Spurlock shimming method.

[Delwin D Fandrich]

----- Original Message -----
From: "Overs Pianos" <sec@overspianos.com.au>
To: <pianotech@ptg.org>
Sent: May 08, 2001 3:56 PM
Subject: Re: Re: Spurlock shimming method.


> While I agree with you Ron N, I can see where Roger is coming from
> here. There must be some minor contribution made by the crowned
> ('arched') sound board being attached to what we hope is a relatively
> rigid perimeter (rim). Del's results tell us that it must be quite
> minor, and while I agree with Del's comment about the large arc
> radius and the lack of rigidity in spruce, the arched panel must
> nonetheless help the strength of the board to some small degree once
> its glued to the inner rim.
>
> It is interesting just how flexible seemingly solid objects can be.
> Try placing a magnetic base dial guage ball-end on the bridge with
> the base attached to the adjacent plate (we adjust plate set bolts
> using a dial gauge in this position). Then lean on the edge of the
> rim and watch the dial gauge indicator move. Even with a heavy
> concert grand rim the movement will amaze you.

This is, of course, the whole point. Given a crown radius of 18 m the
difference between the length of the arc and the length of the chord of a
soundboard rib 1200 mm long is only 0.21 mm. In other words, assuming that
the rib is made of some perfectly rigid material -- obviously not wood of
any kind -- it will only have to deflect the rim by 0.21 mm to go from a
fully crowned condition to a fully flat condition. This also, of course,
assumes that the rib/soundboard panel is not going to distort or twist in
any way whatsoever. In other words, a completely impossible situation.



> Fazioli build their sound boards to a nominal crown specification of
> 9mm per meter (these are [1994] minimum figures). They expect about
> half of the down bearing to be lost when the piano is strung and at
> pitch. Assuming then that the resultant crown at pitch is about 7 mm/
> metre, it follows that there must remain some slight assistance to
> the strength of the board which derives from the remaining panel
> crown (Fazioli also shape the ribs before gluing them to board - a
> known RC sound board producer).

The key word in the above is 'slight.' This is pretty close the
specification given in my example above which works out to 10 mm per 1200
mm, or an 8.3 mm/m crown. In light of the actual numbers I would suggest
that it is so slight as to be of no consequence whatsoever in any
conceivable real-world situation. I know of no real world ribs or soundboard
panels that are this rigid or piano rims that are capable of
supporting them. Perhaps for an hour or even for a day, but for a year? I
don't think so.

The rims you are using in your piano are made by Samick. You can distort
this rim by leaning against it. How can anybody really believe there is
enough rigidity in that rim to resist that 0.21 mm expansion? And remember,
this is the most extreme deflection required to lose all crown assuming a
1200 mm rib -- a rather long rib in most pianos. And assuming a perfectly
inelastic rib and a perfectly inelastic soundboard panel. And assuming no
distortion within the panel. With most ribs and most soundboards in most
pianos the crown radius -- as installed in actual pianos and not in
somebody's dreams -- is going to be considerably less. Hence the difference
between the chord length and the arc length will also be considerably less.
Remember also that a fair number of these ribs terminate on the bellyrail,
all of which are notorious for their flexibility.

The best argument given so far discounting the whole notion of the rim
supporting crown is given above where you point out that when you lean
against the rim of the piano the crown of the board changes. And this is the
same rim that is supposed to be supporting crown by virtue of its absolute
rigidity? I don't think so. Nor do I think there is anything you can do to
the piano rim that will accomplish this. Making it out of maple won't do it.
Making it thicker within any rational limits won't do it. Adding the
Centripetal Tension Resonator won't do it. What else does anyone have in
mind? Keeping in mind, of course, that we still have to end up with a piano
folks can buy and move occasionally.

Have you ever heard the expression, "missing the forest for the trees?"
Well, this is one of those situations. We seem to want so badly to believe
something other than the plain truth of the thing that we get bogged down in
all this minutia which is of absolutely no consequence in any kind of real
world piano. In the meantime, of course, time marches on and the piano
continues to languish as a has-been instrument. Argh!

Regards,

Del