Bridge caps

[Ron Nossaman]

>Terrific! Another field of study.

You didn't want to run out did you?


> But do you actually think that the cap 
>itself is "sliding" up and down the pin? 

Yes, absolutely.


>You measured, as I recall 80 
>thousandths difference or in the vicinity; couldn't this as well be wood 
>fiber expansion instead of, well wait a minute, maybe this six of one and 
>half a dozen of the other. I just have a hard time picturing the CAP rising 
>and subsiding like that. Are we talking about different coefficients of 
>expansion between same wood/different grain orientation? 

Not just the cap, the whole bridge is getting taller and shorter and the
pin isn't. Look at what we have. The overall bridge height is 1.091 when
it's dry, and the pin extends 0.126 above the surface. These are all
vertical measurements. What's the total height of a 1"pin at 20°
inclination, somewhere in the ragged vicinity of .020 shorter? That would
leave the bottom of the pin about 0.980-0.126= 0.845, or 77.45% down into
the bridge. The bridge expands to 1.119 tall, or 102.566% of it's dry
height, leaving 0.117 pin extended. The pin now extends 0.98-0.117= 0.863,
or 77.12% down into the bridge. It doesn't come out exactly because the
body and cap are expanding at slightly different rates, and the bottom of
the pin is the point, with the zero movement point just above that, which I
didn't allow for, but this should be a pretty good indication that the
bottom of the pin doesn't move in the bridge, while the bridge expands and
contracts relative to the pin bottom making the top of the bridge slide up
and down the pin, carrying the string along with it.     



>But, yes, I have 
>noticed the notching on the pin and have attributed it more to string 
>movement than pin movement or cap movement. 

Probably some of both.


>HMMMM. Truly nanopinology.
>PR-J 

Yep.

Ron N