Falsies

[Ron Nossaman]

I know no one's posted it yet, but I expect it will be along about any 
minute now, so I'll go ahead and write this up.

As I said earlier, a 20° side bearing at 160# will produce friction between 
the string and pin that will take over 14# to overcome to move the string 
up or down the pin. That's not taking pin tilt into account because in the 
context of driving bridge pins down, the force (string hitting the bridge 
cap) is parallel to the pin as the pin is driven.

That 14#+ is pounds, not PSI. Given a bridge with a generous 1" row spacing 
between pins, and an existing string groove of a similarly generous 0.015" 
wide contact with the string, we get a cap bearing surface for each bridge 
pin of 0.5*0.015=0.0075 square inches. The load distribution is naturally 
not even across that span, but concentrated much more heavily at the bridge 
pin. Generously ignoring that altogether (though it is a very big factor), 
we find that our PSI load of the string footprint on the bridge brought on 
by driving the pin is over 14/0.0075=1866+ PSI. Rock maple's fiber stress 
proportional limit of compression cross grain is listed at 1470 PSI. Even 
with the unrealistic concessions I have given to the loading involved in 
the above example, driving the pin still exerts a PSI load on the bridge, 
by the string, which exceeds the compression limit of the maple cap. In 
reality, it FAR exceeds the compression limit of the cap.

This is why I said that I can see no positive point to driving bridge pins 
in new pianos. It's functionally no different in effect and damage incurred 
than seating strings on bridges. The only reason I see to drive pins in 
older pianos is to get a fresh pin bearing surface against the string. 
Compression damage still occurs, and if the pins bottom in the holes when 
they're driven in, they'll be back to just about where they were with the 
next round of humidity cycles.

It was once my opinion that tapping bridge pins was less destructive and of 
greater benefit than tapping strings. The math indicates to me that I was 
mistaken.
Ron N