Originally Posted by
southpawboston
I edited my text above to explain this. You're right, if you have a horizontal DO, it's not an issue.
It's actually
more of an issue if the DO is horizontal, because the upper edges of the right and left dropouts have to be perfectly aligned. If one is higher than the other, the wheel will meet the ground at a (mildly) oblique angle. For this reason, Sheldon Brown recommended that any time you file horizontal dropouts, you file only the lower edge.
Originally Posted by
southpawboston
If we can assume that the drive-side sees the axle pushing on the leading edge of the DO, then it should be safe to file the trailing edge of the DO and not mess anything up.
Yeah... but I think I'd take some measurements before filing anything, just to be sure.