Why are external BBs threaded the "wrong way"?
 

Rookie mechanic here, just trying to understand things out of curiosity.

The left side of my BB screws in clockwise and the right (drive) side screws in counterclockwise, both in the direction of backwards pedalling. So it seems, when pedalling forward, any drag transmitted through the bearings would tend to unscrew the external bearing holders.

My intuition tells me the BBs should have been threaded the opposite way. What am I misunderstanding here?

Search 'precession.'

The dominating effect in this case is fretting precession. https://en.wikipedia.org/wiki/Precession_(mechanical), not drag-induced torque transfer.

The torque transfer through drag/friction is insignificant compared to the torque produced by precession. So, by applying cyclical "sideways" loads on the threads you are actually tightening the bearing holders.

For the very same reason the pedals are screwed into cranks with "wrong" threads. In terms of friction-induced torque transfer you are actually unscrewing your pedals as you pedal forward. However, in a properly operating pedal that torque transfer is negligible compared to the opposite (tightening) torque produced in the pedal-to-crank junction by precession.

Also see this article for detailed explanation: Left Hand Threads on Bicycles by Jobst Brandt

Excellent [MENTION=278823]AndreyT[/MENTION].

Better question: Why don't Italians get this? ;)

Got it. Thanks for today's lesson in mechanics.

Campag Pedals are marked D & S .. (BB Fixed Cup If Tight Enough does not come Loose )

D= Destra=Right from Latin, dexter
S=Sinistra=Left from Latin, sinister

Or the French; fortunately the Swiss do, ;)

Yea, I can speak from experience about this. Luckily I was only about a mile from home on the first ride of my first Italian BB build when it worked loose, no damage except my pride duck walking home.

Perhaps for a few reasons. First is that one side of an Itl BB is correctly thread directioned. Second having the same thread direction allows for a single chasing pass during the frame finishing process. Third is that when these standards were being developed there were a few different pressures at hand. Provincial, actual use and maintenance differences then today's (USA) riders follow. Andy.

Make a attempt at a joke, get a serious answer....