string seating - was bridge caps

[Michael Jorgensen]

Ron,
     I'm confused.  When the bridge/board expands/contracts thereby changing
the string tension, do the strings stay in the same spot or do they slide over
the bridge?  If they slide, which way?   If the kink slides toward the middle
of the bridge, it would derail the string from the groove in the cap, hence
appearing to "climb the pin."  Sliding the kink further into the speaking
length would also increase false beats.  Could seating simply reposition the
kink to its' desired location creating a straighter string and hence the usual
pitch drop?  Has anyone ever measured and documented exactly how much and
which way(s) a bridge moves during weather changes relative to the plate and
strings?
-Mike

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