Cassette jumping/rattling only in smallest cog?

[Becky]

Even if it only jumps on the small cog and shifts fine it well could be a cable tension issue. That small cog seems more susceptible than others to any slack in cables. (I just changed my cables and was having the same issue after the second ride - only in the small cog - and the bike shifted just fine. Things had stretched and as soon as we pulled the cable tighter it worked great.)

If you have a new bike build - and new cables - there may have been some stretching.

Hmmmm....I hadn't thought about the small cog being the most susceptible to cable slack. Maybe I'll go tinker with that at lunchtime.

Thanks!

[DebW]

The small cog is susceptible to too little cable slack but not to too much cable slack, assuming the limit screw is set correctly. You should also consider cassette wear. The 12-tooth cog will wear noticably faster than the other cogs, because it is smaller and goes around faster. If you've got more than 1000 miles on this cassette, there is potential for this kind of wear.

[Becky]

The small cog is susceptible to too little cable slack but not to too much cable slack, assuming the limit screw is set correctly. You should also consider cassette wear. The 12-tooth cog will wear noticably faster than the other cogs, because it is smaller and goes around faster. If you've got more than 1000 miles on this cassette, there is potential for this kind of wear.

Brand new cassette and chain. Maybe 75 miles on them, and I doubt that any of those miles used the 12. I'm getting a little befuddled by this problem, and am beginning to wonder if I have a faulty 12-tooth cog?

[mayanorange]

Have you done any adjustments since it was built? 75 miles seems about right for the normal new cable stretch to happen, so you usually need to readjust about then.

[Becky]

I've been adjusting for cable stretch. The upper 8 cogs sound fine and shift perfectly. The 9th one makes the noises and I just noticed (as I sat here tinkering instead of working ) that it sounds somewhat rhythmic. I've let the high limit screw out pretty far, and the noise doesn't go away or change.

[Becky]

I've been adjusting for cable stretch. The upper 8 cogs are perfect. The 9th one makes the noises and I just noticed (as I sat here tinkering instead of working ) that it sounds somewhat rhythmic. I've let the high limit screw out pretty far, and the noise doesn't change.

[OakLeaf]

Is the noise in the same rhythm regardless of which chainring you're on, or does it change?

[lunacycles]

This sounds to me like your cassette lockring has come loose, so the smallest cog (at least) is rattling and not staying totally stationary. Would also explain the chain jumping around in that area.

Another possibility is that there is a spacer missing near the 12t cog, or the wrong spacer installed.