Not Smart Enough to Quit
Square taper BB on an AIR9/EMD
I'm about to pick up an AIR9 or an EMD (undecided now) and I plan to run some old XT square taper cranks in a 1 x 9 set up until I can score a 2 x 10 XO drive train at a later date. I'll be getting a new BB but the chatter I've seen about chainring clearance on these frames has me confused as to what spindle length I need. Anybody got any ideas?
I am in the process of building up an older Air9 frame in a 1x9 configuration using a square taper BB & White industries ENO crankset w/34T ring. My frame is the older 68mm BB shell width. I have a couple of Shimano UN 71/72 BB's to choose from in my parts bin and here is what I have determined. The White ENO cranks are very similar to the earlier Shimano M730 in the ring off-set, and based on trying a parts-bin 113mm BB first (too narrow) and a 127.5mm next (works but probably a little too wide) I am guessing the right size for my combo is 121-122.5mm. This would hold true for the older Shimano XT cranks of the M730 vintage if you are running a single ring on the middle chainring location. Here is the link to Sheldon Browns BB /crank fitment information. Each frame is somewhat different, but this is a pretty good baseline to keep in mind depending on the vintage of cranks.
A lot depends on which vintage of XT cranks you are using. I typically run mid 90s XT 737 cranks in a 1x9 configuration with a salsa ring and I can tell you that those cranks fit very differently than the older 730's. Where the 730 requires a 122.5 mm or greater spindle width to clear, the newer XT 737 square taper cranks have a different off-set and 113mm spindle (or in some cases 110mm) will work fine on most frames.
MTB triple cranks usually have the outer chainring at around 52-57 mm; the middle is usually in there around 47.5-50 mm. My guess is that on an older XT crank with square taper, the middle ring (if that is what you are using for the 1 x 9) is probably around 47.5mm when used as a triple. You can leave it there, or pull it in a tad, or push it out a tad using the bb spindle length. Either way, you should get full use of the cogs in the rear. If you are going middle ring, is it a 32T or 34T ring?
Originally Posted by xray_ed
If you get a high end square taper BB, you'll be able to adjust the chainline left or right a few mm's. If you are using a cheap Shimano square taper - you'll just have to hunt and pick until you narrow down what works best. I assume you have one now (a bb). What spindle length is it? Start with that and see if it works. If not, go from there (longer or shorter spindle length).
All of this is speculation until you tell us which ring you will be using up front and what size ring it is...
Read Uncle Sheldon while you're at it.