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  1. #1
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    38T Outer Ring on a Triple Set up?

    From online shop's description of the chainrings:

    Shimano XT M785 Chainrings

    For 2x precision Dyna-Sys shifting use chainrings in their designated combinations, do not use alternate configurations
    Match (AK)'s together or (AJ)'s together


    Why can't the configurations be changed? I want to run a 38T outer ring on my XT M770 cranks. I know the M785 is 10 spd and my M770 is 9spd, but it's shifting fine. I only want the outer 38T ring.

    What I want to run:
    22t, 32T, 38T

    What I currently have:
    22T, 36T
    Rear ZEE mech
    SRAM 3x10 X9 front mech
    11T-36T XT Rear cassette
    XT M770 cranks and rings
    We Ride In God's Country!

  2. #2
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    It's recommended that you don't mix and match some components because they might not work optimally, from an engineering perspective.

    But first of all, your rear derailleur has a capacity of 28T (DH) or 36T (FR), i'll assume 36T; the setup you want to run requires (38-22)+(36-11)=41T capacity. Also, the difference between 32T and 38T is not that great, practically... (much smaller than 22T to 32T)

    You would be better off with a 26-38T + bash (or true 2X10), but even then you are pushing the rear derailleur's capacity at 37T because of your huge 11-36T cassette.

    All in all, adding that 38T chainring will probably work and shift OK, perhaps very well, but some gear combinations will definitely stress the rear derailleur.

  3. #3
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    Additionally... thinking about it, i'm not sure if you can use an outer Shimano 2X10 chainring as an outer 3X10 chainring... I guess it will be fine as long as it's the correct BCD (104mm), but i could be wrong.

    I'll reiterate that buying one inner and middle chainring is probably a better approach, for example some Blackspire 8/9 or 9/10 designed as inner and middle (ramped) chainrings. An inner chainring is only like $25. Nice colours available, too.

  4. #4
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    Thanks but all I need to know is if it'll fit and shift okay
    We Ride In God's Country!

  5. #5
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    Quote Originally Posted by myitch View Post
    Thanks but all I need to know is if it'll fit and shift okay
    I already responded to your question in the Turner forum. I replied that the M785 38T chainring will need a small amount of modification to fit on the M770 spider. I am also now thinking that the M785 chainrings are thicker where the bolt holes are. They are designed to mount on the backside of a spider... The chain ring bolt hole is recessed on the backside of the chainring which will probably cause spacing issues when mounted on the outside of a spider. I have a feeling that this will cause less than optimal shifting... You may want to look into a non-shimano chainring for this setup.
    Killing it with close inspection.

  6. #6
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    Quote Originally Posted by FireLikeIYA View Post
    I already responded to your question in the Turner forum. I replied that the M785 38T chainring will need a small amount of modification to fit on the M770 spider. I am also now thinking that the M785 chainrings are thicker where the bolt holes are. They are designed to mount on the backside of a spider... The chain ring bolt hole is recessed on the backside of the chainring which will probably cause spacing issues when mounted on the outside of a spider. I have a feeling that this will cause less than optimal shifting... You may want to look into a non-shimano chainring for this setup.
    I have a m785 40t chain ring and just measured the thickness... it's 5mm. I didn't have time to measure the thickness of my outer m770 ring but I believe it is 4mm. This would cause the spacing to be off a little so again it may not shift the best. I also looked at the Blackspire SuperPro rings and they advertise their 38t as a middle chainring with a 5mm thickness.

    Blackspire - SuperPro 104/64 BCD Chainrings | Blackspire
    Killing it with close inspection.

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