this is my first post here so please bear with me if this is too long or of it is a well-known problem. I try to fix all mechanical issues on my own, because I do ultra cycling and need to understand how things work when I'm out in the middle of nowhere with only my multitool and a broken bicycle.
I have a problem which has been present for about two months.
When I use the outer chainring, gears work perfectly. But when I use the inner chainring, the rear gears go into auto mode and start skipping up and down. It is most noticeable when I use force, like when climbing, but it happens even while merely coasting and swapping easily between rear gears.
The first thing I did when the auto gearing started was to change the gear wire and tubes. While doing this I found that my rear wheel had actually not sat 100% perfect, but was slightly off by a millimeter or so. After this "fix" the problem went away for a few days but it soon came back.
I have completed two races while having this problem, the last was a 430 km race with lots of really big hills. It was a major PITA with the skipping gears, but luckily it is much less of a problem when I use the smallest three gears, so the issue is not so bad when climbing really steep hills.
I can't for the life of me understand how the gears can work when using the outer chainring, but not the inner?
I have wondered why this has suddenly happened (my bike is a 2006 Fuji Team Pro carbon). I seem to recall that I experienced a loud bang! on my rear derailleur in the middle of June, but that may be pure fantasy.
I replaced my wheels around mid-May, my current wheelset is Mavic Cosmic Carbone SL.
The problem began about a month after putting those wheels on.
Can there be a problem with the rear wheel? I am using two spacers as recommended by Mavic for this wheelset and this cassette.
Could it be the derailleur that is bent or somehow is misshapen? The derailleur is a Dura-Ace which is original with the bike. The cassette is Ultegra, as is the crankset and chain (all of which have been on the bike for two seasons now).
I compared my setup with a friend's, and the angle of the chain when going from the larger back cog to the outer chainring is much more pronounced on my bicycle.
Any ideas are most welcome. I don't mind buying a new RD but I need to understand what's happened.
The drivetrain seen from above.
The drivetrain seen from behind.