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Can someone help me please. I'm trying to understand why Shimano has created 3 different cranks for it's 1x12 systems. Each one has different chainline and one of them doesn't appear to make any sense.

I'm using XT numbering (FC-M81XX) but question it applies to all Deore, SLX, XT and XTR.
  • FC-M8100 - 52mm chainline - Perfect chainline for 148mm boost hubs
  • FC-M8130 - 56.5mm chainline - perfect chainline for 157mm "super boost" hubs
now, the mystery one...
  • FC-M8120 - 55mm chainline - Perfect for ???
Is there some other standard? Is this one meant for 2x systems? Is there some other reason?
 

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since 4/10/2009
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My Guerrilla Gravity Pedalhead uses 148mm hubs with a 3mm offset to the drive side, and a 55mm chainline spec. Pretty sure all GG bikes spec this way.

IIRC, the Yeti ARC uses 148mm hubs and a 55mm chainline.

I'm sure there are more, but these are the ones I know.
 

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I’m running the SLX FC-M7120 with the 55mm chainline on both my GG Smash and my Rootdown and the chainline spacing is perfect. I think it’s necessary on the GG as stated above. Maybe not on the rootdown but it helps with tire clearance and has no negative effects that I’ve noticed.
 

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Some frames need more clearance for larger chainrings, or even normal chainrings.
Shorter chains stays means that the area around the BB has to be wider, meaning you need to move the chainring outboard.
Same thing with wider tires, larger tires, larger chain stays around the BB, wider chainline.
 
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