Looking at a gallery of bike-related photos, I noticed these on a number of the bikes. The photos seem to be mostly European; the mystery objects are mostly on front wheels but sometimes both, and always look dark. They're almost always in pairs, but not symmetrically placed on the wheels. I haven't seen any closeup shots that would let me identify them.

They aren't reflectors. What are they?

Flabblestrips.

Looks like bike computer inductive sensor and magnet to me.

Picture's not very clear but they look like Reelight magnets to power their head- and tail-lights.
https://www.niagaracycle.com/categor...xoCYtoQAvD_BwE
Also looks like the associated headlight in front of the hub in the picture.

I'm going with advanced alien technology.

Cool! I wondered if that could be what they were; I knew such a thing was possible but didn't know anyone had actually done it. Thanks!

Reelights, from DK, an Inductive pulse powered bike light, you see coming out from the hub.

basic model lights when the magnet passes by the coil of wire in the light itself.

Umm. It's a male fork?

Those are flux capacitors.

Yep.

Good luck getting up to 88 miles per hour, though.