Reverse thread surprise

[Vargas]

I never assembled a bike.

I read that the bottom brackets have reverse (left) threads on one side. To me it made a lot of sense - in case the reverse thread was on the left side. That way when pedaling you would force the botton bracket into thightening the threads.

Yesterday I looked at the kit and realized the reverse thread is on the right side. You have to screw the bottom bracket counter clockwise to tighten; the opposite direction of pedaling.

Is there any reason for that? Doesn't it make more sense to have the right side tighten clockwise and the left side counter clockwise in order to have tightening in the same direction as pedaling?

 

[John Tolhurst]

planetary motion of the cup

Well, pedalling creates impetus for a planetary motion of the cup in the shell, which has the effect of winding the cup in the reverse direction. That is why they don't come out.

 

[Vargas]

Guess I found it...

I beleive I found it! The BB axle spins clockwise. The balls inside the BB bearing spin counter clockwise. This is why the threads are reverse; the balls (not the axle) drag the BB cup or housing counter clockwise.