due ruote 

I recently built up an old Trek 520, and swapped in a cartridge BB. It's a Shimano unit, I believe a UN-72. At any rate, the drive side threads are fixed to the cartridge. The crankset is a Shimano B-124 triple.

When I built it up, I had a couple mm of clearance between the small ring (which I never use) and the chainstay. The driveside thread flange was torqued down tight against the BB shell.

I am guessing I've ridden the bike a couple hundred miles since build-up. Everything was great until yesterday, when the small ring started rubbing on the stay.

There is no play detectable in the BB/crankarm assembly. The bike hasn't been crashed or abused in any way. I am at a loss to think of any explanation other than the spindle shifted a bit toward the NDS. Is that even possible? I tried a google search but couldn't find a reference. Any other ideas?

As for fixing the problem, I will either go back to the OEM cup/cone unit, or add a spacer to the cartridge unit. For convenience I would prefer the latter, but if there is reason to believe I have a faulty cartridge, obviously I don't want to re-use it.

Thanks.

CliffordK

Are your chainring bolts tight?

Can you get a smaller chainring?

Perhaps just remove the triple and block the derailleur out.

melloveloyellow

If it is a square taper BB, then the drive side crank can move inward depending on the torque applied. Just a guess..

dr_lha

Based on the design of the square taper bottom bracket I think there's basically no chance that the spindle moved, especially if there's no play, so I think the only reasonable answer is that the crank has "settled" onto the square taper as you've ridden the bike, moving the chainring closer to the chainstay.

What spindle length does the UN-72 you installed have? From this page on your crankset:

VeloBase.com - Component: Shimano FC-B124, Biopace Touring Triple

It would appear that you'll need one that has a pretty long spindle length, or with an asymettric spindle as the drive side apparrently should have a 5mm offset. If your bottom bracket doesn't comply with that, it's not surprising that your chainring is hitting the frame.

due ruote 

The crankarm has not been removed or re-torqued since I built it up, at which point there was no rubbing.

Chainring bolts are tight.

It's actually the bolt heads from the small ring that are rubbing on the BB lug, so a smaller chainring wouldn't help.

I assume you mean just remove the small ring. I could do that, but I would just as soon leave it in case I take the bike somewhere I need a bailout gear.