When choosing aftermarket wheels for your Challenger, there is a considerable amount of choice. Car wheel technology has evolved considerably, over the years, from steel- once the preferred material- to lighter weight aluminum alloys. Among all types, you have the option of choosing between forged and cast wheels.
In discussing the difference between the two, it is important to distinguish between the advantages and disadvantages of forging and casting. The strength of any metallic product is not only dependent on what it is made of, but is also dependent on how the raw metal is treated. Its internal crystal structure ultimately decides its strength and other characteristics, and this is influenced by the way in which the raw metal is treated.
Cast wheels are created by pouring molten metal into specifically designed molds. The liquid metal is then cooled to create solid wheels of different dimensions. They are usually made from aluminum alloys. The two prime types of casting processes are gravity and low-pressure casting. During the casting process molten aluminum is either poured or drawn using a vacuum into a mold, where it is formed into the desired wheel shape and allowed to cool. Once the wheel has cooled, minor modifications such as drilling and trimming are made, allowing for quick and inexpensive production. A third type of casting is spun-rim, flow-forming or rim-rolling technology. This specialized process begins with a low-pressure casting and uses a special machine that spins the initial casting, heats the outer portion of the casting and then uses steel rollers pressed against the rim area to pull the rim to its final width and shape. The combination of the heat, pressure and spinning create a rim area with the strength similar to a forged wheel, but without the high cost of forging. This type of wheel weighs less and has more structural rigidity than a standard cast wheel.
While a cast wheel may be easier and less expensive to manufacture, the process of allowing the molten aluminum to solidify leads to porosity, which are essentially inconsistencies in the structure. These can lead to cracking, oxidation, pitting in the finishing, and a reduction in the wheel’s structural integrity. To counteract this deficiency, manufacturers are forced to design with larger tolerances. This leads to heavier wheels in order to achieve the desired structural integrity.
A more expensive forged wheel begins as a solid piece of metal referred to as a “billet.” The billet is then subjected to heat and intense pressure, which compresses the material to form a raw forging. These raw forgings are “forged” into different profiles to allow for the creation of different wheel designs. The raw forgings are then lathe turned and milled to form the final shape and application of the desired wheel. Due to the immense pressure that the billet undergoes during the forging process, the metal is less porous compared to cast wheels and has an interlocking and aligned grain structure. Although this can only be viewed under a microscope, this structure results in improved strength, durability, and resistance to corrosion and oxidation. Due to the more consistent forging, the same structural integrity can be achieved using less material in a forged wheel versus a similarly designed cast wheel.
Forged wheels are often made into traditional one-piece monobloc wheels, or due to their inherent strength-to-weight, they can be made into two and three-piece variations to give manufactures enormous flexibility in creating custom offsets and widths.
As expected, cast wheels are cheaper. The higher manufacturing cost of forging adds to the forged wheel cost. However, car owners who desire premium wheels go for the forged ones, because of they are less susceptible to cracking under impact, have better fatigue properties, and have less unsprung weight.
These advantages were readily known to the SRT engineers when they had Alcoa design the retro-inspired five-spoke wheel (20 X 9 inches) on the inaugural 2008 Challenger SRT.