Since aluminum alloy is a fairly easy material to work with, custom wheels are built using several different methods. Today's alloy wheels are created using one of three construction processes: casting, flow-forming, or forging. How a wheel is created affects its weight, load strength, and performance characteristics.
Most custom alloy wheels today are cast wheels, where aluminum alloy is melted down and placed in a mold that gives the wheel its shape. The molten alloy hardens to form a wheel that is lighter than steel, yet durable enough to function as a wheel. There are two methods of casting wheels: gravity-casting and low-pressure casting.
Gravity-casting is exactly what it sounds like, where the molten alloy is poured directly into the mold using gravity and allowed to cool. This often requires thicker, heavier designs for structural integrity. For low-pressure casting, pressurized chambers are used to force the molten alloy into the mold. This compacts the alloy and reduces porosity as it cools, improving the wheel's density.
Gravity-casting has a lower production cost than than low-pressure casting, while low-pressure cast wheels have slightly less weight and improved structural strength than gravity-cast wheels. With either method, cast alloy wheels are far more affordable than forged wheels, and are perfectly suitable for the average driver's needs.
Also called spin-forging or rotary forging, flow-forming combines aspects of casting and forging methods to produce a wheel that is superior to a cast wheel, but less costly to produce than a forged wheel. Flow-formed custom wheels are especially popular with tuner enthusiasts.
During flowforming, a cast wheel is spun on a machine at high speeds while heated. Steel rollers put pressure on the barrel area as the wheel spins, compressing the alloy and stretching the wheel to its desired width and shape. This strengthens the alloy at the molecular level and increases density, improving the wheel's impact resistance and load-carrying capabilities.
Flow-formed wheels are generally lighter and stronger than cast wheels of a comparable size. Due to stretching the alloy, a flow-formed wheel requires far less alloy material to achieve the same tensile strength as a comparable cast wheel. This makes flow-formed wheels great for high-performance applications, where drivers constantly seek ways to reduce their car's unsprung weight.
Forged wheels are the highest class of aftermarket wheels. Unlike cast or flow-formed wheels, a forged wheel is made from a solid block of aluminum alloy called a billet.
During the forging process, a billet is placed under extreme heat and pressure, and gradually pressed into the shape of a wheel. This process modifies the alloy's grain structure and increases its density, giving it exceptional tensile strength. A computerized machine lathe precisely tools the final design into the aluminum, producing a wheel that is extremely dense, lightweight, and strong.
A forged wheel has high strength for reduced weight and exceptional structural integrity compared to cast or flow-formed wheels. Thanks to their superior load-carrying capabilities and reduced unsprung weight, forged alloy wheels are extremely advantageous for performance and racing applications. They are also far more expensive to produce, due to the precision required in the forging process and the specialized machinery involved.
Some manufacturers offer custom wheels with multi-piece construction, such as two-piece wheels where the barrel and center are cast or forged separately and then bolted together. Forging is ideal for multi-piece wheels, as the alloy is more durable and can be tooled into more elaborate wheel styles. When a multi-piece wheel gets damaged, it can be repaired by simply swapping out the damaged piece instead of replacing the entire wheel.
A three-piece wheel consists of the inner barrel, the outer barrel, and the center. Three-piece forged wheels offer the greatest level of customization potential among aftermarket wheels. They are also by far the most expensive custom wheels available on the market. In many cases, three-piece forged wheels are custom-built to order due to their sheer cost of production.
Due to steel's high melting point and extreme hardness, steel cannot simply be cast or forged into a wheel like aluminum alloy. Steel wheels typically have two-piece construction, with the center and the barrel "stamped" or cut from sheet metal. The pieces are then welded together to form a complete wheel.
Note that the overwhelming majority of aftermarket wheels today are made from aluminum alloy. Overall, steel is heavier and less visually appealing than aluminum alloy, so aluminum alloy is a more ideal material for custom wheels.
If you're looking for custom wheels for day-to-day driving, cast alloy wheels are an improvement over stock steel wheels without breaking the bank. For the driving enthusiast, flow-formed wheels provide an edge in performance at a slightly higher price. If you desire the absolute pinnacle of custom wheels and are willing to pay a premium for it, then forged wheels are right for you. Get the right type of wheel for your budget and driving needs!