Jim Merz observed:
"In the very early days of caliper brakes the arms were very flexible. It was possible to transfer the deflection of the rear caliper into the rear stays by making tabs that came very close to the stays. When the brake was used these tabs touched the stays, limiting the flex. In this case the brake needs to be mounted on the rear of the seat stays."
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I always thought along those lines too. And older brake calipers certainly could seem to flex considerably (although perhaps it was not from the arms, but rather from looseness of the bushings or washers around the mounting bolts).
But, has no one noticed there seems to be no perceived flex issues on a front brake. A front caliper is always mounted forward of the fork, in a position where there could never be a possibility of "bracing" even the most flexible of brake arms against the fork blades.
I would think that brazed-on cantilever studs with center-pull caliper arms would be ideal for mounting a rear brake on the front side of the rear stays. This design (whether with center-pull arms or cantilevers mounted) still is sometimes applied to best of Mixte style frames. I always thought that made perfect sense from a functional perspective. And it can also look absolutely superb... on a Mixte frame, that is.
Here is a JP Weigle example of such a mid-stay "forward" (downward) positioned brake:
http://farm2.static.flickr.com/
http://farm3.static.flickr.com/
But, on the far more exposed rear stays of a conventional (Men's) bike, even the most elegant front-of-stay mounting can just look a bit... uh..."unfinished".
I think it is all just about aesthetics and has little to do with function. We are just accustomed to looking at that rearward facing bit of shiny bicycle jewelry.
BOB HANSON, ALBUQUERQUE, NEW MEXICO, USA