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  1. #1
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    650b for new frame, I haz questions

    I'm working towards ordering a new frame, primarily for road with some dirt fireroad stuff (SoCal, so rocky hardpack stuff). I ride a 54-55cm frame and like a compact cockpit. I'm thinking of going with a disc 650b wheel to allow for larger volume tires. Having gone through a few different frames in the last few years seems like a 650b rim with a 38 or 42 tire will me to retain a compact cockpit, run higher volume tires, and without gobs of toe overlap. (I realize frame geometry and stem length play into the overall equation here.)

    So what the question you ask. Question is, is my thinking flawed? Should I be thinking about other factors?
    I thought of that while riding my bicycle. ~ Albert Einstein on the theory of relativity

  2. #2
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    Lots of newer frames allow use of both 700c wheels with road-ish tires and 650b wheels with larger volume tires. The circumferential difference of a 700c with a ~25mm tire and a 650b with a ~45mm tire is going to be just about the same, so while you'll see toe overlap benefits with the large volume 650b tire compared to with a large volume 700c tire, your overlap situation with the big 650b tire will be about the same as it is with a 700c road tire.

    Guy I ride with has the new 3T aero gravel bike. Apart from the fact that I throw up in my mouth every time I think of the phrase "aero gravel bike" and that he's had some significant teething issues with seat post slippage and a few other niggles, it's a super cool bike. If I was in the market for that kind of bike I'd definitely head toward one meant for the 700c road tire/650b gravel tire program. I think you're spot on.

    It will be interesting to see whether those kind of bikes evolve more to shorter (like where 70mm means "long") mountain bike stems or maintain more road-traditional ~100mm stems.

  3. #3
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    The diameter of a 700c/25mm compared to a 650b/42 is less than a half inch different, but that difference will grow with the use of larger tires on the 700c.
    The rim/tire combo of 650b will also be slightly lighter than the same rim and tire combo sized for 700c.

    I don't know what bicycle you're contemplating, but there are a number of low trail bikes being made and that offer 650b in a low trail geometry and that would help with the toe overlap, but gives the bike a different feel than the mid to long trail that most of us are riding.

    I've been running a low trail 650b/42 for a coupla yrs now and really like the ride of those large volume tires and handling of the low trail.
    Too old to ride plastic

  4. #4
    wheelbuilder
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    It's a great setup for road and even most moderate trails. I also run with and without fenders depending on the season.




  5. #5
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    Quote Originally Posted by velodog View Post
    I've been running a low trail 650b/42 for a coupla yrs now and really like the ride of those large volume tires and handling of the low trail.
    What bike is this?
    “Statistics are like bikinis. What they reveal is suggestive, but what they conceal is vital.” -- Aaron Levenstein

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  6. #6
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    Great comments and affirmation of my thoughts. I have read that 700c/25mm is almost equal to 650b/47mm so something like the WTB Horizon or Compass Swithcback Hill tire will work well. The geometry then get's designed around a standard 700c bike. My contemplation is do I really want to run a tire that wide for normal use. The point of this new bike is to run larger volume tire and run disc for greater flexibility with wheel and tire size. The largest tire I use now is 38mm and that is nice, especially compared to the 28mm I use on my racier carbon bike. A 47mm feel oh so BALLOON-like for a bike that will 90%-95% of the time be on pavement of decent quality. (I've read Jan/BQ's findings on tire size not impacting speed so there's that I guess.)

    Frame and geometry wise I will be going with a custom steel frame. I have a low trail sportif-style frame from DeSalvo and a lightning quick Chinese carbon frame with 50mm Chinese carbon wheels. I love riding both for how well they do what they do. I've done a 9 day loaded tour, countless centuries, and a few doubles on the DeSalvo. For a bike that by most accounts should not be comfortable all day, the Chinese bike is a dream to ride all day. It's so light (around 16# dry) and the tight cockpit make it just as or more enjoyable to ride on a 100-150 fast road ride. Only place where is suffers geometry wise is steep fast mountain descents above 40mph - talk about white knuckle! I'm leaning towards like a 60/40 split (60% Chinese / 40% DeSalvo).

