A wheel might look like a single object, but it is made up of several parts. To understand a wheel, you must understand the parts of a wheel. Let’s get acquainted with the most important wheel parts by starting at the center of the wheel.
The center bore is the opening that allows the wheel to fit on the axle. This is the part of the wheel that actually attaches the wheel to the vehicle and bears the weight of the vehicle. When you buy aftermarket wheels, you must ensure that the center bore is at least the size of the OEM wheel. This generally isn’t a problem as most manufacturers make wheels with a large center bore in order to fit as many vehicles as possible. When the center bore is larger, hubcentric rings are used to fill the gap. Center caps will cover the center bore with style.
Going out from the center bore is the center disc. This is the portion of the wheel into which the bolt holes are machined to create the bolt circle. This area is the point of contact to the axle seat, the lug bolts and the lateral surface of the rotor. Everything on the wheel connects in some manner to the disc.
The lug holes create the bolt circle with 4 or more openings. The diameter of the bolt circle is called the bolt circle diameter and abbreviated as BCD. The amount of holes and the diameter of the bolt circle is what defines the bolt pattern. Now that we’ve got the center covered, let’s move outward in our wheel anatomy.
The spokes connect the center disc to the outer ring of the wheel. The spokes give the wheel structural integrity and are one of the major elements of style in wheel design.
The outer lip is the portion of the wheel in front of the spokes. For the most part, the dish only comes into play when it is a large area. When the spokes are significantly distanced from the outer edge, the wheel is considered a deep dish wheel. This is done purely for aesthetic reasons. As the dish gets deeper, the face is more vulnerable to damage from impact.
Now, on the very outer portion of the wheel is the barrel. The barrel is what creates the structures necessary for mounting the tire. The barrel has many parts. The smallest inside diameter of the barrel is the drop center. If the drop center is close to the front face of the wheel, it is a front mount wheel. If the drop enter is close to the back face of the wheel it is a reverse mount wheel. The barrel edges are flared to create the flanges. The flanges keep the tire from slipping off. The outer facing flanges are part of the cosmetic face of the wheel.
Just inside the flanges are flat areas called the beads. This is where the edges of the tire sit onto the wheel. Mounting humps circle the barrel on both the car side and the cosmetic side of the wheel. These ridges separate the beads to keep the tire from slipping away from the edge of the wheel.
Now you know the parts of the wheel, let’s look at how the pieces are put together.