[CAUT] Wire Stretch

[Fred Sturm]

On 4/30/07 5:29 PM, "RicB" <ricb at pianostemmer.no> wrote:

> What is problematic about
> this is that any such change in string position height to the bridge pin
> is governed by the response of the surface of the bridge to climatic
> changes, and hence quite uniform... which in turn dictates a pitch
> change that is directly related to the length of the string. Shorter
> strings will be more effected then longer strings
Yes, and that does happen on some pianos (response to humidity change very
large in the top octave or two). Steinway and Baldwin products come to mind.
Not so on other pianos, as Yamahas for instance. I haven't a clue why there
should be that difference between makes, but I do always know to anticipate
that enormous pitch change in octaves 6 - 7 on Hamiltons and 1098s (and
Everett studios), and to a lesser extent on the grands of those makes. We're
talking within 20 cents in the rest of the piano, and 30 - 50 in that top
octave and a half.

... hence a very 
> graduated and even curve of change in pitch.... which clearly does not
> happen. We see big breaks where the bridge is doglegged which need to be
> accounted for amoung other things.
Yep, lots of strange things happen, and lots of factors are involved. I have
become more and more convinced that bridge growth and shrinkage in all
dimensions (height, width _and_ length) are quite significant, sometimes
more than soundboard. This could explain why some pianos just don't seem to
want to stabilize as much as others with a Dampp-chaser (excuse me, ahem,
Piano Lifesaver) system installed. A thought, anyway.
    So many factors, including lamination/solid (bridge and board),
compression/rib crown (and does it still have crown? Downbearing?), rib
cross section, age of wood (how hygroscopically active is it?), orientation
of grain, fineness of grain . . .  And then you add the wire factors.
    We will never really get a handle on the inter-relations of all these
factors. I'm not saying it can't be done, but nobody is going to go to that
much trouble. We might tease out a few things, though. Ron's epoxy laminated
bridge, for instance, and how much of a difference it makes.
Regards,
Fred Sturm
University of New Mexico