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Can't shift down from 50T with clutch on, but can shift down with clutch off, why?

I'm using sunrace 11-50 and XT RD-M8000 (SGS), finished setting up everything for absolutely perfect shifting, then realised the clutch was off. Turned it on, shifting is still excellent (but with the usual clutch effect) except it will not shift out of 50T. The shifter clicks, the RD stays put, same thing if I do multiple shifts down. I can however backpedal for a while after one downshift to drop the chain into the 42T, the RD then moves, after which I can pedal forward and carry on shifting down perfectly.

This is not a derailleur setup issue, not only have I got everything setup by the book, but I've played around with 4mm to 12mm gaps between the 50T and top pulley with B-screw with no luck.

My calculations (full formula including 2 links for derailleur, not rough estimate) showed that 113.4 (=114) links on the chain should be enough, I installed with 117 links initially, felt that was too much chain, removed 2 links and am using 115 now. Doubt it's chain length, can't see why everything would be fine with clutch off if it's chain length, but worth considering.

RD is just out of the box, so I doubt it's the Nm on the clutch, but going to check that out soon.

It's not a big issue, since there are only 2 to 5 (depending on conditions) places where I need/want the 50T and all of them are during the (mostly) climbing half of my local rides, so I can ride up with clutch off and ride down with it on.

Above all I'm curious why extra chain tension from clutch could cause this; even if the Clutch tension is too high and that is the reason, why would this happen? After all, it shifts even under quite a lot of load with clutch off (I avoid this bad shifting practice where possible, I was just testing!)

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    I've had times where high chain pressure has been enough to stop a chain dropping to a smaller cog. The chain pressure was enough to overcome the rear mech's spring. Is there a tension adjust on the rear mech to increase downward shifts / to a smaller cog ?
    – Criggie ♦
    Jan 5, 2018 at 9:48
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    @Purr did you try to adjust the clutch tension?
    – Klaster_1 Нет войне
    Jan 5, 2018 at 12:05
  • @Criggie some claim the m8000 etc are have over done their parallelogram angle change to improve performance on bigger cassettes, I can increase tension with the B screw, I tried it up to 12mm gap 50T to upper pulley, wasn't enough. Will try adjusting even further to see if it works (up till near max b-screw), can always just file away some of the derailleur hanger to get the 50T to upper pulley gap back down to 6mm, increasing tension all round, reducing parallelogram angle. Have a spare hanger anyway.
    – user20209
    Jan 5, 2018 at 15:21
  • @Klaster_1: I said I would check it, I haven't yet. I'm not going to adjust is blindly, I need to get my torque wrench back from a friend first. Going to go that route before the increasing RD tension and adjusting parallelogram angle via filing mech hanger as per comment above.
    – user20209
    Jan 5, 2018 at 15:24
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    Then I adjusted the clutch, (assumed it should be correct since the mech was new, so it was the last thing I tried, this was the issue. It did not seem to make sense that it could be the issue as I was not aware that the clutch merely provided friction, and did not tension the chain per se.)
    – user20209
    Jan 5, 2018 at 20:31

1 Answer 1

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The RD-8000 is specified for a maximum tooth size of 46 (1x11-46). Likely you are just exceeding its design criteria and reaping the rewards of DIY mix and match.

The clutch does not add tension. It add friction in one direction to resist a forward moving cage, but allows the spring to pull the cage back freely. As the chain drops off a cog onto a smaller one, it has to briefly lift and this causes the cage to move forward against the clutch. The clutch is designed to slow this action - possibly in you setup just enough to keep the chain on the large cog and prevent it dropping on to the smaller cog. A longer chain will make little difference, as the clutch is friction based, so spring tension changes probably won't change the behavior.

Chainline is probably all you have left to play with. (You don't mention the bike, presume its 1x now, was it always 1x). Moving the chain ring out a millimeter or two might make a difference - or it might not.

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  • People report running this with extended B screws in GS cages just as well as shimano, I however got an SGS long cage which SunRace state as being completely compatible with the cassette. The bike was singlespeed till recently and 1x10 before that, but that's irrelevant, it's got a 73mm shell BB, running BB-MT800 exactly by the book, single spacer driveside, chain line is exactly how shimano want it,the only direction I'd consider moving it is actually more inwards (if I cared about 50T chain drop when back pedaling, which I don't).
    – user20209
    Jan 5, 2018 at 14:58
  • Whole setup atm is 1x11. M8000 Shifter and SGS RD, HG701 chain, M7000 crank, 32T CRM70 Chainring, BB-MT800 one 2.5mm spacer drive side on 73mm BB shell, Sunrace MX80 11-50. The frame is an On-One 456 Carbon. Chain drop when back pedaling the largest cog is a "feature" of all standard shimano setups in 11-46, this is due to their official recommended chainline lining up somewhere between 7th-8th gear, which makes sense to me, as that is the middle of where one actually typically uses 1x11, I don't mind since I can back pedal more than enough to deal with any potential rock strikes in the 50T.
    – user20209
    Jan 5, 2018 at 15:44
  • Another though is change the chain to another manufacturer - KMC would be my choice. They have a little more lateral flexibility which may help with chain drop as well as getting off the 50.
    – mattnz
    Jan 5, 2018 at 20:18
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    It was the clutch, too much friction, also your answer explains the difference between friction the clutch adds and the tension in the chain when pedaling which I was unaware of, as well as why this difference can be important. I set the Nm to the lower end of recommended range, this took a little over a whole turn anti clockwise. Now everything is setup and works perfectly, maybe just because it's new but I swear it's better than a friends pure XT 11-46! Accidentally voted your answer down, so give it a quick edit so I can vote it up!
    – user20209
    Jan 5, 2018 at 20:21
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    Update: I've actually increased the friction out of curiosity again, about half way between original setting and where I previously put it. Everything is still working perfectly, I expect this has something to do with chain wear, currently at 3.5k km and the chain tool is showing a fair bit under 0.75 wear at all places around the chain. I expect I will have to reduce the friction again when I finally need to change the chain.
    – user20209
    May 25, 2018 at 23:19

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