For checking the wear-related elongation of bicycle chains for aluminum cassettes or steel
A chain can still be fully functional, but from a certain degree of wear it does not fit exactly in the sprockets. During use, the chain lengthens by stress and wears the chainrings and sprocket. If a new chain is then fitted, it no longer fits the gears/sprockets and jumps over the teeth under load. Therefore, it is important to change the chain early if the length exceeds 0.75% (for light metal cassettes) or 1% (for steel cassettes).
First insert the narrow side into the chain links and then the .75 side. If the tool rests parallel on the rollers over the entire measuring length, the chain is worn out.