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  1. #1
    Senior Member john hawrylak's Avatar
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    BB & Forks for 650B x 42

    I’m gathering information for a custom 650B Low Trail with 650Bx42 tires and fenders.

    Basic Questions:
    Are Bottom Brackets and Fork Crowns readily obtainable for 650B x 42 mm wide tires?
    Do the BB have the correct angles for the chain stay clearance or is a special bending of the stays required to achieve tire clearance?

    Concerns
    #1 A 2009 video with Boulder Bicycles Mike Kone mentions typical BB do not have the correct chain stay angle for good tire clearances with 650Bx42 tires. Kone mentions they build their own BBs. May be the supply situation has changed since then.

    #2 Same 2009 video mentions fork crowns are not wide enough for 42 mm tires and fenders. Again, may be the supply situation has changed.
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  2. #2
    Randomhead
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    there are crowns wide enough for 42mm tires, but supply can be spotty. Pacenti offers a couple of crowns, and there are Long Shen crowns that will clear 42mm. I assume that most crowns used on CX bikes will clear a 42.

    As far as BB shells, I am not sure which angle he is talking about, as far as clearance between the stays, there are various ways of doing it.

    I am just working on my first 650b frame, I am not using tires that big though


    Tubing sizes and styles change things a lot. Are you willing to use s-bend chainstays? Are you set on having a lugged frame? Standard, oversize, or double oversize tubes?

  3. #3
    Senior Member john hawrylak's Avatar
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    Quote Originally Posted by unterhausen View Post

    As far as BB shells, I am not sure which angle he is talking about, as far as clearance between the stays, there are various ways of doing it.

    Tubing sizes and styles change things a lot. Are you willing to use s-bend chainstays? Are you set on having a lugged frame? Standard, oversize, or double oversize tubes?
    Great clarifying questions. Apologize for not providing the info in initial post

    I thought Kone was talking about the angle the attachment points on the BB make with the shell.

    Lugged frame. Standard size tubing (1 1-8" DT, 1" TT).

    I am not sure about S-bends. Looking for guidance on what techniques are possible to use. Can the stays be bent in a near semi-circle at the point where the tire is??
    John Hawrylak
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  4. #4
    Randomhead
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    Maybe I'll take some pictures of my frame later to clarify. I am using the re-issue Columbus SL (standard size tubes) with round oval round chainstays. It looks to me that you could dimple the stays to fit a 42 mm tire with reasonable clearance. The only problem I had with my bb shell was the angle from the seat tube to the chainstays; the bb was a little closer to the ground than I felt comfortable with so I blacksmithed the sockets. There is also the angle between the chainstays. Mountain bikes have problems with chainstay clearance, and there are all sorts of tricks that people use. They are trying to clear 2 1/2" tires though, that's around 17mm bigger than the tires you are proposing to use.

  5. #5
    Randomhead
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    This bike has around 42cm stays (not positive about that, I'll have to measure) and that's a 32mm tire. If you were going to run 42mm tires and use fenders, that's pretty tight at the closest point of the seat tube. I was too lazy to prop the frame upside down to give a better view of the clearance on both sides at once, so I just moved one of the stays so that the tire is hitting a stay. It's hard to judge, but there is about 1cm clearance -- tire is touching on the side you can't see. There is some room to crimp the stays for more. I think I might have to crimp the outside of the stay a little to clear the inner chainring. If you were willing to go with a little longer stay, clearance might not be an issue at all.


  6. #6
    Retrogrouch jd0g's Avatar
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    650B build

    Hi, new to this corner of the forum. I have just built a steel frame based around the Nifty Swifty tire you have pictured (on the same rims too ). The best solution I found had me adding an S-bend to the chainstay to achieve proper alignment at the rear dropout, and a dimple for tire clearance. By adding a few mm to the BB spindle I avoided dimpling the chainstay twice. I think you'll need to get the fender jammed in there before you go much further. Will you be adding another chainstay bridge as a mounting point?

    True Temper OSX - main
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  7. #7
    Randomhead
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    welcome to the forum. The fender might be a problem, and I also want to be able to use Nokian studs, so there will probably be issues with those too. I think I'm stuck dimpling if i need clearance because these aren't going to bend properly. I do intend to add a bridge for the fender.

  8. #8
    Senior Member mudboy's Avatar
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    My 2nd and 3rd frames were both 650b.

    For #2, I used the Pacenti P-B crown, which was perfect, and has plenty of clearance for Fatty Rumpkins,

    For #3, Pacenti was out of crowns, so I just went with a 'cross crown from Nova. Again, no real problems, but I preferred the Pacenti.

    For BB's, on both frames I used oval chainstays and clearance wasn't an issue, even on relatively short stays (417mm for #2, 425mm for #3). Did I have a ton of clearance? No. Did I have enough clearance that I wasn't worried about it? Yes.

    See pics at my flickr account, URL is in the sig.
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  9. #9
    tuz
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    From what I understand the issue is that of the clearance between the (42 mm) tire and the chaintays. You want to increase the chainstay/BB axis angle, but that will increase the rear spacing for a 42-44 cm chainstay lenght, as well as causing some chainring clearance issues if using a low tread crank. So you have to bend the chainstay back a little, around the tire & fender. And you probably have to add a dimple too. That's how Herse did it.

    I'd like to build a 42x650b bike in the near future. If anyone has ideas on how to make that bend... a 0.7mm chainstay doesn't look like the easiest thing to bend...

    Edit. For crown options, there is the Grand Bois crown too. 53 mm between blades. The fenders for 38-42x650 tires are around 58 mm wide.
    Last edited by tuz; 01-31-12 at 11:45 AM.
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  10. #10
    Randomhead
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    Quote Originally Posted by tuz View Post
    I'd like to build a 42x650b bike in the near future. If anyone has ideas on how to make that bend... a 0.7mm chainstay doesn't look like the easiest thing to bend...
    there are s-bend chainstays for cyclocross, that's my only thought

    Note that the tires Pete used are 45mm
    Last edited by unterhausen; 01-31-12 at 02:56 PM.

  11. #11
    tuz
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    Quote Originally Posted by unterhausen View Post
    there are s-bend chainstays for cyclocross, that's my only thought

    Note that the tires Pete used are 45mm
    Right good to know! (you meant 42 mm I guess). Having the wide fenders might complicate things further however. It seems there are some 22.2 ROR stays available with an S or single bend. That should do the trick.
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  12. #12
    Senior Member mudboy's Avatar
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    Quote Originally Posted by unterhausen View Post
    there are s-bend chainstays for cyclocross, that's my only thought

    Note that the tires Pete used are 45mm
    Actually, on Weinmann ZAC-19 rims, they are 42.05mm wide @ 60PSI.

    Both bikes do have 135mm rear spacing, which will buy a little more clearance.
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