There are many videos on chain wear, measurement and when to change your bike chain. Our goal here is twofold:
1. What is chain stretch and does it really stretch with wear?
2. Of the many tools available to measure chain wear, which give you the best results?
Chain parts:
Chains are composed of inner and outer links with protrusion of link material we call bushing (in older chains they may be separate pieces). They are held together by pins (also called rivets). Then we have rollers to slide over the cassette and chainring teeth. A full link an inner and outer plate. From pin to pin is half a link. If we were to measure pin ends of 24 half pins, the total distance should be 12 inches, that is, the distance between pins at the end of the inner and outer link should be 1 inch and the distance from one pin to another (half link) should be 1/2 inch.
Chain wear:
As the chain wears (worn chain), the pins, rollers, bushings and links, and wear. This is not really stretch the metal but rather results in elongating the chain. This creates a problem as we pedal in that the rollers now move up on the cassette teeth and chainrings causing excessive wear (so call shark teeth) and poor shifting.
Chain Wear-Indicators:
Almost all chain-wear indicators ( chain checkers ) are go /no go gauges (we'll explain that in a minute) that push the rollers away from each other. Checking a chain consists of pushing a tang on one end of the gauge into the chain and then pushing a curved tang at the other end. How deeply the curved tang goes into the chain indicates two separate % of wear (go or not go). These chains, however, can give you false readings by simply pushing roller apart. The rollers may wear differently from the links and bushings. In addition, some 12 speed chains have larger roller such as the SRAM AXS and SRAM Red. Using a longer gauge or measuring 12 full links will give better results but you need very accurate ruler and a keen eye.
This is where the three point chain checker comes in. 3 point instruments measure chain length directly and take the roller wear (or size) out of the equation (Park Tool cc-4 and cc-4.2, Topeak and several other name brands). Such tools requires both hands. Place the end without the measure number on an inner link. Push the adjacent tang into place and with your thumb and finger, push up on the chain to make the chain taut. Next slide the marked tag into the chain link. This has the effect of pushing the pins in the same direction as opposed to pushing rollers apart.
Chain replacement depends on the chain speed and, in some cases, the type of chain (SRAM Red or Eagle).
*Single speed or 2 sprocket:
• If wear is 1%
*For 10 speed and below:
• If the tang end goes up to the .5, the chain is 0.5% wear
• If the tang end goes up to the .7, the chain is 0.7% wear
• Severely worn if the length between pins reaches 1% (check your cassette and chainrings for wear). It had been suggested the chain be replaced between 0.5 - 0.75%
*But with the advent of 11, 12 and 13 speed, these recommendations have changed as follows:
• Replace the chain when it reaches 5%.
• Check with the manufacturers recommendation for SRAM RED or SRAM EAGLE.
In summary:
1. Bicycle chains don't stretch but wear of the parts result in elongation between the chain pins.
2. 3-point chain checker will give you a more accurate indication of wear and may be the best chain wear indicator (chain checker).
3.Chains should be replaced as in the previous chart shown.