[CAUT] pre-stretching new string?

[David Love]

Not to speak for Ron but, it is the change in length that is responsible for
the change in tension.  The question is what changes.  I think Ron is
correct that the change comes from changes in not just the board, but the
bridge, pinblock, rim and bracing.  If you can control the moisture content
in all of the wood in the piano, it will stay in tune quite well.  Any
expansion or contraction in the soundboard, bridges and case will result in
a change in the speaking length, a change in tension and an accompanying
change in pitch.

David Love
davidlovepianos at comcast.net 
www.davidlovepianos.com

-----Original Message-----
From: caut-bounces at ptg.org [mailto:caut-bounces at ptg.org] On Behalf Of Scott
E. Thile
Sent: Thursday, June 07, 2007 9:44 AM
To: 'College and University Technicians'
Subject: Re: [CAUT] pre-stretching new string?

Hello Ron and list,

Very interesting ideas in terms of string length, but what about changes in
tension? 

I've always assumed that the main thing going on with seasonal pitch changes
was a result of adding and reducing tension due to the rise and fall of the
board as it reacts to changes in it's moisture content. 

I assumed (apparently correctly) that any string length changes would be
negligible.

Scott

> 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