It seems that a spindle length that is not exactly the same as my current bracket, as long as it isn't too small, will just move the position of the crank slightly. As long as I know it's not too small, and know that the chmaber width is good, is that all fine?

If too far outward, your derailleur may not reach and your chainline may be bad.

Ok. I'm only being imperfect with the measurement because I don't have tools on me right now to take my crank apart to properly measure the spindle length. With the crank still attached, it sure looks to be around 115mm. Does that sound like a totally unreliable measurement? How far into the crank does the spindle go that I am not measuring by not removing the crank?

Sometimes one accurate measurement is worth 1,000 guesses.

Why do you want to know? If it has anything to do with ordering or buying a replacement bottom bracket, do yourself a favor, measure, and get the right one the first time.

Yes, that's exactly why.

If I know my chainline length (45mm), and I know my chamber width length, shouldn't I be able to precisely calculate my spindle length?

Nope, it mostly depends on the design of the crank arms. Years ago, crank arms were pretty much straight up and down and BB spindles were in the 120 mm's. More modern square taper crank arms are offset to the point that some require BB spindles less than 110 mm. You're right in thinking that it's OK to "cheat" by a few millimeters but it's a pain to guess wrong and find out that you bought a part that won't work.

My personal philosophy is, if I OWN parts I'll bolt almost anything up and see if I can adjust it to work to my satisfaction. If I'm BUYING parts I'll always hold out for stuff that I know is compatible.

Yeah, what he said. OTOH, a relatively recent double crank on a square-taper spindle is probably sitting on a 113mm spindle. On the gripping hand, it can be very brand specific. If you know what brand crank you have, you can look up the length here: Sheldon Brown's Bottom Bracket Size Database

If the spindle was symmetric (many are not), then theoretically what you're saying is possible, but measurement of chainline and the relationship of chainline to spindle isn't standardized. All in all, it's much easier simply to take off the crank arms and measure the spindle length. Even this doesn't mean you're totally out of the woods if your spindle isn't symmetric. There are tables that will help you convert a given assymetic spindle length to a symmetric equivalent that will give you the same chainline spec.

Some reading:

Chainline Concepts | Park Tool
Sheldon Brown's Bottom Bracket Size Database

If you give the gurus here more on what you're replacing and what you're trying to replace it with, you'll get better guidance.

- Mark

Thanks for the links, very helpful. I'll actually measure the thing and then check these charts before ordering.

For square taper spindles, the actual measured length is usually a bit longer the the specified length, 0.5 ~1.0 mm or so. e.g., if you measure 114mm, 113 is what you specify.

Nice. That reminded me to order copies of Mote & Gripping. Wife & I read them from the library years ago, they've been among our favorite books ever since, it's time to read them again.

OP, get the right sized BB for your specific crankset.

As your links elaborate on...entirely depends on who made it, in what year, and in what model. Campagnolo has used probably a dozen spindle lengths over the years across their models over the years.