all rims will give the impression of turning backwards at certain RPM's. Your eye does not feed a constant image to the brain, it is more like a movie camera, certain frames per second.
when this happens, and the wheel has made ALMOST 1 full rotation it will give the apearence of turning backwards. Your same reference point on the wheel is behind where it was just before and looks to have moved backwards. let the rmp pick up a bit and it will look to no move at all, let if pickup some more and now it is going forward again.
Check out a car leaving a stop sign or signal. whatch the rims and you will notice what Iam talking about.
useless knowlegde from physics class in highschool