string seating - was bridge caps

[David Ilvedson]

Question:  Why does the pitch on the just seated string sometimes drop 5+
cents?  I don't as a matter of course seat strings but when I hear that
fuzzy, un-focused tone I know what to do...I seat with a 1/4" brass rod
drift and a light tap in front of the bridge pin in the speaking length and
watch that sucker drop...the tone always clears up and I won't have to do
it again for many years.  

David Ilvedson


*********** REPLY SEPARATOR  ***********

On 4/10/01 at 1:30 PM Ron Nossaman wrote:

>Hi Gang,
>
>Just getting back to this because my server's been down...  Again.
>
>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. If anything, seating
>strings hard across the center of the bridge to hammer the string track in
>the cap flat again (if deeper) should do a lot more long term good than
>lightly seating them at the pins. No, I haven't, and no, I don't, but yes,
>I will probably eventually try it.
>
>
>Ron N