Originally Posted by
FBinNY
To answer only the question of why right cups have left threads, which at first glance seems counter-intuitive, you need to understand something of the character of ball bearings.
Draw a quick sketch of a simple ball bearing assembly, consisting of a small circle surrounded by a bunch of smaller circles, and surrounded by a ring. If the inner circle is spins clockwise, each ball will spin counter-clockwise creating a counter-clockwise reaction force on the ring.
It's cute but the ball bearings have nothing to do with it.
To understand this precession phenomenon, make a loose fist, and stick a pencil down the middle of your fist. Then wiggle the end of the pencil around in a circle. If you follow, you will find the pencil wants to rotate opposite the direction you're wiggling its end. In this analogy your fist is the BB shell, the pencil is the BB cup, and its end is being shoved around by pedaling forces (which are the same regardless of whether ball bearings are used.) Same thing happens with pedal threads, so the left pedal is reverse threaded, and with lug nuts on older cars, where the left wheel nuts are reverse threaded (newer cars use conical faced nuts which eliminate this problem.)