Parts of a Wheel: A Complete Guide

Parts of a Wheel

The wheel is composed of the tire, the rim, and the hub. Each of these components has several other parts that complete the functionality of the wheel. The tire is held on to the wheel with tire beads and supports the vehicle while providing traction using the treads.

Rim

The rim is the part of the wheel that the tire wraps around. The rim supports the tire and gives it form while allowing it to fill with air. Rims are often made of steel or aluminum for strength and durability. There are hundreds or even thousands of different styles of rims and they can come in a variety of colors and polishes. They also have different diameters and widths that hold different sizes of tires.

Barrel

The barrel gives the shape to the tire and the mounting structures. The barrel’s surface closes off the tire so that the tire can retain the air pressure.

Outer Lip

The outer lip is the outer edge of the wheel. This is also the part that usually gets damaged or bent when you drive hard over a pothole or a curb. The outer lip faces outward and is measured from the face of the wheel to the edge. A deep outer lip is easier to damage by impact because the spokes are not able to provide as much support.

Drop Center

Most wheels have a portion of the barrel that is bent towards the inside of the wheel. To properly mount a tire with the same diameter as the wheels diameter, a single side of the tire has to be inserted into the drop center. This will allow the tire to move enough so that the other side can adequately hold on to the edge of the rim.

The drop center is near one of the edges on the wheel. Front-mount wheels have the drop center near the front face of the wheel. The tire will be mounted over the face. Deep dish wheels often have the drop center near the inboard edge and these wheels are called reverse-mount wheels.

Flanges

Flanges are the edges of the wheel designed to prevent the tire from slipping unnecessarily. The flanges are located on both ends of the wheel and will keep the tire held onto the wheel even in harsh conditions or uneven road surfaces.

Beads

Inside of the flanges are the beads that help to fit the tire onto the wheel and create a seal. The beads and flanges are also the part of the wheel that will transfer energy. If the beads or the flanges are inadequate due to damage or other imperfections, the energy may be awkwardly transferred and cause the entire vehicle to shake, especially at high speeds.

Mounting Humps

Mounting humps are ridges that go around the barrel on both sides. They seperate the surface of the beads and are there to hold the tire to the wheel edges. Most mounting humps will have a slanted edge so that the beads of the tire can fit over the humps if it needs to be taken off. However, some high-performance wheels do have straight surfaces so that the tire cannot easily be removed. This ensures that at high-speeds, the tire will remain tight against the wheel and has no chance of coming off.

Hub

The hub is the part of the wheel in the center that is attached to the rim. This part holds several components including the brake pads, the rotors, and the calipers that are not actually parts of the wheel itself. This then attaches to the axle. It also holds the mechanisms that allow the wheel to function.

Center Bore

The center bore is an empty area on the back side of the wheel. This component contains a functional space that goes over the axle and supports the car’s weight.This makes it one of the most important parts of the entire tire and it is important the specs and measurements are correct if you are replacing a wheel.

Plate

There is a piece of metal located around the central bore that has bolt holes in it. That piece is the plate. The plate of the wheel is the middle and makes contact to the axle seat, lug bolts, and the rotor. Every other component of the wheel is connected in some way back towards this plate.

Spokes

The spokes are the pieces that are between the edge of the rim and the plate in the center of the wheel. The spokes also support the outer edge and prevent damage from minor impacts. There are thousands of designs for the spokes. The classic style has 5 spokes, but there are some that have intricate designs or as few as 3 spokes. The strength of the spokes varies greatly depending on the material and the design.

Dish

The dish is the outer component of a wheel that contains three parts. The dish is the part of the wheel that extends out farther than the spokes. Some wheels have spokes that are sunken in. These wheels are called deep-dish wheels and are made for cosmetic purposes because the space can be used to show off the finish. The deeper the dish, the less resistant the face is to impact.

Bolt Circle

The bolt circle is the area created by the lug bolts. The Bolt Circle Diameter is the length between the area and is referred to as the BCD. The number of lug bolt holes in the wheel plus the BCD makes the bolt pattern (so 5 lug bolt holes and a 4 inch BCD would be 5x4” bolt pattern). The bolt pattern can vary between wheel and car manufacturers and make it difficult to cross fit wheels that are designed for different car manufacturers.

Valve Stem

The valve stem hole is a place on the wheel that we use to fill the tires with air or to check the current air pressure of the tire. There are a few different types of valve stems on the wheel. Rubber stems are the most common, but there are stems designed for aesthetics and some made out of metal.

Center Cap

The center cap is on the center of the wheel where the spokes meet. On most wheels, the center cap can be removed. However, most steel rims do not have a removable center cap. The function of the center cap is to cover up the hole in the center. It also protects the lug nuts. A wheel without a center cap can still be fully functional, but it does make the wheel look more uniform and complete.

Lug Holes

Lug holes are the empty spaces covered by the center cap. The lug nuts hold the wheel together and determine the bolt pattern. The number of lug holes varies greatly depending on the manufacturer. Some cars have a few as 4 lug holes and nuts while some have as many as 8.

Outboard Face

The outboard face is the side of the wheel that you can clearly see when the wheel is attached to the vehicle. This is the side that also provides the aesthetics of the wheel structure. The outboard face is also vulnerable to impact and needs to be somewhat resistant to damage.