How do you determine the fork oil level? And other questions...

Discussion in 'The Garage' started by Skippii, Mar 30, 2010.

  1. Skippii

    Skippii Milkshakes, my lad.

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    If it says "5 inches" Does that mean with the forks compressed? With the springs in or out?
    If compressed, I'm guessing it means with the springs out, right, since otherwise it would be...impossible to check.

    Why is fork oil level so important, anyway, as long as there is sufficient oil in for the damping rods to work? I hear that half an inch difference in the level can make a bike difference in handling...in what way? Is it more or less important for damper rods or cartridge forks?
    #1
  2. mikek

    mikek Adventurer

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    normally fork oil level is measured in millimeters not inches.

    oil level is set to the specified level by removing fork springs and allowing forks to gently bottom out, maximum travel.

    typically you would overfill the forks and draw out the excess oil with a syringe/tube that is set a specific distance below the edge of the top of the fork tube, in roadracing you will see a lot of bikes running between 90-110mm fork oil level.

    sufficient oil to cover damping valves is one thing, the relationship of oil and air volume inside the sealed assembly is another IMPORTANT thing.

    more critical on cartridge forks than damper rod forks.

    HTH,
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  3. Tseta

    Tseta Lost

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    Typically the oil level is measured with springs out, forks fully compressed.

    The oil level affects the air chamber left in the fork. As you know, oil doesn't really compress (much) under pressure, but air can be compressed quite well. So, when you hit a bumb, the forks compress, and in addition to all the other suspension and damping stuff going on, the volume of air inside is compressed, acting as an additional "spring". This happens most noticeably at the end of the suspension stroke as compressing air gets progressively harder.

    The air chamber/oil level is sometimes even used as a tuning variable to control for instance bottoming resistance.

    -T
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  4. Skippii

    Skippii Milkshakes, my lad.

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    Thanks. Makes perfect sense. Don't know why I didn't think of that.
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  5. Skippii

    Skippii Milkshakes, my lad.

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    Here's a great link for those interested. Also talks about using forks with no springs at all, but pressurizing the air in the forks.
    http://www.4strokes.com/tech/ktm/forkoil.asp

    One more questions: I'm seeing specs for both oil level and air pocket height. These are the same thing, right?
    No one is measuring fork oil from the the bottom of the fork to the level, but rather from the top of the tube down to the oil level, right?
    #5
  6. Skippii

    Skippii Milkshakes, my lad.

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    Also, how important is flushing the forks? I just read to fill them with paint thinner or kerosene before filling them with oil, then draining that out. Never heard that before, but I guess it makes sense. Wouldn't mineral spirits harm the seals, though? Or maybe not for a short duration. Though kerosene would be much cheaper, anyway.
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  7. cagiva549

    cagiva549 whats a cagiva

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    Spirits or varasol is an exelent cleaning agent , if you do that on the bike pour some varasol in the fork and screw the cap back on with out the spring then agitate the hell out of it then drain . Do it again with atf to flush out the varasol then refill with your atf to the proper level . You can also tune your forks by adjusting the oil level . Thats how it was done before adjustable forks . higher oil level for stiffer forks , lower levels for softer and a half inch will make a difference . The levels usally made a difference in the maximum travel of the forks and oil thickness would effect the short travel . SEYA
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  8. Gramp-Z

    Gramp-Z Long timer Super Supporter Supporter

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    Let me add , for bikes with "anti-dive" forks , make sure all the oil is out of the valving on the side with ant dive . (speaking from experience) . The manufacturer will list different volumes for right and left fork . But if you have the oil level for compressed fork with no spring , it is much easier . And a quarter inch makes a big difference .
    #8
  9. fritzcoinc

    fritzcoinc Enjoying my last V8 Supporter

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    Here's what I use. Made it myself. Adjust the length of the tube with a scale and lock with thumb screw, set the plastic block on top of fork inner tube, draw off extra oil into resivor with hand pump. Repeat on other fork and both will have same oil height or air gap. A 10mm reduction in air gap made a big difference on my Honda XR650L. +1 above, be sure to bleed all the air out of the valving by stroking the damper rods before making finial air gap adjustment.

    [​IMG]
    #9