Any cyclist knows that the left bike pedal is left-hand (i.e. reverse) threaded. This is so the pedal doesn’t unscrew itself while you’re pedaling. But go grab a bike and spin the pedal and crank around and you might be a little confused. Last time I did this, I thought, Wait why isn’t the right pedal reverse threaded? When you spin the pedal and crank forward, as if you’re actually powering the bike, the effective spinning of the pedal around its axle (AKA the spindle) should actually unscrew both pedals: lefty-loosey on the right pedal and righty-loosey on the left. Did every bike manufacturer get this wrong?!?
Of course not, and the real answer blew my mind. (Probably because I’m not a mechanical engineer.)
It is not, not mind you, because of the effective unscrewing force from the non-zero friction of the ball bearings. Instead, it is an effect that works in the opposite direction (in this case): mechanical precession:
“Precession is the process of a round part in a round hole rotating with respect to that hole because of clearance between them and a radial force on the part that changes direction. The direction of rotation of the inner part is opposite to the direction of rotation of the radial force.”
The source of the screwing/unscrewing force is thus radial on the spindle—the downward force you put on the pedal—instead of the twisting force from the ball bearing friction. This radial force translates into a screwing/unscrewing force because there is a small amount of clearance between the spindle and the threaded hole in the crank. I picture it like a pencil in a toilet-paper tube: crank the end of the pencil around, and there is a force that wants it to rotate on its long axis (from friction with the wall of the tube).
The screwing force from precession (on a reverse-thread on the right pedal) is much stronger than the unscrewing force from friction of ball bearings, so bike manufacturers ignore the latter.
I wish I could find an animated gif of mechanical precession, but I haven’t found one. Anyone have a book on “advanced thread theory” and want to make an animation?
UPDATE: Here’s a nice animated figure from Wikipedia: