Curve on Bridge Bottom

[Ron Nossaman]

>Yeah, that is what I mean - the bridge's orientation can get distorted. If 
>that bridge is not on the top of the arc, the orientation of the bridge top 
>would change (rotate) from when you planed it down to set downbearing, to 
>when the piano was strung - perhaps not enough to notice.

Unless the board is flat on installation and zero bearing is set, this
happens to some degree in every piano - probably even with a flat board.
The treble doesn't deflect much because the ribs are short and stiff and
the bridge is very close to the belly bar. The low tenor doesn't deflect
much because it's on a shorter rib than the mid tenor, is relatively close
to the case, off center of the rib, and relatively lightly loaded. The part
that deflects the most  is somewhere in the middle third of the bridge. If
the ends of the bridge don't go down much and the center does, because the
center isn't in line with the ends, the bridge rotates. It's just a
consequence of the bridge being curved. There is certainly some distortion
of both the bridge and panel assembly as well, no matter how it's done.



>No compass and straightedge needed for this one. Yes, of course, assuming 
>the inner rim bevel is on a horizontal plane (hope that is not tooooo big an 
>assumption). 

I've seen a few (not counting verticals) that were far from horizontal, but
I'll buy the "plane" part. I've heard a lot of talk about contouring the
rim to the soundboard curve, but I don't know how far off plane (if at all)
that this has been taken. It mostly seems to be just beveling. I never
actually took the trouble to try to measure how far off plane the rim top
was - since it wasn't likely that I'd try to change it anyway. 


>I guess that was my question: No matter what gymnastics one may 
>perform with soundboard clamps, won't everything just re-align itself once 
>you install the board? I am understanding you to say yes! 

Yes, within reason. Again, there has been a lot of talk about not
disrupting the "natural" curvature of soundboards by forcing them into any
shape they haven't already assumed after ribbing. This, after cooking the
panel down, thinning it's perimeter, gluing it to flat ribs, and
compressing it to near the fiber stress proportional limit when it
re-hydrates. Right!  



>The reason I ask 
>that is that I am aware that some very respected and prominent soundboard 
>builders have the Gold, Silver and Bronze in clamp gymnastics.

I find the soundboard response to be more dependent on design. That is, of
course, unless I've just accidentally blundered into a magic method of
clamping that has made the difference instead of all the other changes I
make with a re-design.  


>But on many boards that don't have cut-off bars, or only have small cut-off 
>bars, the bridge is nowhere near the middle of the board, and hence the 
>bridge does not end up at the arc apex. I suspect the only way to improve 
>that, short of installing a well-designed cut-off bar, would be to make 
>un-symmetrical ribs and force the apex on the complex curve of the board 
>underneath the bridge.

Yep. You'd have to go with a progressive radius curve like a parabola or
catenary curve, which should be very interesting in the top two octaves
where you can't center the bridge on the rib anyway. Then there's the
consideration of which bridge gets to claim the apex when there are two or
more bridges on the same rib. I prefer to let the crown height fall in the
middle of the rib unmolested, and concentrate on accommodating load
distribution with a desired response. You can shift the load response of a
rib off center with asymmetric feathering you know, if you decide you want
to - like if you want a stiffer low tenor on the same rib as a more
flexible bass.  


>Is this one of the reasons to put in a good cut-off bar? To get the darn 
>bridge into the center of the board? Or is the cut-off bar strictly to 
>reduce soundboard area and minimize undesirable vibrations of the board?

Centering is part of it, but getting rib length down and eliminating
unproductive areas of the board is the primary reason. As a bonus, a good
cutoff bar helps brace and stiffen the straight side of the rim too. I
wouldn't turn down a freebie.


Ron N