Where's the engineer? - was string seating - was bridge caps

[Mike and Jane Spalding]

----- Original Message -----
From: Bill Ballard <yardbird@vermontel.net>
To: <pianotech@ptg.org>
Sent: Saturday, April 14, 2001 7:28 AM
Subject: Re: Where's the engineer? - was string seating - was bridge caps


> Pardon me for dozing in the back row through most of this excellent
> class and suddenly waking and not understanding what I have missed,
> but......

hey, Bill.  Sometimes I fear that I'll drone on about this nerdy stuff and
put everyone to sleep.   glad to have some feedback.  Good post, good
questions.

>
>
> Wouldn't the calculation of the sideways force need to include the
> speaking length?

The sideways force is determined by the string tension, and the angle that
the wire makes as it goes around the pin.  The offset divided by the
distance between pins is a good approximation of the the angle.  But any
other method for determining the angle would work just as well.



> It seems to me much easier to make a .131" deflection in the middle
> of a segment than 3/4" from one end, and that ease increasing as the
> overall distance from aggraphe/capo to rear bridgepin grows.
> If there's going to be a quiz on this, I'd like to know the right answer.
<g>
>
If there were a quiz, you'd pass, because the angle resulting from a .131"
deflection in the middle of a very long string is a smaller angle than the
angle resulting from a .131" deflection only 3/4" from the end.

> Also, correct me again if I've fallen asleep at a critical moment,
> but isn't the calculation of the force of static friction between the
> bridgepin and string simply to tell us what force would be holding
> the two together should the bridgepin "heave" and the string have no
> compelling reason to do anything other than "stick with it" and
> follow it upwards? But at the very moment when the string follows the
> pin upwards, and a gap opens up underneath the string, wouldn't the
> string, experiencing the increasing elastic forces (of an increasing
> horizontal deflection, and the arrival of a vertical deflection) much
> prefer to reduce the increasing elastic forces of its own elongation,
> by simply sliding down the pin back onto the bridge cap?. Assuming as
> we have in this case a perfectly cylindrical bridgepin.

The wire will slide down the pin (or fail to rise with it) if the downward
force can overcome the friction.  The downward force due to downbearing is
not enough.  The incline of the pin (approx. 20 degrees) also causes a
downward force on the wire, which may be enough, depending on the friction
between the pin and the string.  IMHO this is too close to call based on
calculations, as there is too much uncertainty about the actual incline of
the pin and the surface condition of the pin and string.  I think most of
our experience is that some do and some don't.   There is a limit to
theoretical analysis, and with pianos we get there pretty quickly.  While I
enjoy examining problems this way, I think there is equal or greater value
in sharing our experiences with real pianos - what did we try and how did it
work.   And of course, if theory and experience disagree, it's a warning
sign that we're missing a key factor someplace.

Mike Spalding