Cutting rib radii

[David Love]

Is it true that the tighter the radius the stiffer it is?  I don't think
that's true.  When I'm setting up rib deflection calculations I don't
concern myself with a change in stiffness that a tighter radius will bring,
rather it is the total amount of crown for a given length of rib and the
total amount of deflection that I consider.  A rib of given dimension will
deflect the same number of mm's for a given load, but the amount of
remaining crown will vary as a function of how much you had to begin with.
That's the reason you might consider adjusting the radius.  

As far as how important the type of curve produced is, I defer to those who
have commented that it doesn't probably make much difference.  Also, in a
compression style board, I would imagine the curve formed by the rib being
bent by panel expansion is not a perfect circular arc.  

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

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Will Truitt
Sent: Thursday, August 07, 2008 4:07 PM
To: 'Pianotech List'
Subject: RE: Cutting rib radii

David, thanks for your observation here that the arc will not bend evenly,
it will bend more in the middle and less out towards the ends, more of a
parabola.  So let me ask this question:  Might that not be a Good thing -
the tighter the radius, the stiffer it will be, and the less tight, the more
flexible; all else being equal.  The greatest need for stiffness along the
rib would be under the bridge, where the strings are pressing down.  And we
want the panel and ribs to be be more flexible out towards the rim - which
is why we thin them. I have long wondered if the responsiveness of the
soundboard system might be improved by designing a rib in a parabolic shape
to meet these dual needs.  I don't know if it would be a meaningful
improvement, one that improves it but trivially so, or does nothing at all.
Nor do I know how we would assign real values to the shaping of such a rib.

Food for thought, maybe it's junk food....

Will Truitt