Curve on Bridge Bottom

[Ron Nossaman]

>>In the context of compression crowned boards,
>>diaphragmming the panel would lessen and weaken crown, so they would try to
>>minimize that long grain loading to try to avoid pushing the top half flat
>>immediately upon assembly. I wonder if they really did do that, and how
if so,
>>much they deviated from plane.
>>
>>Ron N
>
>Ron,
>I didn't really understand this.  Why would the top half be pushed flat 
>upon assembly?
>
>Phil F

With the crown along the grain forming as the left rear and right front of
the soundboard are forced down to the rim, it's the rib crown that's
forming the long crown. As this happens, the ribs are necessarily subjected
to some additional load beyond that required to maintain crown
perpendicular to the grain. Steinway soundboards are compression crowned,
by gluing flat ribs to thoroughly dried down panels and letting the re
hydrating panel bend the ribs as it expands. The resulting PSI load on any
given panel and rib is determined by the panel's thickness. The thinner the
panel, the higher the PSI load. Thinning the panel means that there will be
less crown formed by panel expansion in the first place, and panel PSI
loads will be higher while the load carrying capacity of the assembly is
lessened. Given an identical rib set, a diaphragmed compression crowned
board won't support as much load at as high a crown as a compression
crowned board with the panel the same thickness as the thickest part of the
diaphragmed board, throughout. 

So pushing that diaphragmed panel down on the same rim as the thicker panel
will result in less crown in the diaphragmed panel. Since the panel in the
top half of the scale is thinner than the lower half, the ribs are shorter
and the crown is less, that's the likely place for the board to be pushed flat.

Yes, that would result in less long grain crown, but with the edges of the
board planed down, measuring from the top will show a contour very much
resembling a crown, whether there actually is one along the bottom of the
panel or not.

That's what I meant.

Ron N