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I have a new road bike and when I move into the largest cog in the back, some variation on the above photo happens every couple of rotations. (Has been fixed by adjusting the barrel adjuster). I'm curious: what is this called?

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    Not sure there is a special name for it. Isn't this the result of the rear derailleur not being properly adjusted? – Christian Lindig Jan 22 '17 at 10:08
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    You say the bike is new. Is that new to you, or new as in all the parts are new? I ask because the cluster looks worn in this picture. If it's only new to you then clearer pics of the cluster, with the camera close to the line of the axle will show the problem. – andy256 Jan 22 '17 at 12:46
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    You get the same effect when the chain is worn, except that if one looks closely the chain links are longer than the tooth spacing. In your case the chain is simply riding on the teeth because it's not centered over the cog. It appears, from this angle, to be too close to the camera, relative to the cog. This most strongly suggests that the low-limit adjustment needs tweaking. – Daniel R Hicks Jan 22 '17 at 14:46
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    Note that a decent bike shop should adjust this for free, within the first few months of use. – Daniel R Hicks Jan 22 '17 at 14:47
  • Could you pop a chain checker into the chain and make sure its not elongated beyond 0.75% ? Unlikely, if the whole bike and transmission has done under a thousand km. – Criggie Jan 22 '17 at 21:42
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The photo shows the result of a derailleur being out of adjustment. There are many on line tutorials that explain derailleur adjustment. There is also the chance that the trainer stand is somehow interfering with the derailleur movement.

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You could call that 'skipping,' or something similar. 'jumping' maybe. Typically when that is happening the chain will skip over the teeth of the current gear or attempt to jump to an adjacent gear repeatedly.

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