By Johan Bornman 

Chains don’t stretch, they elongate due to wear in the pins.
 

How much can a chain "stretch"?

The pitch – distance between links – on a bicycle chain measures exactly 1 inch (25,4mm) over "two" links. There is no use in translating to metric here, as this measurement is standard all over the world, and always has been. Worn chains measure slightly more because internal wear elongates the chain by leaving larger gaps between pins and sideplates.

"Stretch" in physics would relate to strain which is either i) elongation under tension (temporary), or ii) elongation following yield (i.e. permanent ) when something was loaded / pulled beyond its ability to return to its original state.

Over time – as a result of wear - the pins of a chain are ground away and the recessed holes within the sideplates become oval. The chain’s sideplates – however - retain their original length. It’s not really “stretching” of the chain’s sideplates that makes the chain longer, but erosion of its cavities and the pins’ diameter. The net result is that the chain – over time - becomes longer, hence elongated (but not really ‘stretched’ as most people would refer to the phenomenon.)

Allthough chains lengthen, they do not really 'stretch'. Not even a 'Tour' rider fortified with race juice can stretch the hardened steel of a chain. Chains elongate / lengthen by the mechanism described above.

Relegate “chain stretch” to your lexicon of forbidden bike terms alongside “cable stretch”, “puncture proof” and "a drug-free Tour winner”!

The effects of a worn chain.

A popular bicycle myth is that a worn chain somehow affects gear shifting, but this is not true.

Bicycle chains, unlike other drivechains that run on a single pair of sprockets and remain in perfect alignment when operating, are designed to flex sideways to accommodate gear shifting. The jockey, after all, deflects the chain onto the next gear. However, a worn chain does not tend to overshift (which would have been the case if the bike-lore was true). In fact, if the chain is very badly worn and the B-screw setting on the derailleur has the top pulley too far removed from the sprocket, you will experience sluggish shifting. (This is an extreme and unlikely case, but poor shifting remains an unreliable indicator of a worn chain).

The real effect of a worn chain remains hidden until its too late;  it wears sprockets, ruining (expensive) cassettes.

A worn chain works perfectly well on a worn cassette. However, a new chain does not mesh with a worn sprocket.  The trick is to change your chain often enough to prolong the sprocket’s life, but not so often as to outweigh the cost of new sprockets.

Sprockets don’t last indefinitely. Just like a human tooth, a cog (a single tooth on a sprocket) also has a hard protective layer - that equates to a tooth’s enamel - encasing a softer material (dentine in the case of your teeth). On sprockets the protective layer is case-hardened steel that protects the softer - yet more tensile - steel inside. Once the thin, case-hardened layer is worn away, the cogs deteriorate quickly.  A rear sprocket wears by widening the U between teeth, whilst a front sprocket wears by eroding the tooth from the root.

It is possible to replace just the 3 or 4 of the smallest sprockets, if you can find the exact same sprocket. (There will be a "J" number or similar stamped on the side...)

And remember: if it ain't broke, fix it!