An important component of any wheel is its bearings. Car wheels, skateboard wheels and motorcycle wheels all have bearings. Wheel bearings bear the brunt of the friction between the axle and the wheel so that the wheel rotation is smooth. A wheel bearing is a disk with a ball bearing inside, which rolls and enables the movement of the wheel, all the while reducing the friction. The wheel bearings must be greased and repacked regularly to function properly [source: Frantz]. If your car is vibrating or screeching, it's time to clean and grease the wheel bearings. Here's how.