Fork oil strangeness

Discussion in 'Thumpers' started by johan, Dec 23, 2006.

  1. johan

    johan Been here awhile

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    Hi,

    Decided to give my bike an early christmas present and change the fork oil.

    Shouldn't be too hard even for me, but I am confused now. (I've read the posts in the LC4 thread)

    After removing the springs I pumped and drained approx >400 ml from each leg. I then added the spec 420ml to each leg, but this is where I got stuck. In one of the legs, no oil seem to be visible, no matter how I pump (it comes up through (24) in http://www.ktminfo.com/partscat/Parts.asp?Page=03582013&Cat=320899&CatID=556 , but then flows back and dissapears)

    Just go and buy some more oil to add, or is something fubar with that fork?

    /johan
    oh, and merry christmas to y'all
    #1
  2. laramie LC4

    laramie LC4 crash test dummy!

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    add more oil and keep pumping till it stays. then set your air chamber height to what you want. stock is 150mm (15 cm) from the top of the outer tube with the springs removed.

    good luck,

    laramie :beer
    #2
  3. creeper

    creeper Still alive...

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    What Laramie said about the oil level from the top of the tube... that's all that matters.

    You can vary the "air chamber" capacity by changing the oil level. A higher oil level, say 110mm from the top of the tube (the minimum air chamber volume for a 4860 MXMA WP fork) will make the fork firmer, especially in the last 1/2-2/3 of travel... inversely, a lower level such as 150mm (the maximum air chamber volume for a 4860 MXMA WP fork) will make the suspension firmer, again in the last 1/2-2/3 of travel.

    You always start out with atmospheric pressure in the air chamber of 14.7 psi (at sea level), but changing the volume of the chamber will determine how quickly the rate of pressure rises.

    As an example...

    Suspension ‘A’ has a 100mm air chamber with atmospheric pressure. We compress the suspension 75mm, reducing the air chamber to 25mm. The pressure of the air is now 58.8 PSI

    Suspension ‘B’ has a 150mm air chamber with atmospheric pressure. We compress the suspension 75mm, reducing the air chamber to 75mm. The pressure of the air is now 29.4 PSI

    For a given amount of travel, suspension ‘A’ pressure numbers demonstrate that, although it starts out with the same pressure as suspension ‘B’, the rate that it increases will be faster and the end pressure higher due to the smaller starting volume.

    C
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  4. itsatdm

    itsatdm Long timer

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    A related "what if question" I have Kayaba USD forks with a threaded air bleedoff screw. What affect would there be if the screw was retightened while the fork was compressed?
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  5. creeper

    creeper Still alive...

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    :lol3 "What if" questions are the best kind of question... and frequently how people come up with spiffy new ideas and ways to do things.

    Much like a fork builds positive pressure from use, a temporary negative pressure (vacuum) would be created.
    Either would need to be "released" to reestablish atmospheric pressure... which is the pressure value manufacturers consider when establishing air chamber volume baselines and range of adjustment.

    Air volume/pressure has a substantial effect on fork function... the reason why fine tuning the stock chamber volume is a worthwhile endeavor, and why add-ons like fork subtanks are becoming increasingly popular with multi-purpose bike riders.

    C
    #5
  6. itsatdm

    itsatdm Long timer

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    SOo theoretically, you could temporarily soften the fork, but mostly in the last 1/2-2/3 of the stroke?
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  7. creeper

    creeper Still alive...

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    I wouldn't swear by it, but yes... or possibly slow down the rebound. Hard to say as I've never thought about it before until you posed the question.
    With enough vacuum, you'd shorten the fork travel... but I suspect that atmospheric pressure would sneak in past the seals eventually.

    Try it and let us know what happens. :wink:
    #7
  8. johan

    johan Been here awhile

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    OK, added some more oil, but still a no show after some pumping.

    Is there something to think about concerning the rod and the needle. Should the needle be in or out? Is it super crucial to keep the hydrualic stop (24) and the outer on the same level when pumping?

    I am not familiar with the inner workings of the fork, so it is hard for me to mentally see what could happen with the oil, so sorry for being a wuss about this
    #8
  9. johan

    johan Been here awhile

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    I forgot: Is it supposed to flow through (24) when pumping?
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  10. creeper

    creeper Still alive...

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    PM me your e-mail addy and I'll send you a PDF rebuild manual for your fork. It's about 50-60 pages so it will go thru in one lump.

