Shimano 12 speed chain suck?

Hi all – I have an issue with a bike I just bought (second hand) and am looking for technical/emotional support…

When I pedal fast in the highest gear then stop, the chain sticks/sucks for a moment, especially on the smallest sprocket 2 sprockets.
It also does the same thing when I place stationary and I spin the pedals backwards really quickly – the chain sags a little between the chainring and cassette and the derailleur arm moves slightly before pulling back to where it should be.

The setup is:

• Shimano 1 x 12 7100/8100 mix
• SLX Cassette (10-51)
• SLX 12 speed Chain fitted the correct way with the writing facing outwards (it’s worn according to my park tool  but there’s no slipping or skipping when pedalling hard, I’ve seen far worse)
• XT chainset 32T
• XT long cage derailleur (correct for cassette rage)
• B-Tension is correct according to the  line on the cassette
• Freehub replaced with a new one (bearings were mush)
• Any washers/spacers/in the freehub to wheel interface also replaced
• End caps  also replaced
• Wheel bearings are fine and run super smooth
• Axle is straight AFAICS
• Deraileur pulleys are running smooth
• Gear indexing is fine across all sprockets both up and down
• Does the same thing with the derailleur clutch on and off

I initially thought it was the freehub bearings but after replacement it was the same, so I bought a full freehub and it’s still the same – I am confident I’ve ruled out the wheel and freehub bearings.

• Worn chain/cassette causing the issue?
• Poor derailleur adjustment?
• Wheel bearings or bent axle?
• Something else I haven’t thought of!

I’m mechanically competent having fixed bikes and anything else that breaks for 40+ years but before shelling out for a new chainring/chain/cassette, or using the LBS to diagnose it and pay a fortune I was hoping for some feedback of things to test or just a “yes it’s the drive train” based on experience or “LBS it mate”… Thanks 🙂

Correct chain length is important with 12sp Shimano.

Chain length is correct (imo)

Did you follow the instructions about adding 5 or 6 links.

What hub is it? Does it spin very freely or does it feel like the pawls/ratchet creating too much resistance?

Given OP’s first comment, it suggests a grunged up cassette freehub.  Some are easier to do than others.

My first stop on a used bike. I replaced the four cartridge bearings in a used CX bike I bought last year. Runs super smooth now – both hub bearings and the freehub bearings. Easy.

It’s a syncros hub (formula) with an FH904 freehub, which is brand new OEM from syncros themselves (I bought the full rear hub service kit). Spins totally freely,. Spinning the wheel it just keeps going with the freehub clicking away nicely with no rumbles or roughness in the bearings at all.

I bought the new freehub as I tried changing the old freehub cartridge bearings and still having the same issue.

It’s got me totally beat other than charging everything in the drivetrain as the next step!

Freehub issue.

So when you pedal fast and then stop the only thing moving at this point is the wheel right?

And when you stop the chain goes saggy at the top?

Doesn’t sound derailleur related.

If you pedal backwards in the stand the slack is at the top again? Does it do the same if the brakes are on? Sometimes yes, sometimes no then possible bent axle. Always yes, then freehub/bearing related? Are the bearings pushed in too tight so there is axial load on them causing them to bind at speed?

I would check the chain length using the Shimano procedure first of all.

if you take the chain off and fit the wheel to the bike is there any play at all if  you try and move the wheel side to side?

Wait, is it a FS bike? If so, anything worn in that area?

Is it just the inertia of the massive cassette?

Have you checked the clutch on the mech?

Thanks for the responses!

It’s a hard tail so no worn Sus bits.

No play in the wheel bearings at all.ajd they’re not fitted too tight.

New freehub does exactly the same thing so I’ve ruled that out.

I’ll definitely measure the chain later using the Shimano method though it’s the one that came with the bike so I’d assume it’s correct.

I’ve stripped cleaned and regressed the whole thing so it’s totally clean including the chain, cassette, and i don’t think it’s cassette inertia as I wouldn’t expect that.

I don’t have another 12 speed boost wheel to try or get would be doing that next to rule the wheel out.

I’m having real trouble thinking of this as anything other than a hub/freehub issue tbh. (And that’s not just because my experience of Formula hubs has been exclusively terrible)
I’d take the chain off, then fit the wheel, and try turning the cassette backwards and feeling for roughness/excessive resistance. It could be something that only becomes apparent with the wheel when it’s torqued up in the frame.

That’s a good point and something I haven’t tried. I’ll measure the chain this evening and try that also.

Is it just the inertia of the massive cassette?

This. It’s just the weight of the cassette. Unlikely to cause a problem in real life. Stop spinning the cranks backwards so quickly!

and i don’t think it’s cassette inertia as I wouldn’t expect that.

Why not? A couple of my bikes do it, they didn’t when i had smaller cassettes.

Looking at a similar thread about the chain sagging, the guy made an error when installing the rear mech, the loose spinning spacer with the tab that the b-screw contacts was the wrong way up and wasn’t pressed against the gear hanger, his b-screw was screwed against the gear hanger tab and not the tab of the mech, it’s a long shot but worth checking.
Also check the cassette is definitely whizzing backwards freely once the thru axle is tightened up, chain removed obviously or just pulled away from the cogs.
I also regularly give my mech pivots a squirt of silicone spray and make sure they’re moving freely.

