical motion to heat is what shock damping is. No heat produced in response to motion (neglecting contact friction) results in no damping. Most of the heat is produced at the intersection of the piston holes and the closest shims. Also at the bypass hole related to so called rebound adjustment. The oil conducts the heat to the body and shaft.
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  10. Nodabs

    Nodabs Been here awhile

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    I'd recommend going to a NPTF tap for a better seal on the fittings. The F is for fuel fit, a interference fit thread.
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  11. Cascao

    Cascao Adventurer

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    For who want go and save this Sachs/Ollé shocks:


    Tech forks:
    #11
  12. motobene

    motobene Motoing for 46 years

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    If only the Sachs was as easy as an Olle to get apart!
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  13. motobene

    motobene Motoing for 46 years

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    Didn't know about the F= fuel fit. Will check that out, thanks. Have always used NPT. All tapered pipe threads are interference fit.
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  14. Nodabs

    Nodabs Been here awhile

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  15. Vintage pro

    Vintage pro Been here awhile

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    Chris the photo of the bottom of the shock shaft you show with the piston(second photo) you are missing a steel insert to regulate slow speed rebound damping. Here are some pictures of what I'm talking about. You can see from the photos the cavity where the insert is pressed in. The hole size is 3mm, if you don't fix this the shock speed will be too fast like the Showa shock we dealt with. Ohlins have a shaft jet that screws in from the bottom, they range in hole size from 3.1mm to 4.mm. Makes a big difference. Surprised some of you experts out there did'nt catch it, just kidding.

    Attached Files:

    #15
  16. motobene

    motobene Motoing for 46 years

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    That bypass main jet dodad was missing from the factory.

    It's together. We'll just see how she works.

    I was able to increase free shock travel 9.5mm, with 1.2mm of that as negative travel. Should way out perform a stock Ohlins just like last time, given the piston correction, revalve (a modified version of your recommended stacks), this lowly Sachs should significantly out perform a stock Ohlins shock.
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  17. motobene

    motobene Motoing for 46 years

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    Looked at photos of other Sachs ZFs and they all look like the shock of this post. No jet dodad pressed into the counter bore. Blurry example showing the piston upside down (from the factory):
    160829 Sachs ZF Upside Down Piston.jpg

    Anyway, after checking for overnight gas leakage, the shock is on its way back to the owner. I achieved free shock travel increase of 9.5mm by machining three parts. Unlike the Ohlins, increasing travel isn't nearly as brainless on a Sachs.

    Travel increase with the re valve and flipping the piston back over the right way plus fresh oil is going to make this old rag more compliant and less harsh bottom prone than a stock GG45 or 46 Ohlins.

    180803 Sachs ZF Eldon Malone Assembled.JPG
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  18. PMK

    PMK Been here awhile

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    They sure pinch the edge flow on those body to shim clearances.
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  19. Vintage pro

    Vintage pro Been here awhile

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    If it turns out like a pogo stick you'll know what the problem is. Every one I've had apart to fix (50+) has the restricter pressed in that cavity. PMK most of the people don't get what you are talking about, better said that the first shim on piston is 33mm unlike Ohlins would use 32mm to have less restrictive flow into piston ports. The body I.D. 36mm in this case.
    #19
  20. PMK

    PMK Been here awhile

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    Edge effect, the dimensional area of the clearance between the shock body or fork cartridge ID vs the largest shims OD on the pistons opposite face vs the ports area vs area of the notch that allows oil flow into the port and transmit pressure onto the face shim.

    If the edge area is insufficient, this can limit high speed compression or rebound as the flow is restricted. In many cases, likely not trials, this limitation is also cause for cavitation of the oil.

    While I would agree that typically the main piston does appear inverted in these installations, I am not sure it is. It could be, but then again may be by design and tuning what they wanted. To back up my statement, as I watch trials rider accomplish hops or floating the rear tire, if the edge effect creates excessive damping, at those shock shaft speeds, it could hinder the ability to hop on the rear tire.

    Consider that the rebound force is essentially the springs force. There is no means to extend the wheel faster than the spring can push. Compression force is dictated by not only rider weight and bike weight, but the force if the impact with the feature.

    Not saying the factory installed the pistons correctly or incorrectly, granted it does appear wrong by textbook methods, however, finding that many built incorrectly seems odd. True, if the design engineer drew the spec wrong then all may be wrong, but I question it. However, if the mod and revalve works better, then that proof holds up. Consider too, if they simply flipped the pistons, the shim stacks would be grossly incorrect also.
    #20