tapping strings

[Ron Nossaman]

>But, wait! You just said you didn't think it was physically possible for a
>string to ride up on a bridge pin if there's positive downbearing! So, if'n
>there's positive bearing, why would the string want to straighten back out?
>
>I think that, if there's not very much side bearing -- if the offset between
>front and back pins isn't very steep -- it's possible for the wire to climb
>the pin at least to a certain point and *stay* there if the instrument is
>played for extended periods. I've noticed this happening on freshly restrung
>(and brand-new) pianos as well as not-so-recently restrung ones. A very
>gentle tap will make the string pop out of the groove that's formed at the
>zenith of its travels.
>
>We're talking about tapping strings and crushing the front edge of the
>bridge -- isn't this happening, to some degree anyhow, when the piano's
>played? I think it's akin to trying to keep a piano in tune or a child
>seated for extended periods of time. It's just a fact of life that it ain't
>gonna happen. And neither will strings remain seated on the bridge. They get
>too excited and need to be re-settled.

I've gone over this a bunch of times on Pianotech. Check the archives under
"string seating" and such. Meanwhile, here's a clip from the most recent
round...

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My point here is that strings don't hang up on bridge pins in the first
place, so seating them back down isn't curing anything. Sure, they move,
ping, and seem to have been up the pin before seating, but I think that's
just because we're not looking past a basic false assumption (yet again).
Here's the scenario according to the way I see it. 

Incidentally, this does assume a reasonably well set up piano with positive
bearing at both the front and back notches and the notch edges remotely
centered on the pins - for simplicity's sake. 

During the wet cycle, as the swelling bridge is pushing the string up the
pin against down bearing, side bearing, and pin inclination, the wood
surface of the cap is crushed under the string. Since the most resistance
the string offers to being pushed up the pins happens AT the pins, rather
than in the center of the cap, the cap edges crush more than the center.
The string is no longer lying on a flat cap. This is important. In the dry
cycle, the soundboard crown is less, the bridge is shorter, so the
downbearing angle is less across the bridge. At some point, it's likely
that the string isn't touching the bridge surface at the edge of the notch
where the pin is, but it is touching a little further back on the bridge
because the edge is crushed down below where the pin inclination can force
the string under tension. The string hasn't climbed the pin. It's
horizontal termination support just no longer coincides with the notch
edge. If the pin is even a little loose at the bridge surface, it will
flagpole and produce false beats. Seating the string will knock it down on
the bridge, creating a slight negative front bearing angle between the
speaking length and the length of string that wasn't touching the bridge
prior to seating. The false beat may go away temporarily, but it isn't
fixed, and will return as the piano is played and the string tries to go
back to it's natural straight line between termination points. 

The false beat is there in the first place because the bridge pin is loose,
the bearing angle is low, and the cap is deformed so the horizontal
termination point is behind the (-20°) vertical. The notch in the pin may
eventually be a factor, but these kind of beats will often show up very
early in a piano's life before any significant wear damage accumulates on
the pins.

Seating strings won't cure any of these conditions, so why is it so
universally insisted upon? It's quick, easy, and gives the immediate
illusion that the tech is improving something. At least that's why I used
to do it before I decided it wasn't a long term fix. 
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That's the gist of it. When a tech can touch a screwdriver to the side of a
bridge pin opposite the string and make the false beat disappear as long as
a light pressure is applied (never touching the string directly), what
indication is there that seating the string will fix it? 

Ron N