r cielec wrote:
> Ahoy !
> The recent discussion of Swiss threads has prompted me to expose another empty space in my head where knowledge should have been filed many years ago.
>
> Bottom bracket cups and threads:
> Any mechanical engineers out there who can explain why the cups remain tightened or why they loosen ( I mean with proper installation not because of error, cross-threading, neglect) ?
> Re: Threads are opposed RH and LH ? Does it matter which side is RH or LH ? Can LH on right with RH on left still keep the bb cups tight?
> Would a LH-LH loosen up?
> Why Italian RH-RH and only one lock ring?
>
The secret word is precession, easily demonstrated if you hold a pencil loosely in two closed fists. Make a rotating motion with a hand, and the pen rotates in the opposite direction. This is what happens with pedals, and you don't want the pedal to unscrew. With the BB it is a bit more difficult to envisage , but here the same principle applies
-- mvg
Marten Gerritsen