Chainline question Shimano 1x11 BSA on 036 frame

[gj-070]

Hello,
Plan was to create a 036 build topic when i am finished, current status is that i am almost finished, almost ready to take some pictures. Without pictures no nice build topic

But i have a question .. some specs:
- Frame 036
- Bottom bracket BSA, 68-73mm, with a 2,5mm shim ring on both sides installed.
Spec : Model: BB-MT800, BB-Type: HOLLOWTECH II, BSA, Threaded BB shell width: 68/73mm, Shell diameter: BSA (BC 1.37)
- Crank Shimano 1x11, chainline 50.4mm according to specifications, can not find a particular part number or other marking referring to the proof that it is a 50.4mm. Roughly measuring the 50.4mm looks OK
Specs: Shimano XT 11-speed Crank FC-M8000-1 32 175mm, Model: FC-M8000-1, Gradation: 32T, Crank Arm Length: 175mm
Q-Factor: 176mm, Threaded BB shell width: 68 / 73mm, Gear: 11-speed, Construction: Hollowtech II, Compatible Chain: HG-X 11-speed, PCD: 96mm, Chain line: 50.4mm
- Rear side 12x142mm

When i hold a straight bar next to my front chain wheel, i would expect my bracket chain wheel to be exactly in line with the rear cassette 6th chain wheel. So during chain wheel 1,2,3,4,5 have chainline offset to one side and chain wheel 7,8,9,10,11 have offset to the other side.
Unfortunately the straight chainline looks close to the 5th sprocket instead of 6th.
Do i have something wrong in my setup ?
Anybody checked this at his 036 build with Shinmano 1x11?

Best regards, Gerjan

[MTB2223]

No, you don't do anything wrong. The chainline is on the 5th cog for the XT M8000.
But Yes, I think it's better to have it on the 6th cog, but it wont solve the back pedalling issue on the 11th cog (42T).

[gj-070]

Thanks a lot for the reply, yes it looks similar like the movie.
Strange but understandable, since the 42 tooth cassette chain wheel has moved inwards.

So are there people who have added shims like in the You Tube movie to their front chain ring? Results?
Do you need extra long Shimano chainwheel bolts (i don't even know if available) ?
 

[Carbon_Dude]

I have an XX1 setup on my 036 but it would generally be the same as your Shimano 1x11.  For the -036 I needed to add the spacers that came with the bottom bracket to get the proper spacing for my chain line.  I believe the BB width on the -036 frame is 68mm, I needed to add a 2.5mm thick spacer on each side of the BB to make it like the 73mm BB on my -057.  When I back pedal, I don't have any issues, just as smooth as any other drivetrain.

[MTB2223]

With a SRAM chain on a XT M8000 cassette almost no back pedal issue

[gj-070]

Nice info.
Any experience, how a KMC chain on Shimao 1x11 performs during back pedalling ?

[MTB2223]

Any experience, how a KMC chain on Shimao 1x11 performs during back pedalling ?

See the youtube video.

[Carbon_Dude]

Makes me wonder why I don't have any problems with the chain dropping of my 42T cog on my XX1 when pedaling backwards.  Maybe because SRAM did their homework and engineered the XX1 group to not have this problem?  Maybe I have a better chainline than the guy in the Youtube video who is running an aftermarket cog on a 1x10 cassette?  Maybe I have a problem and don't even know it?

I think the SRAM chain works a little better in the Youtube video because it's more forgiving when flexed sideways, most likely the chain is being pulled too far towards the big cog in relation to the front chainring and it just wants to move toward the center of the cassette.

Another possibility is the guy needs to put one or two links in his chain for the larger 42T cog or maybe adjust his B-tension screw.  Either way, just because he made the problem a little better by switching chains, doesn't mean he has fixed the problem.  He still doesn't understand root cause.

