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I've got a '70s-era ten-speed road bike with (what I think is) a Huret Eco rear derailleur which shifts well, but for some reason a loud grinding sound emits from the rear of the bike when using the second-smallest cog and riding using this cog feels much less smooth than the others. However, the cog itself doesn't look worn or damaged, and the derailleur tracks quite well.

Of course, since this bike has down tube shifters, some grinding can occur in any gear. Nevertheless, it is very easy for me to trim the derailleur with any other cog in order to eliminate this grinding... except for the second-smallest one: No matter how much I try adjusting the derailleur's trim using the shifter, I can't eliminate this grinding on this one cog and only this one cog. Even more strangely, there is no significant difference in feeling/sound when running the chain on the small chainring as opposed to on the big one.

What is going on here? Is some part somehow worn/damaged or simply needs adjustment?

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    How many miles does this cog have on it? Any isea? You may need to change it. Mostly, with modern bikes, youd just replace the whole cassette. But can you source this for your bike? – PeteH Apr 10 '16 at 17:54
  • What do you see when you put bike in a repair stand, sit next to the rear tire and spin the crank by hand while in that gear? You should be able to see exactly what's going on. – zipzit Apr 12 '16 at 15:15
  • You don't mention how (or whether) this problem is affected in any way by front shifting (while keeping the rear on the second cog). Is it better or worse on the large chain ring? – Kaz Apr 13 '16 at 19:12
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    One possibility is that, in certain cog configurations, the derailer jockey wheels are actually rubbing/bumping against the cogs. This would typically due to chain wear and a resulting chain that's a hair too long. (For a bike of that age it's just about a given that the chain is worn out and should have been replaced 10 years ago.) – Daniel R Hicks Apr 13 '16 at 21:35
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    It's hard to say. The old 10-speeds tend to be a bit more tolerant of the cog wear problem than newer ones. And if it can't handle the new chain you can try removing one link pair from the old one to shorten it (if it appears that chain length is the issue). – Daniel R Hicks Apr 13 '16 at 22:38
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This could conceivably be a cross-chain issue. Hypothesis: when the chain is on the second cog, it extends toward the chain ring at such an angle that there is interference from the teeth of the larger, third cog.

This kind of thing can't be diagnosed remotely without detailed pictures at suitable angles showing the chain line, derailleur and cassette details and so forth.

Like user zipzit commented: get it in a stand and try to pinpoint the source of the noise. When you find it, slow down to a crawl and try to find the exact location of the metal-on-metal action. If it needs load to reproduce, have someone apply the brake to the wheel.

  • I could try to get some photos (I've got a halfway-decent DSLR and close-range lens for it) if that would help but I haven't got a clue as to what exactly I should photograph... – errantlinguist Apr 13 '16 at 22:37
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Since the chain looked quite worn, I bought a new one and installed it. Funnily enough, the grinding sound/feeling has completely gone away, but now the chain slips on the rear cogs... and can you guess on which cog it slips the most?-- Yep, on the second-smallest one.

Judging from this and from previous experiences, I suppose this means that diagnosing all drivetrain-related issues should look like this:

  1. Is there something wrong with the bike's drivetrain?

    1. Yes → Is the chain badly worn/damaged?

      1. Yes → Replace the chain with a new one. Is the problem gone?
        1. Yes → Is there now yet another problem?
          1. Yes → Go to step 1.1.2 below.
          2. No → Be happy and go ride.
        2. No → Go to step 1.1.2 below.
      2. No → Is the cogset badly worn/damaged?
        1. Yes → Replace the cogset. Is the problem gone?
          1. Yes → Is there now yet another problem?
            1. Yes → Go to step 1.1.2.2 below.
            2. No → Be happy and go ride.
          2. No → Go to step 1.1.2.2 below.
        2. No → Look for help online. Did you find a solution?
          1. Yes → Be happy and go ride.
          2. No → Post the problem on Bicycles SE. Did someone provide you with a solution?
            1. Yes → Be happy and go ride.
            2. No → Just go to a bike shop: This is getting ridiculous.
    2. No → Go look for another flowchart: The only thing I know about is drivetrains.

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