>Good question Ron. My guess is it's a compromise. If it was a structural >thing, wouldn't the stiffness of the rim and the ribs would make the board >too stiff ( too much impedance?) around the perimeter reducing the effective >area of the board? Hi Keith, I wasn't suggesting not feathering ribs at all, for the reasons you mentioned. But why feather a load carrying beam in a way that kills so much of it's load carrying capacity and puts such tremendous strain on the feathered sections? Maybe it made sense with compression crowned boards, because the panel is the structural member in that system, rather than the ribs. In a rib crowned system, if it really is a rib crowned system, the ribs are most definitely load bearing structural members and should at least try to act accordingly. Make a typical rib out of scrap pine or something, and feather it in a more or less "conventional" manner. Then prop up the ends and load the center to bend it. Does it make a nice even curve? No, it doesn't. What parts are bent the most, and are therefore under the most strain? Right, the feathering.And not even at the ends, but somewhere between the end of the rib, and the beginning of the feathering. About the spot where the feathering takes the rib to minimum thickness, continuing to the end. That's the point taking most of the fiber abuse. Why not feather the rib in a way that the fiber strain (judged by curvature under load) is more evenly distributed along the rib? The rib can still be as flexible as you want it, but the entire rib will be working for you (like a longbow) instead of just the terribly abused ends. As many concave crowned soundboards as I find with no measurable string bearing in the killer octaves of pianos with "rib crowned" boards, I suspect that these are indeed compression supported boards with crowned ribs. Doing load analyses on these ribs, with string scale and bearing schedules taken from the piano (allowing for what was probably in the killer octave before it went flat), and considering the feathering of these ribs, I see no way those ribs are holding up even half of the string load placed on the soundboard assembly. They may be rib crowned, but they're not rib supported. And as long as the ribs are feathered that way, they won't be. Ron N
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