I'm running a Shimano Tiagra drivetrain with a 105 rear derailleur. The derailleur has approx 16000 miles on it. The cassette and chain have about 8000 miles. I'm on my second chain, cassette, and crank. Original rear derailleur.

Over the past month or so, the shifting from my 14t, 15, and 16t cogs (on a 12-23t 9-speed cassette) has slowly degraded. When shifting to larger cogs the cable throw seems inexplicably less than it should be. Pressing just slightly beyond the click, as is usual, often doesn't engage the next cog. The return shift to smaller cogs seems to always work, and I don't have the issue in the upper and lower gears that I use less often.

I've used the barrel adjuster to make sure the chain is centered on a cog when in that gear. It is as dead center as I can get it in all cogs once shifted to them.

The cassette is worn. In another 500 miles or so I was planning to replace the chain, cassette, and derailleur pulleys. If, however, the shifter is wearing out, I'd rather upgrade the entire group to 105 or Ultegra.

So, does it sound like I've worn down a tooth on the ratchet and that I'm not getting enough cable throw between a few of my often-used gears, or does something else sound off?

I'm pretty sure indexing doesn't really deteriorate as STIs age; they were designed well in that respect at least. Your levers are prolly fine.

However, your RD has some serious miles on it, and the issue you describe could easily be attributable to a worn one. Fresh pulleys won't do much for it either, since wear at the upper and lower pivots, and the parallelogram pivots, all adds up too and can't really be fixed. Check for RD wear by wiggling the bottom of the cage from side to side and looking/feeling for slop at the various pivots. Compare that amount of slop to what you can feel on a new bike for some perspective.

It's almost certainly worth grabbing yourself a new RD; Shimano ones weren't designed to be all that durable (recent ones look better, with wider parallelogram arms) compared to Campy. If you do that before putting on a new cassette and chain, you'll isolate the issue. I'm tipping you'll see an instant improvement.

Still, I might be wrong, and it might actually be shifter wear. If you think that's the case, maybe you can borrow a newish RD to diagnose it before you start buying parts.

DAB: It would probably be worthwhile investing in a new set of high-quality cables first. It might help with the shifting (could hardly hurt!) and if you decide to replace other components you will already have new cables ready to go.

Your well-worn cassette is probably not helping matters either.

I agree that it is unlikely that the shifters are the cause. If they are still clicking the right number of times then they are likely working properly and the problem lies elsewhere.

Thanks for the input. After further inspection, and another ride, I do think it is less likely the indexing is screwed up.

After more fidgeting, I got the slop to be equally distributed across all the cogs, which would indicate the cog teeth, the chain, or the derailleur. It still hits all gears, but there is a delay and rough shift in maybe 1 of 20 shifts, more commonly in the central cogs. The width of the cog teeth are down to about half their original width in the central 7 gears. Only the extreme low and high are close to the original width. I haven't measured the chain wear, but I'm assuming it must be getting time to replace that as well.

I agree with the replacing of the cables idea. I should try that too. I'm fresh out of cable and housing and have been lazy getting to the shop. Greatly appreciate the ideas. Keep em coming.

I would bet that if you replace the chain and cassette that shifting performance would improve greatly. Replacing the cables and especially the cable housings could also be the bigger problem. Derailleurs are actually very dependable for many thousands of miles with reasonable maintenance and if not bent. Flushing out the shifters with WD40 often cures shifting issues and it won't hurt to try that but in your case it's more likely something else.

My shifters act that way when they fray at the bead inside the shifters. You can's see it because it it hidden inside.

Click the shifter all the way to the small cog. You can see the bead and maybe a few frayed cables with a flashlight. I've replaced cables and thought I had new shifters.

DAB: When I suggested replacing the cables I should have been more specific that you should change both the inner wires and the cable housings. Don't skimp on quality, get die-drawn stainless wires and lined housings. Prep and install them properly with the correct ferrules and pay special attention to the loop which enters the rear derailleur. If as suggested by Mr. Beans you have fraying inside your shifters you will be glad you replaced them before they broke rather than after.

The brifters are fine. Replace the cassette and chain 8000 is alot of miles. Then put on new cable housing and cables. The RD will last that is not the problem I am pretty sure. For about $125 you can have a bike that shifts just perfect. This all sounds like a combination of chain wear, cog wear, and housing/cable that will make ever better performance. Put a good metal ruler on the chain and take a measurement for 12 inches. Do not guess you have issues get the facts with good accurate measurement on the chain. A good machinist ruler with 1/32's will be precise and better than any chain checker you can buy. Make sure you take the time to get the housing and cable routed in the proper way. Just read Sheldon Brown........do it right the first time. This will take a few more minutes and is easily worth the time spent to not be fustrated with the shifting.

One thing to remember, cassettes, chains, and cable/housing have limited life spans and are usually the problems. These are things that replaced over time and wear. No need to keep digging because they need to be changed periodically when worn. If when you do this it still acts up the same way then you to rethink. It is not a waste because the other parts are worn and would need replaced anyhow.