I am tempted to leave well alone, but I don’t want to risk a chain breaking! Should the rear cogs always be replaced at the same time as the chain? What about the front chainrings?
Chain and sprocket service life will depend on factors such as environmental conditions, frequency of cleaning and lubrication and, as suggested by the question, replacement strategy. Expect between 1,500 and 3,000 miles from a 10-speed chain.
It helps to establish what counts as ‘worn out’. A chain is worn enough to affect transmission efficiency when it lengthens by 0.75% but has some life left if extended mileage is the aim, in which case it should ideally be replaced when it lengthens by 1%. There are several chain-checking devices available that make a fast, accurate assessment of chain wear. Or you can check with a ruler by measuring across 24 rivets: unworn, they will cover 12 inches; worn by one percent, 12.12in (12 and one-eighth inches).
At less than 1% wear, the chain, if clean, will still run smoothly on the sprocket teeth. Sprocket wear will have been low and a new chain will ride the barely-worn sprocket teeth without jumping. If the chain is allowed to lengthen further, it will quickly wear the sprocket teeth into a hooked shape and a new chain will jump on them until it wears to match, which it will do rapidly. Should a new chain jump, the sprocket may be considered worn out.
Some cyclists go to great lengths to extract maximum service life by operating chains and cassettes in rotation in the hope that components will wear out slowly but simultaneously. A typical scheme might be to use, from new, a cassette with three or more new chains consecutively, each replaced when worn as noted above but retained for later. The three used chains are then re-used and replaced in sequence after wearing to some further measurable degree. All sprockets and chains get replaced when the last of the three (or more) chains reaches the chosen wear limit.
As a way to extract maximum mileage, the system has some merit but it does mean riding around on well-worn, inefficient parts for a long time. It is simpler to replace the chain when worn by 1% or 0.75%. Early replacement extends sprocket life to four or five chains. When, eventually, a new chain jumps on any of a well-used cassette’s sprockets, the cassette needs replacing immediately. Chainrings should be replaced when their teeth become noticeably ‘hooked’.
Cycle’s Technical Editor
This was first published in the August / September 2015 edition of Cycling UK's Cycle magazine.