Originally Posted by Jobyl
No, it's not.
Sheldon says:
"The right pedal has a normal thread, but the left pedal has a left (reverse) thread.
The reason for this is not obvious: The force from bearing friction would, in fact, tend to unscrew pedals threaded in this manner. The fact is, however, that it is not the bearing friction that makes pedals unscrew themselves, but a phenomenon called "precession".
You can demonstrate this to yourself by performing a simple experiment. Hold a pencil loosely in one fist, and move the end of it in a circle. You will see that the pencil, as it rubs against the inside of your fist, rotates in the opposite direction."
Another way to put it: As you drive your car or bike forward, the wheels push dirt backwards. If your wheel locks up while you're moving, it pushes dirt forwards. Similarily, as you drive your right pedal counterclockwise (relative to the clockwise-moving crank), the balls inside it drive the spindle clockwise, screwing it in tighter. If the ball bearing joint freezes up due to long neglect, then pedalling forward will unscrew your pedal.
Robert