Why do left pedals have inverted thread?

Question

We all know right pedals are screwed regularly in the cranks. Whereas left ones are screwed in the other direction.

Why is it like this and not the other way around?

Here is how I see things: under normal circumstances, the direction of the thread matters little if at all since the bearing allows pedals to spin freely regardless of how they are attached to the crank.

But when the bearing has troubles, I would think the thread direction is here to protect the pedal from unscrewing from the crank as one is pedaling.

But when you pedal forward, the right pedal is spinning counter-clockwise (since the crank is spinning clockwise) so you would unscrew the pedal from the crank.

Same works for the left: when you pedal, the pedal is spinning clockwise so in the direction to unscrew the inverted thread on the left.

I must be missing something,... but what?

Answer 1

The reverse threading is to counter precession, not friction from the bearings. The wiki page explains it well (and I doubt I could): http://en.wikipedia.org/wiki/Precession_(mechanical)

Answer 2

superdesk's link to Wikipedia is excellent, but let me also add Sheldon Brown's explanation, which dismisses some other theories (like the ankle saving theory).