Curve on Bridge Bottom

[Farrell]

Thanks Ron. I do see your point. I'll have to think and play on this a while. Seems like the bridge can really be subject to some distortion! Likely what was going on with a Young Chang G-213 I tuned this evening for a concert tomorrow. Top two octaves were just as quiet as a mouse. The upright bridge that was subject of my tests had a reverse crown, just as the board did!

I know you're going to want to shoot me for this, but after all you said about the curved bridge finding its place, or rather conforming, to the curvature of the board, this leads us to the folks that align the apex of each rib to the long bridge line - and maybe half-way between the long and bass bridge at that end. This always seemed kinda goofy to me - wouldn't all the ribs just rotate end-to-end and even out after you take the think out of clamps? I trust you do not do such a thing?

Terry Farrell
  
----- Original Message ----- 
From: "Ron Nossaman" <RNossaman@KSCABLE.com>
To: <pianotech@ptg.org>
Sent: Saturday, May 04, 2002 11:11 AM
Subject: Re: Curve on Bridge Bottom


> >The only bridge I had laying around was from an upright and it was notched 
> >at the tenor/treble break, so I followed your suggestion over the 39" tenor 
> >segment - it bowed 5/16" with 100 lbs applied in the middle (my son stood on 
> >it).
> >
> >I'm not sure what I was supposed to observe. Kinda seemed to act like a big 
> >rib.
> >
> >Terry Farrell
> 
> Holding one end of the bridge with one hand, setting the other end on a
> bench top or something solid, and pressing down with the other hand in the
> middle of the bridge, you were supposed to observe that you couldn't hold
> the bridge top level. Using just the relatively straight tenor segment
> didn't illustrate the principal. The rotation is the key point. When you
> find a piano known to have a crowned bridge, with a nice 3mm soundboard
> crown in the low tenor and negative crown in the killer octave, what
> happened there? Why didn't the bridge support the crown in the killer
> octave? Why is there negative front string bearing and positive rear
> bearing in the killer octave? Lay a straightedge from low tenor to high
> treble of the bridge and locate the point of the bridge that furthest
> deviates from that line. Where is that point in the scale? When you tear
> the soundboard out and look at the bridge, you will find it is still
> crowned. So why didn't it support the crown in the killer octave? Consider
> that the string bearing loads in the top half of the scale are typically
> over twice those in the lower half, and the loading per mm of bridge length
> is much higher in the top half of the long bridge, where the bend is most
> extreme, than in the bottom half.
> 
> Lay your upright bridge on a flat bench surface. Does it conform to the
> surface pretty closely? Now prop the killer octave up with a pencil. Don't
> the ends of the bridge still touch the bench top, while the middle is a
> pencil's width high? How is that different from a crowned bridge?
> 
> So my point is that, while the bridge does distribute and support load
> locally from rib to rib, it doesn't from end to end because of it's shape.
> The killer octave gets the brunt of the load, relative to rib dimensions
> and load capacities, because of the bridge curvature, and regardless of
> bridge crown. That's the way I see it, for what it's worth.
> 
> I would love to see a real live pedigreed structural engineer's analysis of
> this thing, to see just where and to what degree the board loading is
> affected by the leverages from that curved bridge. Any FEM enabled,
> qualified and curious soul(s) out there who are willing?
>  
> Ron N