In addition to coming in many different sizes and designs, wheels come in all different styles of construction and composition. Here are a few of the most important compositions and methods for wheel owners to know about.
Steel is both heavier and stronger than aluminum, and has been used for wheel construction a great deal longer. Steel bends and becomes damaged much less easily than alloy. Because steel is already so strong, further casting or forging methods are generally not necessary. Most steel wheels are stamped out by massive presses and then welded together to form the wheel, as in these steel racing wheels. The downside to this is that steel will not allow for the kinds of spoke and face designs that make allow wheels such an artistic platform on the car. For the most part all one can do with steel faceplates is to stamp some windows in them for brake cooling purposes. However, several companies nowadays are working hard on creating steel wheels that are chrome-clad, meaning that they have a thin overlay, usually made of tin, which has been chromeplated and then glued on the face of the wheel.
Many Ford and Chevy pickup trucks now come with chrome-clad wheels as standard options.
Aluminum alloy is a mixture of aluminum and nickel. The proportions of metal in the alloy determine both the strength and weight of the wheel. Less nickel in the alloy means a lighter wheel, but one which is more pliable and easier to bend in an impact. More nickel means a heavier wheel, one that does not bend easily, but may be more brittle and prone to cracking.
Cast aluminum is just what it sounds like – molten alloy is poured into a mold and allowed to cool. Several types of casting methods exist, but what they have in common is that cast aluminum is not very dense, and so greater weight of metal is needed for strength.
The simplest form of casting metal is to pour the molten metal directly into the mold. This also creates the least dense metal, as only the force of gravity is pushing the metal into the mold. Gravity-cast aluminum alloy must therefore be thicker and heavier than other methods in order to have enough strength to be used safely for wheels.
There are two types of pressure casting in use, low pressure and counterpressure casting. Low pressure castings uses air pressure to force molten metal into the mold. This causes the molten metal to pack itself into the mold with more density and greater strength. Counterpressure casting uses the opposite process – creating a mild vacuum inside the mold, which literally sucks the molten alloy into it. The results are basically the same for either process.
Flow Forming is a hybrid process in which low pressure cast aluminum is stretched and formed using heat and high-pressure rollers to shape the wheel. The stretching and forming process creates a thin and dense metal which has properties similar to forged aluminum.
Forged Aluminum is created by taking a solid “billet” of aluminum alloy and subjecting it to a tremendous amount of heat and pressure, usually about 13 million pounds of pressure, in fact. The pressure simply crushes the metal into the desired shape. The forged blank can then also be flow-formed to shape the barrel. This creates a wheel that is extremely dense and enormously strong, but also very light. Pound for pound, forged aluminum is orders of magnitude stronger than a cast aluminum alloy.
Rotary Forging is a brand new process. In rotary forging the aluminum billet is forged under the same kinds of pressures, but is done while the forge is spinning at high speed, and often at an angle. The centrifugal force involved causes the molecular structure of the metal to reform in circular chains that are strongly bonded together. This creates a wheel that is even stronger against radial impacts than conventionally forged aluminum. TSW is rather cagey about their process, but it seems to also involve some variant of flow-forming, with rollers on each side of the barrel that forge the metal still further.