Chain skips on rear gears when using inner crank chainring
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.
Last edited by tormodg; 07-30-11 at 09:40 AM.
I would check your hanger ,from your photo , it might be off ( bent ) just enough to give you your problem . it look like it bent inward and need to be pull back out. if you have a hanger alignment tool use it ,if not take it to you LBS and have them do it.
Is the chain skipping teeth on the cogs or moving between two cogs? Two different problems with different solutions. Rarely does any solution require a new rear derailler though.
The best picture you could post is of the chain on a cog/chainring combo where it's skipping. If the chain is obviously not centered on the cog, your problem has to do with derailler adjustment or hanger alignment. If the chain is centered then the problem is likely a worn out cassette/chain.
As to the problem being with the wheel, I have encountered wheelsets where the cassette ends up sitting closer or farther from the derailler hanger due to differences in the freehub. In those cases, the fix is adjusting the high and low stop screws (and possibly a slight tweak to the cable tension).
Thanks a lot guys. I will check for all these things, and probably visit my LBS!
I have a dumb question, what part is the hanger?
Ok, thanks. I just checked my bike. It seems like this is a pretty thick part. Is it common for them to get bent?
Originally Posted by tormodg
yes it is common for them to get bent ,even new ones can be bent . mount it onto the frame ,mount the wheel ,use a hanger alignment tool to check and make any correction than need to be done . remount the derailer and ride.