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Thread: Open Chamber Pain

  1. #31
    KTMTalk Member wfopete's Avatar
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    Default Re: Open Chamber Pain

    By going to 160mm the only thing I can think of is that by doing so we are taking the oil out of the equation to get the forks to mechanically bottom and then work up from that point with clickers and oil level to get to that happy bottoming spot. I would suppose at that point you would need to re-visit the stacks.
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  3. #32
    Lifetime Team KTMTalk KTMLew's Avatar
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    Default Re: Open Chamber Pain

    Quote Originally Posted by wfopete View Post
    By going to 160mm the only thing I can think of is that by doing so we are taking the oil out of the equation to get the forks to mechanically bottom and then work up from that point with clickers and oil level to get to that happy bottoming spot. I would suppose at that point you would need to re-visit the stacks.
    You are dismissing the air pressure rise. Lower oil height slows/reduces the rise. That's also how subtanks work. They add air volume. Feels like lower oil level but is adjustable to make air flow speed sensitive.

    But you are correct. the oil level is easiest to try too. You can even remove the bleeder and pump some oil out into a measuring cup. Myslef I would put bike on stand, hang some weight near rear of seat so bike doesn't endo, remove bars/fork caps. Remove springs and suck out some oil. Just lowering it to 150 will make a big difference. But it also depends on how you measure it. Do you dump the oil between the tubes like WP says? Makes about 20mm difference. So your 130 might be my 110.

    Confused yet?
    Last edited by KTMLew; 10-17-2020 at 10:55 PM.

  4. #33
    KTMTalk Member wfopete's Avatar
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    Default Re: Open Chamber Pain

    Quote Originally Posted by KTMLew View Post
    Remove springs and suck out some oil. Just lowering it to 150 will make a big difference. But it also depends on how you measure it. Do you dump the oil between the tubes like WP says? Makes about 20mm difference. So your 130 might be my 110.

    Confused yet?
    Slightly, but I'm still following with your line of thought. I have a few questions. Last time I had them apart and added oil, I blocked off both the rebound adj rod and piston rod. But why would that make a difference in oil height after purging out the air? As long as you keep it out of the rebound rod and get the oil between the tubes, oil height is oil height...yes? I set oil height like in this Slaven's video which he claims is the WP method.

    https://www.youtube.com/watch?v=glG3gUjxjEU&t=45s

    Does cavitation come into play from the 160mm oil height? BTW, when I changed BV stacks I simply raised the front end off the ground so the bottom of the tubes where higher than the tops and popped the valves out/in with no oil loss/mess; big time saver.

    Click image for larger version. 

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    Last edited by wfopete; 10-18-2020 at 06:48 AM.
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  5. #34
    Lifetime Team KTMTalk Dale's Avatar
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    Default Re: Open Chamber Pain

    I found the easiest way for me to inject oil is with a helper and a lift stand. One person lifts the bike while the other squirts the oil in.
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  6. #35
    Lifetime Team KTMTalk KTMLew's Avatar
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    Default Re: Open Chamber Pain

    Quote Originally Posted by wfopete View Post
    Slightly, but I'm still following with your line of thought. I have a few questions. Last time I had them apart and added oil, I blocked off both the rebound adj rod and piston rod. But why would that make a difference in oil height after purging out the air? As long as you keep it out of the rebound rod and get the oil between the tubes, oil height is oil height...yes? I set oil height like in this Slaven's video which he claims is the WP method.

    https://www.youtube.com/watch?v=glG3gUjxjEU&t=45s

    Does cavitation come into play from the 160mm oil height? BTW, when I changed BV stacks I simply raised the front end off the ground so the bottom of the tubes where higher than the tops and popped the valves out/in with no oil loss/mess; big time saver.

    Click image for larger version. 

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    i don't do the last step where he dumps the fluid. Just lower you level by 20mm and see how it works.

  7. #36
    KTMTalk Member ij616's Avatar
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    Default Re: Open Chamber Pain

    ...also check your methodology. I found I got much more accurate level settings by letting the forks sit a few minutes after pumping the cartridge and pumping the entire fork with my hand over the tube, then measuring.

