Alcoa^® wheels come in different shapes, sizes and looks. The diagrams below illustrate the typical features of a wheel, which include:

• A bore used to attach to the axle of the vehicle.
• Bolt holes used to assemble the wheels onto the vehicle.
• Hand holes, which allow heat dissipation as the vehicle drives on the road.
• The flat side of the wheel called disk face encompasses a mounting flange that not only gives the wheel its aesthetics, but more importantly provides the robustness of the wheel.
• Bead seat, which is a critical surface as it is the interface to the tire. The bead seat of the wheel is the flat area around the wheel, just inside of the flanges, where the edges of the tire “seat” onto the wheel. The bead seat can affect how the tires seal. Any major imperfection, such as a bend in the wheel, will transfer vibration from the wheel/tire combination directly into the suspension and can make the vehicle shake at speed.
• The rim width and bead seat diameter simply indicate the size of the tire to be mounted on the wheel.
• Depending on the application, the flange thickness can vary to ensure it meets the design and application intent. 
• Alcoa^® medium and heavy-duty wheels are produced and assembled with valve stems. The valve stem is the universal mechanism used to inflate or deflate the tire. The Alcoa^® wheels portfolio includes two wheels that have dual valve stems. With a second valve stem mounted at 180 degrees, the tire pressure monitoring system (TPMS) sensor can operate on one valve while air pressure is serviced on the other.

Typical Wheel Assembly Configuration

While all wheels and tires appear to be alike, round and somewhat simple, they are quite complex. There are many products to be transported and every application requires a specific solution:

• The wheel connects the tire to the axle of the vehicle. It transmits forces from the axle to the tire and vice versa. The wheel carries the vehicle, as well as the load in it. Also, since the wheels are connected to the axle, the wheels allow the vehicle to accelerate and decelerate.
• The tire contains the air and therefore allows for a comfortable ride. It also provides the vehicle the ability to handle speed and traction during maneuvering, and provides the ability for braking.

Now that we have defined the basic purpose of a wheel and a tire, let’s review their practical application.

Front Wheel Assembly

The automotive wheel/tire application is somewhat straight forward, as there are two tires per axle; one on each side. 

In medium duty and heavy-duty applications, there are many variations. The front tire assembly is mounted on either side of the truck axle through a hub using the mounting holes, which can be as few as six and as many as 10 depending on the wheel design and application. As such, the hub is the intermediate component that attaches to both the wheel and the axle, allowing the tire to rotate freely.

Rear Wheel Assembly and Wide Base Assembly

For medium and heavy-duty applications, the rear wheel assemblies are called “duallies,”as they are often assembled face to face as a pair, on each side of the rear axles. In weight sensitive applications, two duallies, or duallable wheels, can be replaced with one wide-base wheel, providing the customer significant weight savings. In both situations, the wheels are mounted onto the hubs of the axles in two ways.

Offset Overview

During the assembly process, the wheel is mounted flush to the hub. The distance between the flush surface of the wheel and the center of the wheel is called “offset.” 

1. In the front assembly, since the hub is mounted flush against the inner face of the wheel flange, the offset is called “inset.” Likewise, the outset is the distance between the middle of the rim and the outside face of the flange.
2. In the rear assembly where duallies are typically used, the distance between the two midpoints of the wheels is called “dual spacing.”
3. In the rear assembly, where a wide base is used in place of two duallies, the offset can be positive, negative or zero depending on the position of the hub-flange relative to the middle of the wheel.

Learn more about what tires fit which wheels and why on the Behind The Wheels podcast episode, “There is a Rhyme and Reason for Every Fit.”