[jstanley]

This is called "hunting tooth". It applies to any system of gears or timing belts.

In gears, if you make sure the number of teeth of mating gears is coprime then you wear the teeth of one gear evenly against the other gear. If there are low factors, then each tooth of one gear only engages with a small number of teeth on the other gear, exacerbating wear.

With belts, if the number of teeth on the belt shares low factors with the number of teeth on either of the pulleys then you get the same effect: any given tooth on the belt only ever meshes with a small number of teeth on the pulley.

[dredmorbius]

You've just made me realise that several of the more popular freewheel / sprocket combinations are in fact prime: 11, 13, 17, 23.

Usually contrasted against the decidedly non-prime 52 (2 * 2 * 13) and 42 (2 * 3 * 7) chainrings. 38 if you're old-school triple (2 * 2 * 7).

My chain lengths would vary and I never counted them specifically, though I'd typically remove a few links for fit.