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Thread: Some nitrogen info on works shocks

  1. #1
    Join Date
    Apr 2002
    Location
    Nampa,Idaho
    Posts
    1,267

    Some nitrogen info on works shocks

    Here is some info on how to check your nitrogen pressure on you works shocks with cans.Pressure in Works Performance shocks.
    First of all the normal pressure that we put
    in most of our shocks is 250lb of nitrogen.
    If a shock has our small 1 1/2" diameter body which
    is typical of our twin shocks and a reservoir and
    a 5/8" shock shaft we often use 175lb of pressure and
    also on some BMW applications. It would not actually
    hurt anything to run 250lbs in these applications
    but would feel stiffer and or a little harsher.
    If you have tried to check the pressure in one of our shocks
    you have let out at least most of the pressure and will need
    to repressurize, if it was an emulsion shock and you lost a
    few drops of oil(which can make a pretty good little
    mess) that should be no problem. The shock is mostly full
    of oil but we leave 1/2" of air space in emulsion shocks.
    The pressure in an emulsion shock when filled to 250lb
    will normally drop to about 175lbs and that is its normal
    running pressure and may vari with temperature (sort of
    carbonation in reverse). A reservoir shock, floating piston
    or bladder type will pretty much hold the pressure you
    put in.
    If you are really serious and want to check the pressure,
    it is almost impossible to put any kind of pressure gauge
    on the valve and not lose the pressure. There is another
    way, the area of a 1/2" shock shaft is about 1/5 of a
    square inch (14mm 1/4 sq in, 16mm or 5/8" shaft 1/3 sq in).
    So with spring removed compress the shock just a little
    against a bathroom scale and see how hard it pushes back.
    Multiply this by 5 for 1/2" shaft and you have a good
    approximation of the pressure (40lb on scale would be
    200lb of pressure in the shock). On a 1/2" shaft
    emulsion shock 30 - 40lbs on the scale is a more or
    less fully charged shock. 1/2" with reservoir should
    show 40 - 50lbs on scale. A large body reservoir
    shock (5/8" shaft) should show about 80lbs on scale
    that would be about 240lb pressure.
    Where to get nitrogen, of course Works Performance,
    almost anyone who services shocks, most Yamaha
    dealers, and some other motorcycle shops. Many welders
    have nitrogen and can do this, you may substitute Argon
    but avoid Welding Argon if it has carbon dioxide in it.
    Last (I have never done this but) people who service
    commercial fire extingishers pressurize normally with
    nitrogen, use about this pressure, and the ones I have
    seen charge through a schrader valve (tire valve). That
    means if you can talk the guy into it, he can charge your
    shocks (not normally the fire department).
    Why don't we just fill the thing with oil and forget
    about the air space? Actually we do on some of the Softail
    Harley shocks that are pull shocks but that means when you
    pull the shaft out of the shock there becomes a blank space
    inside the shock (cavitation). On a normal push shock
    if you filled the shock all the way with oil and tried to
    compress the shock, the shock shaft would become a hydraulic
    ram and blow the shock appart in some way. Oil will not
    compress (exept on neutron stars and black holes, places I
    don't go) and the shock shaft does take up room inside the
    shock when you push it in. We must leave an air space
    and if we do not pressurize than the air space it is very
    soft easy to compress and we have to use stiffer valving
    to make up for that. Or we can pressurize and make the
    air space stiffer and valving softer which works much better
    overall. Even though the air space is small if we put in
    a bunch of pressure we are putting a bunch of gas into
    the shock and we do not want a bunch of oxygen (to help
    burn up the oil) or much moisture (to mess everything up).
    Nitrogen is clean, dry, and not expensive, and does not
    react with anything in a shock.
    What about a reservoir shocks? Proper reservoirs sperate
    the gas and oil, they normally are about 2/3 gas space
    and 1/3 oil sperated by a bladder or floating piston.
    The shock has access only to the oil and is full oil itself.
    The shock works in oil only, not an emulsion of gas and oil.
    If the reservoir is not pressurized the shock piston
    will actually push oil ahead of it into the reservoir
    when you hit a bump, no compression damping, and a
    big cavitation behind the shock piston so the shock
    just slaps back. Basically while an emultion shock
    can work kind of OK without pressure, the reservoir
    shock can NOT work at all without pressure. Now
    with your reservoir normally presserized and working
    properly when the shock compresses the amount of shaft
    that you push into the shock, that amount of oil has to
    go into the reservoir and compresses the gas space by
    that amount. The pressure pushes the oil back into
    the shock as the shock extends. Air could be substituted
    for nitrogen in an emergency but should be let out again
    and replaced with nitrogen within a few days. (truck stops
    should have better than 100lbs of air pressure available).
    People think that different gases have different expension
    rates but that is not true. All gases expand the same with
    temperature, the exception in a shock is something like
    water which can go back and forth between liquid (not a gas)
    and steam (which is a gas) and does unmentionable things to the
    pressure as it goes between the two states, a very little
    bit of moisture can mess up a shock that is being used
    hard and therefore subject to a very wide range of operating
    temperatures. Nitrogen is a very good gas for a shock but is
    not magic in any way it just does not do anything bad.

  2. #2
    Join Date
    Oct 2001
    Location
    south beloit IL
    Posts
    4,636
    great info! this belongs in the FAQ forum.
    1985 LT250R < stolen!
    2002 400EX

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