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I am somewhat puzzled, because one of the promises of Boost standard was "you can run wider tires". But it does not seem so -- I try to figure out my next (?) setup, 2x10, 29x2.6" tires and I am looking on Alivio FD non-boost version with clearance of 2.3" tires and Boost version with... 2.3" tire clearance.

  • I have something like 20-years old bike, non-Boost of course, 3x9, 26x2.35" tires and I have around 1 cm of space between tire and the chain in the closest position to the tire
  • my naive guess is chainrings in 2x crankset are placed in "free spaces" between chainrings in 3x crankset, so in the case of this Alivio FD the chain is pushed further outside,
  • Boost also adds additional push outside.

So I am totally lost here, I have 26x2.35" tires, I push and push chain outside in order to get more space, and as the result I have room for 2.3"?

Granted, the wheel size plays some role here, looking from BB perspective, the chain in 29" tires will run more parallel.

So maybe I rephrase -- should I be afraid of this 2.3" official spec and take it seriously, or is it just safe play on Shimano part and I can get away with something wider? Like the mentioned 29x2.6"? Or at least 2.4" to see some progress :-).

Background info: the next setup is supposed to be Ritchey Ascent which supports 29x2.6", at least as the frame+fork is considered. Now the question is about rest of the gear.

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  • One idea - have a close look at a fatbike next time you come across one in the wild, and see how their BB is set up. Example mtbr.com/attachments/image00016-jpg.1112679
    – Criggie ♦
    Nov 11, 2022 at 21:12
  • @Criggie I see the picture and at the same time I don't see what is so special there :-). Can you point out what should I pay attention to?
    – greenoldman
    Nov 12, 2022 at 4:41

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The FD limits the tyre width by moving the cage to the left, to access smaller chainrings.

If you could move the FD, right crank/spider/chainring to the right then you'd have more room for wider tyres, but then the cassette's neutral position would be shifted to the right and low/easy gears would have more-extreme chain bend.

One solution is to move the cassette to the right, along with the derailleur. That's how we got wider standard OLD measurements, and the newfangled BOOST and variants.

Example - "Super Boost Plus" has an Overlocknut dimension of 157mm where road bikes are 135mm. (but many of those bikes are 1x and lack the FD completely)

You'd think another option might be to move the wheel backward so the FD and tyre are apart, but in the smaller chainrings the chain still passes close to the tyre.

enter image description here

So this doesn't help either.

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  • Thank you. I understand this all. Maybe I was not clear -- let's start from my old bike. I have X space (in mm), then I change FD to double to make more space, change to Boost to make even more space, and the end I have less space. It does not compute. So something took that space. I am looking what was it or if Shimano specs were way off.
    – greenoldman
    Nov 12, 2022 at 7:58

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