I run an Ultegra 6600 RD with a nine speed cassette. I recently put on a 26 tooth cog for a hilly criterium, and I noticed something strange when I took the chain off. It seems like the upper pulley touches the cog when I'm in the lowest (biggest) rear cog, no matter how much I tighten the B screw. There's no rubbing/pinching when the chain is on and actually tensioning the RD. Is this normal?
I also like to check my limit screw adjustments with the chain off, how do I do that with this bigger cog? Thanks in advance.
The B screw adjust is not a static adjust. It alters the jockey wheel clearance to the cassette while the drive train is running and it works with the length of chain and spring in the take up motion of the cage. So the answer to part 1 is yes, its normal.
With the chain off, and the cable loosened also, the limit screws can be roughly set or checked by sighting down the plane of the smallest cog for the high adjust and the largest cog for the low adjust while manually holding the derailleur against the appropriate limit. The jockey wheel should be directly in line with the cog at the limit. The final adjustment is still best made with the drive train running.