I have an older Nishiki hybrid bike that I got new in 1999. I used it through the winter this year with studded tires and, over the weekend, started stripping it down to clean off all the grit and salt. The original crankset (a tourney stamped steel 48-38-28) is in pretty bad shape and I'm planning to buy a replacement. I'm planning on getting an Altus m311 48-38-28. The specs I've seen say that I should use a bottom bracket with 122.5 mm spindle. The BB I have now is a BB UN26 with 117 mm spindle. According to bikepedia, the original spindle was 120 mm, so I think my chainrings have been too far inboard by about 3 mm. (but maybe the old spindle was symmetrical so same length as the 117 mm on the driveside, but 3 mm longer on non-drive? I don't know. I don't have the original spindle anymore.)

As it is now, I'm not really having any problems with chainline. Even in big-big and small-small combos, the chain rub is not so bad that I completely avoid using them. Well, I never use small-small because it's never really useful, but if I'm coming up on a short steep hill in the big ring, I'll shift to the big-big briefly if I need to to get up it instead of shifting front and rear.

If I get a new crankset that's spec'd for a 122.5 mm spindle am I correct in thinking that that should move the chainrings inboard by an additional 2-3 mm? I'm just trying to decide if I should order a new BB along with the crankset. The old one still turns smoothly and there's no reason to replace it other than possible chainline issues. I'm much more likely to use the big-big combo than the small-small, so if my chainrings are a little too far inboard, I don't think it'll be a problem at all... Am I thinking about this right or am I missing something?

Not all the cranks "climb" the spindle equally. Shimano gives specs for using their BB-s, which usually results in correct chainline without cranks being to far out, or hitting the frame.
However, you can sometimes go with a few mm +-. If you ride on the largest chainring most of the time, it's even better to have a slightly shorter spindle (as long as the FD can accomodate the shifting to the smallest chainring and the cranks don't hit the frame). That way chain will be straighter when used with mid-cassette cogs.

Also, some Shimano BB-s have a sort of a "stop" at the right hand side. If the crank bottoms out against that stop, before having gotten a proper preload against the spindle, you can expect the bolt to get loose over time, as well as the spindle to get damaged from "play" (even before you can sense it).

The best way to determine this kind of stuff is to try the cranks on a known BB axle and see what might be able to be made "better" with a different BB. In theory a BB axle that's 5.5MM shorter then current will be expected to change the chainline by about half the length difference. But a little tolerance drift, wear, or other variables can make chasing down these last few MMs impossible without testing. Andy

And the various designs have a variety of profiles.

80s XTR M900 came in to meet a shorter spindle M730 XT(same years) much less so

right angle straight arm cranks spindle had to be longer still , TA cyclotourist a classic , are in that category...








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Thanks everyone. That helps. I'll try the new crank on the old BB and go from there.

When I swapped out my BB loose bearings for a sealed cartridge, I ran into a problem with the front derailleur not pushing the chain out far enough to climb the tall gear. I ended up having to shim out the derailleur to make it work. After that, everything else fell into place chainwise. I went with a 117mm spindle when the Parker chart was recommending a 122mm. I never would have been able to make the 122mm work because it would have pushed the gears out even farther away from the frame. But if it's working good like you said, why fix what ain't broken?