Cars are reflections of their owners, and few things reflect quite like a set of polished aluminum wheels. The American passion for aftermarket wheels really kicked into gear back in the 1990s, and catapulted to a whole new level when Cadillac debuted the world's first set of dubs on a production car. For a while it seemed like there was no hoop to large, no rim to ridiculous in proportion, design or cost to be unworthy of consideration. At the end of the day though, substance always needs to come before style -- especially when you're talking about the material of the rim itself.
There are three basic types of rim on the market today: steel, cast aluminum and forged aluminum. The cheapest aluminum wheels are cast, meaning that they're made by pouring molten aluminum into a mold, and then machining it into shape. More expensive forged aluminum rims are made by hammering semi-molten aluminum into shape between two massive hammers. This process is similar to the forging process used to make swords like the Japanese katana, and makes for a finished product that's very strong and tightly woven together on a molecular level. Cast materials, on the other hand, look more like tightly packed sand under a microscope. That makes cast materials brittle and prone to shattering like glass under impact. Stamped steel wheels are exactly what they sound like: wheels made of steel pieces stamped into shape, and then riveted or welded together.
Despite what some have claimed, steel is by far a stronger wheel material than any aluminum alloy -- especially cast alloys. The misconception about aluminum's ultimate durability comes first from the fact that aluminum is stiffer than steel, given the same weight of materials. So it will resist denting better. It will also bear a great deal of load; the best aluminum alloys have about the same tensile strength as steel. But that's not the end of the story, because aluminum fatigues very quickly after repeated load cycles, whereas steel will last almost indefinitely. Steel can also take a blow and bounce back into shape, and can be repaired if it gets dented. On the downside, for the same strength and wheel dimensions, steel is about a third heavier. But it also costs about half or a third of what aluminum does. Steel's low cost and high durability make it the favored material in heavier applications, but it's the most practical choice for almost all applications.
But who cares about practicality? The fact is, aluminum wheels look better, come in far more sizes and designs, and can offer a serious edge in performance and ride quality. While spinning, your rims act like flywheels, absorbing kinetic energy from acceleration and storing it. The lighter the wheel, the better your vehicle will will accelerate, brake and handle. The advantage in handling comes with a secondary benefit in terms of ride quality, as well. Because aluminum wheels are lighter, they have less inertia, which means they can respond to road imperfections faster. First, that means better grip for cornering on real-world roads, and fewer bumps transmitted through the suspension to the driver. It also means that you can get away with using softer springs for better ride quality without sacrificing handling and suspension control. This is why you'll rarely find a set of steel wheels on a BMW or other modern, upmarket luxury car. Second, the lighter wheels also translate to a theoretical improvement in city fuel economy, since you're not expending as much energy to accelerate them.
Steel wheels are adequate for most applications as long as they're not installed in place of factory alloy wheels. Steel's durability, low cost and ease of repair make it the default choice for many, but it's going to make for a harder ride and somewhat lessened performance prowess. But steel is the material of choice for most off-road applications, and not just because it's tough; heavier wheels with more "unsprung mass" can enhance traction at low speeds. Lightweight, forged rims are definitely the way to go if you're looking for on-road performance and don't mind spending at least twice as much as you would for cast aluminum rims. Cast rims are fine for most applications, as long as you bear in mind that they are far more prone to damage. If you're planning on running extremely low-profile tires, you may end end up spending more replacing bent cast rims than you'd have spent buying a forged set in the first place.
