Tangentially related, I have recently become aware of a sound similar to a bag of potato chips being crushed coming from the rear wheel of my mountain bike. Closer inspection showed that the rear wheel hub exhibited an unwanted additional degree of freedom with respect to the rest of the wheel. The hub was able to be moved slightly back and forth radially. "An excuse to get my hands dirty, and an easy fix", I thought foolishly -- it is only mechanics, after all, nothing that will not surrender immediately to the agile mind and nimble hands of a computer programmer! Fast forward 300$ in tools of and 4 weeks of attempts to fix this, which typically resulted in the purchase of a new specialized tool, and the bike runs again. I learned more than I had hoped about the vast number of very small metal balls that reside in a modern freewheel hub, and it was a good lecture in humility.