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Old 07-31-10, 01:44 AM   #1
Roasted
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Do 73mm BB shells offer more strength than 68mm?

Naturally I would think the default answer would be yes, however I was thinking more about the design of how bottom bracket shells are. It's only a 5 mm difference, and not only that but I would think the weld quality in that area which holds the shell to the actual tubing would weigh in heavily in terms of the strength factor.

I was just curious because as I was looking around at full suspension XC/AM frames, the vast majority of what I've found that didn't cost an absolute fortune tends to be 68mm. I just wasn't sure what the advantage was of 68 vs 73. The only thing I could think of for using 68 in this case would be to obtain better chain line, due to the way rear triangles are sometimes set up to provide the rear suspension.

Anyway, I was just curious what users had to say about this. I couldn't find any definitive reasoning behind what's what so I figured I'd ask. Thanks guys!
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Old 07-31-10, 03:32 AM   #2
Torchy McFlux
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No. It's not stronger.
73 was commonly used on many aluminum bikes where the diameter of the stays being welded to the BB shell were larger. Some companies (like GT) used them because they felt it allowed for more mud clearance between the rear tire and the chainstays.
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Old 07-31-10, 06:36 AM   #3
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Thanks for the response. Is that the same reason 80mm popped up too?

(I have no idea what 80mm bottom brackets are, never seen them, but I saw some downhill frames that were listed to have an 80mm BB shell...)
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Old 08-01-10, 10:30 PM   #4
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I have to wonder - wouldn't the wider diameter stays make for a stronger frame? I know that wouldn't be in direct relation to whether or not a BB shell width makes for a stronger setup, but I would think the stay diameter counts for something. Then again, when comparing my two hardtails (one uses 68, one uses 73), there's physically no difference in the stays. The only difference I see is the hardtail with 68mm shell, the stays are welded right up to the edge of the shell, whereas the other hardtail with the 73mm shell, the stays are not welded on the edge, and the shell itself has a bit of a lip on each side. Basically, I could sheer off a few mm on each side of the shell on the 73mm hardtail and not effect the welds.
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