What's your preferred BB drop?- Mtbr.com
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  1. #1
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    What's your preferred BB drop?

    I am in the market for a new hardtail frame. I want to go down one size from my XL hardtail Fisher, but I think I want less BB drop than the Large Fisher's 76 mm. My XL has 71 mm, and I am hitting rocks and roots a little too often for my liking - not only with the 180 mm cranks but even sometimes with the 44T ring. I would think the Large's 16 mm shorter wheelbase perhaps reduces hitting a little, but not enough. I haven't really tried any 29" frame with other BB drop so I would appreciate hearing what could be a good number, in someone else's opinion. Intended use is XC and marathon racing.

    It seems that 11.7" is the industry standard stated XC hardtail BB height (at large frame sizes), which Fisher states only for its XL size. As a side note, I am not sure why most companies don't state BB drop but BB height, at best, when the height depends on (not stated) tire size (then both drop and height also depend on not stated fork sag). Anyway, therefore I assume that the industry standard BB height would correspond to 71 mm 29 inch frame BB drop. From that, "my" (custom?) frame would, compared to the Large Fisher, just need -5 mm to reach 71 mm and -5 mm to compensate for 180 mm cranks, total -10 mm BB drop. But could really 5 mm higher BB and a little shorter wheelbase, as compared to today, make that much difference for me? Or should I accept going 5 mm further down from today and just get a Large Fisher frame, and think that I only need to improve riding technique? If I went with 60-65 mm BB drop, would the center of gravity be too high? I assume there's a reason why XC bikes don't have that high BB. For you who ride both KM and Fisher: does the KM's less BB drop (68 mm) make any noticeable differences to the better and the worse? If anyone is riding a Large hardtail Fisher with 180 mm cranks: what's your experience with the BB height?

    Any thoughts to share about anything of all this?

  2. #2
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    I would go with less drop. I have ridden 29" hardtails with as little as 30mm drop ( a converted 26" bike) and have not noticed a lack of stability, what you do notice is the extra clearance in rocks and roots (I run 170mm cranks also).

    A 26" bike with an 11.7" bb height has only about a 35mm drop. For 29" I like a 12" bb height which fits into your 60-65mm drop nicely. Plenty stable and noticeably more clearance for your longer cranks.
    A bike by any other name is still a bike.

  3. #3
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    50-60mm

    That's what I've been building for myself and others. I think the Fishers go a little overboard - with large wheels, you can still keep the BB quite a ways below the axles (for stability) and also get some extra pedal clearance. I think once you're past about 40mm of drop, you're not going to notice any increased stability. Of course, some people like to sit a little lower to the ground, and standover is another consideration. So the answer, as usual, is "it depends". Long or short cranks also come into consideration, as does riding style, favored terrain, etc.

    Most people I've talked to feel the fisher's 70-76mm is too much. YMMV.

    -Walt

  4. #4
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    My XL Fisher does hit pedals more often than I bike I've ever ridden, but still that's only once every few rides. I run 180's. I agree the low BB of the Fishers is a bit overboard, but not extremely so. When I'm clearing an ultra-slick climb or rolling down an aburb steep DH, the low BB does do miracles for me. It's just that the pedal striker are easier to count than advantages, plus they are indeed worse than the added stability does good. I never had a single problem with my KM at a claimed 68mm of BB drop. Then again, it's my singlespeeder with a tall gear, pedal strikes may therefor be less likely to happen, even if the BB drop were the same.
    Klok - XC - Skate - Ski

  5. #5
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    I wish my BB was a little higher on my Tam as well. Thats really my main gripe about the bike. I'm constantly hitting pedals, cranks and the big chainring. So much that I've replaced the big ring with a bashguard.

    If the 2005 Fisher HT's have a higher BB, I may just have to buy one!

    The low bottom bracket does help, but Fisher went too far (at least for my terrain and riding style).

  6. #6
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    For some riders, the coming of the announced 2.3" WTB tire may both fulfill a long wish, as well as compensate for the slightly too low BB. It won't make it high, but perhaps normal in stead of really low.
    If I were Fisher, I'd start an actractive money-back action on those who trade in their pre-'03 niner for a 2005 one, to get them out on the trails in all their beauty. Early adopters are rewarded for their faith in Willit's and Gary's ideas, by getting something even better for a price no-one else can get. Just a thought, Fisherguy, you listening??? Wow, I wrote that even before I realized I just might take Fisher up on such an offer, believe that?
    Klok - XC - Skate - Ski

  7. #7
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    50mm...'nuff said.