    C
    #10
  11. johan

    johan Been here awhile

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    Thanks Creeper, got the manual. Seem to have sorted out the issues now (no idea what went wrong yesterday)

    It was just one of those days, planned to take a long ride through the gravel roads to my parents place via gravel roads approx 150km. Net result

    1. Drops the bike after one minut when dismounting for fuel, broken mirror and looking like a complete ass

    2. Notice I have almost no rear brake after 100 meters. Emergency bleeding and I have some braking at least

    3. Gravel roads turns out to be ice roads. No fun with knobbies

    4. Gets lost due to last minute rerouting and missing maps

    5. Drops the map after finding my way back to the planned track

    6. It gets dark, have to slab it the remaining part. 6 hours to go what normally is 150km (but at least 120km was through the woods)

    7. Change fork oil, runs into problem, get the manuals, it works!...

    8. until I drop the fork and spill the oil on the garage floor.

    9. Notice that the brake light is on without any breaks applied. It must be related to my previous work on the adjustment of the rear break when I changed to TEch 6 boots a couple of days ago ( or the bleedling). Tries to find the problem for an eternity. Turns out it is the switch *front* brake lever. WTF:puke1



    Ahh, merry freakin christmas :D
    #11
  12. laramie LC4

    laramie LC4 crash test dummy!

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    that makes me smile :lol3 :lol3 :lol3

    sounds like a few of my adventures....

    glad ya got the forks figured out. they can be kinda confusing at first but after you do it once its no big deal and the next time it will be easy.

    laters,

    laramie :beer
    #12
  13. meat popsicle

    meat popsicle Ignostic

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    After looking at creeper's post (again) I wanted to see the influence of air chamber through the fork's travel so I made a graph:

    [​IMG]

    Mid-stroke there is about 50% more pressure, but at full compression there is 100% more - not linear - interesting but probably only useful to a tuner... :scratch
    #13
  14. Luke

    Luke GPoET&P

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    The problem is you're starting from the end. The oil level is measured with the forks fully compressed and the spring removed, however the actual air chamber is created with the spring and spring guide installed and the forks at full extension. In your/creeper's analysis, you're starting from full compression and compressing even more, which isn't possible.

    If the oil level is set to 100mm, the initial height of the air chamber is 300mm. That's 100mm + 275mm (suspension travel) - 75mm (spring and guide volume). At full compression, the air height is 25mm. So the air pressure is now 14.7 * (300/25 - 1) or 161 psi.

    Repeat for 150mm oil height and you get 75mm compressed, 350mm extended for a compressed pressure of 53 psi.

    These numbers aren't quite right because they ignore the volume of the damper rod and the gap between the fork lowers and uppers.

    But, to take these numbers further....
    The area of the lower fork tube is 2.8 square inches, so the force on the wheel from 161psi is 450# in each leg. In comparison, the force from the spring is 319# (.5kg/mm rate, 15mm preload) With the stock oil level of 150mm, the force is 151#
    #14
  15. Dotbond

    Dotbond Africa, Africa

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    Rocket scientist eh??? :D
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  16. NSFW

    NSFW basecamp4adv

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    I'm glad someone started this thread. I will do mine soon since my left fork has been leaking.
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  17. meat popsicle

    meat popsicle Ignostic

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    So the difference is not 2x - it's 3x more pressure with the highest recommended oil level (100mm) vs. the lowest (150mm), although my manual says the highest recommended level is a bit lower at 110mm.

    Thanks for the additional information Luke. Where did you get the numbers?
    #17
  18. Luke

    Luke GPoET&P

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    Don't take my results as exact. As I mentioned, there were a couple of things that I ignored. I mainly did it to show the method used to figure out the pressure.

    I figured the spring volume from measurements I made the last time I opened the forks. There had been a couple of discussions on spring-cutting here and I was wondering about coil binding, so I measured the wire diameter and the coil gaps. I used those numbers (and spring length, ID and OD) to calculate the volume. The volume of the spring guide was a guess from what I remembered of its size.

    Next time I have my forks open I'll do enough measurements to get more accurate numbers. If anyone else knows the ID of the fork upper, OD of the spring guide or depth of the fork cap, post up.

    Really, the best way to do this is to slap one on a suspension dyno, but they're a bit beyond an amateur's reach.
    #18
  19. Luke

    Luke GPoET&P

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    Electrical Engineer

    :hide
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  20. meat popsicle

    meat popsicle Ignostic

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    I was interested in the relationship between pressure and oil height through the fork's travel so "exact" does not help me much. Although I am not trying to become a suspension tuner I thought it best to at least understand this relationship so that I don't consider it linear and over adjust the air spring during my own hack-tuning sessions.

    A little (change) goes a long way eh? It may also help me understand the effect of fork oil height changes on different parts of the stroke as well as fork handling in specific situations. But I am day dreaming now...
    #20