Bicycle chain wear (stretch)

This is the simplest way with no special tool. Tension the chain (shift onto the big front ring), lay a ruler so that the centre of one pin sits on zero. On a new chain the centre of the pin 12 full links along lands exactly on the 12″ = 304.8 mm mark (or 152.4 mm over 6 links). However far the pin has moved past the mark is how worn the chain is. Measure along the top, unloaded run; repeat in 2-3 places along the chain and take the worst reading.

More precise than a ruler and handy over a short section. Pick the preset for your tool size and dial in the shown starting value - it is a couple of millimetres below the new-chain gap. Slot the inside jaws between the two rollers at the ends of the section and open them until they stop against the rollers: enter the resulting number as the measurement, and the difference from the new-chain reference is the elongation. The chain type switches with a button: a standard 7.65 mm roller or the oversized 7.72 mm one on SRAM Flattop and Eagle. It shifts the result by only ~0.05% (below the tool's precision) but is accounted for. Spreading the rollers with the jaws takes up a little of their play, so the reading runs slightly on the safe side - treat a borderline result as a reason to replace.

Wear is (L − L₀) / L₀ × 100%, where L is the measured length of the section and L₀ is its new-chain reference (number of links × 25.4 mm). The chain pitch is always 1/2″ (12.7 mm), so 0.5% of wear over 12 links is only ≈1.5 mm - too small to catch by eye. A ruler is at the edge of its precision here; for policing an 11-13 speed drivetrain a calibrated checker or a caliper is more reliable.

The narrower the chain and cassette, the sooner you must replace it: 2-10 speeds - at 0.75%, 11-13 speeds - at 0.5% (13-speed Campagnolo Ekar and Rotor included), while single-speed and hub gears tolerate up to 1%. It is not about the chain itself but the cassette: on 11-13 speed the teeth are thinner and packed closer, and a worn chain eats them faster. Catch it at 0.5% and you replace only the chain; let it reach 0.75-1% and the cassette (sometimes the chainrings too) goes with it, otherwise a new chain will skip under load.

The quick way is to drop a go/no-go gauge into the chain. But cheap one-sided checkers that spread the rollers over-read: they add roller play on top of the pin wear, so the chain looks dead before its time. Use a tool marked 0.5% and 0.75% that measures pin wear specifically (such as the Park Tool CC-3.2).

Their pitch is the standard 1/2″, so the ruler method works as usual. Ordinary checkers, though, can lie: SRAM Flattop (road RED/Force AXS) and T-Type (Eagle Transmission) chains have oversized rollers, and a gauge like the Park Tool CC-3.2 simply will not seat - you need the Park Tool CC-4.2 or a dedicated SRAM/Pedro's checker. On its own tool SRAM quotes a limit of around 0.8% - that is a roller-based reading on a different scale, but the point is the same: do not run it to the point of skipping and cassette wear.

How long a chain lasts depends heavily on conditions: dry dust and grit act like an abrasive and kill a chain in a couple of thousand kilometres, while a clean, lubed drivetrain lasts three times longer. Clean and lube the chain regularly and keep an eye on wear - replacing the chain at the limit is far cheaper than the cassette and chainrings later.