    -tk
    wow, this hurts more than it used to.

  8. #8

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    I made my first 29er with 52mm drop. Drop means nothing unless tire diameter is added into the equation. With the 52mm drop and 2.1 Nano's I've got about an rch over 12.5" bb height. I'm running 175mm cranks and find myself tagging rocks right and left. I could go 170mm cranks and probably have no problems because it feels like I'm actually just scraping over the top of the rocks - and off-camber sections.

    Other considerations: 100mm rear and 80mm front wheel travel. I'm waiting for a BW1.0 that will raise the bb up enough to solve my problem, but it will also change my head angle - we'll see how it rides then. For a suspension bike with 4" front and rear, I would probably go to 44mm drop. I think the extra 8mm would be the ticket for a roughly 12.75 - 12.875" bb height. Add in the new "tire" coming out and that could boost the bb height to 13" - still very liveable.

    A hardtail could live with a 55-60 drop.

  9. #9

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    Bottom bracket frop is dependent on lot's of things within the frame design and to be fair I don't really see the point on getting hung up on it.

    I'm finding that a BB height of around 12.75 seems to be ideal or at least for me it keeps the bike short at both ends and allows me acres of clearance, with the bike being very stable.

    Geometry and bike design is so much more about the whole rather than individual angles, and every frame is a compromise.

  10. #10
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    I know what you mean....

    Quote Originally Posted by Vibramicide
    Bottom bracket frop is dependent on lot's of things within the frame design and to be fair I don't really see the point on getting hung up on it.

    I'm finding that a BB height of around 12.75 seems to be ideal or at least for me it keeps the bike short at both ends and allows me acres of clearance, with the bike being very stable.

    Geometry and bike design is so much more about the whole rather than individual angles, and every frame is a compromise.
    Having played about with 29er geometry I know that you have to take a "holistic" approach to it as if you change one thing, say seat angle, a load of other things happen to the whole frame.

    A high BB makes a lot of sense, my 26" MC Ute is running a 12.5" BB and with 170mm cranks I have only had a handful of pedal strikes (I run fixed )

    I would want a 29er to have a similar (@12") height BB with, say, a 2" tyre (so slightly less with a set of CX tyres fitted, and more with some meaty Bonty tyres)

    A frame is a lot more than a handful of numbers

    SSP
    "Put any one on one of these singlespeed bikes and they could not help but have fun"
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    Otis Guy talking about klunkers c1976

  11. #11
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    Thanks everyone,

    Good to hear that I probably could have the BB about 20 mm higher without really
    compromising the ride in other aspects. I am already longing for that better flow I will get.

    BB drop plus height is equal to 622/2 + 55 (Mythos height over bead seat) = 366 mm

    With my 71 mm drop, the BB height becomes 366-71 = 295 mm = 11.6"

    With your feedback, I am not surprised that 11.6" BB height in combination with my 180 mm cranks may be a bit problematic in the rocky sections.

    You running a Large Fisher have 366-76 = 290 mm = 11.4" BB height (if claimed drop is correct).

    To reach 11.7" BB height, we would need BB drop 366-11.7*25.4 = 69 mm

    Here's the average of the average of your preferred drop numbers, as I understand them (sorry if I have misunderstood you) and assuming 55 mm total tire height (I know that assumption may be incorrect):

    BW: 366-12*25.4 = 61 mm
    Walt: 55 mm
    Clox: 68 mm
    TM: 50 mm
    SSM: 52 mm
    Vib: 366-12.75*25.4 = 42 mm
    SSP: 366-12*25.4 = 61 mm

    Average: (61+55+68+50+52+42+61)/7 = 56 mm (Hint for bike company product managers)

    Those 56 mm correspond to BB height (366-56)/25.4 = 12.2"

  12. #12

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    Quote Originally Posted by anden
    BW: 366-12*25.4 = 61 mm
    Walt: 55 mm
    Clox: 68 mm
    TM: 50 mm
    SSM: 52 mm
    Vib: 366-12.75*25.4 = 42 mm
    SSP: 366-12*25.4 = 61 mm

    Average: (61+55+68+50+52+42+61)/7 = 56 mm (Hint for bike company product managers)

    Those 56 mm correspond to BB height (366-56)/25.4 = 12.2"
    You might also want to sort it based on hardtail and rear suspension. My 52 on a f/s is too low. My next build will be 44mm with 100mm rear and front suspension.

    Another good source for geometry is the Ox Brand site. If someone knows 29er geometry, it's these guys. http://www.oxbrandbikes.com/frames.html

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