Seating strings

[Cy Shuster]

Geez, Ron, if you're going to blue-sky some crazy wild-ass idea, at least 
put together some semblance of a cogent argument, with at least a touch of 
logic and some teeth in it.... :-)  :-)  :-)

--Cy--

----- Original Message ----- 
From: "Ron Nossaman" <rnossaman@cox.net>
To: "Phillip Ford" <fordpiano@earthlink.net>; "Pianotech" 
<pianotech@ptg.org>
Sent: Friday, April 15, 2005 7:29 PM
Subject: Re: Seating strings



> I'm not sure I understand the mechanism that you're describing here.  Are 
> you talking about the bridge cap trying to move up on a wet cycle, but the 
> pin holds the string in place, crushing it into the cap?

Exactly. I've directly measured an 0.011" difference in pin height
above the cap surface in a bridge model I cycled through a couple of
cycles of from 4% - 12% MC. The force required to push the string up
the divergently slanted pins is half again that required to render a
string straight past a pin. That's a heck of a PSI load on a maybe
0.010" wide groove in a maple cap. Take a pin out of the speaking
side of an old bridge. Hold a short length of straightened piano
wire in the existing string groove with a screwdriver of knife blade
at a right angle to the wire. hold down in the middle of the bridge
first and note that the wire is pretty much parallel to the bridge
top. Now hold it closer to the notch edge, and closer, and at the
bridge pin hole and watch the tangent angle of that groove go to
maybe 15° at the notch edge. Now imagine a wire under tension
resting on the center of the bridge, and going down at a generous 1°
angle to the agraffe. It will pass the pin hole some distance above
the cap. The string has NOT climbed the pin. The notch edge simply
doesn't reach the strings natural path any more because it has been
crushed too low to make contact unless the bridge is in the
expansion cycle. Forcing it down to contact by tapping the string or
driving the pin has no chance of keeping it down there. It will once
again seek it's straight line path and lose contact with the notch
edge until the next expansion cycle.


> Any ideas about why a flagpoling bridge pin causes false beats?
>
> Phil Ford

Yes. The pin has nearly no stress on it below the surface of the
bridge, so it hasn't greatly deformed the wood that surrounds it. At
the cap surface, the side stress of the cap pushing the string up a
slanted pin (combined with the normal side bearing) pushes the pin
against the side of the hole. The deformation the pin makes in the
hole is like the deformation the string makes in the bridge top. It
curves, because the pin is slightly sprung by the forces involved
just like the string is, only less because the pin is stiffer. So
here's a pin, tight in the bottom of the hole, and looser at the
top. At some point below the surface of the cap is the place where
the back side (away from the string) of the pin parts company with
the cap and is free to flagpole. It becomes a spring. The pin is
still the string's vertical termination point, just like it always
was if the bridge was notched deeply enough at manufacture. But the
horizontal termination is a spring with a more solid termination
somewhat beyond the spring on the bridge surface. The beat is the
difference between the vertical excursion frequency and the
horizontal excursion frequency of the string caused by the lossy
horizontal termination. Tapping the string or driving the pin clamps
the string down to the cap closer to the pin , courtesy of friction
between string and pin and string and cap, and restricts horizontal
movement of the springy pin. This makes the horizontal and vertical
excursion frequencies close enough to the same to kill, or minimize
the beat. If the friction between string and pin isn't sufficient to
hold the string tightly against the notch edge, or if the pin or
notch edge is too damaged to provide the necessary clamp, it doesn't
"fix" the false beat. The unfocused sound that seating clears up is
the string grazing, but not clearing the cap enough to produce a
clear beat. Seating these, especially by driving the pins, should
produce really nice false beats soon enough. In all these cases, the
mechanism that produces the beat is the bridge pin that is not solid
in the cap at the surface and can flagpole. It doesn't take much. I
think we see this only in the top two sections because the
differences in the two apparent speaking lengths the flagpoling pin
provides quits producing clear beats as that difference becomes a
small enough percentage of the speaking length and resulting
frequency. The longer strings absorb the difference better, like
they will in unison tuning.

I need an animation. This would be so much simpler and more obvious
to watch than to describe.
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