To answer my own question from above, my SRAM XX1 does behave similarly when in the 42T cog and I pedal backwards.  However, my crank needs to go around two times before the chain jumps down to the next cog.  This is very interesting, I guess I've never been in the 42T cog and pedaled backwards at least two crank revolutions.

I wonder if changing my HIGH LIMIT screw to allow the rear derailleur to move over slightly would help?  It might but it doesn't matter, I adjusted my HIGH LIMIT so that the chain does not jump of the 42T cog on the back side (even when I push hard on the shifter) and really cause a problem.

I'd much rather have the chain jump forward on the back pedal (something I've never noticed before), than have the chain jump off the backside into the spokes.

[cmh]

Maybe I have a problem and don't even know it?

That was my guess. I've got the X1 group and probably have this problem with my KMC chain on there - but I couldn't help but think - how often do I really do anything more than a quick backwards reposition of my feet when I'm in that gear? That gear's reserved for the big climbs, and usually you've got to keep the power on to keep moving. If you're going fast enough to be able to backpedal enough for the chain to come off, then you should be shifting up anyway.

Also, in all instances, the chain remounted the proper gear when it was pedaled forward - so again, I'm wondering how much of a problem this REALLY is.

I mean, I can be pretty damn anal about how a bike works on the stand, but this is a gear designed to shift under power which means it's DESIGNED to let the chain go when instructed. I'm far from convinced this represents a real problem in any actual riding situation.

[Carbon_Dude]

I agree, it's really a non-issue.  I must never backpedal, even for fun (or maybe bordem), when I'm in the big cog.  I'm also fairly anal and it doesn't bother me.  Now I can go obsess over something else.

[carbonazza]

I meet the problem quite often.
On steep climbs, when it is muddy I try to climb them anyway, and sometimes I fail.
I unclip, and walk until it becomes a bit flatter.

To position the pedal on the strong foot, we do this by pedalling backward.
But sometimes in the action I forget the issue, and jam seriously the chain between the cassette and the derailleur, even with an half rotation.

On my late 256 I used for a while a 30 teeth ring that had a more pronounced offset.
Reducing the chainline. The problem disappeared.
But I'm not sure, if this caused the 4 smaller sprockets of the cassette to wear prematurely by having a bigger chain angle.

So now when stopping in the middle of a muddy climb, I try to remember to lift the seat/back wheel and position the pedal by pedalling forward.

[cmh]

I avoid that problem by living in an area where if I ride when it's muddy, there is a healthy and well established guilt distribution network. 

I just got an Absolute Black chainring, and it's kinda interesting because they shift it inboard a mm to "fix" the chainline:

"All 3 sizes have built-in M8 threaded standoffs to provide the ideal 1X chainline. They improve stock Sram chainline by 1mm."

I understand the reason for having the chain line up off-center, but it seems like the big cog issue could be reduced by just getting the chainline on the center of the cassette. Dunno.

[trekcarbonboy]

I avoid that problem by living in an area where if I ride when it's muddy, there is a healthy and well established guilt distribution network. 

HAHA! TRUTH!! 

[karstenhorn]

Is it only the 1x11 groupsets that are having this issue with back pedalling on the biggest cog ? I do have a 2x10 setup with a wide range setup 11-42 and I do not have any issues whatsoever. I can even be at the big cog at the front and the biggest cog at the rear and no jumping off at all.

As I also sometimes have to step off in the middle of a climb(if I'm quick enough to click out before I crash), I would find it rather irritating not being able to put my pedals into position by turning them backwards.

Karsten

[cmh]

Is it only the 1x11 groupsets that are having this issue with back pedalling on the biggest cog ? I do have a 2x10 setup with a wide range setup 11-42 and I do not have any issues whatsoever. I can even be at the big cog at the front and the biggest cog at the rear and no jumping off at all.

My guess is the 2x10 - if the cassette is narrower, as I think I've read that it is - then it might be just far enough inboard to lessen the likelihood of the backpedalling issue.