Bubble gage

[Ron Nossaman]

>Faulty reasoning on my part I think.  I had been thinking of the bridge 
>cross section with loads applied to it and with reactions occurring at the 
>cross section CG.  If you look at it that way, then having different load 
>magnitudes at the front and back faces of the cross section, with their 
>attendant moment arms from the CG, would cause a rolling moment about the 
>CG, as well as a downward load at the CG.  That sounds OK in 
>theory.  However, on a real bridge, the reaction really isn't just at the 
>CG, but along the entire bottom of the bridge.  Load applied at the front 
>face will probably go straight down and be reacted by the ribs, and the 
>same with the load on the back face, so there would be no tendency to roll 
>the bridge just from those loads.
>
>Phil Ford

If the bridge was short in length, tall, and straight, you would get some 
torsional force from a severely off center load. The treble bridge is 
neither short, tall, nor straight. to get a torsional rotation in the 
middle of a long curved bridge, you would have to twist the bridge in the 
middle, with the ends staying flat. You can't look at a long curved bridge 
as just a cross section, because it's supported over a much wider footprint 
from end to end than is obvious from the footprint at any one place. It has 
outriggers. It's the curve that does that, just like it's the curve that 
prevents that curved treble bridge from acting as a rib and providing beam 
support of crown. Treble bridges don't roll locally (except maybe at the 
extreme ends if they're undercut too far (high treble), or cantilevered on 
an apron (low tenor). The whole bridge rotates down in the killer octave as 
the soundboard support fails.

Ron N