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Alignment of the bottom bracket shell on low-end aluminum road frame...

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Alignment of the bottom bracket shell on low-end aluminum road frame...

Old 08-04-11, 05:01 PM
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antlrscauzflats
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Alignment of the bottom bracket shell on low-end aluminum road frame...

Really dumb question, should the bottom bracket shell be perfectly centered in relation to the seat tube? It's a pretty generic frame. Need to make sure before I post more about this...
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Old 08-04-11, 05:12 PM
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fietsbob
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By how much? can you measure the situation
and quote the dimensions you got, by measuring ?
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Old 08-04-11, 05:26 PM
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After noticing it visually, I used a profile gauge. I rested the handle part of the gauge against the shell and pushed the pins to touch the seat tube. Then I checked for symmetry on each side of the frame, comparing the pins to the seat tube. So it's like using a small dishing tool. If either the handle or the pins don't just touch the shell and the seat tube then something is off. On the non-drive side there is at least a millimeter gap between the pins and the seat tube. I haven't actually measured that gap yet but it looks no less than 1mm. I also checked the seat tube a little for roundness with some vernier calipers and it seems pretty round.

So what do you think of this method? The only pitfall I see is that it could be misguiding if the seat tube is not round enough b/c the relevant measurement should relate to the axis of the seat tube. In fact, I didn't even bother using this method against the down tube b/c it's not round enough. Also, the shell is indeed 68mm wide. Lastly, if you're bottom bracket shell was off-center by one millimeter would you care?
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Old 08-04-11, 05:56 PM
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Originally Posted by antlrscauzflats View Post
Lastly, if you're bottom bracket shell was off-center by one millimeter would you care?
Depends on what I paid for the frame. If it was an inexpensive frame and everything else was OK, I wouldn't worry about it. I'd check the overall frame to see if the seat tube is centered relative to the rear DOs and head tube to make sure the frame is straight. If it is, the BB can be shimmed if necessary.
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Old 08-04-11, 06:27 PM
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A variation in the 1mm range between the right and left side distance to the face isn't going to make any difference at all. You'll correct for more than that to get your ideal chain line anyway, so it becomes meaningless.

What's more important is the squareness of the shell, or the BB axis to the centerline of the frame. A shell that isn't square to the frame, sets the chainrings at an angle, and increases chain wear, and can make it harder to keep the chain from derailing in certain gear combinations.
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