    I will likely go with something like an 80-90mm stem and short reach/shallow drop bars to keep the cockpit tight. I've found if I can get the right saddle/bar drop I can keep from being too stretched out whilst not being too upright.
    I thought of that while riding my bicycle. ~ Albert Einstein on the theory of relativity

  7. #7
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    Quote Originally Posted by ergott View Post
    It's a great setup for road and even most moderate trails. I also run with and without fenders depending on the season.
    Rad bike and wheel sir!
    I thought of that while riding my bicycle. ~ Albert Einstein on the theory of relativity

  8. #8
    wheelbuilder
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    I run 38mm Pacenti Pari Motos (run 40mm on these rims). The handling difference is there compared to same bike with 700X25 but I adjust quick enough. At 40psi it's a pretty fast setup and I've dropped into pacelines with the bike. Fun keeping up with all the road bikes. Compass also makes some fast smaller rubber so you don't have to go as high as 47mm.

    Sent from my SM-G950U using Tapatalk

  9. #9
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    Quote Originally Posted by Lombard View Post
    What bike is this?
    Boulder All Road

    650b for new frame, I haz questions-img_0931.jpg
    Too old to ride plastic

  10. #10
    wheelbuilder
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    Quote Originally Posted by MTBMaven View Post
    Rad bike and wheel sir!
    Thanks!

    Sent from my SM-G950U using Tapatalk

  11. #11
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    Quote Originally Posted by velodog View Post
    Boulder All Road

    Click image for larger version. 

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    Classy!
    I thought of that while riding my bicycle. ~ Albert Einstein on the theory of relativity

  12. #12
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    You gents seem to operate in similar vein as me with collective cycling interests. When designing the geometry of the new bike, it seems the frame should be designed around the smallest/narrowest tire I expect to run to minimize the risk of pedal strike? Maybe other things as well?

    With all else remaining the same (frame/fork/rim), the only changing variable is tire circumference/diameter. I did a little research, and if my math is correct, the difference in diameter between 650b/32mm and 650b/48mm is 33mm or 1.3 inches. That seems like a fair bit. So question is, are there any negative effects from raising a bike 33mm and/or changing the wheel diameter by 33mm? Mostly concerned here with handling and not rotational weight and such.
    I thought of that while riding my bicycle. ~ Albert Einstein on the theory of relativity

  13. #13
    wheelbuilder
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    It's going to effect trail and wheel flop. You can't really have a bike handle well at both those extreme tire sizes. I'm not sure why you'd want to. 650X32 is pretty small, I won't go that route for my bike. These 38s are the lowest I'd go. I might even try some 42mm tires at some point.

  14. #14
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    I think that if I would stick with 700c wheels for 32mm tires. There are a coupla 32mm options for 650b but I would just go 38mm as the smallest for 650b.
    The only reason I see for a 32mm\650b would be a conversion where there isn't clearance for a larger volume tire, and that wouldn't be the case if your having a frame built.
    Too old to ride plastic

  15. #15
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    Ergott and Velodog,

    Thank you both so much for your feedback. Very helpful discussion! I'd buy you both a beer if I could.
    I thought of that while riding my bicycle. ~ Albert Einstein on the theory of relativity

  16. #16
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    Quote Originally Posted by MTBMaven View Post
    Ergott and Velodog,

    Thank you both so much for your feedback. Very helpful discussion! I'd buy you both a beer if I could.
    Sparkling water for me, if you could.
    Too old to ride plastic

  17. #17
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    From my reading it seems that a number of folks settle on 650b x 42 as the best allrounder size (rather than going to 47). I'm starting on 47 to 2.1 though, as I'm using the bike for mainly gravel/rougher track rides.

  18. #18
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    Quote Originally Posted by tangerineowl View Post
    From my reading it seems that a number of folks settle on 650b x 42 as the best allrounder size (rather than going to 47). I'm starting on 47 to 2.1 though, as I'm using the bike for mainly gravel/rougher track rides.
    Thank you for the input tangerineowl. I think I will start off with 42.

  19. #19
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    MTBMaven...hit you with a PM.

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