Quality bikes are astonishing things. Engineered by MIT graduates, developed in wind tunnels and designed to squeeze every possible watt from a rider’s legs, a high end bicycle has more in common with an actual rocket ship than you might imagine. The tolerances that match a chain to it’s cassette are extremely tight, and in the quest for lighter everything, modern cassettes are made with less material to shave weight. Over time your chain will stretch, and the cassette teeth will no longer match the chain. If you ever feel that your gears “slip” when under load, this is often the case. Not only is this unsafe, it indicates that you’re causing damage to the expensive drivetrain of your bike. Think of all the bits your chain touches. Cassettes, chain rings and even jockey wheels can suffer if you continue to ride on an old chain. The worst case, is that you wear away the teeth of these items and when a new chain is installed, it no longer matches. Needless to say, that’s one heck of an expensive tune up.