Originally Posted by squirtdad
The way I visulized it finally was like a set of gears. the axel (bb spindle ) moved forward (clockwise), but it rotates on the bearing, which then rotate backwards (counter clockwise). It is the bearings that exert force on the cup so even the the apparent (pedal direction) force is clockwise, the actual force (bearing) is counter clockwise...which would unscrew a right hand thread.
But it's not like a set of gears because both the cups and the crank rotate on the same axis, unlike a set of gears. I won't claim to understand why precession happens though.
[edit] After trying the experiment on Sheldon's website, the "phenomenon" is most likely caused by the tiny clearances that exist in crank and pedal bearings (obviously cheaper BB's and pedals will thus suffer worse from this condition). If you actually hold the penicl tightly and try to rotate it, all the force is exerted in the same direction as it's applied. If you allow the pencil to wobble, that's when you get the force in the opposite direction. Anyone care to comment? [edit]