This is a very old, but still fascinating animation I found. The mechanics of it are pretty impressive, and I hear some folks even built working versions of 'em. Probably not going to go anywhere because ICE engines are being phased out, but still an interesting take.
My most proud Covid impulse buy.... the Icebear Maddog. A 150CC chinese knockoff of the Honda Rukus. It's super fun, goes up to 50mph on the flats, and because it's chinese I had to learn a bunch of motorcycle repair tricks (which was actually quite fun).