Bridge pin angle

[Ron Nossaman]

> It could happen that way.  I suppose it depends on the strength of the cap 
> material and the size of the string.  With a pristine bridge cap made of 
> material that is not strong enough to keep the string from indenting it, as 
> the cap rises the string will crush itself into the cap, increasing the 
> contact area between string and cap and lowering the bearing stress.  If 
> the bearing stress drops below the allowable before the cap has risen all 
> the way, then for the rest of the cap's rise the string would go with 
> it.  So, the cap would be indented and the string would be pushed up a 
> bit.  One thing to note with this situation - the force required to push 
> the string up and the bearing force (the bearing area times the bearing 
> stress), have reached an equilibrium.  This means that for the cap to push 
> the string up there should be no further indentation.  So, if this string 
> was tapped down in the dry cycle, on the next wet cycle it seems to me that 
> it would be pushed up again without further indentation of the cap.

Once the cap edge is indented enough, the string won't even touch it 
at least half of the time. Losing moisture, even with the string 
following the cap down, the edge will always remain below the 
string. At equilibrium, edge contact will depend on having just gone 
through an expansion cycle or having been seated by a tech, and 
whether or not the pin clamping forces and friction are enough to 
maintain contact against the string's tendency to straighten out 
from it's own stiffness and tension. If we assume that the string 
will overcome these forces in time and take the straight tangent 
from the bridge top, which I think it does, we get this. Going from 
a dry cycle through a wet cycle, the cap surface will have to rise 
on the pin whatever the gap distance is at the notch edge before 
contact with the string is even made. So edge crushing only occurs 
when the present wet cycle expands the enough to close the gap. The 
more important aspect of this, I think, is that the increased side 
forces on the pin from the expanding bridge   cap compress the cap 
around the pin and make it loose at the top. And this happens with 
every wet cycle of whatever duration and severity.


>>I agree, and tapping in bridge pins would be even worse.
> 
> 
> Why?

Friction levels. Seating a string with, say, a fingernail will do a 
minimum of immediate damage to the cap (discounting whatever may 
happen at the next wet cycle). Your math showed over 11 lbs of 
static friction between string and pin. It seems to me that driving 
the pin would be little different than seating the string directly 
with a tap from an 11 lb hammer.

Ron N