A chain is a row of tiny hinges
A bicycle chain has to do two jobs that seem at odds: wrap around small toothed wheels and transmit a strong pull without coming apart. Its repeating links solve this with joints. Pins connect the plates, while rollers around the joints meet the sprocket teeth. As the chain moves, each joint pivots slightly, letting the chain curve around a gear; the plates themselves remain stiff enough to carry the pull.
Why the fit matters
The teeth push on the rollers, and the chain’s tension turns the sprocket. Because the distance between neighboring pins—called the chain’s pitch—must match the spacing of the teeth, the chain needs to bend at its joints without changing its effective link spacing.
That distinction explains why a worn chain can seem to “stretch.” The steel plates usually have not lengthened like elastic. Instead, friction gradually wears material from the pin-and-joint surfaces, increasing the clearance at each link. Across many links, those small gaps add up, so the chain no longer fits the sprocket teeth as precisely.
A small wear pattern, a costly result
A loose fit makes the chain ride less cleanly on the teeth and can accelerate wear on both chain and sprockets. Replacing a worn chain in time can therefore help preserve the more expensive gears. The chain’s cleverness is not that it bends everywhere: it bends at a long series of controlled, pin-sized hinges, while staying firm along the span between gears.
