jcb3

Seems to me that a french (right hand thread) fixed cup (tighten clockwise) will get tighter during pedaling, as the axle moves clockwise as well.

I'm clearly missing something here - Sheldon Brown also says the same thing, that left hand thread fixed cups tighten with pedaling action and that a right hand thread fixed cup will tend to loosen.

Further thoughts or explanation?

Thanks, Joe

verktyg 

It's the Coriolis Effect! In the southern hemisphere, the left side comes loose!

What happens is as you pedal, the crank and spindle rotate in a clockwise direction. The ball bearings rotate in a counterclockwise direction. Friction from the balls and road vibration can cause a right hand threaded fixed cup to work loose!


Note: the following applies to steel bottom bracket shells for lugged construction frames from the C&V era - not welded, lugless or plastic frames. Most modern BB shells, lugs and fork crowns are precision investment cast steel.

The threads in most C&V era bottom brackets (from the top holy of holy Italian bikes on down) were not precision cut! They can be undersize, oversize, egg shaped and so on.

One cause is that some BB shells were made from a flat piece of metal that was rolled into shape and welded with a large internal seam. The metal in the seam area is a lot harder than in the rest of the shell which can cause the taps to cut out of round threads!

Another is that the taps used to cut the threads were worn out. Most BB shells came prethreaded with an undersized thread. The heat generated from brazing will warp the shell. The partial threads allowed finishing taps to easily start into the BB shell in a finished frame.

The steel used for BB shells was usually a soft gummy low alloy type which allowed the "spigots" where the tubes fit in to be drawn out of the original tube.

Using worn taps in the soft steel frequently left burrs in the threads. Chasing the threads with special bottom bracket hand taps can easily re-cut the threads oversize. Loctite Blue is the solution for that problem.

This link show re threading BBs:

Bottom Bracket Tapping, Threading, Chasing, and Facing | Park Tool

verktyg

Chas.