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My bike has 12 gears on the rear wheel. When I put the last gear (12) it starts constantly changing from 12 to 11 gear without me doing anything. For example, I put the last gear and it works; right away it changes to 11 gear for a very short time; then back to the last gear and so on. All the other gears work fine. I don't known what's possibly wrong. Sorry for my English.

Thank you!

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    Can you show a clear picture of your cassette and rear derailleur when in the 12th sprocket? From behind and from side. Did you try basic cable pull (barrel adjuster) adjustments? – Vladimir F Jul 21 '20 at 14:47
  • Sounds like a small rear derailleur adjustment is needed like Andrew says. What brand is it? – mvreijn Jul 21 '20 at 21:21
  • This is ordinary deraileur adjustment. Look at the park tool tutorials. – whatsisname Jul 22 '20 at 4:36
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First, it's not clear which gear you mean by "12th". Is it the largest or smallest one?

It sounds like your rear derailleur is not properly adjusted. As 11 of your 12 gears apparently work fine, it's most likely a problem with the limit screw for derailleur. It's probably not allowing the derailleur to travel far enough.

Park Tool has quite a few videos covering how to perform most bicycle-related maintenance.

In this case, Rear Derailleur Adjustment is the appropriate video.

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    Make sure you make the adjustments in small increments. 1/2 a turn extra can result in the chain coming off the gear and wedging between the frame or over the large gear and jamming in the spokes. – mikes Jul 21 '20 at 21:14
  • I just adjusted my rear derailleur using the linked video as guidance. Took just a few minutes and it worked great. I thought this would be a tough job, but it was super easy. – Ralf Jul 22 '20 at 4:50

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