Fork midvalve question- and other DIY suspension tuning

Discussion in 'Crazy-Awesome almost Dakar racers (950/990cc)' started by slackmeyer, Sep 5, 2011.

  1. slackmeyer

    slackmeyer Don't mean sheeit. .

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    I've been more and more disappointed with my forks lately, as I ride my 99 LC4 more and more. I put in heavier springs (that helped), for a while I ran ATF for fork oil, now I've switched back to Maxima 5w oil, and the lack of damping is really bugging me. So I'm going to try to do something about it.

    Low speed damping seems particularly lacking- when I'm in a turn on the pavement, the front end will sink by an inch or more even while I'm on the gas turning in. Small pavement seems will use up a couple inches of travel- very disturbing while cornering. I haven't been able to find any obvious problems with the parts or assembly of the forks, so I think I'll start playing with the shims.

    First- here's where I am:
    <a href="https://picasaweb.google.com/lh/photo/gM2XeHMgUmKPt7BkAPgnqu4BpFZPVMsAsIdfA-OyIm8?feat=embedwebsite"><img src="https://lh3.googleusercontent.com/-2l2ke_xrzL8/TmUwWC4lpdI/AAAAAAAAESA/D2_zTaKcHEg/s640/IMG_20110905_115110.jpg" height="478" width="640" /></a>

    If I'm measuring the midvalve float correctly, I'm getting about 1.60mm of float with the stock stack of 4 (8mm x 24mm x 0.1mm) shims. Everything I read (on thumpertalk and ktmtalk suspension forums) recommends having the float at 1mm and below. Seems like all that float means I have almost no low speed compression damping. Is there any reason not to tighten it up to .8-1.0mm?
    #1
  2. Johnf3

    Johnf3 Long timer

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    With four of the same-sized shims, it's really just a check valve. Changing the set up to a real mid valve with progressive shims and then reducing the float would help for sure.

    I'd love to see some discussion get going on valving these forks. I know there are some good tuners out there that people send these forks off to, but some DIY discussion would be cool.
    #2
  3. slackmeyer

    slackmeyer Don't mean sheeit. .

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    Same here. It seems hard to correlate from what a 525 woods bike is running to what we need, but when I compare shim stacks, it sure seem like the 950 got the short end of the stick.

    I can see that the midvalve is essentially acting as a check valve, but what still confuses me is the amount of float- more float means less oil that has to go through the base valve, especially at low speeds. I definitely want more low speed damping.
    #3
  4. slackmeyer

    slackmeyer Don't mean sheeit. .

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    #4
  5. Johnf3

    Johnf3 Long timer

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    What's the compression stack on the base valve? Do you have it apart?
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  6. slackmeyer

    slackmeyer Don't mean sheeit. .

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    Here's a google doc with all the stock shims on my bike: 950 shim chart.

    I've been studying zerodog's 640 fork mods, I'm going to start with something like his work, but with more on the compression stack. In other words, beef up the compression a little, make the rebound a bit more progressive, and change the midvalve quite a lot, by making it progressive and reducing the float to about 1 mm.
    #6
  7. slackmeyer

    slackmeyer Don't mean sheeit. .

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    Just added what I intend to do to that link. Feel free to comment- this is just a starting point for me, I don't expect it to be perfect, but I hope it's better, and hopefully I learn something from it.
    #7
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  8. Johnf3

    Johnf3 Long timer

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    I think that once you actually install a real mid mid-valve set up like you are looking at, your new base valving may be harsh. I know you are trying to firm up the low speed, but reducing the float will help with that (and actually bring the mid valve into play), and you might be able to actually go a bit softer than the stock single stage valve stack that you posted.

    I've actually been happy with slightly heavier oil and much heavier spring rates, but then again I Iike the feel of a bike with heavy springs, light valving.
    #8
  9. knobbyjoe

    knobbyjoe Adventure and dirt rider

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    slackmeyer,

    Last week I rode new friends 950 with "Super Plush" forks on a pre run of the Dust Devils Reno 200. Man! What an improvement over my 990's forks. No bottoming and super solid in the rocks and ruts.

    My bike has the settings at max and bottom out all the time in the dirt and it dances in the rocks. So I'm going to have Super Plush do mine before the KTM Rally in Tahoe.

    And Super Plush is just across the bay from you. Prices look very reasonable too.

    http://www.superplushsuspension.com/adventure.htm

    KJ
    #9
  10. slackmeyer

    slackmeyer Don't mean sheeit. .

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    Yeah, I'm actually planning to take my shock to them soon. But I'm a DIY guy at heart, and I'd really like to know what's going on in there. So I've got $40 worth of shims on the way, and maybe I'll gain some information regarding the dark art of suspension tuning.

