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  1. #1
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    Help me understand Bottom Bracket sizing

    I apologize in advance for asking a basic question but I'm returning to mountain biking after a long hiatus and a lot has changed! One major change I see is that Bottom Brackets aren't as "standard" as they used to be.

    The frame's bottom bracket tube is simply one of many tubes on a bike. It has three dimensions: Length, outside diameter, and inside diameter. Knowing they are threaded, I assume there are also different thread pitches and that the depth of the threads can vary.

    So when I see a frame's bottom bracket listed as "73mm" or "66mm," I assume that's talking about the length (width, really, since it is perpendicular to the frame's main axis).

    Is that all that varies on a frame's bottom bracket tube? Does OD or ID also vary?

    how are the actual bottom brackets (the bearings and axle) now sized? Do you buy them in specific lengths? Diameters? Styles?

    TIA

  2. #2
    dru
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    There are a bunch of new pressfit BB shells that aren't threaded at all. I don't know the diameter, but they are easy to identify bcause the BB bearings are pressed into the BB shell.

    What is way more common is the threaded shells you are talking about. Almost all of these will be english threading (ISO/English or BSA, 1.37 in x 24 TPI), in a 34.6-34.9 mm inside diamter shell, although more than a few european road bikes use Italian threading (36 mm X 24 tpi).

    The shell widths for the English threading is either 68 or 73 mm.

    BB shell wall thickness does vary, depending on the shell chosen. A skilled builder can use a thinner wall and avoid warping. Someone unskilled at frame building needs to use a thicker shell to avoid warping (I warped mine anyways when I built my frame). What is important to you is the inner diameter and threading.

    Pressfit (BB30 is one type) are becoming more common, however more than 90% of what's out there still uses threaded english shells.

    As for your question about sizing BBs, the old stuff is based on spindle length to establish the correct chain line with the crank arms you've chosen. 68 vs 73 mm is most often accomplished by a spacer that comes with the BB itself.

    I have no clue about the newer outboard bearing stuff aside from the fact that the cups thread into the BB shell. I think the cranks use some type of spacers to set things up?

    I'm sure there's a tutorial here somewhere?

    Drew
    occasional cyclist

  3. #3
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    Sheldon's BB section covers a lot of the options (aside from the newest stuff):
    - Sheldon Brown's Bicycle Glossary Bo--Bz

    And as noted above, the outboard bearing ones like the Shimano HollowTech-II thread into the BB shell, spacers adjust for width and chain line, wider shell = fewer spacers:
    - DSCF1320


    I think it all comes down to threaded or not.

    - If threaded, then figure out the size and thread pitch (likely english thread as noted above unless you suspect otherwise)

    - If not threaded (i.e. the press fit type), use the appropriate kind.

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