[CAUT] Wire Stretch

[RicB]

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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