The vast majority of custom wheels today are built from aluminum-based alloy, and for good reason. An aluminum alloy wheel combines aluminum with small amounts of other elements that enhance the material's hardness, tensile strength, and overall durability. The exact composition of a custom alloy wheel varies from manufacturer to manufacturer, but often includes traces of silicon and magnesium, among other elements, with aluminum often making up 95 to 99 percent of the alloy.
Thanks to aluminum's low mass density, aluminum alloys are fairly lightweight for their level of tensile strength, making them highly suitable for building a wheel. Most custom alloy wheels are cast or forged. Cast alloy wheels are easier to produce, while forged alloy wheels are extremely light and strong, but also more expensive than cast wheels. Some custom alloy wheels, such as High Tech Racing rims, are produced using an advanced "flowform" method to create a wheel that is lighter and stronger than cast alloy wheels, but more affordable than forged alloy wheels.
Aluminum alloy wheels are favorable due to their high strength-to-weight ratio, as lighter wheels are better for performance. A car's wheels are "unsprung mass", meaning that their weight is not supported by the suspension springs. Unlike sprung mass, unsprung mass requires more energy to move, turn, or stop, which impacts the car's performance and fuel economy. The heavier the wheels are, the more slowly the car responds to driver input, and the more fuel it needs to burn. Since aluminum alloy wheels are lightweight, they help to minimize the car's unsprung mass, improving its handling response and fuel efficiency.
Older and low-priced cars typically come with stock wheels welded from steel. While steel is cheap, hard, and strong, it is also significantly denser and heavier than aluminum alloy. Steel wheels generally weigh far more than alloy wheels, making it more difficult for a vehicle to accelerate, steer, or brake. To overcome the increased wheel mass, the vehicle has to burn more fuel when accelerating or decelerating, decreasing its fuel economy and producing higher CO2 emissions. Increased unsprung weight also lowers the vehicle's center of gravity and puts excessive strain on the suspension system, which can wear out the shocks and springs faster. Steel is also a poorer conductor of heat than aluminum, so steel wheels do not dissipate heat buildup in the brakes as efficiently as alloy wheels.
Aluminum alloy is also easier to work with than steel, allowing alloy wheels to come in a wide range of creative, eyecatching designs. Since aluminum has a much lower melting point than iron, aluminum alloys can be cast or forged at lower temperatures and with less difficulty than most steels. Aluminum alloy can also be drilled or machined with relative ease compared to steel, allowing for a variety of patterns and wheel styles. Due to this aesthetic variety, alloy wheels provide a car owner with a high degree of freedom and customization over their car's appearance and style. Due to the clean, polished surface of aluminum, alloy wheels are generally more visually appealing than steel wheels, which are often covered by hubcaps to hide their appearance.
Aluminum also resists corrosion fairly well compared to steel, maintaining its appearance and sturctural integrity longer. When steel rusts, its iron content reacts to air or water to form iron oxide. Iron oxide, or rust, is not only flaky and brittle, but also permeable to air and water, allowing the iron underneath to oxidize as well. On the other hand, when aluminum corrodes, quickly forms an extremely thin layer of aluminum oxide, which is extremely hard and durable. This thin layer insulates the underlying aluminum and protects it from further oxidization. This characteristic of aluminum allows custom alloy wheels to outlast their steel counterparts. That said, even aluminum alloy can eventually corrode over time, so regular maintenance is key to keeping your wheels in good condition.
During the hot rod era of the 1950s, custom wheels were usually made from magnesium-based alloys instead, also called "mag wheels." Magnesium alloys are even lighter than aluminum alloys, and are slightly better conductors of heat, which led to their use in racing sports. However, mag wheels are also more fragile and prone to rapid corrosion, making them less desirable for the everyday driver. In comparison, aluminum alloy wheels are also lightweight and have heat dissipation capabilities, yet are also less likely to crack and hold up well against tarnshing.
Due to their poor durability, mag wheels largely declined during the 1960s in favor of aluminum alloy wheels, though they still see some use in professional competitive racing. Some wheel manufacturers, such as American Racing and US Mags, offer classic wheel designs that originally debuted as mag wheels, but are now available today in high-quality aluminum alloy. Thanks to its advantages in performance, visual appeal, and corrosion resistance, aluminum alloy is the choice of material for today's custom wheels.