Originally Posted by Andrew R Stewart

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.