(Note: In what follows, I refer to the drive side bottom bracket cup as the "fixed cup" and the non-drive side as a the "adjustable cup". This is archaic, a relic of the time when the drive-side cup was indeed fixed, and one adjusted the bottom bracket by rotating the non-drive-side cup with a spanner)
This is the sequence of events that took the frame out of commission. One day last summer, I was riding, when suddenly (-- !! --) the cranks locked to forward motion. When I looked down, I saw the adjustable cup had threaded out up against the crankarm; further pedaling would push the cup even tighter against the crankarm, so the crankarm was locked.
That night, I put the bike on the stand, and removed the BB (Ultegra English cartridge, Octalink) from the bottom bracket shell, to get a look at what was going on. When I went to re-install it, I found the fixed cup threads, as well as the corresponding drive-side bottom bracket shell threads, were completely stripped. My best guess at what happened is:
- The fixed cup side stripped
- When it did, this allowed the entire cartridge to rotate clockwise (looking at it from the drive side)
- This rotation caused the adjustable cup (which acts as a snug sleeve around the cartridge) to rotate counter-clockwise (looking at it from the non-drive-side). This threaded the adjustable cup outward.
- What would have caused the fixed cup threads to strip?
- Once the fixed cup threads were stripped, what would have caused the cartridge to rotate clockwise?
And that is one of the reasons why Aaron is Aaron, and I'm not. I never would have thought of this, and there is something profoundly incisive and analytical about the thinking that leads to this solution.
- Aaron has lost the beard, and, although those of you who have seen me know I have no sense of aesthetics, I'd say it's a good look for him!
- Cargo Bike Ride, March 23rd, Alaskan and Broad (Myrtle Edwards Park. Umm -- can't see a time on the flyer -- will try to find that out.
- For the cartridge to rotate clockwise (when looking at it from drive side) once it was unimpeded by those pesky threads is a complete surprise to me. My understanding is that the force of precession will cause the cartridge, when free, to rotate counter-clockwise when the BB axle is turned clockwise (i.e., forward pedaling). This makes me suspect that somewhere in this long, sordid tale of lust, greed, and hubris is an impediment to the free motion of bearings over their contact surfaces (think: "freezing"). I can see this causing the cartridge to rotate in the same direction as the axle.
- Another minor mystery that touches on this story in two ways is how Italian bottom brackets stay threaded. The force of precession I mentioned in the above bullet would cause a counter-clockwise force on the fixed cup in response to clockwise rotation of the axle (forward pedaling), and since the Italian fixed cup is right-threaded (lefty-loosey), I'd think the fixed cup would tend to unthread from the bottom bracket shell.
In fact, that is exactly what happened to me on the maiden ride of the very first true racing bike I built up, in 1985. It was a beautiful Palo Alto blue Columbus SL frame, built up with Campy Record gruppo, including Italian threaded BB. About 3/4 of the way through the ride, the cup unthreaded. My LBS (I was in Ann Arbor) suggested I use threadlock. Aaron, however, emphatically says not to use thread lock, but just to install the cartridge to the proper torque. In 1985, we (or, at least, I) did not know from torque. If you'd've said "torque", we (I) would have thought "that guy who played in the Monkees."
- Although Aaron's solution of not tapping the apparently undamaged non-drive-side threads is compelling, I am going to go ahead and tap it Italian, so it matches the drive side. My motivation is that I now have a tap set, and I'd like to practice chasing and tapping (and just for the hell of it, facing). This is a good frame for me to learn on, since it has other problems (excessive flex in the rear triangle), and I won't be upset if I ruin it.