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GRX 812 rear derailleur shifting problem from 8th to 7th gear

vwillrockuvwillrocku Posts: 19
edited 21 November in MTB workshop & tech
Hi all, I have a tech question to all familiar with Shimano rear derailleur with clutch and hope someone could help as I am not a tech guy. My bike with 1X GRX groupset works fine expect when the RD-RX812 shifting from 8th to 7th gear (smaller cog to larger cog), it sometimes refuse or slow to shift. Other than that upshift or downshift of any cog is smooth.
I tried to turn the cable tension thumb screw on the derailleur but the problem still exists. What do I miss? Do I need to decrease the tension of the clutch or make any adjustments?
Thanks for advice in advance!

Posts

  • Have you checked the hanger alignment. If the hanger is bent your changes will never be perfect. If all other gear changes work ok then I would get the hanger checked. A bike shop can do that for you or you could buy a hanger alignment tool.
  • steve_sordysteve_sordy Posts: 2,318
    If you have a hanger alignment tool, it will take 10 mins at most. If there is nothing wrong with the alignment it will take you 5 mins. You do not need to even unfasten the gear cable from the mech, just undo the mech hanger bolt and move the mech down the chain out of the way.
    But mech hanger alignment tools cost a lot of money (Google it). If you take the bike to your LBS they will do the job very quickly and therefore cheaply. The last time I took mine, which was several years ago, it cost me £10 and there was something wrong!

    My last use of my alignment tool, was because of a very similar fault to yours. It turned out that the alignment was spot on, but during the process I got a good look at the underside of the mech and noticed that the jockey wheels were at an angle to each other, about 10-15 degrees out of alignment. In other words, my lower mech arm was twisted. (It was looping out on that steep climb that did it, the bike travelled a fair distance before hitting the ground). A carefully calculated and restrained twist applied by hand sorted it out. Well, actually two twists and I was still not absolutely spot on, but I thought that one more twist might take me too far and then I'd be into yet more twisting and at risk of permanently damaging the mech. So, I stopped there and gave it a test ride, to find it was perfectly OK. :)

    When it comes to shifting problems, if the usual quick fix of cable tension does not sort it out, then if the cassette and chain are in good condition, then it is probably alignment. You should not need to start adjusting your limit screws or your B-screw.

    The more gears you have on the cassette, the more likely it is to have alignment problems. In over twenty years of biking (7, 8, 9,10-speed), I NEVER had an alignment problem until I got a 1x12 bike. Then I seemed to be having them regularly, hence the purchase of an alignment tool. As is the way of these things, once I had set up the alignment myself, it was more than a year before I needed it again. This time on a 1x11bike, and now again on another 1x12 bike.
  • @aberdeen_lune @steve_sordy
    Thanks for all your valuable recommendation. Then I think it is time for me visit a LBS for a alignment check. I will get back to you after checking. Thanks for your help again!
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