Originally Posted by
Newspaper_Nick
Why? I am currently using a square taper triple crankset. These are almost identical. Why will i need a shorter BB?
122.5 is a pretty old but common spec. Newer cranksets tend to have more offset and consequently require shorter BB spindles.
If it was my bike and you're not in too much of a hurry, I'd wait for the crankset to come in and test fit the new crankset to your existing spindle. That way you can see if your front derailleur will reach the biggest chainring or not (my bet}.
If you are a chainline purist you can hold a straight edge against the middle chainring and see how closely it comes to bisecting your cassette. If you're more like me, you'll just eyeball how far you think you need to move the chainrings inward and buy a BB with a spindle short enough to do that. It doesn't have to be perfect. If you're a risk taker you'll probably need something in the 112mm to 116mm range. The lowest cost solution is to find a donor bike so you can hunt around while you're waiting for the crankset.
Don't forget that BB spindles have two ends. In most (but not all) cases, if you want to move your chainrings inward 5mm you'll need a BB spindle that's 10mm shorter.