why do Shimano do that with their BBs?

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pliptrot
Posts: 705
Joined: 12 Jan 2007, 2:50am

why do Shimano do that with their BBs?

Post by pliptrot »

20+ years ago I bought a mountain bike with a 739XT group - including a 110mm UN72 BB and a triple chainset. The left crank was about 5mm further out than the right. Having been through 2 of the same chainsets, 2 frames and 3 BBs I can confirm that this is not a 1-off. I recently found a new UN72 on ebay, so that is now fitted, and had the same effect. I put ~4mm of spacers between the frame and the drive-side flange to get the cranks equally offset. But now the chain line is such that the big ring offers a perfect chain line with the 11 tooth (smallest) sprocket (8 speed). So why do they do that? For a while I used the bike with only the big ring and no front derailleur, and a 107mm BB from Tange offered a much better oreintation of the crank and chain line. But if I stick with what I have the largest 3 sprockets are not usable with the big chainring - the chain derails because the chainline is so canted. Even the middle ring to largest sprocket is no-go for the same reason. Weird.......
hamster
Posts: 4131
Joined: 2 Feb 2007, 12:42pm

Re: why do Shimano do that with their BBs?

Post by hamster »

Shimano 110 bottom brackets are asymmetric. But they always seem to leave loads out on the left.
slowster
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Posts: 4628
Joined: 7 Jul 2017, 10:37am

Re: why do Shimano do that with their BBs?

Post by slowster »

According to Brucey's list of Shimano UN55 measurements in the link below, a 107mm bottom bracket has the same right hand stickout as a 110mm, but the left hand stickout is 3.5mm less. I don't know if the Tange 107mm bottom bracket would have had the same stickout measurements compared to a Shimano 107mm bottom bracket.

viewtopic.php?f=5&t=105385
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