string seating - was bridge caps

[Ron Nossaman]

>I made this point and today decided to pull out the old calculator and at
least
>contribute with this one factor...Given a 1 degree string downbearing... equal
>bearing front and back of the bridge and a say... half inch distance from the
>middle of the bridge to the edge one should expect that the string would
indent
>the bridge such that at that edge the indentation should be sin 1 * 0.5.... I
>would think. That would give a result of about =.0087 inch... a little under 2
>hundreths of a millimeter.

That one degree overall bearing would be the total of two half degree
angles in this example, so your figure should be half of that. Also, the
groove isn't a straight line, but curved across the bridge top, so the
actual point of departure (tangent?) of the string from the bridge top
isn't in the center. Lay a thin straightedge in the groove and rock it from
level until contact is made at the notch edge. Observe the angle difference
from front to back. I just did some measurements on an old Mason & Hamlin
bridge, at the low tenor where the bearing would have been fairly light. I
measured overall height at the notch edge, 1/4 distance between pin rows,
and 1/2 distance between pin rows. Then I took the same measurements in the
same spots with a straightened 0.025" (smaller in diameter than the string
that made the groove) length of music wire in the string groove, and
subtracted for groove depth. Averaging a half dozen trials, I find that the
half way and quarter way grooves are very close to the same depth at around
0.010". The groove at the edge, however, is over 0.019". I have done this
on several other bridges and gotten similar figures. That's a difference of
0.009, where the figure sin(0.5)*0.25 should be more like 0.0022. 

This same situation is visible all through both the bass and treble
bridges. The relative uniformity of the damage at all points in the scale
lead me to conclude that this isn't deformation from bearing angle, since
these angles change considerably across the scale.  


Ron N