BB Drop Discussion- Mtbr.com
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  1. #1
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    BB Drop Discussion

    I saw a little bit of bb drop talk going on in another thread, but was hoping to get more in depth. Im about to start drawing up my own frame out of pure necessity, since there's nothing on the market that really meets my needs. Ill be building a very simple, old school geo 29er that will be ran fixed gear, so bb drop is very important due to pedal striking.

    Im starting with the geo from a couple frames I liked riding, but both have relatively low bottom brackets so I was hoping to nudge mine up a bit. Based on the other thread, its not as simple as just pushing it up 20mm. What other things need to be adjusted around the bb so as not to make things go sideways.

  2. #2
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    I'm not a frame designer or builder. Just 2 bits of insight.
    It sounds like you are after bb height more than bb drop. Right?

    I think 2 things to consider is what will your tire height be. You then know the axle centerline. Next figure out what the shell height will be, along with the crank arm length. Do the math on what the drop will be based on the height off the ground you are looking for. Adjust as necessary.

    With that said, I have no idea how the handling characteristics change with more/less drop. I mean, does less drop handle like crap when compared to a greater drop? Obviously your CG is altered, but I think there is more to it than just that.
    Does chainstay length make a difference depending on the drop. Meaning, does a 10mm drop handle the same on a long chainstay as it does on a shorter stay? Or would one need to drop 12mm for a longer stay instead of 10mm to have the same ride characteristic?

    So maybe it's going to be a compromise between how much drop you can achieve and still keep a decent height.

    Sounds like a fun project you're taking on. Good luck!

  3. #3
    pvd
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    BB height.

  4. #4
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    May want to consider shorter cranks as part of the package also.

  5. #5
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    As others have noted, what you care about is height. If you know the tires you'll use, that makes it easy. If you're unsure, go measure some tires.

    If you are new to riding fixed offroad, or just want to be able to run some different sized tires, use an EBB for chain tensioning duties, so you can throw up/down as needed to change BB height.

    -Walt

  6. #6
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    BB Drop Discussion

    I’d not worry about the cascading effects changing BB drop will have on other parts of geometry for now. Old school 29ers had around 60mm drop, assuming non-plus tires, which puts you at about 12.1” with 2.2’s. But I personally wouldn’t go below 12.5” or even 13” for a dedicated fixie with 175 cranks.

    Ps-Walt’s suggestion of using an EBB is excellent


    Sent from my iPhone using Tapatalk

  7. #7
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    Thanks guys, I meant height not drop. The plan was already to go EBB since I want this thing to be very clean and minimal looking.

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