Why are these wheels made like this?
I'm unfamiliar with this! Explanation?
I'll probably completely butcher this, as I'm at work and in a bit of a hurry, but at the risk of making a fool of myself I'll take a stab at this.
First, have you ever looked at (prop type) fan blades, whether in your house or in your car, and noticed that blades are never spaced evenly? That's done on purpose so that the fan doesn't generate an annoying tone. Secondly (and this one's a little "out there"), you can generate a tone by spinning a toothed wheel in close proximity to a magnetic pick-up (think of a pick-up sensor in an electronic distributor, but with a LOT more tabs on the reluctor), and then amplifying the signal that is generated by it. (Ha! This is also VERY loose correlation to how a Hammond B3 organ works.). Or maybe think of vibrating guitar strings....
Anyway, any regularly patterned "pulse" will generate a tone, and depending on the frequency, that tone may just be audible. And annoying.