Originally Posted by
cny-bikeman
Different cranksets are offset differently from the end of the BB spindle, thus take a different length of BB spindle to put the chainwheels in the proper position. The chainwheels would be out too far - would not line up properly with the rear and front derailleur might have a hard time reaching the large chainwheel.
This, or the problem can go in the other direction. The chainring could rub against the frame if the BB axle is too short for that crankset. Crankset & axle need to match, or at least be "close enough" for the application. Not too close & not too far.