Friend of mine brought his bike over for me to look at. Apparently the bottom bracket fell out during a ride. Rare as I felt it maybe, I offered to tighten it for him. Initially it looked like it'd be an easy job. just take out the arm, tighten BB, put arms back. done.

Upon taking off the crank arms and inspecting, i found out that the threads on the drive side of the bottom bracket shell was stripped. so much that i can just slide the BB (which visually still have all the threads intact) all the way in so the lip is flushed against the shell.

here are some info:

- cannondale aluminum frame, 2011 HT model. forgot exactly the model name. around $900 full retail
- English thread
- taper BB
- not sure what brand BB it was, but not shimano.

he says after he noticed the crank is loose and the bb fell off, he tried to push it back in into finish his ride. he is not very mechanically inclined so i don't think he was fixing it nor did i think he could've done much damage either. it was just a local dirt trail, nothing steep nor technical.

what i found strange and mysterious is how could this have happen. the only confirmation i know is the bb became loose and fell out. there are two possibilities:

- some how the bb wasn't tighten correctly and became undone. and somehow him trying to continue riding (just a few miles) managed to completely strip the thread - is it even possible for a loose BB to strip the shell's thread completely, in such a short amount of time?

- the thread was stripped due to poor installation or defect. perhaps it was overly tightened to start with, and over time the bb just worked it way out.

i have seen and known many people with bikes, this is the first time i have ever heard a BB can fall out in the middle of the ride. and i am having trouble understanding what could've caused it.

anybody has any experience with something similar or have a possible explanation?

Only help I can offer is that I've seen a friend trying to "climb" a log (landing with the biggest ring/bash guard on the obstacle with a pedal in an up-and-forward position, then pushing down on the pedal to clear the obstacle) which caused the BB to seize momentarily and unscrew itself enough to throw the front shifting off. The messed up shifting what caused him to notice that something had happened. We adjusted the shifting and the BB worked well enough to let us finish the ride.
But this was a steel frame. I can imagine if something similar was to happen to an alloy frame, a couple of miles of continued riding might possibly trash the threads.

BB's loosen (as within the threaded shell) over time and not all at once. I doubt it was climbing over a log that did it, perhaps this was when you noticed it? There is a tendency to blame the moment just before awareness as being the when something happened. The first fix I think of is to use a repair BB which does not rely on the shell's threads to secure it in place. Andy.

Perhaps the steel cup was cross-threaded during installation in past? This could damage the threads enough to make the cup slightly loose, after which wobbling during riding could erode away the rest of the threads.

Fortunately, there are a couple options to fix this: a threadless cartridge from e.g. Velo-Orange, or reaming the shell and tapping Italian threads to use an Italian thread bottom bracket.

Doubt away to your heart's content if you wish, I'm very confident in my/our interpretation.
Immediately before the bike stayed in gear just fine, immediately after it wouldn't. There was no gradual going out of trim or chain scrape first.
And the whole cartridge BB had shifted sideways. While it still turned, it felt very notchy and was replaced later.

I just can't see how, without previous damage, the BB would suddenly shift over from the kind of forces that log climbing offer up. The force to pull the BB sideways/outward that would tear off the many threads of the shell (even being softer Al.) is REAL significant. What is much more likely the case was that the BB had begun to loosen up without your friend noticing anything. Then with the BB unit loose it would auger out the shell with each pedal stroke. Bit by bit the shell's threads were mashed and molded. Then when an otherwise containable twisting/sideways force was applied (the body english needed to get over the log and the large ring digging into the wood providing a fulcrum with which the crank/BB would be yanked side to side) the shell's now damaged threads couldn't hold the BB unit any longer.

Yes, the majority of the side ways movement happened in short order but the enabling was the previously loose BB damaging the shell's threads over time. This is but one reason bikes need periodic servicing. To nip problems in their bud, before the small and easily fixed (simple retightening of the BB) becomes a big and frame damaging issue.

I have seen this kind of shell damage many times before and each time the rider had little or no clue to a impending issue until they couldn't shift any more, the chain would auto drop or the rings/cranks would hit the stays. Just because we don't perceive a problem does not mean there's no problem. Andy.

+1 Andy.

BB thread carnage doesn't just happen spontaneously, even with steel in aluminium. There has to be a sustained period where it's ridden while loose, or at least been cross-threaded, or had the wrong-side tap run through it or something.

If the the threads are lubed and undamaged already and the cup is tight, nothing can happen to the threads.

Again we see how little value the correct answer has.

This isn't all that rare. If threaded BB cups are loose, they can move up and down slightly with every pedal stroke. The process wears threads away by friction. Usually the process is stopped soon enough that the damage is minimal, as the owner notices the play, or hears the creaking with every stroke.

This is simply an extreme case, and your friend was probably riding it loose for a long while.

AnkleWork- Not sure if this is a dig at my answer. But I'm comfy with my Experiences. Andy.

Take a 90 kilo lad going for a bunnyhop over a log but missing, landing on the big ring. Direction of thread matches the pedaling direction. BB seizes momentarily and pedaling action breaks it free. No damage to threads, but bearings were shot.

I repeat - impact causes BB to seize, pedaling action shifts it over. I support my theory on the massively notchy feeling in the BB afterwards.
But if you who weren't there still insists on having a clearer view of the event I'll leave you to it.

Weird filter. I think your answer is correct.