Measuring Chain Wear Accurately
It’s important to measure your chain for elongation. A worn chain will quickly damage the teeth of your gear sprockets, because the distance between chain rivets exceeds the spacing between the teeth. When pedaling a worn chain, a rider’s force at any instant is concentrated on the tooth that happens to be at the top of the chainring, and atop the rear cog, rather than distributed evenly across the other teeth.
Almost all chain-wear indicators are go/no-go gauges that push the rollers at each end of the gauge away from each other, thus measuring the wear on those two rollers, and the distance between them. 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 into the chain a number of links away. How deeply the curved tang goes into the chain ostensibly determines a chain’s elongation.
These gauges, however, can give you a false reading by simply pushing worn rollers apart. One way to minimize this inaccuracy is with a longer chain gauge. The longer the gauge, the less important is its measurement of wear on two rollers relative to the length of chain between them. Also, since SRAM AXS 12-speed road chains have larger rollers (7.95mm vs 7.75mm on most other chains), these two-point gauges will be off by 0.2mm from the get-go.
The most accurate chain-wear method is direct mea- surement. Since chains are on an inch standard, if the distance between the pins on either end of a set of 24 links measures 12-1/8”, rather than 12”, the chain length has grown by one percent (since 0.125” is approximately 1 percent of 12”). Measuring 24 links of chain requires good light, sharp eyes, and sometimes reading glasses. You need to clean the chain and measure it with a rule or a set of calipers. This method, quite obviously, is less convenient than simply using a chain gauge.
For simplicity and accuracy, I recommend Pedro’s Chain Checker Plus II, as well as Shimano’s TL-CN42 Chain Wear Indicator. Both instruments measure chain length directly, and take the roller wear out of the equation. Both tools require two hands to use. So, one hand will get dirty on the chain. Both tools are also slower to use than a traditional chain gauge.
Both tools are more accurate because they use three tangs to measure the chain; two tangs are close together, and when pushed into the chain, push the rollers firmly away from each other. The third tang contacts the same side of a roller approximately 10 links away as the nearest tang. Since both of these tangs push the same direction against the rollers, they evaluate the distance between the pins those rollers surround.
If the third tang goes into the chain, it indicates that the chain has elongated by 0.75 percent and should be replaced (Pedro’s Plus II tool has two steps on the third tang, indicating either 0.5 percent or 0.75 percent elongation); if it won’t go into the chain, the chain is within specification. These three-point tools are the only gauges I know of that accurately measure SRAM AXS 12-speed road chains.