stretching wire -- an anecdotal analysis

[Ron Nossaman]

> I haven't done the test but I would be willing to bet that the amount of pin
> movement (a measure of the total elongation of the string) necessary to pull
> up the 150 cent flat piano (and it's subsequent tunings to get it stable)
> would exceed the total movement of all the second piano's tunings (i.e.,
> 10-15 tunings early in its life, 1 tuning after 60 years). Why?

I'd say that's a SWAG, without anything to substantiate it. If 
I were to bet, I'd bet the total tuning pin movement of both 
pianos would be very close to the same. Again, a guess, but 
based on my current understanding (based on what little real 
information I've been able to find) that strings don't stretch 
over the long term.


> The wire that never got the early tunings seems to me to require more pin
> movement to finally get it to the point of equilibrium because it was not
> brought back up to pitch during the first cycle of relaxing. At that point
> the wire was at a lower yield strength because it had not gone through the
> cycles of work hardening, therefore it elongated more. 

It's been my experience that pretty much every new piano I've 
tuned since it's sale will need a quarter tone pitch raise at 
some time in the first two years (likely when the winter heat 
comes on some months after a mid summer tuning), and will be 
pulled up to a lesser degree with most of the next two years' 
tunings as well. It adds up to a fair amount of pin turning.


> Now if I understand what Ron N is saying, he says that once the wire is
> initially tensioned, it has yielded and will not yield any more. 

It's not me that says that. Everything even approaching a 
credible source that I've been able to find says that is the 
case. I found hundreds of references to high temperature 
creep, but nothing saying there is measurable creep at room 
temperatures and the stress levels in piano strings.


>There are
> other factors causing the pitch to drop. I presume these other factors would
> be  bridge movement, soundboard settling, wire bending around bridge pins &
> beckets, etc.

Yes, I expect so, and something we haven't discussed yet.


> It seems to me the practice of changing a broken string would tend to
> discredit this theory. A new wire is placed on an already tensioned scale.
> It won't cause any bridge movement or soundboard movement, which isolates
> deformation of the wire as being the sole cause of pitch dropping. Now when
> I put a new string on, no matter how careful I relieve all the bends around
> the pins, that string will drop in pitch in relation to the older strings
> over the course of the next few months. It will require a couple of "stops
> in the neighborhood" to bring it back up to tension and introduce stability.
> Apart from wire elongation I don't know how else to explain this phenomenon.
> And my understanding of steel loading is consistent with this paradigm.

And as Ed Foote pointed out, string a rebuilt (or new) piano 
with new soundboard, bridge caps, and pinblock, and you'll be 
chasing the tuning for months. Replace a string in an already 
stable piano, and you can get it stable in a week. Same wire. 
Why would it keep "stretching" in one instance and not in the 
other?
Ron N