Join Date: Aug 2008
Location: Somerville, MA and Catskill Mtns
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There's something of a black art to sizing up replacement BBs, especially when switching from asymmetrical to symmetrical. It's never obvious. You can try your best at estimating what will work and what won't, but at the end of the day it often comes down to trial and error. I would start with the 122.5 and use a 2mm spacer on the drive side. But then you will also have to file down the flange molded into the plastic NDS cup so that it can thread into the BB shell the additional 2mm to compensate for the 2mm shift caused by the spacer.
Even though this won't, on paper, recreate what you have now, it might not matter. Oftentimes those older asymmetrical BBs didn't even get it "right"-- I've found more often than not that on OEM spec'd bikes, the L and R crank arms aren't symmetrical with the bike's centerline, and the cranks sit too far out. I think sometimes the bike manufacturer just wanted to play it safe and go with a larger BB than necessary to make sure everything fits. Finagling with a non-original BB often improves things. That's happened to me several times when switching from asymmetric to symmetric BBs, I've actually improved the crank symmetry and chainline.
As an example, my Schwinn Voyageur came originally spec'd with a 124mm asymmetric spindle and a 110/74 triple crankset. The cranks clearly stuck out at least 10mm farther away from the bike's centerline than they needed to, with the right crank farther away from the BB shell by about 4mm relative to the left one. Also, gobs of clearance between the granny ring and the chainstay. So, I went with a 113mm symmetrical UN55 and a 1.5mm spacer on the drive side. Now, the crank arms clear the chainstays by about 5mm, are symmetrical with the centerline, and the chainrings aren't as far out anymore. Chainline is just fine, and I have a narrower Q factor (about 151mm, down from 162mm... pretty decent for a 110/74 triple crankset):
Last edited by southpawboston; 02-09-12 at 10:20 PM.