Originally Posted by
TullySteve
Alright maybe this is a dumb question but here goes: Why would a RH thread (as on this BB) unscrew itself on the drive side? It screws in clockwise. You pedal forward clockwise. What am I missing? It seems the potential problem would be on the other side: pedal direction counter-clockwise, BB RH threads unscrew counter-clockwise.
If you look at the axle, bearings, and cup as a planetary gear system it's easy to understand. Picture the drive-side, and focus on the ball at the top of the axle. The top of the axle goes forward as you pedal, it drives the bottom of the ball forward. The top of the ball drives backward, driving the cup against which it bears backward (i.e. anti-clockwise). So a left-hand threaded cup will be driven tighter by normal pedaling, a right unscrews.