Help me Understand GRX chianring for chainstay clearance- Mtbr.com
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  1. #1
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    Help me Understand GRX chianring for chainstay clearance

    I had a gravel frame built by a builder and they got it perfect (below) Ultegra chainset. (olive green)
    But when I told them I want to go with the GRX they gave me
    (green and orange)
    the location of the chainrings are roughly in the same place. shouldn't they be more outboard? Is it as simple as adding 2.5mm?

    what do Shimano's chainline numbers mean, and how they translate for frame clearances.

    I want to make sure the chainrings don't interfere with the chainstay.
    How do I know where the chainrings are in reference to the center-line of the bottom bracket.
    Attached Thumbnails Attached Thumbnails Help me Understand GRX chianring for chainstay clearance-2020-09-15-09_04_30-autodesk-autocad-2021-%5Bmuur_4.dwg%5D.jpg  

    Help me Understand GRX chianring for chainstay clearance-2020-09-15-09_11_57-autodesk-autocad-2021-%5Bmuur_2.dwg%5D.png  

    Help me Understand GRX chianring for chainstay clearance-2020-09-15-09_39_51-chainsets-google-sheets.png  


  2. #2
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    For GRX, Shimano moved the cranks outward 2.5mm, but not the cassette. That doesn't change the chainline, it's still going to be based on the rear cassette. It's really terrible.

    As a result, you get a horrible chainline in most gears so that constructors don't have to try to make good bikes. The customers that are shopping GRX are not technically savvy so they can do this and get away with it.

    Worse, with a wider q-factor, most folks will find pedaling for long periods of time less enjoyable than a narrower q-factor.

    Basically, Shimano phoned this in so that clueless tards can put giant tires on junk bikes.

    I make all-road bikes with q-145 cranks that have clearance for double chainrings and 622-46 tires. This shouldn't be too hard but Shimano thought (correctly) that it would be for most builders.

    I haven't checked, but if you are going to use a GRX crank, you should probably be using a 148mm hub.

  3. #3
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    GRX moved the road chainline (formerly 130mm hubs) out 2.5mm to match the 135/142 disc hubs that are standard on "gravel" bikes.

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    I am going to be using a boost hub.
    I am asking about frame clearance for the chainstay.

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    Quote Originally Posted by dr.welby View Post
    GRX moved the road chainline (formerly 130mm hubs) out 2.5mm to match the 135/142 disc hubs that are standard on "gravel" bikes.
    Thanks I understand that, but I am concerned about the chainline clearance (interference with the chainstay)

  6. #6
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    You have the correct understanding, they moved everything out 2.5mm from their road groups.

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    Note that one of the reasons to move the chain line out 2.5 to catch up to 135 hubs was to reduce the chances of the chain hitting the big ring when crossed over in the small ring and small cog. This was mostly a problem with road length short chainstays and bigger big rings.

    Moving the cassette further out with a Boost hub essentially undoes this fix, but odds are you are still OK with the smaller GRX ring and (likely) longer chainstays.

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    Quote Originally Posted by dr.welby View Post
    Note that one of the reasons to move the chain line out 2.5 to catch up to 135 hubs was to reduce the chances of the chain hitting the big ring when crossed over in the small ring and small cog. This was mostly a problem with road length short chainstays and bigger big rings.

    Moving the cassette further out with a Boost hub essentially undoes this fix, but odds are you are still OK with the smaller GRX ring and (likely) longer chainstays.
    Thank you, I am interested in clearance with the chainstay though.

  9. #9
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    As I said, everything was moved out 2.5mm

    Help me Understand GRX chianring for chainstay clearance-screen-shot-2020-09-15-10.52.41-am.jpg

  10. #10
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    Quote Originally Posted by dr.welby View Post
    As I said, everything was moved out 2.5mm

    I do get that that ,and thank you.

    If you look at the chain lines in the data sheet, there isn't a 2.5mm increase. it goes from 43.5 to 46.9, or for the FX810 49.7
    If you look at the first detail you'll see the first ring is 40.7mm from the center of the bb. The second ring is 48.6mm from the center. That's for a 43.5 chain line. How are the 40.7, and 48.6 calculated from the 43.5? How would I get those numbers for 46.9 or 49.7?

  11. #11
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    I can't tell from that drawing what those numbers represent, so you should be talking with your builder.

    40.7 + 48.6 / 2 = 44.65, so those numbers aren't chainring centerlines for a 43.5 chainline anyway.

    Either you need to get the cranks and measure them yourself, or send a pair to your builder.

  12. #12
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    Quote Originally Posted by dr.welby View Post
    I can't tell from that drawing what those numbers represent, so you should be talking with your builder.

    40.7 + 48.6 / 2 = 44.65, so those numbers aren't chainring centerlines for a 43.5 chainline anyway.

    Either you need to get the cranks and measure them yourself, or send a pair to your builder.
    Sheldon Brown:
    Road Double 43.5 mm Shimano spec, measured to the midpoint between the rings.
    with typical 5 mm chainring spacing, this puts the inner at 41 mm, the outer at 46 mm.
    I think the numbers the builder is using is the outer edges of the chain rings.
    So an assumption of a grx minimum clearance for the
    FC-RX810-1 would be 47.2 and 52.2 ?
    And why is the RX810 the only one with the wider chainline?

  13. #13
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    2.5mm is a smidge more than the thickness of a chainring, it is not much but it does make a difference if the bike is built and assembled correctly.

    Having clearance with the chain stay is but one part of the equation.

    Could the drawings you provided be from two different builders?
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  14. #14
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    810-1 is a 1X crankset, so it would make sense that chain line measured to single ring in the outside position is about +2.5mm.

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    Quote Originally Posted by dr.welby View Post
    810-1 is a 1X crankset, so it would make sense that chain line measured to single ring in the outside position is about +2.5mm.
    I didn't pick up on that, thank you! So 49.7 is most likely the outermost dimension for all of the grx chainsets. I can understand why they say +2.5mm instead of 2.8, but 43.5 +2.8 is 46.3 not 46.9.

  16. #16
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    Quote Originally Posted by shoo View Post
    2.5mm is a smidge more than the thickness of a chainring, it is not much but it does make a difference if the bike is built and assembled correctly.

    Having clearance with the chain stay is but one part of the equation.

    Could the drawings you provided be from two different builders?
    Same builder, but it's been 5 years between drawings, so I am guessing tighter tolerances, their chainline is a difference of .25mm

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