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I replaced my 3x8 system with 2x10 system on my Trail bike. My BB is of 73mm size and as a standard I left one spacer at drive side and none at non-drive side of the chain-ring as of now. But I could see the median of chain rings is not aligned with middle of my cassette and the smaller chain ring is aligned in line with 5th or 6th gear on cassette.

The model is Shimano Deore 2X chain ring.

What would be the best solution? Adding spacer(1 or two) at non-drive end of the crank and dropping the spacer at drive side? Or additional spacers to smaller chain ring bolts? Or both?

  • Have you installed the latest Shimano Deore M6000 groupset? That group is designed for the modern MTB 142mm or 148mm rear dropout spacing and runs a wider chainline. – Argenti Apparatus Apr 4 at 12:19
  • Yes. I ordered Deore M6000. Thanks for the reply. Any solution? I put it on Cannondale Trail 6. – Anand Apr 5 at 15:03
  • But as per the specification, rear dropout spacing remain 135mm for M6000 compatibility. – Anand Apr 5 at 15:20
  • Where did you get that bit of info from? Shimano spec says 142/148 rear spacing productinfo.shimano.com/#/spec/MTB/Front%20Chainwheel – Argenti Apparatus Apr 5 at 15:23
  • I guess O.L.D stands for spacing. The compatible M6000 shimano hubs are of type 135mm. Could be wrong. Any solution you may suggest for this? Referring this specification for hub -Shimano Deore M6000 productinfo.shimano.com/#/spec/MTB/FREEHUB%20(Disc%20Brake) – Anand Apr 5 at 15:54
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I can give an explanation of what is going on with the setup you have, and why the chainrings seem offset from the cassette. I think that perhaps there is not any problem and you don't need to adjust the chainline.

An older bike with a triple drivetrain will have 73mm BB shell (as you said) and a 135mm rear dropout spacing. Standard chanline was 47.5mm. The M6000 group cranks are designed for the modern MTB 142mm or 148mm rear dropout spacing and runs a wider chainline: 48.8mm or 51.8mm respectively. Shimano specs are here. The FC-M6000-2 model crank has the 48.9mm chainline, the FC-M6000-B2 crank has the 51.8mm chainline. Hopefully you have the former. In that case you're chainline is only 1.3mm wider, and your large ring on the new double crank will actually be closer to the centerline of the frame than the large ring on the old triple crank.

If you have a -B2 model, hopefully you can return it and exchange for the -2 model.

Updates:

Can't believe I missed this - if you have a 73mm BB shell you should not need the two 2.5mm spacers that come with Shimano mountain BBs. Those are for use with MTB frames that have 68mm shells. Check the BB instructions.

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  • Mine is FC-M6000-2. As of now I I did not place the drive side spacer. But added a spacer to non-drive side. Is it okay to keep like this and ride? – Anand Apr 6 at 5:53
  • You have a threaded external bearing BB I assume. No I don't think that's OK because you have reduced the number of threads the NDS cup is screwed into the frame, and increased stress on it because its further outboard. – Argenti Apparatus Apr 6 at 10:50
  • What if I dont use no spacers either side? – Anand Apr 6 at 12:40
  • @Anand then overall BB width will be narrower than the crank axle needs – Argenti Apparatus Apr 6 at 12:56
  • You are right. Sorry to bother you. Appreciate your help on this. What will be an option if I plan to replace BB and 2X crank to make sure it will work smoothly with my existing axle and new 10 speed (11-42) cassette? – Anand Apr 6 at 13:14

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