Fork subtanks... are they for you?

Discussion in 'Thumpers' started by creeper, Aug 27, 2006.

  1. sixer

    sixer I suffer from Ainrofilac

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    Woah very trick setup. I'll be looking into these soon. I honestly was getting ready to cringe before I saw the price but then actually saw it and sir I will be ordering a set when I get things worked out. :clap
    #41
  2. Zerodog

    Zerodog Long timer

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    Thanks for the compliments Creep. I am glad you are liking the system on your bike. I need to just set aside some cash and get you to make me a seat this winter.
    #42
  3. creeper

    creeper Still alive...

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    Well, this is my perspective... there are lots of subtanks on the market, and they all serve a similar purpose, but you're the only company that has addressed the 'simpler' needs of the DS rider, or casual off-road rider that doesn't want to screw around with a variable screw.

    Between your excellent pricing, ball valve control and the fact that your a member in good standing of ADVrider and KTMTalk who's here to participate instead of just sell stuff, I can't imagine any DS rider in these forums choosing another brand of subtanks.

    I may sound like a Infinity Machine 'promotional spot', but that's my take on things.

    C
    #43
  4. dagwood

    dagwood Banned

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    you were in my hood and didn't call? you're dead to me:fitz
    #44
  5. meat popsicle

    meat popsicle Ignostic

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    Paging Matey Peeps to the White Courtesy Phone.
    #45
  6. creeper

    creeper Still alive...

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    U'huh... tell me something I don't know. :wink:

    U'huh... ... ... I got nuthin'. :dunno
    #46
  7. meat popsicle

    meat popsicle Ignostic

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    OK, working on it.

    [​IMG]
    #47
  8. creeper

    creeper Still alive...

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    Underwriters Laboratories? Keep working.....
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  9. Jan from Finland

    Jan from Finland Been here awhile

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    Hi Creeper,

    I wouldn’t like to argue with you, but I have to ask. Why the fork subtanks work? All the suspension graphs I have seen show that the air gap doesn’t have much effect in the early stage of fork travel. The effect increases only as the forks compress. The effect is about the same as you had progressive springs.

    So, to get more compliant suspension in early stroke, you would still have to adjust your basic (high speed) damping settings to soft. Right? If you adjust your oil level in that case such a way that you get a nice progressive damping when subtanks are open, what happens when you close the valve? Your end stroke would be even more progressive, but the early stroke would be softer than you original setting. As you likely know, you can’t adjust the high speed damping by the external adjusters on the fork. They mainly adjust the slow speed damping. So your “hard” setting (subtanks closed) would have softer early stroke than normal and therefore you suspension travel would effectively be shorter.

    Is that what you want? I can’t see the way, how you would get the best of both worlds with subtanks. Am I missing something?
    #49
  10. Zerodog

    Zerodog Long timer

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    Here is the theory page from my site. This should explain why subtanks work.
    http://www.infinitymachineanddesign.com/Theory.htm

    The added benifit for a Dual sport is exactly what you have said but the other way around. If you adjust you oil level and valving so you have a perfect ride on the road you will have a punishing ride offroad. Opening the tanks in effect drops your oil level. This makes your forks softer and more compliant for a better offroad ride. Or you could have it the other way around depending on your setup.
    #50
  11. creeper

    creeper Still alive...

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    Curious... Did you read my first post Jan, with the explanation of how being able to juggle between two distinct air volumes is a good thing?

    Air functions as a spring. Assuming the same starting pressure (14.7 psi @ sea level), the size of the air chamber determines the air springs rate and end pressure relative to a fork travel value.
    The smaller the air chamber size, the faster the air spring rate rises and greater the end pressure is... again for a given travel.

    Add a secondary chamber with a flow control valve and you effectively increase your individual fork air chambers by the volume of the subtanks, which when open, greatly reduces the rate of rise and end pressure for a given fork travel by combining their capacity with that of the existing fork capacity.

    The idea is to start out with the correct (in my case quite firm) springs for your weight and intended 'worst case' use.
    My worst case is logging/fire roads at 80 plus mph... surprise pot holes around every tree shaded sweeper. :eek1

    When I do ride single track, I want some additional compliance and I don't want to change springs or oil levels to do it. I simply open the subtank valves and away I go... a kinder, gentler fork.

    Regarding compression and rebound damping... my damping is always set on the light side as I like a rapid response.
    Stiff springs and light damping settings are a popular set-up here in the PNW. It works well in the terrain we're likely to encounter.

