Steel wheels are most often made up of two pieces and then combined into a single part that you buy. The inner piece, which is called the center, bolts onto the car. The outer piece, known as a barrel, holds the tire. The two pieces are welded together to form the wheel. Most original equipment wheels are assembled in this fashion. Many aftermarket wheels are created this way as well, especially those made for trucks. The benefit to using steel is that it is durable and easy to repair, but it can have limited design variations. For these reasons, it has remained a popular material for wheel manufacturing.