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Same chain, different slack?

This deep secret I figured out a few years ago. Uneven tension as the wheel is rotated is caused by sprocket-wear. A few years ago on a Chinese quad - the chain displayed the inconsistent tension when rotated; I decided to replace the chain but keep the old sprockets. When I installed the new chain it did the same thing until I changed the sprockets, then put the old chain on the new sprockets and the tension was still consistent which confirmed the problem was caused by worn sprockets.

What you observed there was probably from incorrectly-made sprockets on the ultra-cheap Chinese machine. The ‘bottoms’ of sprockets really don’t wear at all, which is where they’d have to wear to cause the effect you describe.

OTOH, if a poorly-controlled sprocket-stamping line makes a sprocket (or sprockets) whose mounting bolt circle, sprocket tooth perimeter circle, and hub cut-out are not quite concentric, you’ll see exactly what you saw.

At the same time, even only mildly-kinked links will make a chain shorter than it is without the kinks. When those kinked links are in the free air between sprockets, they make the chain tight. When they’re wrapped around the sprocket(s), that effect is removed because the sprocket tends to straighten them, but mostly because slack in the free section of the chain is a function of where the pins on the ‘first’ and ‘last’ teeth of the sprockets are.
 
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