The wear of the chain is variable, and this is why it is not easy to change when it needs to be changed: in short, there is no general rule. Wear is influenced by many factors, such as the technical qualities of the biker (you will understand later), the time of use, the type of chain, the trails traveled and the level of maintenance.
LEARN TO CHANGE
It happens that a high level athlete or rider, despite the many kilometers put under the wheels and the power developed, tends to change chains less than a beginner e-bike rider. How come? Because often, for beginners, shifting is done under stress, perhaps in Turbo mode, and this leads to rapid wear of the chain (and risk of breakage).
OTHER VARIABLES
The other variables at play, in addition to the biker's technical shifting qualities, are the cyclist's own power: the more power we give on the pedals, with our legs or with the engine, the sooner we will wear out the chain. The usage time: of course if we use the bike every day it is very different from using it once a week. The type of chain: on modern bikes with 12 speeds the chain is always thinner and lighter, in this case the material tends to wear out sooner than on older, more robust bikes with less speed. The chosen routes: The trails traveled influence wear, as if you face particularly uneven terrain the transmission will be subjected to multiple jolts and vibrations compared to using it on a cycle path, perhaps cemented or asphalted. The maintenance: a neglected chain, i.e. not cleaned/oiled (read this article for more information), degrades faster than one that is lubricated and maintained properly.

HOW DO YOU CHECK THE WEAR OF THE CHAIN?
The real wear of a chain cannot be perceived by eye. To measure wear you must use a simple and economical tool. Just search for 'Chain wear tool'.

The meter indicates the state of wear of the chain and is used to change the chain before it reaches such wear as to compromise the transmission. Because remember: the more the chain wears out, the more it will ruin the friction components, i.e. the front sprocket(s) and the rear sprocket set.
The instrument is equipped with a measurement scale, which goes from 0 to 1 cm, where 1 indicates the bottom of the scale, with a maximum chain elongation. When the chain reaches the bottom of the scale it has lengthened by a total of 1 cm and in this case it has hollowed out the pinions and crowns to make room in the transmission.

To keep a transmission in good condition, without always having to change front and rear chainrings, it would be advisable to change the chain before reaching the full scale (1cm of elongation).
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