I'm beginning to shop for parts for my winter project (an 82 Trek 614 with a triple) and was considering swapping in a UN-55 since the races of the old BB are in rough shape (and may do the same on my wife's Trek). I've read the threads about sizing and still I'm not sure I have it straight. The old spindle is an asymmetrical one, stamped "3U" and has the exact dimensions listed in Sheldon's table - 124.5 overall length, 32 non-drive side, 40.5 drive side. In the market, I'm seeing only 122.5 and 127 as choices in the UN-55. Can those of you who have swapped in lots of these on 68mm shells with a triple crank point the way for me a little bit? I think one of the things I'm missing is the logic of how spacers are used (these are also mentioned in Sheldon's table). I know I should've measured the chainline to the middle ring before I broke the bike down, but it slipped my mind. Anyway, the LBS told me 127 for my case, and another guy whose opinion I trust told me 122.5. Maybe this is one of those things that becomes obvious to you when you actually do it?
Terraferma 650b, Mondonico SL and ELOS, Masi Gran Criterium, Trek 610, Breezer Liberty, Georgena Terry Classic
Go to http://icelord.net/bike/ and download pdfs of the Sutherland's and Barnett's manuals, and check out the chapters about replacing spindles and replacing spindles with cartridge BBs. Both will show you how to figure it out, and have tables of data on JIS and other apindles, and on the varieties of cartridge square-taper. It'll probably be beneficial to put the parts you have back together so you can get an initial set of measurements.
It's pretty involved, and makes me think that LBSs can do no better than guess if they haven't tried to sort it out.
'71 Mercian, '72 Jeunet, '82 Jack Taylor, '13 Rawland
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 09:20 PM.
Thank you kindly, gentlemen. That is what I thought...lay in a stash of various width spacers and make it work.
southpaw, thanks especially for the 122.5 with 2mm spacer suggestion (and nice Voyageur, btw). That is very helpful as a start. I'll slap the old BB/drive side triple back in there and take a few measurements just for my own education. I really appreciate the honest advice, and I hope it will be an improvement when I'm all done. Thanks again.