tapping strings

[Ron Nossaman]

>So, the pin stays put in the base of the bridge while the expanding wood of
>the cap (alone) advances upward, pushing the string ahead of it? I thought
>quarter-sawn wood tended to expand laterally more so than vertically. (Your
>earlier figures seem to say otherwise....) I'm gettin' quite the education,
>here! I hope the professor doesn't get impatient..... :-)
>
>R. Torrella

Ah but I didn't give you lateral dimensional changes. The whole bridge
shrinks and swells, just like any piece of wood. Flat sawn or quarter sawn,
it will still do it, it's just a matter of how much in which direction.
Bridges get wider and narrower with seasonal changes too, but so far nobody
seems concerned about that - including me. They also get longer and
shorter, which may very well be important, but that's another lifetime of
discussion. As for the vertical dimension: When the pin is initially
installed, it is already tighter at the bottom of the hole than at the top,
simply because the bottom of the hole hasn't had most of the length of a
pin driven through it. Once installed, the compression stresses imposed by
the string side bearing are vastly greater at the surface of the bridge
(where the string is), than at the bottom of the pin. As a result, over
time, the pin gets loose toward the top while remaining much tighter at the
bottom. So the pin stays put (mostly, it may migrate minutely over time) at
the bottom, and the bridge grows and shrinks around it - carrying the
string up and down the bridge pin. The one I measured showed around 0.009"
vertical travel on the pin at the cap surface. 

Ron N