[dasho]

I had a chain installed Friday and have had a strange shifting issue since. I have a 50-34 chainring with a new 12-27 cassette. When I rode the bike for the first time since the chain was installed, while on the 50 tooth and middleish of the cogs (17 or 19 tooth?) the chain would start making a loud (constant) rubbing noise one the one cog until I shifted to the next. Sometimes it would jump over the one tooth. When I was in the 34 tooth and went through the gears, there wasn't a problem.

I then changed the cassette to an 11-23 and everything worked as normal. I put the 12-27 back on and played with the rear der adjuster a bit both loosening the cable and then tightening it but still have the issue. I made sure the cogs were in the correct increments. Any idea what could be causing it and how to fix it?

Thanks in advance.

 

[danl1]

How many miles (and/or chains) on the 12-27? Sounds a little bit like a few cogs are worn.

 

[joe_blow]

with a new 12-27 cassette.

^^^^^^^^

 

[Al1943]

Do both cassettes have the same number of cogs?
Are they the same brand?

 

[dasho]

Yes....

They both have 10 cogs and are Shimano Ultegra's

 

[cyclust]

It sure does sound like that cog is worn. If the cog is severly worn, putting a new chain on will sometimes make the problem worse, since the old chain had worn and would mesh properly with the equally worn teeth. Now that the chain is fresh, the worn cog may want to skip. If you have a lot of miles on that cassette, especially if you use that cog a lot [ likely since it is towards the middle of the cassette] then I would replace the cassette.

 

[dasho]

The cassette and chain are brand new

I thought maybe a spacer was missing but they are in the correct place. Maybe it's a defective cog.

 

[cyclust]

Oh, well maybe that one cog got put on backwards?