BB for 1985 Cannondale ST500?
 

Thanks in advance for any and all insights.

I picked up a stripped Cannondale frame and fork, a black 58cm 1985 (I think) ST500. It's missing (among many other things) a bottom bracket and its headset bearings.

BB shell is 70mm. The ST500 had a triple and i think the original spindle was quite wide (127mm?), but I'm planning on using it in the city as a 1x5 or with an IGH wheel I've got hanging around - in any case, a single on the front. I have a Sugino AT and I suppose could position the chainring on the inner face of the spider, but does it make sense to shave off a few mm to get appropriate chainline? It seems to me that without parts on hand trial and error is the method I've got available, but I'd love it if there were a better approach.

I'm generally quite ignorant about BB and spindle choices - is a Shimano BB-UN26 just fine or is there something more durable I can invest in (weight is not a serious concern for me)?

Cheers!

That’s a 68mm English threaded BB tou have there.
The correct spindle length is completely dependent on crankset and ring placement.
An inexpensive Shimano cartridge BB is an excellent choice for the intended purpose.

I think a Shimano cartridge BB would be a good choice. If going with a triple you are correct that it will probably end up rather wide. I have had a 1984 and with the diameter of chainstays/clearances, it needs a rather long spindle for most triples. With the sealed units having symmetrical, non-offset, spindles probably especially so...

Good luck with it, so worth getting back on the road, you're going to love it!

The Sugino AT cranks feature removable spacers for the small ring position, so should be used with a shorter spindle and with the single ring on the large-ring plateau.

This would require something like a 115mm bottom bracket I'm guessing.

This is a guess-timation. Hopefully a symmetric bottom bracket of known length can be test-fitted before making a firm spindle length choice.

Thanks to all. I went with too wide (127.5mm) to start and figured I could use the aforementioned spacers to get decent chainline in the short term and only be out about $20 if it's seriously off. Tight clearances on those pinched chainstays seem to post their own challenges.

And @Sandstrom, I'm genuinely excited, too. The past year since my 1987 ST600 was stolen has been rough as the otherwise lovely steel bikes I've been riding feel... flexy and sluggish by comparison.

Hope you can get it dialed in soon. For reference (only) with that crankset, I believe for triple I had a 124mm spindle and when running a double I used 121.5. These were were traditional cup and cone BBs so probably not too helpful for comparison with the sealed, symmetrical units. I have brought bikes to the LBS in the past to get their assistance in selecting the appropriate sealed bottom bracket replacement.

Are you sure it is 70mm? Dales were almost certainly English thread and 68mm.

FWIW, my 85 ST400 had a 116mm spindle with the original iwo ring crankset. A triple I purchsed for possible hill/mountain use(SOTR?) is speced for118mm spindle. Do you have the spec for your crankset? 127mm seems quite wide! For me a 118 mm spindle tried on my double provided noticeable knee displacement to the right and eventually knee discomfort. Thead carefully!