Downbearing, string angles and such: "Thump's Theorem"

[gordon stelter]

    I see it this way: The primary action conducting
string energy to the soundboard is NOT the waving of
the bridge up and down, as if it were a feather tied
to the middle of a plucked cothesline. Nope. It is the
yanking forward of the bridge's top front edge, as the
string travels to maxinmum vibrational amplitude, and
then its release to a neutral state as the string
reaches the middle of its vibratory path. Therefore,
the bridge acts as a "bell-crank", pumping the board
in and out with a twisting motion at the glue joint
under the bridge body. The primary cause of "S" curve
breakdown in boards, over time.
     The string's "backlength" must be adequately long
to allow flexibility in the fore-and-aft motion of the
bridge's top surface, and should not impede it with
any more than a minimum angle of deflection between
rear bridge pins and plate bridge. But the front
string angle should be adequately acute to insure that
the "Bell Crank" action is effected, but not so acute
as to put a severe static twisting stress into the
board, which would impede tonal production ( as it
would bring the board closer to  elastic limit ) and
cause premature board failure.
    As to height of bridge: A high bridge would 
increase the bridge's length as "lever arm"  in this
pumping motion, having the effect of increasing
impedance aginst the string vibrations, and resulting
in a tone which dies more quickly, but would provide
greater dynamic enhancement on fortissimo playing. A
short bridge, on the other hand, would draw the energy
from the strings more gently, resulting in longer
sustain, but with less dynamic variation on heavily.
    That's my general take on the situation, which I
realize does not take into account many other factors
in belly construction.
     Fire away!!!
     Thump



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