I have a issue with my Marlin 7 (2022). It has a Shimano Deore M5120 rear derailleur.

When the bike is on the bike stand the bike shifts perfectly. When I'm riding the bike the bike shifts fine until 8 to 10 gears then will change randomly.

I have made sure back wheel is not loose and the clutch is engaged on the derailleur.

I'm unsure how to adjust, when I can't replicate the issue on the stand.

Could any one give any tips.

Thanks Sam

  • Try to get shifting to work perfectly on the stand, then it usually also works fine while riding. Play around with the tension in both directions.
    – Michael
    Commented Jun 29, 2022 at 10:05

1 Answer 1


This type of indexing issue can be particularly frustrating to try and resolve.

One solution is to try and fine tune it whilst taking short rides up and down a quiet local street.

Start by establishing which direction the 'ghost' shifts are happening in. For example gear 8 shifting by itself into gear 9. Then make very small adjustments (1/4 turn) of the barrel adjuster to tweak the cable tension - in my example adding a little tension to pull a fraction more towards gear 8. Make a mental note of how many turns you have done incase you want to undo your changes later.

Then ride up and down the street trying all the gears (in both chainrings) and see if the problem is resolved. Repeat the process until you get a result you are happy with.

  • 1
    Thank you for the response. Managed the sort it with you advise. The B adjust was way out. I had only just brought the bike. I thought the shop would of ensured it was working well. Commented Jul 4, 2022 at 17:43

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