Belly rail crown - Why???

[Delwin D Fandrich]

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Re: Belly rail crown - Why???
  ----- Original Message -----=20
  From: Ron Overs=20
  To: Pianotech=20
  Sent: November 18, 2002 4:13 AM
  Subject: Re: Belly rail crown - Why???


  To listees concerned with sound board installation,


  Again I find myself questioning a long standing tradition in piano =
making, and why it should be so done.


  The theory of sound board curvature (whatever that might be) tells us =
that the shape of a sound board panel ideally should conform to that of =
a sector of a sphere. Since it has been decided that the sound board =
should be installed so that it experiences less stress, makers =
historically have decided that there must be a rise in the centre of the =
belly rail to accommodate the spherically shaped 'board (6mm or 1/4" =
would seem to be a typical figure).

As well you should.

In the above you bring up three assumptions I have questioned over the =
years. First, that the soundboard should be installed 'less stress.' =
Second, that there should be a rise in the center of the belly to =
accommodate the spherically shaped soundboard. And, third, that the =
shape of the soundboard should be spherical.

The whole idea of soundboard crown and its subsequent physical =
distortion by the effect of string loading is to distort -- stress -- =
the thing.=20

  I am wondering about the validity of this practice, since the board =
does not in reality assume a spherical shape. It would appear to be =
crowned more or less perpendicular to the 'line' of the long bridge, ie. =
along the line of the ribs. Furthermore, the treble corner tends to curl =
up like a potato crisp after the ribs have been fitted, which would =
cause the raised centre of the belly rail to place the glued down belly =
under even greater stress than if the belly rail was made straight.
As you say, no soundboard panel is crowned to a true spherical shape. =
More typically it will take on either a cylindrical shape or, possibly, =
a slightly conical shape.

With only one exception that I am aware of no manufacturer has shaped a =
rim to accommodate a spherical soundboard shape and that was Kimball in =
its later years. Of course by then they were using laminated soundboards =
exclusively. Some vertical piano makers do shape their soundboard liners =
to a more-or-less spherical shape but it is generally only a fraction of =
the shaping that would be required to really match the claimed shape of =
the soundboard assembly.

At best crowning the bellyrail takes care of only one relatively short =
span of the soundboards edge. What about the rest?

  Just sitting at an empty case thinking about this, I can see no =
disadvantage in setting the belly rail straight. In fact with the piano =
I am setting up at present, the cut off bar extends so far across to the =
treble end of belly rail that the 'board will not be supported by the =
belly rail until the highest treble section is reached. The cut off rail =
I am building is a continuously curved member of 30 mm depth and 40 mm =
wide. It will be glued to the back posts, and the entire non-sound board =
bass corner will be filled in with a 19 mm thick plywood piece which =
will butt up to the sound board panel over the cut off bar. This is =
intended to provide a high level of lateral support for the cut off bar =
and sound board perimeter. I can see no advantage in setting any part of =
the sound board perimeter at a height other than that of the inner rim.
It is my practice to set the rim and bellyrail, including the soundboard =
cutoff rail, flat. Since the soundboard will be stressed -- i.e., have =
considerably less crown -- once the strings go on I prefer to have a =
flat, consistent mounting surface for the parameter of the board. =
Besides, my cutoff bars tend to be so shaped that very little of the =
soundboard is actually glued to the bellyrail.

And, while we're on the subject, for the same reasons I fail to see any =
advantage to beveling the inner rim to (supposedly) accommodate the =
shape of the soundboard. It is claimed that this somehow aids the =
maintenance of crown but I surely don't see how. To illustrate simply =
clamp a piece of spruce to a bench and see how easily it is bent.

Del


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