Cassette jumping/rattling only in smallest cog?

[Becky]

Looking for some ideas here. I have a SRAM PG-970 cassette (12-26) on my commuter. The chain jumps and rattles, but only in the 12-tooth cog. The rest of the cogs are nice and quiet. Everything shifts fine, so I'm thinking it's not a cable tension issue. The jockey pulley seems to be properly aligned below the 12 when in that gear.

Any ideas? Anything I'm overlooking? I don't often need the 12 on a bike that weighs 30+ lbs. fully loaded, but it's bugging me that it's not right.

Thanks in advance for your help!

[OakLeaf]

Limiting screw on the rear derailleur?

Did this start when you changed something? Can you see whether the jockey wheel is too far toward the hub, or too far toward the drop-out, in that gear? Try turning the high-gear limiting screw accordingly, in small increments (like 1/4 turn, paying attention to how much you turn it each time so that you can go back if needed).

A work stand is a huge help in making derailleur adjustments. Alternately, if you have exposed joists or a ceiling hook in your garage or basement, you can use tie-downs to hang your rear wheel off the ground; some car racks allow the cranks and rear wheel to turn; or recruit a friend to hold the rear of the bike off the ground while you shift, turn the crank, and adjust.

Shimano's website has adjustment instructions for all their products, not sure about SRAM.


Other thoughts:

If the derailleur is very old and/or dirty, maybe the spring isn't pulling it all the way out? Even a small rock caught in the spring I suppose?
If the chain is old, maybe a very slightly sticky link that's only noticeable when it's under the least tension?
I'm assuming that your 12 tooth cog itself is in reasonably good condition since you said you don't use it often.

[Becky]

It's a new bike build, so nothing has really been changed out. The derailleur, cassette, and frame (among other things) are all brand new. The drivetrain was dialed in after the install and I've been adjusting for cable stretch as needed. I've been tweaking the high limit screw, but I'm at the office at the moment, so it's difficult. I'll put it in the workstand and tinker some more tonight.

Could it be a bent tooth on a brand new cassette? Bent derailleur hanger? B-screw adjustment? I'm brainstorming at this point, and I appreciate any and all suggestions that y'all may have. Thanks!

Edit: FWIW, the drivetrain is Shimano, except the the cassette, but they should be compatible with each other, right?

[maillotpois]

The chain jumps and rattles, but only in the 12-tooth cog. The rest of the cogs are nice and quiet. Everything shifts fine, so I'm thinking it's not a cable tension issue.

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.

[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.