Originally Posted by
JohnDThompson
Are you sure the brake bridge is mitered evenly? From the picture it appears the left side of the bridge may be shorter than the right. If so, this is a cosmetic issue. A frame that shows 2mm difference in the "string test" is only 1mm out of alignment; well within manufacturing tolerance.
Actually the string test reads roughly 1/2 the actual error. So if it reads a 2mm (high to low) difference, then each side is 1mm off at the seat tube, or roughly 2mm at the dropout. (This is a highly rounded analysis, the actual multiplier depends on the relative distance of the seat tube and dropouts to the head tube).
Either way. The string method is not gospel. It's more analogous to a screening test in medicine, where a positive (error) would indicate the need for more definitive testing.
Over the years, I seen too many perfectly good frames "corrected" into scrap metal, so I'm a strong advocate of a measure twice, cut once approach to frame straightening.