Il pedale sinistro è filettato "al contrario" perchè altrimenti si sviterebbe. Quello destro invece ha la filettatura normale perchè tende naturalmente ad avvitarsi.
La spiegazione del perchè di questo comportamento era già stata linkata nei post precedenti (
https://www.sheldonbrown.com/pedals.html) ma non esplicitata:
"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. 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 with the other hand. You will see that the pencil, as it contacts the inside of your fist, rotates in the opposite direction. This occurs because the pencil's diameter is smaller than that of the opening in your fist. The external threads of the pedal axle are slightly smaller than the internal threads of the crank -- or they wouldn't fit!"
Anche le scatole movimento sono filettate nello stesso modo (ovviamente se non pressfit). Io ho avuto una bici con movimento a cartuccia e passo italiano (filettato destrorso su ambo i lati) e nonostante il
frenafiletti la calotta sinistra tendeva ad allentarsi.
Pedali e pedivelle son tutti fatti così.