Originally Posted by habilis

I have a similar problem and came up with the following procedure. I'd like to know what others think of it.

If the problem is matching a new BB to a new crank, I suggest acquiring the crank first. Then try taking the following measurements and putting them in a simple top-view diagram of your drive train:

1. Measure the distance from the center of your rear cogset to the middle of your rear axle between the dropouts (middle of "over-locknut distance"). That measurement establishes your chain line.

2. Measure the same distance from the center of the BB shell to the middle of your chain set. On a double, that will be the space between the rings; on a triple, it will be the center ring.

3. To the distance found in step 2, add the distance to the crank hole in the crank set. Measure only to the point where the crank bolt will actually seat, because that's where the BB spindle will end.

4. You now know half the length of the BB spindle. Double this length to get total spindle length, assuming the spindle is symmetrical, as most are.

5. The width of your BB shell (usually 68mm) and the thread type are the final determinants in selecting a BB. Otherwise, buy a threadless BB for your shell width, and with the spindle length you calculated.

I think I got it right, but if others disagree, I'd appreciate input. I'm sure others have had this chicken-and-egg problem.