[CAUT] Wire Stretch

[Andrew Anderson]

Ric,
Could string seating procedures also overcome friction at the bridge 
and move tension back behind the bridge?  I done it and heard "ticks" 
as the string let go of its oxidised bonds and passed over the 
bridge.  I've assumed that (in humid, rusty climates) the area behind 
the bridge can become a well of instability when pianos are tuned 
annually or less and these bonds can form.

Andrew Anderson

At 06:21 AM 4/29/2007, you wrote:
>Hei Tim
>
>I agree this is an interesting question, and also was on my mind 
>when I wrote my bit about vertical displacement of the 
>string.  Seems to me that there are some self conflicting ideas 
>being tossed around by some folks.... and I've yet to get a clear 
>idea of what is meant or not in some instances.
>
>But this much seems clear to me because of my own looking into 
>things.  Vertical displacement of the string in degrees enough to 
>cause significant pitch change form all but the highest part of the 
>scale  has got to be nearly negligible as a factor in all this.
>
>On the other hand, the idea that a strings length changes by virture 
>of a change in offset angle through the bridge pins resulting from 
>climatic effects is interesting tho. And these length changes can 
>certainly account for significant pitch changes.  But the problem I 
>have with this is that as explained it precepts the strings moving 
>up and down the bridge pins in response to the bridge surface 
>pushing up and retreating from the strings due to climatic 
>changes.  Three things bother me about that :
>
>1: This comes from the same corner that has also previoiusly 
>demanded that strings do not move up and down the bridge pins in the 
>first place. That the friction of the pins will instead cause 
>crushing of the bridge cap.
>2: The amount of height change on the bridge pins needed to cause 
>the pitch change also changes downbearing pressure to a point where 
>either it pushes the limit of what a soundboard can actually hold up 
>thereby probably causing the panel to flaten a bit in response.
>3: The result of a bridge surface response to climate would be quite 
>even growth or shrinkage.. i.e. an even change in the offset angle 
>and and even increase in tension of the strings which will be 
>directly related to their length.  In other words we'd see a very 
>evenly graduated curve of increase/decrease in pitch relating to the 
>strings length.   And we dont see anything like this in the real world.
>
>Still.. One DOES notice that seating strings results often enough in 
>very significant pitch drops... which could very easily be explained 
>by the resultant decrease in string offset angle through the bridge 
>pins.  I like the idea myself... but it does have some head 
>scratching bits about it.
>
>As far as the surface of the bridge crushing..  This gets even more 
>complicated because this doenst happen evenly over the length of the 
>string on the bridge suface.  At the edges, assuming the indentation 
>was deep enough to be significant... the middle would still be 
>higher. Strikes me that tho on the one hand you would be decreasing 
>offset angle by forceing the string into the indentation, you would 
>on the other hand be changing the distance the string has to travel 
>over the surface of the bridge by way of how well the strings 
>horizontal path conforms to its theoretical line of deflection.
>
>Cheers
>RicB
>
>I think Jeff Tanner asked a question that was interesting.  There 
>was a lot of discussion about crushed bridge surface causing the 
>pitch to  drop, but then the thread changed to wire stretch causing 
>the pitch  to drop.  I thought his question was: which is it?   I 
>don't believe  I've seen anyone answer his question yet.
>
>
>Tim Coates
>
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