    John: You may be right. Perhaps I'll change the mid valve and rebound stacks, and leave the base valve for later, since I can take it out and change it much easier.
    I'm running .56 springs, btw (I weigh around 160-165 all geared up to ride), and until recently I was using a heavier weight fork oil, which helped some. But I was still bothered by the fact that I could confidently ride much faster on my 640 enduro with dual sport tires than I could on my 950 with dedicated street tires- and the 640 worked better off road, too.
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  11. Johnf3

    Johnf3 Long timer

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    Yeah, it's gonna be hard to "valve away" the near 200 pound advantage the 640 has. At some point, physics won't be denied.

    Let us know what you come up with as you experiment. This will be great information for us DIY'ers.
    #11
  12. trailtrick

    trailtrick goat trail rider

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    First replace those 4x 24mm shims because are cup and not seal anymore .
    Mid valve compresion shims cup in abouth 30 to 45 hours on a 640 0 950/990 bike

    is best to run something like :

    2 24x.15
    1 22x.15
    1 20x.15
    1 16x.15
    light mid valve spring and float in between .6mm and 1.2 mm

    few thinks you want to think in consideration is
    #1 how much plushness you want to sacrificate for precision and find the balance you want mid valve/base .

    # 2 find the stacks the make the base and mid valve work in armony together

    #3 the terrain you ride , honest skills( this is like going to the doctor )so not lie to yourself here

    # 4 were you want the bike to perfom best dirt o street o compromise o both

    #5 it will take few tries to find something decent and few more to make it good and understand the changes you made , make lots o notes


    # 6 lots o guys buy DIY kits from me o other company's for reference and base line ,and by doing that cut you test and trial and error time by 1/2
    #12
  13. Johnf3

    Johnf3 Long timer

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    Thanks Javier. I was hoping you would chime in.
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  14. trailtrick

    trailtrick goat trail rider

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    for a 640 enduro is no critical run the .15 shims , for the 640 adv, 690, 950 or 990 adv o SE is the only way the run the forks for more than 50 hours with out cupping the shims

    I service mine every 7k miles now and the shims look ok but i replace theme , in my 12k miles trip in south america were abouth 70 % was dirt the shims were pretty much toast by the time i service the forks with 13.3 k miles on theme ..
    stock shims last maybe 2 k miles on a SE o ADV and maybe 4k miles on a 640 .
    iff you look at the picture on the first post here you see the were marks on the shims .
    #14
  15. slackmeyer

    slackmeyer Don't mean sheeit. .

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    Thanks a bunch Javier-

    I went down to look at the midvalve shims- I wasn't sure if any damage would be visible, but it's clear that at least one shim is pretty well cupped:

    <a href="https://picasaweb.google.com/lh/photo/uOfzjjgZdjWThMU2MyMYi-4BpFZPVMsAsIdfA-OyIm8?feat=embedwebsite"><img src="https://lh6.googleusercontent.com/-jl7hLHkNquk/Tma9Rkn1vbI/AAAAAAAAESE/Kkn36FmaQGM/s640/DSCF5474.JPG" height="481" width="640" /></a>

    I'm planning on keeping notes over a few changes.
    #15
  16. trailtrick

    trailtrick goat trail rider

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    they are all cup just the face shim is the most noticiable.
    Iff you are in bakersfield ,drive up to frazier pick up some shims, bring the mid valve holders and i machine theme for you and set you up.
    #16
  17. slackmeyer

    slackmeyer Don't mean sheeit. .

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    Wish I could, but that's a 9 hour drive for me. . . . . Guess I should order some more shims. . . .
    #17
  18. slackmeyer

    slackmeyer Don't mean sheeit. .

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    Quick update:
    Just got the forks back together today (finally), took a short ride on the street. Much improved. After some looking around though, I think I want to find some better fork oil- this stuff (Maxima 5w) feels too thin, especially when warmed up. I looked at some info on fork oil tonight, looks like I should track down some silkolene rsf or some redline suspension fluid, both much more stable.

    As far as the feel today: high speed compression seems to be about where I want it- much more controlled, but not harsh. Low speed- I could still use a bit more damping, but I'll try a different fork oil before I change shims around. I ended up with about 1.05mm of float on the midvalve, and I got the sense today that I may have been happier with something less than that, or the heavier shims that Javier recommended.

    more later. . . .
    #18
  19. Chuckracer

    Chuckracer Jerkus Maximus

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    Excellent thread - thanks for taking us along. I'm learning...that I don't know crap!


    But that's good. That's why I'm here.
    #19
  20. slackmeyer

    slackmeyer Don't mean sheeit. .

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    Feel free to ask questions. I'm not sure that I can answer them correctly, but we'll all learn from the experience, I'm sure. I put this thing together cribbing off of other threads like this one, plus some figuring some stuff out based upon the problems I was having with stock valving. It's starting to seem a lot more straightforward to me at this point.

    Ordered some Redline medium suspension fluid today, should be in tomorrow. It's supposed to be around a 10 weight, but the key is that it's much more stable with temperature increases. My takeaway from fork oil research was to use redline or Silkolene rsf, or Mobil one ATF (if you want a heavier weight oil, it really is pretty good). Mobil1 seems to be around 15w, heavier than a person should need if the valving is doing it's job.
    #20