Originally Posted by
Dominae
The chain does not appear to be too long. It was not sagging at all when I had it in the small front chain ring and small rear cog. I also pulled back the rear derailleur cage as suggested by cny-bikeman and there is no observable twist to the derailleur cage. I have not yet checked the derailleur hanger, but my assumption is that if it were a bent derailleur hanger, it would affect the shifting in the large front chain ring as well as the small.
Please don't ask for advice if you are going to dismiss it. Several of us have explained that it is not the chain length per se, and sagging in the small-small combo is irrelevent in any case. You may or may not be able to detect the twist, as you are not accustomed to what to look for, and your "assumuption" about the effect of hanger twist is flat out incorrect, as I took pains to explain with an example. As it happens I had
exactly that problem on my own bicycle, and the only thing that fixed it was hanger alignment - although that is hardly the first time I've seen the issue.
I don't generally trot out my "credentials" but rather let my posts stand on their own. This is an exception. I was a mechanic, service manager and business owner in the bicycle industry for over 20 years. I was considered the best person in my city for frame and fork alignment, and did more fitting work than anyone in the area, short of a framebuilder about 30 miles away. I designed and taught a 60 hour bicycle mechanic course, raced, commuted and toured, including a 3 month 10,000 mile trip in 1976. I have solved many problems that have stumped other mechanics in shops where I have worked, and some on here as well. I gave you the procedure to follow to solve the problem, and what I deduced was the cause (accident/impact) was most likely spot-on. And yet your "assumption" trumps all of that.
I do not come to conclusions and recommendations I post from my opinion or some mental exercise imagining bizarre reasons for problems, or "what happend to me one time," but rather from all of that experience coupled with a logical approach to problem solving. The same holds for FBinNY, mrrabbit, hillrider and others. If you want to discard all that then don't ask the question in the first place.
At this point you are not going to solve this online or on your own. You need to take the bike to a competent shop and have it checked by the head mechanic. You probably won't listen to this but it would probably help to advise the shop that you have been told that the most likely problem is twisting of the hanger or rear derailleur/pulley. Unlike you I would not be bothered if I were told I was wrong and that there was something I did not consider or overlooked.