We are looking at one side of the wheel only. Calculate each side
    seperately.

    For a 36 spoke wheel we use 18 spokes on one side.

    All units are typically mm but can be whatever you like.

    ---------------------------------------------------

    o = offset of the flange from the center of the hub 

    Radiuses:

    h = radius of the hole circle of the hub
    r = radius of the Effective Rim (ERD / 2)
    s = radius of the spoke hole

    L = SQRT ( oo + hh + rr - 2hr cos(a) ) - s       [ general ]

    ---------------------------------------------------

    a = 360 ( crossings / spokes )                   [ degrees ]
    a = 2PI ( crossings / spokes )                   [ radians ]


    For a 36 spoke cross 3 wheel:
      a = 360 ( 3 / 18 ) = 60 degrees
    Luckily cos(60) = 1/2, so
      for a 36 spoke cross 3 the equation is:


    L = SQRT ( oo + hh + rr - hr ) - s               [ 36 spoke cross 3 ]


    Example: 

    DMR Revolver SS hub:

      h = 58 / 2   = 29
      o = 39       = 39
      s = 2.6 / 2  = 1.3


    Specialized Saturae HX22 rim:
      r = 555 / 2  = 277.5
      L = 265.8


    Mavic XM719 Disc Rim
      r = 539 / 2 = 289.5
      L = 





    L = SQRT ( oo + hh + rr - .765(hr) ) - s         [ 32 spoke cross 3 ]