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@oddsock
I think my frame might be slightly bent.
I measured my chainline front and back as best I could: it's between 42 mm and 42.5 mm at the front which I measured 2 ways. Firstly, using a metal ruler, I held one end against what looked like the centre of the seat tube and read off against the centre of the chainring. Secondly, which involves less guess work I think, I measured from the chainring side of the seattube by butting the ruler up against it (28mm), and then from the far side of the seattube using a flat surface held against both the tube and the end of the ruler (57mm). Chainline = (28+57)/2 = 42.5mm. It would be better to measure the diameter of the seat tube but I don't have a vernier caliper. It should be 57-28=29mm.
At the back, the spacing is 120mm while the cog centre is 19mm in, so the chainline is 120/2 - 19 = 41mm.
So in theory, it's out by 1 or 2mm. But when I check it by eye, sighting along the chainring, it's much more, 4 or 5mm. So does this mean my frame is bent? Or maybe just the track ends? Does anyone know a good way to check?