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  1. #1
    650b me
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    target BB height for 29'er hardtail?

    So, I have to fill out a worksheet in advance of my framebuilding class at UBI. (See my post on TIG vs fillet brazing for the rest of that story).

    They want dimensions taken from my best fitting bike to use as a starting point for designing the frame that I will build. I am going to build a 29'er hardtail. I don't have one built up right now. I disassembled my Niner MCR because I wasn't happy with the ride characteristics, so I can't take a measurement from it. Niner lists BB drop, but not BB height on their geo specs. I know PVD hates this number, and I agree, it's meaningless.

    Regardless, I'm not sure I'd want to use the BB height of the MCR as I am less than satisfied with the geometry. I was going to buy a Canfield Bros Nimble 9 frame to replace the MCR. Very different geo: shorter chainstays, slacker head angle, etc. I THINK this is more what I'm looking for in terms of ride characteristics. Of course, Canfield doesn't list BB height either in their geo specs!

    So, short story long, I'm soliciting opinions on a target BB height for my upcoming 29'er hardtail build. If it helps, I mostly ride Colorado Front Range trails: steep and moderately rocky. Thanks!

  2. #2
    Nemophilist
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    Well...

    I don't know much, yet, but I'm not sure how BB Drop is a worthless measure? BB Drop and Height are two sides of the same coin as it pertains to ground clearance. If you have a frame & fork, you can fairly easily measure BB/D. If you know the diameter of your wheel/tire combo, you can calculate BB/H from there. With a hardtail, you do not have to worry about factoring in sag in your BB/H calcs. BB/D CERTAINLY has fairly significant effect on handling, so it seems worthwhile to consider it.

    My RIP9 with 2.25 tires has a 13.5" BB/H, but factor in the sag and it is a peddle basher. Of course, you are not likely to be building a frame with a 46.6" WB, so that will automatically improve the BB clearance. I am laying out a Fatbike right now, and am looking at about 12.8" BB/H, with a 2.0" BB/D. I think that is a good compromise between low CG and ground clearance.

    I'd think that somewhere around 13 would be good, unless you are riding incredibly technical terrain with lots of logs and boulders to ride over. Hopefully someone who really knows this stuff will confirm that, for my sake as well as yours.

  3. #3
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    Depends on the rider

    Generic number for hardtail mountain bikes is about 305mm/12". If you like really long cranks or like to pedal through lots of rocky stuff, go up. If you ride up smooth fire roads and bomb down steep singletrack with the pedals level, go lower.

    It's a matter of preference as much as anything. Given that this is your first frame, I'd just go with 12" - there will be lots of things you'll want to change about it in 6 months anyway, so no need to obsess.

    -Walt

    Quote Originally Posted by golden boy View Post
    So, I have to fill out a worksheet in advance of my framebuilding class at UBI. (See my post on TIG vs fillet brazing for the rest of that story).

    They want dimensions taken from my best fitting bike to use as a starting point for designing the frame that I will build. I am going to build a 29'er hardtail. I don't have one built up right now. I disassembled my Niner MCR because I wasn't happy with the ride characteristics, so I can't take a measurement from it. Niner lists BB drop, but not BB height on their geo specs. I know PVD hates this number, and I agree, it's meaningless.

    Regardless, I'm not sure I'd want to use the BB height of the MCR as I am less than satisfied with the geometry. I was going to buy a Canfield Bros Nimble 9 frame to replace the MCR. Very different geo: shorter chainstays, slacker head angle, etc. I THINK this is more what I'm looking for in terms of ride characteristics. Of course, Canfield doesn't list BB height either in their geo specs!

    So, short story long, I'm soliciting opinions on a target BB height for my upcoming 29'er hardtail build. If it helps, I mostly ride Colorado Front Range trails: steep and moderately rocky. Thanks!
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  4. #4
    Eric the Red
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    The thing about bb drop is that it's a good way to isolate the bb location from the wheels. You can say it's got 60mm and the bb will always be in the same place relative to the rest of the frame. If you say 12", you have to ask what tire that's with.
    Granted, when the bike's built the only thing that matters is how high the bb is off the ground, but when you're comparing frames prior to purchase, or building one, bb drop can be useful.

  5. #5
    Old school BMXer
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    Quote Originally Posted by edoz View Post
    The thing about bb drop is that it's a good way to isolate the bb location from the wheels. You can say it's got 60mm and the bb will always be in the same place relative to the rest of the frame. If you say 12", you have to ask what tire that's with.
    Granted, when the bike's built the only thing that matters is how high the bb is off the ground, but when you're comparing frames prior to purchase, or building one, bb drop can be useful.
    Walt's generalization is correct, but wether you're talking drop or height, you need to know what travel fork you're using. Long fork generally equates to a higher BB.
    May the air be filled with tires!

  6. #6
    650b me
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    I thought someone might ask the fork question. In this case, it will be a Fox F29 100mm.

  7. #7
    Rock and/or Roll
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    So then, is BB height measured while sitting on the bike, to allow for suspension sag, with the correct tires, inflated to the correct tire pressure?

  8. #8
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    Quote Originally Posted by turbogrover View Post
    So then, is BB height measured while sitting on the bike, to allow for suspension sag, with the correct tires, inflated to the correct tire pressure?
    sure, if thats the way you want to do it

  9. #9
    pvd
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    297mm for 100mm fork.

  10. #10
    650b me
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    Any decimal places I should be concerned about?

    Thanks, Peter.

  11. #11
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    Quote Originally Posted by golden boy View Post
    So, I have to fill out a worksheet in advance of my framebuilding class at UBI. ...

    They want dimensions taken from my best fitting bike to use as a starting point for designing the frame that I will build.
    At any rate, the worksheet is just that, a worksheet - a moving target. It's just to get you thinking about which way you want to go, but it's by no means set in stone. The instructors will also be more than willing to discuss it with you and let you make appropriate changes.

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