Playing around with shim stacks

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  • Playing around with shim stacks
  • You may struggle to get a Fast cartridge, I thought I’d get one but they’ve been on back order for months…

    I wouldn’t “play” with a shim stack, I think you need to be quite scientific about it!

    Premier Icon Onzadog
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    Without a dyno though, playing is all you can do. Make a change, try it, see if you like it. See if you want more or less of the change you made.

    Surely it’s how every suspension tuner gets started isn’t it?

    Didn’t know FAST cartridges were in short supply though.

    Premier Icon Onzadog
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    There’s actually quite a few upgrades for the Pike now which was heralded as a wonder fork when it was first released.

    Compared to the Avalanche cartridge in a Fox 36 that I had before, the compression feels too harsh and it seems that’s not an uncommon complaint.

    I’m tempted by either a Fast or Avalanche cartridge upgrade. However, the whole point of a shim stack is the ease of tuning.

    Has anyone tried playing with the shim stack rather than just dropping in a new cartridge?

    I’m guessing it will take about an hour each time I strip the charger damper down and fiddle with shims so it’s weighing that up against the high cost of upgrades.

    Premier Icon Onzadog
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    Any fettlers in this evening?

    deanfbm
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    No harm in giving it a go.

    Got to start somewhere

    Fiddle then hook it up to a shock wiz.
    You’d get some good usable feedback

    Premier Icon BlobOnAStick
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    I done it on a Marzocchi fork.

    The hardest bit is finding someone to sell you some shims, and you’re likely to want them ready to go before you crack open the forks. So you need to know what shims you’re likely to want and get them ordered before you start.

    Premier Icon Onzadog
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    Might be a bit fanciful, but I’m wondering if I can make a difference by shuffling around the existing shims.

    There’s a preload shim quite close to the piston. Moving that out should make a difference.

    iffoverload
    Member

    Manitou made a laymans guide for the ABS+ damper tuning kit.

    examples of XC, Trail, “jump” and linear stacks.

    relevant to all MTB shimstack tuning

    ABS+ Tuning REV 3-10-2011.pdf

    drop me a line if you cannot find a copy.

    Premier Icon Onzadog
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    Thanks guys. Bruneep, there’s similar (identical?) In the Pike manual. Both are for the rebound stack but obviously the principle is the same so may well serve as a guide for the experiments.

    Premier Icon Neb
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    Did try a modded shim stack on your pike?

    I’ve recently put a TF tuned coil in and I’ve noticed that the fork seems over damped and had come to exactly the same conclusions. I’ve thought about moving the ring shim one or two positions back from the piston face.

    Just wondering if it’s worth a go.

    Cheers

    skaifan
    Member

    Adjusting the shim stack only affects the rebound and is only worth doing if your already running your rebound very close to fully open or fully closed on the factory tune.

    Premier Icon Neb
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    Sorry, I didn’t make myself clear. I was talking about modifying the compression shim stack to reduce damping, the fork feels ok, just a bit harsh over roots / rock gardens. It doesn’t feel as supple as previous coil forks.

    I have to ditto this my forks feel the same and a remedy would be great.

    Premier Icon tillydog
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    @Neb:

    Have you read this post on MTBR? (And the whole thread).

    And the Radtechnik wiki page? (In German, but should be able to make sense of it with google translate and a bit of patience.)

    Doesn’t seem to be much feedback on the suggested improvements…

    Premier Icon Neb
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    I’ve read all the threads above thanks, I think I’ll try dropping the ring shim down a few steps first, then if that doesn’t work removing it all together but stiffening the stack with a larger clamp shim.

    I was thinking about getting and Andreani piston, but it’s £190 delivered from Italy which seems a bit steep! I just had the thought of whether it’s possible to reverse engineer it and copy it with a 3d printer…

    leebaxter
    Member

    What about some thinner oil?.

    It would be under damped, rebound too fast and compression too soft but with the same characteristics.

    Premier Icon tillydog
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    I think I’ll try dropping the ring shim down a few steps

    That seems to be the common-sense conclusion that drops out of the MTBR thread, but if you feel that the whole fork is over damped, then thinner oil might be worth a try.

