There are so many parts to skates, we know it can be overwhelming, but they are all important! The bearings are what allow your wheels to spin!
With roller skates there are two bearings in each wheel, so you need sixteen bearings if you buy new ones for your skates. They are inserted on into each side of the skate wheel, and you slide that onto the axle, then you put on the lock nut on, and that keeps them on there!
The inside opening is the only part that varies on the bearings we sell, that is the part that goes on the axle. So you either have an 8mm or 7mm axle, that tells you which size bearing to purchase. The outside of our bearings are always the same size and will work for inline or quad roller skate wheels.