Considerations for plus-sized frames- Mtbr.com
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  1. #1
    650b me
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    Considerations for plus-sized frames

    I'd like to embark on a project to design a steel frame for 26+ wheels. What width bottom bracket do most builders use for plus-sized bikes? I only have experience with 73mm BB's, so I'm curious about the cascading effect that using a wider BB would have.

    Let's say I use an 83mm BB. Do I then need a crankset with a wider Q-factor? If so, who makes such a crankset? Chainline would be affected, so that will impact the rear wheel spec, correct? Do I then need to go to a boost hub? Other considerations?

    If such a frame can be readily executed with a 73mm BB and 135/142mm hub, just let me know and I'll probably stick with that. Seems like it would get awfully tight though. Thanks!

  2. #2
    650b me
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    I'll correct myself before someone else does: I don't mean Q-factor, I mean....spindle length.

  3. #3
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    I haven't really been paying attention, but it seems like you don't need any extra wide faffery if you commit to 1x. A quick look at what production builders are doing with 650+ should tell you; 26+ is the same thing with a small extra margin for error.

    If you give me your intended tire/chainstay dimensions and whatnot in a PM i'll mock it up in bikecad pro. I'm kinda bored.
    "Things that are complex are not useful, Things that are useful are simple."
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  4. #4
    650b me
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    Thanks Scott. I failed to mention it, but I am thinking a 1x drivetrain. I was just on Surly's site and they use a 73mm BB on the Instigator. Although they do use a chainstay yoke to make the clearances work. Hell, maybe I should just buy an Instigator frame.

    Thanks for your mock-up offer, but I don't have exact specs in mind. That, and I have Solidworks, so I'm well covered when it comes to CAD. Cheers!

  5. #5
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    In theory, the only thing stopping you from using any BB standard/chainline you want is chain/tire interference. In practice, for most frames chainstay/tire/chainring clearance is the bigger issue, of course.

    Solution: draw up what you want to do and see if it'll all fit. If not, look at moving the chainring/chainline outboard (there are lots of ways to do that) and make sure you move the cassette (either by offsetting or using a wider hub standard) to match your desired chainline.

    -Walt

  6. #6
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    Quote Originally Posted by golden boy View Post
    I'd like to embark on a project to design a steel frame for 26+ wheels. What width bottom bracket do most builders use for plus-sized bikes?
    My 1x1 with 40mm rims and Dirt Wizards has plenty clearance with its 73mm BB, but I only ride singlespeed, so I don't know if a derailleur setup would need more. Probably not if it was a 1x.
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  7. #7
    650b me
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    Thanks guys. Sounds like 73mm BB will work. I'll mock it up in Solidworks as Walt suggested. At this point, it is a design exercise, not an actual build, so diving into drawing/modeling is exactly what I should be doing. I hope I don't regret having sold my Anvil jig! But truth be told, I like the idea of designing and building my own 80/20-based beam-style frame jig.

  8. #8
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    Others have mentioned that you may be able to fit everything into a standard 73mm BB and 135/142mm hub. For reasons that will become clear below, I'll mention that uses a 6mm offset direct mount chainring if you go that way. To go over the wider stuff briefly:

    Boost moves the chainring out 3mm and the hub spacing out 6mm to 148mm. There are a couple of companies (Marin, others?) that use a QR hub with 141mm spacing. You need a special direct mount chainring with 3mm offset (Raceface, SRAM) or boost crankset (Shimano) for an ideal chainline.

    If you want a 83mm BB, you probably want to step up to a 150/157mm hub to get the right chainline. You need a special crankset for this. And you might need to hunt and/or spend a lot of money to get a 175mm crank since most of these are DH cranks and in 165 or 170mm length. If you want to use a 150/157mm hub with a 73mm BB, you can do so with a single ring by putting the ring on the outside rather than middle position, or using a zero offset chainring (like a BB30 chainring).

    The really confusing thing is to keep the spacing groups together:
    -142 thru/135 thru (rare nowadays)/135 QR
    -Boost 148 thru/141 QR (rare)
    -DH 157 thru/150 thru

    Point being, a boost 148 hub is not only 2mm different from a 150mm hub, it's really 9mm different because you should be comparing it to a 157. Those extra 7mm are for a lip for the end caps to sit on.

  9. #9
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    Surly has some great options as mentioned. I just did a Troll build, which gives you the option of going single speed or running a rear mech along with a 73mm BB with normal cranks, 135mm rear hub...

    You could also look at the Crust Evasion.

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