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I don't have a LBS (there are BS, but not L), so I generally order everything I need online. I need to replace the bottom bracket on my commuter bike, which I know is a VP-BC73 but that BB's available in sizes from 107 mm to 127.5 mm and I'm not sure what size I'll need. The bike has a 28-38-48 triple front chainring.

I plan on using a Sunrace BB as the replacement, which is available in 103, 107, 111, 113, 118, 122, 124, and 127 mm lengths (italics are out of stock at my preferred supplier).

The obvious answer here is to take the BB out and measure it; trouble is, there's no crank puller or BB tool available—I'll be ordering these with the new BB. I've also tried contacting the bike's manufacturer (a bicycle chain store—I know), but they've been non-responsive.

The best I've managed so far was an attempt to measure the distance from the outside of one crank to the other (at the BB), then remove the crank bolts and subtract the distances from the outside of the crank to the end of the bottom bracket axle. This gave a (rudimentary, at best!) measurement of 129 mm.

I can possibly order a couple of BBs and return one, but I'd really like to be able to narrow it down that much, at least. Is it better to be too long, or too short (or does it really need to be exact)?

UPDATE: I measured the chainline yesterday, it's 53 mm at the front (I know that's wide, but that's what it is) and 48.5 mm at the back. The rear cassette is a 7-speed Shimano 14-28.

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Be aware that BB can have a different chain line for the same length, so its not as simple as getting the same length BB. Refer (as always) sheldonbrown.com/bbsize.html –  mattnz Nov 3 '13 at 6:45
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Actually, it's pretty hard to measure BB length even with the cranks removed, since the "length" (at least on a square shaft) is relative to some fictitious location on the shaft (which, at least on some Shimano units, are not symmetrical). Your best bet is to remove the cartridge and hope that the markings on it are still legible. –  Daniel R Hicks Nov 3 '13 at 13:57
    
@DanielRHicks My plan was to measure the entire BB axle, from one side to the other using a pair of vernier callipers; is this not the 'length' they're sold as? –  Calrion Nov 5 '13 at 0:26
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At least some Shimano cartridges do not fit that mold -- one axle end is longer than the other (for reasons that are not entirely clear), and the official "length" is a couple of mm short of the actual overall measurement. –  Daniel R Hicks Nov 5 '13 at 1:51
    
Well that's certainly a trap for new players. At this point I'm thinking I'll just have to order the tools first and put up with the extra postage. –  Calrion Nov 5 '13 at 2:35

1 Answer 1

up vote 1 down vote accepted

As, if not more important than length, is the chain line. Obviously related, but enough variation to cause problems, as I found out not when I got into my own bike maintenance. Sheldon (as always) shows a way to measure chain line here. The same page has table of various Bottom brackets and the length and chain-line measurements.

Essentially you measure from the center of the chain rings (triple is easy, double measure half way between the rings) to the center of the seat tube to get your chain line. Then you should be able to match up a BB that will suit.

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I hadn't considered the chainline; thanks. Looks like I've got some reading to do. :) –  Calrion Nov 3 '13 at 8:06

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