RS Forks - Negative Air Chambers

KonaKurt
KonaKurt Posts: 720
edited May 2010 in MTB workshop & tech
Another quick question from me!

Someone told me that whenever I adjust the pressure (pump!) of my negative air chamber, I should always have the fork & bike upside down.

Is this true?? And if so, why?

KK.

Comments

  • The Northern Monkey
    The Northern Monkey Posts: 19,174
    Stops oil coming out of the valve i think.
    Santa Cruz Butcher
  • KonaKurt
    KonaKurt Posts: 720
    Hmmmm. Sounds logical to me. Thought I remember he said something about getting innaccurate pressure readings...???

    KK.
  • nicklouse
    nicklouse Posts: 50,675
    well upside down is more extreme than necessary but yes it stops the oil coming out.

    be aware that you may need to cycle the fork a few times if you get an oil lock in the compression side.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • stumpyjon
    stumpyjon Posts: 4,069
    Don't bother prersonally, I've two shock pumps so just put one on the positive and one on the negative.
    It's easier to ask for forgiveness than for permission.

    I've bought a new bike....ouch - result
    Can I buy a new bike?...No - no result
  • KonaKurt
    KonaKurt Posts: 720
    So in other words, bouncing the fork a few times is better to get an accurate negative reading, then turning fork upside down...? I imagine it's something to do with oil flow??

    I have noticed myself sometimes, when I set a pressure to the negative chamber, remove the pump, refit pump to valve, and then get a slightly HIGHER reading for negative pressure!! Yes you read correctly - a higher value, with forks right way up.

    KK
  • stumpyjon
    stumpyjon Posts: 4,069
    You should get a significantly lower reading when you reattach the pump to the negative chamber. The air volume is very small so loses a lot of pressure when it pressurises the shock pump.
    It's easier to ask for forgiveness than for permission.

    I've bought a new bike....ouch - result
    Can I buy a new bike?...No - no result
  • KonaKurt
    KonaKurt Posts: 720
    StumpyJohn, in theory I agree - negative pressure should be significantly lower when valve is detatched, which is why I usually allow for it by adding 10psi more then I want.

    But strange though it may seem, turning my forks upside down with pump attatched seems to alter the psi reading on my pump gauge.

    Oh well - close is good enough for me!

    KK.
  • a.palmer
    a.palmer Posts: 504
    You have misunderstood. You lose no pressure when you remove the pump. The loss of pressure you notice is due to air entering the pump when you reattach it.
  • nicklouse
    nicklouse Posts: 50,675
    you should NEVER add more air to make up for the loss.

    as you are not doing that the air you here is the air coming out of the pump.

    the reason the pressure is less when you attach the pump is because you have to replace that air that was "lost" from the pump.

    whey it changes when you turn the fork over. oil clogging things and causing the gauge to miss-read.

    or the pump leaking or the pump arm moving out changing the volume.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown

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