    Used to be 2 different methods, one where you didn't dump the outer chamber. I'm assuming with the 130mm height that it is a dumped measurement?
    Heck it's only been 14 years since I messed with OC forks. sorry.
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  8. #37
    Lifetime Team KTMTalk KTMLew's Avatar
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    Default Re: Open Chamber Pain

    Quote Originally Posted by ij616 View Post
    ...also check your methodology. I found I got much more accurate level settings by letting the forks sit a few minutes after pumping the cartridge and pumping the entire fork with my hand over the tube, then measuring.

    Used to be 2 different methods, one where you didn't dump the outer chamber. I'm assuming with the 130mm height that it is a dumped measurement?
    Heck it's only been 14 years since I messed with OC forks. sorry.
    Exactly. I lift the cartridge rod about an inch, fill them to the top and let them perk after a few hand sealed pumps of the tubes to purge the air. Fil, fill, fill. Let perk. Then set to 150 for off-road. Use comp clicker to control bottoming. If you get under eight clicks then either adding shims to BV or add some oil.

  9. #38
    KTMTalk Member wfopete's Avatar
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    Default Re: Open Chamber Pain



    Ok because it was EZ to do and I was curious, I set up a quite un-scientific test for the relationship between compression settings and the amount of travel used. I pulled a 8" thick 12 foot log off my wood pile and blocked it up with some rock and cinder blocks to help keep it in place. The plan was to make successive runs over it with different compression settings to see if going to a harder setting resulted in more travel used. After each series of runs at a particular setting I would turn the compression clickers in 5 turns until eventually I got to full hard compression. I made 3 runs across the log at each setting and measured the amount of travel left over after the three series of runs. All the runs were made in second gear and I tried to keep my throttle/speed consistent to keep the test somewhat legit. Here are the results:



    • 30 (full soft) - 60mm
    • 25 52mm
    • 20 43mm * note this series of runs is a throwaway (Because I did a big stoppie on the way back).
    • 15 48mm
    • 10 44mm
    • 5 25mm
    • 0 34mm (full hard)
    • 0 55mm * note on this run I had purged the air out of the forks that had built up during the pervious runs. I didn't hear much come out.


    Ok you can shoot plenty of holes in this test. The log might have moved or rolled, did I hit the log harder on some runs, was I on or off the throttle when hitting the log and on and on. I get it. BTW; when I was done with the test I had pushed the log about 15 feet from where it was initially placed; cinder blocks and all. But it was interesting to note that there was a downward trend in unused travel until I went to zero clicks out...I didn't see that coming.

    In the end I had made over twenty-five trips over an 8" log in second gear at various compression settings. So what does this mean? Hell if I know. After I made the last run at full hard compression, I turned the clickers back to full soft and made three more runs, the impact transmitted back was less. So despite the forks using LESS travel, when set to full soft/30 clicks out, the impact to the rider was less than at stiffer compression settings using MORE travel. I got a lot more comfortable riding over logs too but I do believe I will take a few Ibuprofen now.

    On second thought you know what I'm thinking about?

    Bottoming cones and drill bits.




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  10. #39
    KTMTalk Member kenpowell's Avatar
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    Default Re: Open Chamber Pain

    Only drill one side / one cone. I'll do it if you want a little help in ruining one cone. But I think it's the cure. It's interesting to note that you never did get anywhere near bottoming.
    Last edited by kenpowell; 10-19-2020 at 07:06 PM.
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  11. #40
    KTMTalk Member gmoss's Avatar
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    Default Re: Open Chamber Pain

    Imo, you shouls not get anywhere near bottoming hitting an 8" log in the first place. JMO.

    My concern here is what happens on the subsequent 4 or 5 following big roots or rocks in the trail. That is where things get interesting. I never thought, worried, or wanted to bottom out. Not sure I ever have.

    I notice the same thing with my mtn bike, go out on C and it gets a little softer, but harsher. I end up back at halfway and have better control and comfort at speed hitting stuff. Susp is like golf, often what you think you need to do is the opposite of what you should do, adjustment wise.
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