GRX crankset with Shimano MTB derailleur

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  • Hello,
    I was reading about the spec of GRX groupsets.
    Shimano says about their cranks:
    "chainline has been shifted outwards by 2.5mm compared with existing road components for increased tyre and frame clearance"
    And about the front derailleur:
    "the front mech must be used with the matching GRX chainset which is similarly pushed outward from the frame by 2.5mm."

    I guess the left crank arm is 2,5mm outward as well, for symmetry.
    So if installed on a 68mm BB, it would be like normal cranks installed on a 68+2x2,5 = 73mm frame, MTB standard.

    My question is, would a 2x GRX crankset shifts correctly with a MTB front derailleur?

    I'm especially thinking about an XT di2 front derailleur on a road, 68mm BB, frame.
    This configuration with road cranksets is known to have the front derailleur too much on the right (problem to reach the lower chainring) due to the derailleur being made for 73mm bb



  • It might just be the rings which are 2.5mm further out. I think the Q factor is the same. GRX appers to be 151mm

    Think to consider. Is the spacing between the rings the same on mtb and grx? Is the chainline the same?

  • I put a GRX chainset on an otherwise ultegra di2 build on a road frame. I believe the GRX q factor is slightly more than the road R8000/R7000, 5mm in total from memory.
    The front mech is fine, but it may depend on the frame, it's right on the limit on the big ring.

  • You're right, I have to consider chainline, not Q factor

  • Thanks for this, I did not imagine that was so many of them!

  • After searching a bit, considering the Q factor is not relevant, we can compare the chainlines given by shimano:
    XT 2x crankset, FC-M8000-2 (non Boost), has a chainline of 48.8mm
    GRX 2x crankset, FC-RX810-2 or FC-RX600-11 have a chainline of 46.9mm
    Ultegra 2x crankset, FC-R8000, has a chainline of 43.5mm

    We can first see that the difference between GRX and road chainlines is 3.4mm, not 2.5mm

    Chainline difference between XT M8000 and GRX 2x cranksets is 1.9mm
    Chainline difference between XT M8000 and Ultegra R8000/DA/105 2x cranksets is 5.3mm

    We can conclude that a GRX 2x crankset will work better than a road 2x crankset with an XT 2x front derailleur, chainline difference being less than 2mm

  • GRX 2x crankset, FC-RX810-2 or FC-RX600-11 have a chainline of 46.9mm

    I was under the understanding the 1x and the 2x options had different chainlines and thought the 46.9 was for the 1x.

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GRX crankset with Shimano MTB derailleur

Posted by Avatar for Cosme @Cosme