Originally Posted by DougG

Everyone knows (or discovers to their detriment!) that the right-side pedal has right-hand threads and the left pedal left-hand threads. Many times, even in repair manuals, it gives the explanation that it has to do with the direction you pedal and the keeping the pedals tight.

But this is demonstrably not so. If your pedal is loose and your pedal bearing is stiff, pedaling in the forward direction will unscrew the pedal, not tighten it. In fact, I use this as a lazy way to unscrew the pedals with the bike on a stand: break them loose, hold the inner shoulder by the flats, and pedal the crank forwards to back them out.

So is there a known valid reason for the opposite threads, or is it something that's just become a standard over the years?

(BTW, anyone remember when some cars had left-hand threads on the left-side wheel lugs? I almost broke off a stud trying to change a tire on my '55 Chrysler before thinking to try the reverse direction.)