Wheel Types and Construction This continues the description of how wheels are made and what processes are used to make them.
Construction Techniques (continued):
Forged Wheels: Forging is the process of forcing a solid billet of aluminum between the forging dies under an extreme amount of pressure. This creates a finished product that is very dense, very strong and therefore can be very light. The costs of tooling, development, equipment, etc., make this type of wheel very exclusive and usually demand a high price in the aftermarket. In forged wheels, computer numerically controlled
(CNC) mills add the cosmetics and the bolt circle to exacting tolerances.
The forging process starts with extrusions of aerospace-grade 6061-T6 aluminum. 6061-T6 aluminum is considered the premium alloy for optimal strength properties optimizing wheel design and production (make sure you ask the manufacturer / seller what aluminum was used). The aluminum billet is prepared and subjected to large forging presses used for all wheel and aerospace applications, and extensive heat treatment and curing processes for optimal wheel strength.
Forging aligns the metal grain in a uniform direction along the contours of the wheel, increasing strength resulting in a finished wheel that is strong and lightweight.
One-piece forging is considered superior to other forms of wheel manufacturing in providing ultimate strength while reducing weight. On on average forged wheels have been proven to be 423% strong than steel wheels and more than twice as strong as cast wheels in comparable load crush testing. This is the same technology that is used to produce wheels for M-1 tanks and 747 jets offering drivers the strongest wheel for tough situations.
In addition, lightweight wheel strength lowers the vehicle’s unsprung weight, allowing faster suspension response, a smoother ride and improved traction when cornering. Additional benefits include reduced brake wear and chassis problems for passenger vehicle owners.
There is the excellent example of the forging process in real life:
Source: Alcoa.com
Die Forging: This type of forging process arguably creates the strongest and the lightest material. This is the process that BBS uses to manufacture their wheels and their summary below shows why.
"Only through the die forged process can we control the flow of the metal to produce a long grain structure inside the spoke and optimizes the mechanical properties of the material. Conventional forged wheels rely on a machining process to create a spoke design and can cut against the grain structure of the material and reducing the long term durability. Although die forging is slower and requires multiple sets of expensive forging dies, our process produces a stronger and more durable structure then any conventional processes."
Source: BBS Japan
Flow Formed Spun Wheels: This specialized process begins with a low pressure type of 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 without the high cost of the forging. Some of the special wheels produced for the O.E.M. high performance or limited production vehicles utilize this type of technology resulting in a light and strong wheel at a reasonable cost.
Why is Flow Forming such a big deal? Because during Flow Forming, pressure applied to the cast rim actually changes its mechanical properties, so its strength and impact values become similar to those of a forged rim. That translates to up to 15% less weight when compared to a standard cast wheel. In turn, less weight is key to record-breaking speed.
Here is an excellent real-world example of how Flow-Forming is done:
Cast Wheels: The casting of wheels is the process of getting molten aluminum inside a mold to form a wheel. There are different ways this can be accomplished and although it sounds simple, this is truly an art when done properly.
Gravity casting is the most basic process of pouring molten aluminum into a mold utilizing the earth's gravity to fill the mold. Gravity casting offers a very reasonable production cost and is a good method for casting designs that are more visually oriented or when reducing weight is not a primary concern. Since the process relies on gravity to fill the mold, the aluminum is not as densely packed in the mold as some other casting processes. Often gravity cast wheels will have a higher weight to achieve the required strength.
Low pressure casting uses positive pressure to move the molten aluminum into the mold quicker and achieve a finished product that has improved mechanical properties (more dense) over a gravity cast wheel. Low-pressure casting has a slightly higher production cost over gravity casting. Low pressure is the most common process approved for aluminum wheels sold to the O.E.M. market. Low-pressure cast wheels offer a good value for the aftermarket as well. Some companies offer wheels that are produced under a higher pressure in special casting equipment to create a wheel that is lighter and stronger than a wheel produced in low pressure. Once again in the quest for lighter weight, there is a higher cost associated with the process.
