Unfortunately Razor scooter's rear wheels develop flat spots from braking.  Both of my son's scooters sounded as if he was riding on a cobblestone road even when he was riding on a smooth road. These wheels restored the quality of the ride to the smooth, like-new experience.  ABEC 5 bearings turn easily, smoothly, and quetly  Since the wheels are made from the same material as the original ones I am sure they will too develop flat spots.