Pianotech

  • 1.  Composite downbearing measurement

    Registered Piano Technician
    Posted 01-02-2025 01:06
    How does one measure composite downbearing?

    If the deflection of speaking length and back scale is positive (i.e., towards the board and plate, respectively,) are the two angles added together? Or subtracted?

    Or, if on one side of the bridge, the string is deflected up and deflected downward on the other side, how is the composite figured?

    Or is it composite downbearing measured entirely differently?


    Joe Wiencek
    NYC


  • 2.  RE: Composite downbearing measurement

    Registered Piano Technician
    Posted 01-02-2025 08:56
    Joe,

    There are different ways to use the bubble gage.  The standard way is to start with the gage sitting on the bridge top string segment where you level the bubble. Next carefully place the gage feet on the speaking string and note which way the bubble moves, and also count the tick marks in the vial. 

    If the bubble moves toward the bridge by, say, two marks, then that is a good thing all by itself. Next move the gage to the rear segment and note the bubble move, which may remain centered, or it may move toward the bridge (positive bearing), or it may move away from the bridge (negative bearing). The final reading is the algebraic sum of the front and rear readings.

    If memory serves, each tick mark on the vial represents 0.003", which indicates that the slope of the segment is rising or declining (stair stepping) at 0.003" pre running inch of string length. So, then, say the front segment reads three positive tick marks toward the bridge, and the rear reads one negative tick mark away from the bridge, then the algebraic sum for total bearing is:

    3 plus marks for 0.009" minus 1 negative mark for - 0.003 = 0.006" total bearing.

    It is easy to get hung up on these details, however. Irrespective of rear bearing (whatever it may be), finding positive front bearing is always a good thing. My long career found me as the service mgr. for Kawai America, or else working in colleges or music stores or rebuilding shops. I have checked downbearing on dozens and dozens of pianos. Many, many times I have found great sounding pianos with positive front  bearing (most important) while the rear bearing was either flat (bubble centered) or even a tick mark negative. Don't freak out if you find this to be the case.

    There is much more to be said here, but I think that this is the basic answer to your question.
    ng



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