    Premier Icon Neb
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    I’m tempted to try a stack similar to the boxxer setup, a linear stack. I don’t often use the rtc3 knob, but I guess the switch will kinda still work with extra preload the lock setting adds.

    I feel I need less LSC but slightly more HSC as I’m using slightly more travel than I’d expect.

    Am I right in thinking that going from an 8mm to a 10mm clamp on an 18mm straight stack, adds about 20% onto the stack?

    Premier Icon tillydog
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    I feel I need less LSC but slightly more HSC as I’m using slightly more travel than I’d expect.

    …together with…

    harsh over roots / rock gardens.

    … would seem to me to imply that *more* LSC and *less* HSC would be the way to go (ignoring all other possible causes / cures for a moment).

    It seems to me that the gist of the complaints on MTBR is that the preload on the compression damper is too high, making it difficult to dial in any compression damping without also adding harshness. Most of the mods discussed seem to be directed at reducing the preload (by moving the ring / 16mm shim up the stack) and/or softening the stack (by moving one or more 18×0.2mm shims to a non functioning position).

    The Boxer compression stacks in the manual are much softer than the Pike stack on MTBR, buth due to less preload distance and thinner shims. Gut reaction is that stiffening the Pike compression damper is the last thing you would want to do…?

    As an aside, I wonder if a Shockwiz would give any useful information?

    Premier Icon Neb
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    As an aside, I wonder if a Shockwiz would give any useful information?

    Unfortunately (or fortunately!) I’ve converted to a coil, which feels better for mid stroke support, it actually feels a touch harsh. My thinking is that an air fork probably needs a touch more lsc damping and can get away with less HSC due to the progressive spring (that can be made more progressive with tokens if bottom out is an issue)

    Premier Icon Neb
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    I dropped the ring shim two positions way from the piston face, so now only preloaded by 2×0.15 shims instead of the 4 shims previously. It’s made a noticeable difference on the open setting, clearly the range of adjustment isn’t as wide (but that is fine). I was a bit worried that the pedal position wouldn’t have enough damping, but that is good too, a little softer which is much better for the riding I do. The lock position is only a little softer, I guess due to the extra spring that preloads the stack.

    It has made a difference, but I guess the fact I’ve changed the damping oil, the lube oil and re-greased the seals would also have made a positive change regardless.

    Premier Icon Onzadog
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    Good feedback. I still need to get around to pulling mine apart for a tweak. I never use the pedal or lock position so no worries there.

    Premier Icon tillydog
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    The harshness* that I had attributed to too much HSC on my Pikes has pretty much gone after changing to the light *rebound* tune.

    140mm RCT3 Pikes with 2 tokens / ~80kg kitted up / Mostly wheels on the ground C-Y-B type riding.

    Previous best settings (Medium tune, as supplied): 50PSI in the air spring which gave ~25% sag (had previously tried higher pressure, but forks felt more harsh), Rebound 3 clicks from open, LSC 5 clicks from open. Didn’t think I needed the light rebound tune because there was definitely too little rebound if I opened the adjuster another click. LSC made harshness a little worse, but needed to reduce dive.

    I had the lowers off for a service and thought I might as well try the light rebound tune to see what difference it made.

    I’ve been running the light rebound tune for just over a month now, and it just seems to make the fork work properly for me. The harshness has gone, and I can run less sag / more air pressure so the fork feels more responsive and there’s still plenty of rebound damping available if required.

    Current settings (light tune, per service manual): Air increased to 60PSI as the fork felt wallowy with 50 after the shim stack change. Rebound is now 8 clicks from open / 12 from closed, still with 5 clicks from open LSC.

    I still think there is room for fine tuning these.

    YMMV.

    *Only on ‘washboard’ type surfaces – usually something like a well trafficked landrover track the front suspension would suddenly feel really harsh and the bike would actually slow down, almost if there was some sort of resonance going on (not headset bearings, wheel bearings, etc.) OK on rougher stuff.

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