One side open The best bearings (for example, Boss Swiss), come with only one side closed. Having one side open makes cleaning faster, because you don't have to remove anything before starting. Most bearings however come with both sides closed, evidently because they're packed with grease. Before cleaning them, you have to open one side, which is often rather difficult. In my first few years as a skater I used to put back the shields I'd removed for cleaning, but it was a complete waste of time. Now I just throw them away. Of course when you install such bearings you put the open side inwards, toward the spacer, and the shielded side outwards.   Nylon cage The cage (also called a retainer) is the part that holds the ball-bearings inside the bearing. Usually the cage is made of metal, but in the best bearings it's made of nylon. Nylon is preferable for two reasons: It doesn't rust... In heavy rain, when the only "lubricant" in your bearings is often just the rain itself, ball-bearings will continue to turn smoothly in a nylon cage, whereas in metal they'll be squeaky and much slower. Oiled, not greased Most bearings are sold with grease inside, which is slower to get moving and slower to roll. For superior speed you need oil, a light oil intended for skate-bearings. You can get a durable skate-oil for training, and an ultralight oil for racing. When I buy grease-packed bearings, first of all if they're double-shielded I remove one of the shields and throw it away. Then I clean the bearings to get the grease out, replacing it with oil. Rod Willmot