Seating strings

[Ron Nossaman]

> There is vibration from play.  I don't know how much variation in the 
> forces at the pin it yields.  I would think it would be fairly small even 
> for a hard blow.  

I really don't know either, but it is a real world dynamic 
complication to your static model.

>It might be enough to overcome the frictional forces.  I 
> suppose it depends on how close the so-called Parallel force is to the 
> Frictional force.  If it's close, then the periodic increase from the 
> vibration should be enough to overcome the frictional force and cause the 
> string to move.  If the frictional force is significantly higher, because 
> of geometry, materials, or surface condition, then the force increase from 
> the vibration wouldn't cause the string to move. I don't know what you mean 
> by movement from thermal effects.

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.


  > 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 would agree (were it offered) that the various segments of any 
given string at any given time will most likely not all be at the 
same tension, but I also think those tension differences go back and 
forth depending on the present climate conditions and the immediate 
past conditions. In other words, I would expect to find the speaking 
length segment of a given string of higher tension than the rear 
duplex of that string one day, and quite possibly lower the next. 
I'm not sure of the time scale, but I expect that everything is 
moving to some degree all the time.


>>There are two possibilities that I can think of, given positive front and 
>>overall bearing. Either the string isn't touching the cap at all, in which 
>>case a feeler gage should slide between the string and bridge cap in the 
>>middle of the section between the pin rows,
> 
> 
> Which would mean what, in your opinion?  That the forces aren't in fact 
> sufficient to keep the string seated against the bridge cap, or that there 
> hasn't been sufficient time for the vibration and thermal movement to cause 
> the string to move down against the cap?

I don't think there is ever a time in a reasonably functional piano 
when the string is not in contact with the bridge. I don't see how 
it's possible, nor any indication that it happens.



> 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. 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.


> It seems that if there were a well defined groove into which the string 
> wasn't firmly sitting (which raises the question of how it became a well 
> defined groove), then tapping it down would push it straight down into the 
> groove which would also seem to be moving it down away from the bridge pin, 
> which I wouldn't guess would clarify the tone. 

It clarifies tone by providing a temporary clamp that prevents the 
pin from flagpoling, which is the probable cause of the false beat 
and lack of clarity (a matter of degree).