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  1. #1
    L1MEY
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    Chainring offset for boost?

    I'm looking into getting an oval chainring for my bike but I'm somewhat confused about the amount of offset required. I tried reading a few articles about it but it's as clear as mud to me :/

    I have a small Ibis HD3 currently running a full GX Eagle drivetrain with GPX bottom bracket. I have boost spacing and I'm running 2.6" tyres (may be switching these to 2.8 at some point). I want to run a 28T oval chainring (yes, I'm a weenie... I ride long uphills in the mountains and I have a dodgy knee). Do I need 3mm or 6mm offset on the chainring?

    Thanks

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    - Jen.

  2. #2
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    Boost uses 3mm offset

  3. #3
    L1MEY
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    Thank you

    Sent from my SM-T800 using Tapatalk
    - Jen.

  4. #4
    L1MEY
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    I've been doing some further research on this and found an article from Wolf Tooth that talks about 1x chain line: https://www.wolftoothcomponents.com/...and-chainrings

    From this article it looks like the ideal chain line for boost is actually achievable with a 6mm offset rather than 3mm, with the caveat being that if you want to run a 32t or 34t chainring you're not going to have enough frame clearance. So in theory it looks like I could probably run a 6mm offset with no problems if I'm dropping down to a 28t oval chainring because it won't interfere with the frame. In my mind, this seems like the ideal solution because I'm more concerned about having the easier gears running smoothly with the chain at less of an angle (I use my 50t granny gear a lot, and don't even remember the last time I used the top gear). Any thoughts on this?

    Sent from my SM-T800 using Tapatalk
    - Jen.

  5. #5
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    Wow. With 28x50 you will motoring along at about 4 mph (6.3 kph) at 90 rpm cadence. That is walking speed. Them is some nasty climbs. Glad I don't have anything like that in my neck of the woods.

  6. #6
    L1MEY
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    Yep, there's some serious climbing around here! The downhills make it all worthwhile though 3000ft descent over 10 miles, yes please!

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  7. #7
    L1MEY
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    Just as a quick update to this in case anyone else was considering doing the same thing...

    I installed the 28t oval with 6mm offset today. It clears the frame by 4mm, so no problem there. Tyre clearance in granny gear (50t) is 8mm at the top where the chain runs between the 50t and the chainring, and 6mm at the bottom where the chain runs between the jockey wheel and the chainring. With 2.6" tyres this seems to be fine, though I'm not sure I would want to run fatter tyres with this setup (my frame can handle up to 2.8"). The chain line looks good and there is less of an angle when in the lower gears, which is what I was aiming for as I tend to spend most of my time at the easier end of the cassette. So far, so good... I'm testing this out on the trail tomorrow and will update this thread if there are any problems or issues.

    Sent from my SM-G950U using Tapatalk
    - Jen.

  8. #8
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    Couple of threads in the Ibis forum about this. On my Mojo 3 and Ripley- both boost I had better chain line with XTR crank and one up ring that put my chainline at 49mm.
    OG Ripley v2
    Carver 420 TI

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