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Hmm, what effect does the caliper not being vertical have? Imbalance in the spring tension on each arm?
I’ve never touched that screw on any brake I’ve owner or worked on.
M_V-
The rim can get pressed laterally by the arm that touches it first. Easy enough to sort out, just move it until both shoes touch at the same time and the rim has no lateral movement. No need for the screw. Never touched it either.
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I don't think moving the caliper will unbalance the spring tension, since the spring only works on one arm of a side-pull caliper anyway and its force is transferred to the other arm via the adjusting screw.
It just bothers me that moving the whole caliper and using the screw have different effects. Moving the caliper actually moves the off-centre pivot such that that arm will move through a different arc, so it feels like there should be a correct position for it (where the caliper is nominally vertical) whereas using the adjusting screw shifts the arms forward and back (one goes each way) through their arc.
TBH the side-pull, dual-pivot caliper is a weirdly asymmetrical design all round especially in terms of how each pad approaches the rim.
Dogtemple
ffm
No, so the caliper itself is nominally vertical. Since there's an adjustment screw you can get the pads evenly spaced off the rim even if the caliper is rotated off-vertical, so I'm never sure.