Trek 7100, pedaling not as smooth in certain gears
I have a 2009 Trek 7100. I had a bent rim, that I had my LBS replace with a double walled rim and upgraded to a 7 speed cassette (bike originally had a 7 speed freewheel). They also had to add a spacer since the rim was setup for a 8 speed cassette. I was told my derailers may need adjustment (which they did). Everything is perfect, except for whenever I am on the two smallest sprokets on the rear cassette. It seems that when I peddle, it doesn't seem as smooth as the other sprokets and I feel a very slight vibration. Any ideas what might cause this on these two sprokets?