Turing’s Bicycle

In Neal Stephenson’s Cryptonomicon, Turing’s bicycle throws its chain whenever one weak link in the chain happens to meet one bent spoke in the rear wheel — so he counts the pedal-strokes and steps off just before it does. The two faults only ever line up on their least common multiple, which means counts that share no common factor hide the fault the longest. It is the same arithmetic that decides how long a cipher runs before its pattern repeats. Pedal the bike, or fast-forward to the next drop, and watch for the moment the chain lets go.