The wheels are one of the most significant parts of a car - generally, four of the most significant parts. Without wheels, cars would be dramatically less efficient, as they would have to use some other form of locomotion such as treads, or perhaps walking. Automobile wheels have come in various forms over time as technology progressed and as needs changed.
The earliest automobile wheels consisted of a metal hoop to which was affixed a solid rubber tire. This rim was attached to a hub via wooden spokes. The hub had a hole drilled straight through it and was fitted directly to a straight axle. In other words, what you were dealing with was a powered wagon.
Progress in technology brought two very significant advances; Metal spokes (which were too heavy and costly) and pneumatic, or air-filled, tires. The pneumatic tire absorbs a considerable amount of shock and significantly increases traction on uneven surfaces because it conforms to the road surface. Early metal-spoked wheels featured a small number of very thick spokes, but this adds entirely too much mass to the wheel. Later models featured a large number of thin steel spokes similar to those found on the bicycle wheels of today. Another extremely signifcant advance in the rolling of automobiles was the tapered roller bearing which transfers stress during cornering from the bearing to the spindle and hub.
Wheels in use today
There are only a few types of wheels still in use in the automotive industry today. They vary significantly in size, shape, and materials used, but all follow the same basic principles.
The first type of wheel worth mentioning, and by far the most-used wheel, is the steel wheel. This kind of wheel consists of several sheets of steel, stamped into shape and typically welded together. This type of wheel is strong, but heavy. They are found on every kind of vehicle from sports cars to the larger pickup trucks; the wheels look different but are essentially the same device.
The second type of wheel to be mentioned is the rally wheel. These are essentially steel wheels but they are made somewhat differently, and tend to consist of a heavier gauge of steel. While the inner portion of a steel wheel is generally welded to the rim along its entire circumference, a steel wheel's inner portion is cut to resemble the spokes of a mag wheel, and is welded accordingly.
Mag wheels are cast and/or milled wheels typically made from aluminum or an alloy thereof. They used to be made of magnesium for their light weight and strength, but magnesium catches fire somewhat easily and is very difficult to put out. This is unfortunate, because it is superior to aluminum in every other way. This tendency also makes it a dangerous metal to work with, because piles of shavings tend to burst into flame and burn through concrete surfaces when they get too hot.
As previously mentioned, spoke wheels (sometimes with more than 100 spokes) are still in use today and are popular on roadsters and low-riders. They tend to be fairly low in weight, and are reasonably strong. They have an "old school" appearance and style which is often highly sought after.
Various combinations of these technologies can be used to produce other, more unusual wheels. Large earth-moving vehicles such as the more gargantuan dump trucks often have some degree of the vehicle's suspension actually built into the wheel itself, lying between the hub and rim in place of spokes. Also, various companies make wheels which are designed like steel wheels but are made of aluminum -- The most famous of these are made by centerline, and the style is actually called the centerline wheel.
Wheels are mounted to the hub by a combination of lug bolts, or studs, and lug nuts. The studs are mounted to the hub, which is attached to a hub carrier or suspension upright. The wheel has holes to match these studs, and is placed over them. The lug nuts are then applied and tightened to the proper tension.
Hub- and Lug-Centricism
Automobile wheels are considered to be either hub-centric or lug-centric, the difference being how the wheel is centered. If a wheel is off-center, the result is a lack of balance and a tendency for that wheel to bounce as the radius changes. Hub-centric wheels are centered by the center bore of the wheel matching the protruding portion of the hub, and lug-centric wheels are centered simply by the position and diameter of the lug bolts. Adapter rings are available for some wheels to center them to the hub, though it is generally not necessary. Some lug-centric wheels are centered by a beveled edge on the lug nuts matching a bevel on the wheel's holes.