Originally Posted by Kimmo
Has anyone who installed one seen it happen? I struggle to understand how rolling balls can unscrew anything halfway tight.
IMO the only reason you need to avoid Italian-threaded frames is the BBs are like 1 in 50, if that.
The process through which the threads loosen is called precession
It is the same reason left pedals are reverse threaded.
Precession works similar to how a spirograph toy
works - when a smaller cylinder rolls around the inside of a larger cylinder, it is actually turning the opposite direction to the rolling relative to the larger cylinder. Int he case of a bottom bracket, the larger cylinder is the bottom braket shell and the smaller is the bottom brakcet cups. Pedalling forces will tend to make the cups roll around the bottom braket shell. THe shell remains stationary and the cups rotate in the reverse direction of pedalling. English BBs have reverse threaded right hand cups so the reulting action of the cup turing opposite the pedalling will tend to tighten both cups. On Italian (and other obsolete both-sides-right-hand-threaded BB styles) the right hand cup can loosen itself off. I would expect if the BB shell is made with excessive clearance and the cup has too loose a fit this will be more likely. On properly made Italian threaded BBs, if the cups are properly torqued then loosenign off is unlikely.
Very few modern frames are made with Italian threading. More modern road bikes come with some style of pressed-in bearings than Italian threads these days.
Last edited by DCB0; 08-01-12 at 02:12 PM.
Reason: added link to spirograph and precession