Another vote for the inertia of the cassette, I get exactly the same effect on the workstand (maybe out on trail as well, have never noticed) with bigger cassettes, you’ve got a lot more weight AND it’s further from the axle so if it’s spinning quickly it’s maybe not a surprise that it can briefly overcome the resistance of the derailleur spring.

Noeffsgiven – I did take the derailleur off and check that little b-screw tab was correctly oriented, and it was,  I also recently stripped and serviced the derailleur completely so it’s silky smooth and clean as a whistle.

Interesting point on the large cassette inertia, that seems like a design flaw as this issue/chain suck in general  increases the risk of the chain being pulled off the sprockets and.on the bottom one jamming between the frame and cassette.

Interesting point on the large cassette inertia, that seems like a design flaw as this issue/chain suck in general increases the risk of the chain being pulled off the sprockets and.on the bottom one jamming between the frame and cassette.

I would say in general that 1x drivetrains in particular and any drivetrains with large cassettes in general will always ask a lot of setup, if it’s not the inertia of the cassette it’s the chainline (I still don’t understand why 1x is touted as a solution to poor chainline as the chainline is worse at the useable extremes). Have you tried backpedalling fast in the largest sprocket on the cassette? It’s 50/50 whether my chain drops down to the next one or not, but happily I never actually do this out on the trail.

I’m still scratching my head over the inertia thing as I have a 10+42 1x 12 on a gravel bike that’s fine, I wonder if I can try that wheel…

If you’ve got another 12s mech to compare against – does the spring in it seem weak? I tried Deore 12s but gave up as I was experiencing the same problems, once I found out my Solaris could take a front mech I changed it to 2×10. But my thinking was that it was inertia/spring not being up to the job.

I’m planning to check the torque on the rear mech later tonight and check for play in the derailleur mount when on the smallest sprocket….

Have you tried it with the mech clutch off?

Does the clutch need cleaning and re-greasing?

Jockey wheels worn, or bushings/bearings (in the jockey wheels) stiff?

Does the BB move freely (take the chain off and check that there’s no binding/drag in the BB)

Any stiff links in the chain?

You say the chain is worn, and whilst it might ‘work’ when the system is under load, when free-wheeling / back-pedalling, the chain and cassette may just not be meshing properly.

I’m still scratching my head over the inertia thing as I have a 10+42 1x 12 on a gravel bike that’s fine, I wonder if I can try that wheel…

You know 10-42 is, well, different from 10-51?

One never experienced this with a 12x Deore or 2 xt mechs, combined with any one of 10-51 Deore, xt, or XTR cassettes.

It feels hub related to me.

Checked all of the above, chain is the correct length, freehub spinning freely when the wheel is fitted and fully tightened in the frame but with no chain on, torqued up the derailleur clutch.  I’m thinking I’ll regrease it all again put it back together one final time and run it for a while to see what happens, other than that I’ll go to a bike shop and try the same thing in a new bike and if that’s the same so be it, if not I’ll replace the chain, cassette, chainring, jockeys.

Maybe my expectations of 12 speed performing better than 10 or 11 are a just bit misplaced.

Did you get this fixed?

Ive got the same issue, if in cogs 11 or 12 and i stop pedalling abruptly the chain sags for a split 2nd and hits the chainstay

Everything is brand new and has been double checked and works fine on their own

Checked everything over and swapped wheels, cassettes and rear mechs from other bikes and it only does it when its got the 11-52 Shimano fitment Brand X cassette fitted, if i fit a Shimano 105 11-36 cassette then there are no issues (drivetrain is all Sram apart from the cassette)  so looks to be the cassette causing the issue

Strange as ive other bikes with 10-52 Sram XD cassettes and they dont have this issue, im wondering if its the combo of a Sram chain and the Brand X cassette causing this

Next thing to try is buy a Sram XD freehub body for my wheels and fit a spare Sram 10-52 cassette and see if that fixes the issue

I’d try adding a wafer thin spacer to shift the cassette away from the hub a gnat’s – might be just too far onto the freehub and dragging a tiny bit on the seal which would give those symptoms. If you fit the cassette but don’t torque it up fully does it still misbehave? (wouldn’t ride it like that though!)

I had this and posted about it at the time – https://singletrackworld.com/forum/topic/chain-sagging-xt-12-speed-hope-pro-4/

I thought it might be the freehub as mentioned above, I even called Hope to chat about it, they recommended I take off the freehub seal and see if it changed anything which it didn’t.

I started having issues with the clutch on the XT mech a few months later and took it all apart and found the clutch was corroded (apparently not uncommon) I replaced it and was slightly better but not perfect, eventually I replaced the mech for an SLX and it’s improved but still not fully resolved.  I do wonder if what was said above about the inertia is a factor, but none of my friends who have the same set up have had this issue.

I really can’t see how this can be anything other than the freehub fundamentally. IF the freehub is pulling the chain then the derailleur cage spring and/or clutch could resist it, hence mitigate the issue, which would explain why the derailleur can make a difference to the behaviour. However, it absolutely must be the freehub pulling the chain initiating the behaviour – there is no other force at work to move the chain when freewheeling. Smaller cogs makes sense as for a given torque the force on the chain will be higher closer to the axle.

had the same with an XT derailleur  on the missus’ ebike, it was sticking the clutch was fine but the bearings for the lower cage needed some love