thingsthatgo

Hi all. My chain is jumping very slightly on a cog, and I'm not sure whether it's the chain or the cassette that needs replacing, or something else. Here is the situation:

SRAM PG1070 10-speed cassette, 11-32, with SRAM 10spd chain. Both the cassette and chain have about 3000 miles on them (much less than the cassette is supposed to last). For the first time today, spinning the crank backwards on the 15t cog I can see/feel a roller catch on the edge of a tooth, causing a slight jump, about every 2 revolutions of the cassette (no correlation with # of crank revolutions). The jump occurs only on that cog, and only when running the chain backwards (I think the jump is just too slight to be felt when the chain is running forwards under tension). The jump is definitely on the cog, not the chainring - I can see the roller catching on the tooth. But oddly, the jump goes away if I shift into the big chainring.

According to my Parktool gauge, the chain wear is between 0.5% and 0.75%. I usually replace at 0.75%, and SRAM's documentation for the chain says 0.8%.

If it were chain stretch, wouldn't it be evenly distributed across the chain, i.e. wouldn't there be a mismatch each time a tooth meshes with a link, rather than once every couple of revolutions?

And if it were a particular tooth, wouldn't it happen every revolution, instead of every 2? And it's not always on the same tooth.

The teeth on the cog don't look obviously worn to my inexpert eye, for what little that's worth.

Could this be something other than chain or cog wear? Is it OK to just keep using both, checking regularly and looking for further developments to show which one it is, before spending money to replace possibly the wrong one?

Thanks!

rhenning

It is probably both wearing as both should wear the same. If you replace either one it will probably get worse. Roger

bikeman715

It could be a tight link in the chain and a chain tool will solve that problem , if that doesn't work then I would change the chain first and see what happen , then the cog or cassette if it contuse .

AnkleWork

Maybe cross-chaining -- which gear combination?
Maybe the shifter cable barrel adjuster needs a slight turn.
Maybe characterize the "problem" better.
Maybe ignore slight perceptions of a problem when turning the crank backwards.

DaveSSS 

Chain skip should never occur if you haven't changed to a different chain, lately. Chain skip occurs most often when you install a new chain on a used cassette. Only the most worn cogs will skip, while pedaling with a high chain tension, like when climbing out of the saddle.

Bill Kapaun

Odd number of teeth on the cog.
1 tooth interferes with either an inner OR outer link.

Retro Grouch 

Turn your crank backwards slowly while you watch the chain as it emerges from the jockey pulley. If you see the derailleur arm move, examine that link on the chain. You have a tight link. Work that link free with your fingers and you're good-to-go.