Bicycle Mechanics - new cassette cog skipping

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I have DA 7800 brifters, a 12-27 DA cassette, Ultegra compact crankset, and a DA chain with about 1000 miles on it. I live at Lake Tahoe and do lots of climbing so need the 27, but I was getting dropped on the downhills to buddies with 52-11 or 53-12 tall gears. So, I bought an 11t Shimano cog, put it on and removed the 16t cog. I can shift into the 11 fine but it makes an awful racket and skips almost continuously. Did I miss something? The 11t has an "HD" stamped on it while the others have "FD". Does that matter? Thanks for any help.
Is that even possible? I though you had to swap the 11 for the 12, or get an 11-compatible 12? But if you didn't, I didn't think the thing would even go together? :twitchy:
Are you SURE that you have an 11T lockring? Does it have a smaller outer diameter than the 12T lockring?
Yeah, it sounds like you've got the wrong lockring. The 11 cog needs a special lockring.
Al
rumrunn6
08-13-09, 03:02 PM
replace the chain as well
rumrunn6
08-13-09, 07:10 PM
worn chains cause chain hop too
this year on one bike for chain hop I swapped a chain form another old bike and that fixed it
on another bike I bought a brand new chain but still had the chain hop so I had my LBS install a new cog and that fixed it
some say if you have chain hop just change both chain and rear cog at the same time
my boss (avid cyclist) says the cog lasts almost twice as long
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