[CAUT] pre-stretching new string?

[Ron Nossaman]

> Hi Ron,
> If that has been determined fairly reasonably, I am not aware of it. I, 
> personally, think soundboard rise and fall are significant factors. 
> Otherwise, I fail to see why those little heating rods and pads and 
> whatnot, installed under boards in grands, have a measurable (there's 
> that word again) impact on pitch stability. But maybe you know 
> different. Maybe it isn't "rise and fall" but some other dimensional 
> change. I await enlightenment <G>.
> Regards,
> Fred Sturm

A little trig, a little Pythagoras, and Sanderson's scaling 
formulas answer a lot of questions. Downbearing deflection 
angles in pianos are so small, that soundboard rise and fall 
would have to be enormously more than we observe in actual 
pianos, to account for observed pitch changes. Dimensional 
changes in bridge caps, moving strings up and down bridge pins 
slanted at 15-20° in opposite directions with bearing offset 
angles around 10°, produce string length changes ten times 
that of soundboard rise and fall.

Take a 406mm speaking length, with a 130mm back scale and a 1° 
downbearing angle.

Bridge cap expansion pushing strings up 0.2mm produces a 
length change of about 0.018mm (0.0007"). To produce a similar 
length change, the soundboard would have to rise 0.75mm, 
taking the bearing angle from 1° to 1.6°. I've measured over 
0.2mm cap rise on pins through an MC change from about 6% to 
about 11% or so, but I've never seen a bearing angle gain over 
1/2 a degree with humidity increase. My current conclusion, 
pending more sensible data, is that soundboard rise and fall 
is not a major contributor to pitch fluctuations, and that 
bridge cap dimensional changes are more significant.

Bridges are in the Dampp-Chaser environment as well as 
soundboards.

The cumulative string length changes from observable long term 
wood compaction of pin block, bridge capping, and soundboard 
flattening, still appear to me to be better evidence for long 
term pitch drop than speculation that string stretching, 
ignoring the observable wood damage, is to blame. Note that 
the length changes I indicated above are less than a 
thousandth of an inch, and that long term compression damage 
in bridges and pinblocks is tens of times that.

I'm not selling theories here. I don't care what the answers 
turn out to be, I just want to know what they are. Lacking 
authoritative answers, I'm trying to go where the evidence leads.
Ron N