Material: rubber and metal. Quality rubber and more stable bearings, wear, mute can work well with the case. Size: 40(outer diameter) x 18(wide)mm and the middle bearing inner diameter is 6mm, inside diameter of the bearing: 21mm. With 30mm and 36mm axles and washers to meet different needs. EASY INSTALLATION: Luggage compartment spare wheels are easy to pack, easy to install without manual. When installing, also pay attention to the tightness of the wheel so that the luggage roller can roll better. Perfect solution: 4 x Suitcase Wheels, 8 x Axles, 8 x Axle Bolts, 2 x Wrenches, 8 x Spacers Before ordering, please carefully measure the diameter and width of your old wheel to choose the good size. When installing, pay attention to the tightness of the wheel so that the casters can roll better.