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  1. #1
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    Building 650b MTB with 29er Fork

    I'm just curious to get thoughts on using a 29er fork for a 650b singlespeed mtb I'm going to be building. The bike is going to be a hardtail singlespeed and I'm looking to put a 100 mm fork on it. The axle to crown difference (650b fork vs 29er fork) is so minimal that I would imagine if factored into the frame design it would be fine. Thoughts or comments?

  2. #2
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    It'll work fine

    In terms of axle-crown, just build the frame for the length of fork you'll use and you're good. You will want to think hard about trail/HTA/offset/front center, though, as the 29er forks run more offset than 26" or 650b. That will mean you'll probably need a slacker HTA and shorter effective toptube to keep the right trail and prevent FC from getting too long.

    -Walt
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  3. #3
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    Thank Walt.

  4. #4
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    If I may interject;

    Walt;

    - I assume there is a "window" or range where front center could be considered ideal for a given rider. Everything else remaining unchanged, how big (long) would you generally consider that window to be before ergonomics and/or handling began to change noticeably? Can you quantify that?

    - Given a frame in the proper range assumed above, it would be my further assumption that lengthening FC would "slow" the handling. Is that correct, or is that even the proper change that would occur?

    Sorry for the hijack, but it seems to be of at least peripheral value here.
    Most people ply the Well Trodden Path. A few seek a different way, and leave a Trail behind.
    - John Hajny, a.k.a. TrailMaker

  5. #5
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    Quick response

    The most useful thought exercise may be to think about very long (ie 3 meters or something - chopper from hell) and very short (say, no fork rake and 80 degree HTA for like 500mm front center). I am making those numbers up, just for the sake of explanation.

    On a mountain bike, you care about 2 things involving the front wheel:
    1: Can you put weight on it to make sure you don't crash in turns (your front wheel is the one doing most of the work when you're turning)?
    2: Can you keep weight off of it so that you can lift it over things and when you run into stuff, you don't go over the bars?

    The tradeoff is obvious - you will *never* go OTB on, say, my longjohn cargo bike. On the other hand, if you try to make a hard turn, the front wheel may very well decide not to cooperate. On the other hand you can get a SUPER nice tight turning radius and very precisely weight your front wheel on a super short front center, but you might not be very happy if you try to ride down a steep gully full of roots.

    So with that said, for 29ers, I think the shortest I've done is around 625mm and the longest is about 750mm or something like that (for some super tall person). The problem is that there's really not a formula - the best approximation is going to be based on saddle height, probably, but really we care about what the bike does when the rider is out of the saddle, mostly, so since different folks ride VERY differently (bent legs vs straight, "attack" position vs leaning back, etc) for me it generally comes down to a judgement call.

    I know that's never what anyone likes to hear, but I've had no luck with formulas for determining front center. I have a range I like for any given size or style of rider and I try to hit the sweet spot by intuition based on my experience.

    I do have REAMS (500+ frames worth) of data on this, so maybe it's time to do a multivariate regression and let the data tell me what the formula is!

    -Walt


    Quote Originally Posted by TrailMaker View Post
    If I may interject;

    Walt;

    - I assume there is a "window" or range where front center could be considered ideal for a given rider. Everything else remaining unchanged, how big (long) would you generally consider that window to be before ergonomics and/or handling began to change noticeably? Can you quantify that?

    - Given a frame in the proper range assumed above, it would be my further assumption that lengthening FC would "slow" the handling. Is that correct, or is that even the proper change that would occur?

    Sorry for the hijack, but it seems to be of at least peripheral value here.
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  6. #6
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    Quote Originally Posted by Walt View Post
    In terms of axle-crown, just build the frame for the length of fork you'll use and you're good. You will want to think hard about trail/HTA/offset/front center, though, as the 29er forks run more offset than 26" or 650b. That will mean you'll probably need a slacker HTA and shorter effective toptube to keep the right trail and prevent FC from getting too long.

    -Walt
    Walt, I've been sitting here looking at my BikeCAD drawing for the past 30-minutes trying to understand this, so I figured I'd ask and maybe you can help me get a grasp on it. My problem is understanding why the HT angle should be slacker if I'm using a 29er fork (with more rake) than a 650 fork. I'm only on frame #7 so please excuse my confusion. When I look at the drawing it seams like it should be the opposite and I should make the HT steeper to compensate for the longer rake of the 29er fork. Any help in explaining this would be appreciated. Thanks Again.

  7. #7
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    Steering trail

    More fork offset reduces steering trail, slacker head tube angle increases it to nullify the effect.

    If you aren't familiar with steering trail, time to dust off the geometry book, because it's just about the most important variable affecting how your bike will steer:
    Bicycle and motorcycle geometry - Wikipedia, the free encyclopedia

    -Walt

    Quote Originally Posted by thomasauction View Post
    Walt, I've been sitting here looking at my BikeCAD drawing for the past 30-minutes trying to understand this, so I figured I'd ask and maybe you can help me get a grasp on it. My problem is understanding why the HT angle should be slacker if I'm using a 29er fork (with more rake) than a 650 fork. I'm only on frame #7 so please excuse my confusion. When I look at the drawing it seams like it should be the opposite and I should make the HT steeper to compensate for the longer rake of the 29er fork. Any help in explaining this would be appreciated. Thanks Again.
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  8. #8
    pvd
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    It makes no sense to build a 650b bike around a 29" fork. It will be jank right of the bat and never be right. Total waste of time and money. It's best to build around a 650b fork then run the 29" fork lowered until you get a proper fork.

    Also, in mountain, head angle and front center are far more important than trail. It's really not worth thinking about trail. These days, forks have the proper offset for the wheel sizes.

    RS Revelation 650b Spec:
    150 AC - 539mm
    140 AC - 529mm
    130 AC - 519mm
    Offset - 42mm

  9. #9
    pvd
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    Sid 650b Spec:

    120 AC - 507mm
    110 AC- 497mm
    100 AC - 487mm
    90 AC - 477mm
    80 AC - 467mm
    Offset - 42mm

  10. #10
    Harrumph
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    Thanks for sharing PVD, I've been looking for those #'s!

    Are you implying that 42 is 'the' proper rake for 650's?
    Slowly slipping to retrogrouchyness

  11. #11
    pvd
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    It's in the range.

  12. #12
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    Actually, I've built quite a few of them, and they work just awesome.
    At least 15, maybe 20.
    ATC is ATC and rake is rake, *IF* you know what's up & how to make it all work together. 29er forks are not going anywhere soon. .
    - Steve Garro, Coconino Cycles.
    Attached Thumbnails Attached Thumbnails Building 650b MTB with 29er Fork-img_0388.jpg  

    steve garro el jefe/el solo. coconino cycles www.coconinocycles.com www.coconinocycles.blogspot.com

  13. #13
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    I managed to get the full set of drawings from the local Sram distributor here in Norway, so for those of you looking for the full spec sheets for the 650b Rock Shox forks I've made them available for download here: http://dl.dropbox.com/u/10488411/Roc...specs_650b.zip

    Truls

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