That's not just any old rubber band, that a power band!!

They come in various widths. The NSR250 has a standard width band fitted, whereas the 300 has a wide one. Steve's F3 on the other hand has a much narrower band stretched on there, which is prone to snapping if you don't replace it regularly.

The power band fitted to a Hardly Davidson is so wide that you have to fit a insanely large rear tyre (allegedly), which does of course stop the thing from turning corners.