[Closed] Chain jumps 2/3 sprockets with (even slight) backwards rotation of cranks...why?

Chainline and the compromises of 1x systems with wide range cassettes. Possibly exacerbated by drivetrain wear.

Park Tool did a video on it on their YouTube channel last year I think.

Thanks @frogstomp 👍🏻

I had wear in mind, although there's nothing obvious...

Could worn jockey wheels be a culprit in this instance? 🤔

Will check out the Park vid. - ta for the heads-up 👍🏻

Could worn jockey wheels be a culprit in this instance?

No, the problem is that derailleur systems aren't designed for backpedaling. When you're pedaling forwards, the chain is guided into the correct sprocket. When you backpedal in the extreme gears, the chain is angled so it tries to jump gears to align itself. The jockey wheels can't guide the chain when you are backpedaling, only when you are pedaling forwards.

it's a known issue of chain line and large sprockets, just learn not to pedal backwards when in the easiest gears

Absolutely, @thols2 - agreed 👊🏻

However there are some occasions where I choose to backpedal slightly - maybe just 1/2 a rotation - in order to get my pedals/cranks into a better position...

The reason I'm baffled is that it's not done it before now, even in extreme gears (sprockets 1 & 2). Why all of a sudden... after cleaning?!

Likewise, it's never happened to me on a 1 x 10 nor a 1 x 12! 🤔 I've always been able to take 1/2 a backwards rotation, even in sprockets 1/2/3, to set my foot position...

#confused

Worn cassette and all the gunk was making chain less flexible or sticking chain to cassette slightly. Now you've cleaned it, it can come off the sprockets easier

That's a wild guess, BTW.

XT m8000 was notorious for this, I ran 40t and 42t cassettes with medium cage mech and it did it from new even when I put the chainline at 48mm, a bit extra on the b-tension helped slightly but it still did it, I recall something about using 46t cassette with long cage didn't have the problem, strange how it's only just started doing it.

Yeah I have seen this from new also, I always fixed it with B tension and found in one case it was caused by chain too short.

Are the bearings in the derailleur jockey wheels seized? This has caused problems for me before.

Many thanks for the thoughts & suggestions, folks 👍🏻

Indeed it's the random occurrence of this issue that's baffled me?!

Jockey wheel bearings seem fine 👍🏻

@endomick - what adjustment did you make to get the chainline to 48mm? From...? 52mm?

@5plusn8 - what adjustment did you make to the B tension to alleviate the problem? ... and, just one link out of the chain, or more?

Cheers all 👍🏻 Anyone else with similar issues? 🤔

M8000 chainline was 50mm the boost was 53mm, 2mm chainring washers/spacers from superstarcomponents, a blackspire snaggletooth will also reduce chainline by 1mm giving you another option. Plenty ways to reduce chainline.

more B tension, move the jockey wheel away from the cassette. You have to be careful though,
because too much ruins shifting too.

The too short chain was 2 links too short. I dunno why it made the chain ride up when pedalling backwards, but putting 2 links back to get the good length as per Shimano guidance made it go away. I just fixed it cos I found it that way on a mates bike he had paid a bike shop to fix his gears, new chain cassette, cable, and the bike shop dude cut the chain too short.

This is/was a well known 'feature' of XT 11speed. OH's bike suffered from it. I did a bit of looking and found suggestions that Sram PC-1170 chain would cure it. It did.

… XT 11speed. …found suggestions that Sram PC-1170 chain would cure it.

Same here (on my bike). Unfortunately the SRAM chain didn’t make any difference. You just sort of get used to it eventually.

As above, it's a "feature" of XT 11 speed. For me it went away as soon as I stuck a Sunrace cassette on in place of the Shimano one.

However there are some occasions where I choose to backpedal slightly – maybe just 1/2 a rotation – in order to get my pedals/cranks into a better position…

get into the habit of frontpedalling half a turn, is my experience of how to fix it properly I'm afraid. Also have less of a dominance to your dominant foot as that's the only real reason for a half backpedal - go into things 'wrong foot forward' deliberately to get more comfortable at doing it.

Like you I found the situation worse in the low gears and tbh with wide range cassettes that means slow uphills mainly where maintaining momentum is usually key so the extra halfturn if you absolutely have to get chocolate foot forward doesn't hurt.

Since you've just cleaned everything, could it be possible that the freehub bearing is a little stiff and thus when back pedalling, the resistance caused by the stiff bearing is causing it to jump?

For me it was a bike stand annoyance, it never came into play when I was actually climbing unless I got out of my depth and had to bail out on a oh oh too steep, what was I thinking section.

The other culprit is the directional nature of shim 11sp chains. SRAM or KMC chains help.

SRAM chain and cassette will fix it, it seems to be a Shimano thing

How thorough a clean are we talking - did you remove the chain? Any chance you might have refitted it backwards? Shimano chains are directional and will jump when backpedaling if backwards.

I would say there is a reasonable chance that you haven't caused this to happen by cleaning it at all and that if you'd have tried this in a stand or whatever beforw cleaning it would have done the same. Someone alluded to this, it simply might not actually happen when riding.

Does it do it whilst riding now? That's the real question you need to answer.

I'd ride it and see before messing around with adjustments..if it does it whilst riding by all means tinker with B tension etc etc.

I’ve had that with my xt 1 x 11 since new, nothing to do with wear - suspect something to do with its design.