tapping strings

[Ron Nossaman]

>
>  If I understood correctly the main cause of the string creeping up off the
> bridge top is the bridge pin climbing up out of it's hole. 


Nope. The string doesn't climb off of the bridge top, and the pin doesn't climb
out of it's hole. As the bridge swells with humidity increase, it pushes the
string up the pin against side bearing and down bearing friction, and against
the slant of the bridge pin - crushing the edge of the bridge cap at the notch
and rounding it down so the actual string termination on the bridge cap (in the
dry season when the cap has gone back down (with the string following))  is
somewhat back from the edge of the notch. This allows the loose bridge pin to
flagpole, making false beats. I don't think it's physically possible for a
string to ride up bridge pins if there is positive downbearing, a realistic
side bearing offset across the bridge, and a reasonable cant to the bridge
pins. The side bearing angle and pin cant alone will keep the string on the
bridge cap even with zero downbearing.



>
> I've seen this kind of thing in wood quite frequently. If you open up the
> wall of an older home you will almost always find the nails being somewhat
> proud of the wood. 


That's because of the heads. Without heads, the nails wouldn't walk out. Why
don't they continue to come out after they get far enough out that the
expanding wood no longer contacts the bottom of the heads in the wet cycle? Why
only slightly proud instead of lying on the ground?


>
> I believe that the same thing is happening in the bridge and the pins need
> only to be lightly tapped back in place. 


Tapping pins in does maybe three things that will affect false beats. First, it
drags the string down with it as it goes in (bending it around the crushed cap
edge to make enough contact to quiet the false beat). This is temporary. The
pin stays tapped down, but the string straightens back out and loses contact
with the cap edge again. Second, it gets the string bearing against a new
section of pin instead of the dished out flat spot it had originally worn.
Simply turning the pin with pliers will have a similar effect, though I don't
habitually do this. Driving the burnished flat spot of the pin (with it's
slightly raised edges from the burnishing) into the bridge cap will sometimes
get those raised edges in contact with the bridge cap and keep the pin from
flagpoling enough to quiet the false beat. There is a fourth result - bottoming
the pin in the hole. I see no evidence whatsoever that this makes any
difference at all. I consider tapping pins in a little to be a less destructive
and much more effective approach to quieting false beats than tapping strings.
Do it every twenty years or so if you think it needs it. Meanwhile, the
majority of false beats are caused by loose bridge pins, not string seating, so
seating strings does little to fix the problem in the long term.

So when you tap the pin in and the false beat clears up some, which of these
changes in the string termination quality made the difference?  


>
> I personally think that this phenomenon also occurs in the pin block too.
> I've seen older pianos where it seems obvious that the pins were originally
> deeper in the block. I won't ever drive them to the point where the coil is
> touching but if I find them with the coil a considerable distance from the
> plate I will drive them in some and retune. I hope this helps. 
>
> Greg 


I can't say one way or another on this one. I haven't noticed it happening.

Ron N