While most chainrings are circular there are also a handful of oval/elliptical chainrings on the market.
A round chainring offers consistent power throughout the pedal stoke.
The argument for oval chainrings is that a cyclists pedal stroke is not consistent throughout the full 360 degrees. Cyclists are able to apply more force on the pedals through the downward part of the stroke and less force on the way up so it only makes sense that chainrings are shaped to accomodate that power variation.
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