Teardown Tuesday: Bicycle Bottom Brackets (virtual)

You know by now that I re-did my bike over the weekend. The part that scared me the most (for no really good reason) is the bottom bracket.On the face of it it's probably the simplest part of the bike. It's nothing more than a bearing to hold the crank while you pedal.The part that confused me was why there were reverse threads on one side -- in fact the non-intuitive side.Let's start with the pedals. The right side (with the chain) pedal is normal right-hand thread. You screw it in like any screw. The left side is left-hand threaded; you screw it in counterclockwise. Even this is unintuitive. If the bearing locks up it unscrews itself... why is that?I was looking around and finally found why: precession. The reasoning given at the late Sheldon Brown's site is:

The right pedal has a normal thread, but the left pedal has a left (reverse) thread.The reason for this is not obvious: The force from bearing friction would, in fact, tend to unscrew pedals threaded in this manner. It is not the bearing friction that makes pedals unscrew themselves, but a phenomenon called "precession".You can demonstrate this to yourself by performing a simple experiment. Hold a pencil loosely in one fist, and move the end of it in a circle. You will see that the pencil, as it rubs against the inside of your fist, rotates in the opposite direction.Ignorant people outside the bicycle industry sometimes make the astonishing discovery that the way it has been done for 100 years is "wrong." "Look at these fools, they go to the trouble of using a left thread on one pedal, then the bozos go and put the left thread on the wrong side! Shows that bicycle designers have no idea what they are doing..."Another popular theory of armchair engineers is that the threads are done this way so that, if the pedal bearing locks up, the pedal will unscrew itself instead of breaking the rider's ankle.The left-threaded left pedal was not the result of armchair theorizing, it was a solution to a real problem: people's left pedals kept unscrewing! I have read that this was invented by the Wright brothers, but I am not sure of this.

But here's where it gets tricky.The bottom bracket is backwards to that. The right side is reverse-threaded and the left side is normal threaded. What gives? All I ever found was "it's like that because the other way would tend to loosen."It's the same thing applied to a larger surface. It's as if the force applied to the outside of the bearing is opposite to the real force applied to it -- as if the bearing shell was turning opposite to the spindle. It's like you're looking at a planetary gear system and the outer bearing race has the opposite force applied; the opposite force serves to screw it in further!