One rear cog doesn't shift down

Question

Recently, I have replaced a chain after noticing that the old one gained 0.75-1 percent of elongation. After that everything worked fine, i.e. shifting was consistent and smooth, drivetrain worked fine. But after only a few ride (say 5-10 km total) I started noticing chain skipping and jumping on the cassette.

The solution was kind of obvious to me. Although I don't have a cassette wear gauge, I thought that it was time to change the cassette as well. My bike is an older hardtail, but it still had decent components and I take a lot of care to preserve well. I've bought a very lightly used higher end cassette SRAM 980 (photos below).

I fitted everything, tried to adjust rear shifter, but still have one pretty annoying problem. Limit screws are adjusted as they should, no problem with that. Cable tension is reasonable, no noise problems or anything on most gears. Shifting up is basically flawless, works every time and is quick. Almost everything going down (towards bigger cogs) is smooth too, with one very noticeable exception.

When I shift from speed 8 to speed 7 (2 to 3 smallest cog) the shift tends not to happen. A very noticeable chain rub can be heard, you can see and feel the chain trying to climb and failing repeatedly. I suspect that cassette or RD are the culprit but am not too sure.

To solve the issue there are 3 solutions that work so far:

1. Increase cable tension significantly. It makes shifting levers feel stiff and clunky, and also makes larger cogs (1st and 2nd in particular) behave weirdly, with inconsistent shifts, shifting multiple gears at once and such. This is clearly not a good solutions
2. Shift to 7th speed from 6th exclusively, works every time but is very annoying. Suboptiomal
3. Push the shifting lever with a little more force when going from speed 8 to speed 7 and holding it there until the shift happens and only then letting go. Not 100% success rate, have to retrain my brain to remember this strategy. Suboptiomal

So, I understand that my issue is sort of minor all things considered, but only a couple of weeks ago I had a perfectly working drivetrain and the only reason it all broke down is the need for new chain. I'm at least trying to figure out what exactly might be the problem and best case fix it.

What I've already tried:

• Refit the cassette, I've taken it off, cleaned, greased and tightened it properly
• Change the shifting cable. Changed it, cleaned the cable housing, lubed the cable. No change. The old one was perfectly fine tbh.
• Clean and relube the chain. Done. I use Smoove. No change after that operation

Technical stuff

• Bike: Norco Manik 2008, MTB, freeride
• Crankset: Husseflet Truvative, original
• Derailleurs: SRAM X7 3 front, 9 rear. All from stock config, though I've maintained them to the best of my ability.
• Chain:
  • Old: unknown, probably some cheap SRAM like 931, came with a bike, this was my first replacement and sadly I haven't saved the chain
  • New: SRAM 971, new, fitted to the old one, under 50 km wear in moderate conditions
  • Option: SRAM 951, mildly used, came with my used cassette
• Cassette:
  • Old: SRAM, a bit lower end
  • New: SRAM 980

Answer 1

Ok, so I figured thigs out in the end. The culprit of the issues was incorrectly adjusted B-screw on the rear derailleur. Once I fixed the adjustment according to this famous Park Tool video about RD setup, the issue vanished. I needed to get the mech much closer to the cogs by moving it counter-clockwise by loosening the B-screw until the gap between the upper pulley of the mech and the cog (when chain is on the largest rear sprocket) is about 5-6 mm (tip: measure it with a hex key).

As an added bonus I got much firmer grip of the chain to the cassette since now many more teeth are engaged at any time, so no more skippping and slipping on steep climbs

What unfortunutely didn't help

(in the order of increasing difficulty)

• Messing with cable tension adjustment
• Cleaning, checking and relubing the chain
• Retightening the cassette
• Changing the cable

I don't say that operations above are pointless in all cases, I just mean that it usually makes sense to start with simplest and least invasive solutions such as simple tweaking of settings and screws

The end solution is pretty simple, yet in several years of adjusting over a dozen different RDs I never once had a case where B-screw was so function-critical. Hope it helps in your case

Answer 2

Other possible reasons for under-performing shifting: The cassette is worn and needs replacing. Usually cassettes last three chains before a replacement is needed. Chainrings normally last two cassettes.

After a chain replacement indexing is quite often in need for adjustment. As said in a comment, turning the barrel adjuster on the RD one or two clicks up or down and doing a short ride with some shifting often eliminates the problem.

EDIT: just forgot this, check the chain for a stiff link, especially if you're using a riveted chain instead of a quick link. But still, some chains come with a stiff link from the factory. They're often a cause for chains jumping on cassettes.

Another way to help with the problem could be a change of shifter cables that have stretched or become sticky (rust, etc). Before the chain replacement there was a certain equilibrium in the system that a new chain has disturbed.

BTW: from the pictures, I see that the lower jockey wheel is rather toothless. If that's normal on this RD don't worry. But in the other case, take a look at both. They are parts that wear out and replaceable at reasonable costs but often responsible for poor shifting performance.