Its because they swing around a pivot, as the pad wears the contact moves towards the pivot axis,
with U brakes , that was outside.. Ignored, the Pad wears thru the tire sidewall ...
downside of the under the chainstay U brake.. people ignored that..
Original Mafac Cantilevers are pretty easy to adjust, IF the post on the frame was carefully fitted.. but that is not always the case..
...
Last edited by fietsbob; 03-07-18 at 12:16 PM.