Seating strings

[Phillip Ford]

Phil F writes:
>>.....I don't know what you mean by movement from thermal effects.

Ron N writes:
>Strings change length with temperature changes. I would expect a change in 
>length of the various string segments to have an affect on the friction 
>points. Another dynamic complication to the static model.

Certainly the real situation is a dynamic one and the static model is a 
(gross) simplification.


>  > They will, but not always.  I think once again it's a matter of whether or
>>not the forces are high enough to overcome the frictional forces.



>I disagree. I have no data to support it (can't be everywhere at once), 
>but I think the strings are continually rendering back and forth across 
>the bearing points (including the bridge pins) with both thermal cycling, 
>and humidity changes over longer time periods.

I'm skeptical.  There is the static friction at the bridge pins that has to 
be overcome.  If my numbers were in the ballpark that amounts to about 10 
pounds or more at each pin.  I don't think that the small changes in length 
of the string segments due to the sort of temperature ranges that a piano 
is normally exposed to would cause that much change in string tension.  So, 
I think the string will stay put at the friction points and the string 
segments will grow or shrink between them due to temperature changes.  How 
would humidity change affect the string?  As I see it the change would mean 
soundboard movement up or down, which would change string tension and would 
probably affect the speaking portion and backscale portion 
differently.  Perhaps enough to cause a 10 pound tension mismatch if the 
movement was great enough.  However, if there is some other mechanism, such 
as string vibration causing some minute movement of the string on the pin, 
this may break the static friction and cause the strings to move past the 
pins with much smaller tension mismatches.


>>If side bearing and down bearing are sufficient to keep the string 
>>against the cap, why wouldn't they also keep the string firmly down 
>>against the notch edge?
>
>Because a past high humidity cycle, or fifty, with the friction of the 
>string on the pin, has crushed the edge of the notch. The string under 
>tension will attempt to describe a straight line between support points. 
>Vertically, the support point on the bridge cap being somewhat back from 
>the edge (because the edge was crushed by the pin friction BENDING the 
>wire between the center of the bridge and the pin and putting the stress 
>on the notch edge), the string no longer touches the notch edge.

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?

>  Note that during wet cycles, few false beats are noticed, because the 
> string is once again clamped to the cap close to the pin by the expanding 
> bridge cap. The real screamers always appear in the dry cycles, where the 
> (nominally) vertically straight shot between the string support point on 
> the bridge, and the agraffe, doesn't contact the bridge cap at the pin. 
> Note that if the pin was solid in the cap, no false beat would be 
> manifest regardless of the condition of the notch edge. The pin is the 
> termination, not the notch edge, and there are plenty of pianos out there 
> with bridges notched well back of the pin that don't exhibit false beats 
> because of it. The friction numbers dictate that. It's only when the pin 
> isn't firm in the cap at the cap surface that we get these noises.

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

Phil Ford