These versatile black plastic wheels have silicone tires and measure 80 mm (3.15?) in diameter. What makes them special is the included set of interchangeable collets that can be inserted into the wheel to firmly grab four different shaft types: 3mm D, 3mm round, 4mm D, and 4mm round. The collet inserts start with a comfortably loose fit on their intended shafts, and unlike simple press-fit wheels, their grip on the shaft gets progressively tighter as they are compressed into the wheel disc, reducing the chances that the shaft will be able to slip within the wheel. This product is a pair of wheels.