Shimano SLX 1x12 HG+: dropping gears when pedaling backwards

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Shimano SLX 1x12 HG+: dropping gears when pedaling backwards
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Posted: Jan 7, 2021 at 10:27 Quote
Been having this issue for a while and out of ideas on how to solve it. I have ~700km on an SLX 1x12 system that was new last August. In difficult technical climbs where I'm in the largest cog and need to pedal backwards to ratchet, the chain frequently drops down to the 3rd or 4th cog. If I can keep it going it makes horrible noises shifting back, or more frequently I get stalled by the suddenly harder gearing.

Any ideas on how to avoid this? I've checked that B tension matches the line on the derailleur. Hanger looks straight. Limit screws are set correctly as far as I can tell and shifting is otherwise fantastic.

Posted: Jan 7, 2021 at 11:57 Quote
CaMKii wrote:
Been having this issue for a while and out of ideas on how to solve it. I have ~700km on an SLX 1x12 system that was new last August. In difficult technical climbs where I'm in the largest cog and need to pedal backwards to ratchet, the chain frequently drops down to the 3rd or 4th cog. If I can keep it going it makes horrible noises shifting back, or more frequently I get stalled by the suddenly harder gearing.

Any ideas on how to avoid this? I've checked that B tension matches the line on the derailleur. Hanger looks straight. Limit screws are set correctly as far as I can tell and shifting is otherwise fantastic.
in my experience (assuming everything else looks good like you say) it was a worn cassette.

Posted: Jan 7, 2021 at 12:04 Quote
brianl wrote:
CaMKii wrote:
Been having this issue for a while and out of ideas on how to solve it. I have ~700km on an SLX 1x12 system that was new last August. In difficult technical climbs where I'm in the largest cog and need to pedal backwards to ratchet, the chain frequently drops down to the 3rd or 4th cog. If I can keep it going it makes horrible noises shifting back, or more frequently I get stalled by the suddenly harder gearing.

Any ideas on how to avoid this? I've checked that B tension matches the line on the derailleur. Hanger looks straight. Limit screws are set correctly as far as I can tell and shifting is otherwise fantastic.
in my experience (assuming everything else looks good like you say) it was a worn cassette.

This has been happening since day 1, so not likely.

Only thing I can think of is limit screw being too far in, but I had to add a half turn from how it was tuned from the factory after it dropped the chain in the spokes. Thankfully I noticed immediately and didn't take out a bunch of spokes...

Posted: Jan 7, 2021 at 12:16 Quote
Sounds very much like a chainline issue. The position of the front chainring (offset) could be too far out than it should be which causes this kind of chain drop in biggest cogs when backpedalling.

Could be sorted with some chainring bolt spacers and possibly longer chainring bolts too, providing it's only out by a few mm's.

Posted: Jan 8, 2021 at 10:02 Quote
From memory Shimano 12 speed chains need to be fitted in a specific direction. I don't know what happens if it's backwards but worth a look.

Posted: Jan 8, 2021 at 10:30 Quote
Same thing is happening on my wife's bike.
2020 Marin Hawk Hill 3
Deore 12 spd drivetrain w/sunrace 12spd hg cassette.
Quickly threw a friends wheel in that had a 12spd SRAM NX cassette (Norco Optic, w/ no chain dropping issues SRAM NX drivetrain) and the problem still persisted.
So I think that it is either in the derailleur adjustment which (the bike shop and myself haven't been able to figure out) or in the chain line, which is tricky to adjust with direct mount chainrings.

Posted: Jan 8, 2021 at 10:57 Quote
Pratley wrote:
From memory Shimano 12 speed chains need to be fitted in a specific direction. I don't know what happens if it's backwards but worth a look.

This. I think I it may also matter which links are on which tooth of the chain ring but I only have a vague memory of reading that.

Posted: Jan 8, 2021 at 11:28 Quote
Recently fitted an Absolute Black oval chainring onto my Buddy's SLX 12 speed which has an assymetric crank spider and isn't direct mount.

To clarify my Buddy's is the SLX MT610 crankset which can be adjusted with crank bolt spacers, not the M7100 version with a direct mount chainring.

Posted: Jan 8, 2021 at 11:59 Quote
https://bike.shimano.com/en-NZ/product/component/slx-m7100/CN-M7100.html

Dealer manual 1. Last page shows chain orientation.

Posted: Jan 8, 2021 at 12:50 Quote
alot of modern bikes with 1x12's do this. there is no tension on the top when pedaling backwards so chainline will cause it.

Posted: Jan 10, 2021 at 8:19 Quote
Hey there, a little of topic but I was gonna get an 11 spd SLX derailleur to put on a 10 spd cassette. Will this work? Thanks.

Posted: Jan 11, 2021 at 9:07 Quote
p-m-z wrote:
Hey there, a little of topic but I was gonna get an 11 spd SLX derailleur to put on a 10 spd cassette. Will this work? Thanks.

Shifter and derailleur combinations of specific speeds do not pair. So, no.

Posted: Jan 11, 2021 at 9:29 Quote
How old is your chain? I have a GX 1x12 that was doing the same thing. Used a chain gauge to check and was around 0.75% stretch. Swapping to a new chain fixed my problem.

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