Just to muddy the waters a bit...
I did some experiments a few years back looking at the relative contributions of string height off the top and break angle to sound. I modified a Classical guitar bridge by drilling more holes in the tie block to make it an 18-hole bridge. This allowed me to tie the strings in two different ways to produce different break angles over the same saddle. By passing the string through the center hole ('normal' tie) I got a 25* break with the strings 11mm off the top. By passing the string over the top of the tieblock before going through the center hole ('high' tie) I got a 6* break over the same saddle. By then putting in a tall saddle, which put the strings 18mm off the top, I got back to the 25* break over the saddle. To give away the punch line on the original question; changing the string height off the top made it sound diifferent, but changing the break angle without changing the string height sounded the same. But, for purposes of this discussion...
One of the things I measured was the displacement of the bridge, both the rotation and the vertical displacment, as the strings went from no tension to full tension. What I found was interesting.
When I tightened up the strings the top was pushed downward, and the bridge rotated forward, with the back edge coming up. There is a point somewhere behind the saddle I'm calling a 'centroid', which was pushed down by the 'average' amount. This vertical displacement was the same (+/-) for the two cases where the string height was the same, and greater when the strings were higher off the top, as you'd expect. However, the 'centroid' location was different, depending on the break angle: with the low 6* angle it was 20mm back from saddle, while with the 25* angle it was 17mm back for both string heights. Of course, with the higher saddle there was both more rotation and greater vertical displacment.
Here's the model I came up with.
Suppose the bridge was glued to a top that was not strechy, but was perfectly flexible; something like a thin, strong piece of paper. As you put tension on the string it will try to form a straight line through five points: the nut, the top of the saddle, the tie block where the string goes in and comes out (to the extent that's possible), and the top of the tail block. Ultimately the string tension is being carried between the nut and the top of the tailblock, assuming those are 'fixed' points. The saddle is pushed down and the tieblock is pulled up. The stiffness of the real top prevents the line of pull from becoming straight, but that's where it looks like it wants to go.
So, it seems that the strings do pull the back of the bridge up as they push the saddle down. Overall the bridge on this guitar was pushed downward by the same distance (more or less, given measurement uncertainty) when the string height was the same and low, and more when the strings were higher off the top, and there was more rotation with the higher strings.
I suppose now we can throw a few bricks at each other about how to modify this for a steel string pin bridge, or a pinless one. Somebody ought to repeat this experiment just to show me where I screwed up. I'm going to bed. Merry Christmas everybody!
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