Ron Nossaman wrote: > >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. Ok... so we have a .00436 inch indentation figure as our lower limit in this example then. > 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. That would depend on the point of departure being below the level of Sin 0.5 * d where d is the distance of the point from center bridge... Otherwise the bridge at that point will still be higher or as high as the bearing angle... agreed ? > 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. No no no no no... you cant use the linear sin expression to make that comparison. You need an exponential equation for that. I used the Sin function just to get to a low point for the edge relative to the bearing angle. Any indentation at this point lower then the Sin function would negate string seating as per your earlier description. As for how the string indents over the whole width of the bridge...well..one would expect the indentation as one approaches the middle of the bridge to be farther away from such a straight line... the string is going to want to try and curve its way over the bridge, not cut a linear straight line. As for your particular example...it would be nice to know what the real bearing angle was for the points you sampled... what did you measure with by the way....... hard to be all that accurate with measuring such small distances without some nice tools. I'd like to check this out a bit closer myself so if you have a neat trick for exact measurements please share it. What your measurement does show is that the indentation at the edge is well below the string bearing angle....(assumming a 1 degree total bearing angle) so your point about the uselessness of string seating holds up well here if that bearing angle is the correct one. In fact we would need a total bearing angle of about 4 degrees to account for an indentation at the edge that deep... so for regardless of the cause of the indentation... your argument regarding string seating is strengthened. > 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. I certainly see the sense in this... if the pins hold the string firmly enough down, then the same "curving around the bridge surface" indentation would be created. I have, as I said before no doubt that this is at the very least a contributing factor. My only point is that string seating does not become superfluous until the indentation at the bridge edge is below the bearing angle line... thats where I brought in the Sin function.. as it can be used to find that exact depth any given point on the bridge. > Ron N -- Richard Brekne RPT, N.P.T.F. Bergen, Norway mailto:rbrekne@broadpark.no
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