A while back, I respaced the rear dropouts on a steel frame from 120mm to 130mm following Sheldon's instructions. This year I decided to use the bike on my trainer, and I noticed that the bike very noticeably leans to the left in the trainer, which causes my right sitbone to get sore. To get the saddle level, I have to put almost an inch of stuff under the left trainer foot. My other bikes are level in the trainer, so I am pretty sure it is this bike.
I have been going kind of crazy trying to fix the problem. I tested the alignment using Sheldon's string test and made some small adjustments, but with no luck. Using that test, the alignment is within a millimeter. As the frame has semi-horizontal dropouts, I thought that it might be caused by the rear wheel not being properly dished, causing me to misalign it. I redished the wheel and made sure the axle was seated in the same location in the dropout on both sides, again with no luck.
I am sort of at a loss of where else the problem may lie. Again, the trainer seems fine and the floor is level. Any suggestions?