Question re bottom bracket length- chainrings hitting chainstay

Trying to finish off my Rocky Mountain project MTB and of course I keep hitting small obstacles along the way.
The latest issue is that when I fitted the bottom bracket and triple Shimano crank, the small chainring is just touching the chainstay. The reason is that the cranks are actually Biopace so they are quite a large diameter at their widest point. It looks like the bottom bracket just needs to be a couple of mm’s longer on the driveside.

The setup is:
Bottom bracket 122.5mm 68mm shell cup and cone

So what would be the easiest solution?
a) Longer spindle bottom bracket- would this affect the chainline much? And how long would I need?

The cranks look similar to this:


Add the (amount of clearance you want) x2 to the current spindle length. (Assuming similar symmetrical / asymmetrical spindles)

Or if you have a bunch of clearance on the NDS, add a spacer on the DS cup.

I have the SLX top pull mech from DICE to give to you too. You owe him cash monies for it.

Thanks Blakey I have fixed DICE up, I’ll grab it off you this week if possible.

So assuming I need another 4mm, a 127mm length spindle would be right. Shimano would make a UN cartridge BB for this?

UN54 comes in 68 x 127.5mm. Don’t know if that many places would keep such a long spindle in stock, but maybe they would - for some commuter bikes?

I’m having the same problem, thanks for some ways ahead.

The crank is designed to work with a certain length BB spindle to attain a certain chainline. Long BB spindles only mean large chainlines on low-profile cranks. MTB cranks typically have to sit out further for the crankarms to clear the chainstays, so the chainrings sit further inboard relative to the central taper, to maintain the chainline. Thus, longer BBs, especially for older touring and mtb cranksets. It’s all happy. 127.5mm is very common.

  • Joel

Thanks tomacropod. Yes I ended up buying the UN-55 127.5mm BB- new and improved with alloy cup!
Works like a charm, and I’m strangely enjoying the Biopace cranks…