    Hope that explains it... or at least explains something. :lol3

    C
    #51
  12. Jan from Finland

    Jan from Finland Been here awhile

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    OK, I probably missed the idea of air flow control. That might make the difference. I was thinking the subtanks just as a two-size air tank. Actually you are controlling the air flow and creating an air damper, not just a spring. My analysis applies only for the spring function. In that case it would effect only in the late stage of fork travel.
    #52
  13. enduro-ince

    enduro-ince dirtslave

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    I just have to say, theres no one more confused on this subject than myself!!! Time to go back and re-read....AGAIN!!!
    #53
  14. creeper

    creeper Still alive...

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    Air... who knew it could be so complicated huh? :lol3
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  15. Zerodog

    Zerodog Long timer

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    The airspring part of your fork really does effect the entire length of travel. This is why if your oil is too high your fork feels harsh on small bumps. If you don't believe it try it. Having a small air chamber/ high oil level makes the pressure ramp up faster effecting a larger area of the fork's stroke.
    #55
  16. Jan from Finland

    Jan from Finland Been here awhile

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    Yes, I did. I see the benefit in trail riding vs. motocross track. There you would need a linear vs. progressive spring rate. But I can’t see how if works in dual-sport. You would need a hard linear spring for road and a soft linear for trail. Two-size air volume doesn’t work like that.

    Yes, but you have to compress the air volume significantly, before the pressure overcomes the spring rate. That should happen somewhere around 1/3 – 1/2 of the fork’s stroke.

    I have no intentions for systematic research. I rather spend my time riding (even if my set-up might be sub-optimal) [​IMG]and just believe in what professionals are saying.

    True, but see my first comment. You can calculate the effect on spring rate easily. You just need to know the air volume on the forks and the dead air volume, which will remain at full stroke.

    Some up-side-down forks can be sensitive for the air volume (because of small dead air volume), but it still appears only when the forks are somewhat compressed. Here is more info about the air spring function. See: Ohlins manual page 8:

    http://www.ohlins.com/Portals/0/documents/manuals/07282-04A.pdf

    and 4Strokes.com KTM Tech: How to set KTM fork oil level - By Mike Rosso

    http://www.4strokes.com/tech/ktm/forkoil.asp

    And please note, I am not saying that your system doesn’t work. I just suspect that if the effect is noticeable at early in the stroke, it might have more to do with the control valve diameter and gas dynamics (= stroke speed depended air damper) than the air volume. Have you experimented with different size of tank volumes or valve diameters to determine the effect? Do you have any suspension stroke vs. force graphs for different set-ups and stroke speeds?
    #56
  17. mars

    mars Starbucks anyone?

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    I didnt get the remote tanks but I installed 52 springs on my 05 640 adventure last night and took it for a ride, wow its a whole new bike. After reading much of this thread I lowered my fork oil to 145mm and took the time to change the fluid to 7.5 weight with a quick cleaning of the parts.

    you are right about the tight stuff, its not quite as good but I punched some ruts and washouts that would bottom out the old stock setup, it just foats over them now. wheww.
    #57
  18. creeper

    creeper Still alive...

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    Yep... 52s seem to be a good rate for a full tank, 195-225 lbs, a fast pace and the occasional hidden surprises.
    Sure does make a big difference over the stock 44-46 springs... taking out all the 'wallowing' and sloppy feel in the fast, rough stuff.
    Does make things a little on the 'non-compliant' side in the slow, tight and nasties though. :nod

    I had my oil level set at 140mm and 5wt before the subtanks.
    I'm still using 5wt. but now the level is at 120mm.
    A bit firmer than your set-up with the valves closed... and most likely quite a bit softer in the tight and uglies than your 145mm set-up with the subtank valves open.

    Without the subtanks, any kind of set-up is a compromise if you want a fork that can handle fast and ugly as well as slow and ugly.
    The nice thing about the subtanks is that even though there is still no such thing as an 'optimum' set-up for all occassions, you have the option of essentially two distinct fork set-ups.

    Ah well.... if you ride in a large enough variety of conditions, most everything is some sort of compromise.
    Hell, the whole DS bike thing is some sort of compromise. :lol3
    #58
  19. mars

    mars Starbucks anyone?

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    I destroyed my original rims and now have numerous dents in my new excells, hopefully this will stop the carnage. Wish I had put the 52's in from the start, I would have saved myself a lot of grief. I now can feel the valving working like it is should. I have a 76 rear spring to replace the 71 that on it now, hopefully that will get both ends working together.
    #59
  20. creeper

    creeper Still alive...

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    What do you weigh? Your stock spring is a 71? :huh

    The stock shock spring on my '03 was a 76. I ended up with a 92 and weigh 205 dressed.
    Now that I got rid of my hard bags, I could probably drop down to an 88... but the static and laden sag set-up perfect with a 92, so I'm not too concerned.

    Oh yeah... I was ass-backwards on the fork oil. My shock is 5wt... the fork is 10wt. :tb
    #60