Because I thought I could
Can't shift to largest cog - Did I bend rear derailluer or hanger?
I had a real bad day last week - I dumped my road bike Thursday night Got too close to the raised curb lip in a construction area (repaving) and went down on the right side. Bike was rideable (after I recentered the brakes and straightened the handlebars). I spent this morning cleaning the bike and going over all the mechanicals. Right brifter is now scraped and scratched , tore the bar tape, and I had to readjust the front derailluer slightly. Drivetrain is Shimano Ultegra 12-27 9-speed triple. Front derailluer and brifters were replaced at the end of last season, rear derailluer and cranks were bought used (unknown miles) and have about 2 1/2 seasons of additional use (approx 4,000 miles).
I have one remaining problem - I can't get the rear derailluer to shift to the big (27) cog. Even with the low limit screw backed all the way out and pulling on the cable (see Park Tool website for their example), it won't quite make the jump. High limit is fine, indexing is otherwise fine, and I tried various setting of the B-screw just in case. I can get it to shift if I move the pulley cage itself manually. Shifting is fine in the big chainring, seems OK on the middle and small. So I have a 8-speed 12-24 for now.
I'm wondering if I bent the derailluer cage (between the guide and jockey pulleys) or worse, the hanger. The hanger alighnment looks OK visually, but I'm not sure and it could be quite subtle. Any tests I can make at home before taking it to my friendly LBS? For example, should the jockey and guide pulleys be in vertical alignment with the cog (that's something I can check).
Crashing is never good for derailleurs .... the fact that you have opened out the low stop screw all the way and its still not getting to the 27T is a worry ... yes the jockey wheels should sit nicely under one another and generally run on the same plane as the spockets. I think you may very well need a closer look at the hanger ... and unfortunately unless you can make up your own alignment tool ... you'll have to pay a visit to the LBS.
Most likely you've bent the hanger. Shops will have a special tool for realigning the derailleur hanger. Actually, that's my favorite tool because using it has made a lot of people think that I'm smarter than I really am. It's not a big deal.