By Richard Fong
DSPORT Issue #171
Less is MoreVehicle dynamics can be helped or hampered depending on the wheel selection. Wheels contribute to unsprung weight, which is any part of a vehicle not suspended by the springs. In addition, they contribute to rotational inertia, which must be overcome in order to accelerate or stop a vehicle. Subsequently, heavy wheels pose a hindrance on performance. It’s been said that for every pound of unsprung weight that can be removed, the vehicle handling will improve as if a reduction of 15 pounds of weight were removed from the chassis. Reduced weight contributes to improvements in vehicle dynamics and improved reliability of suspension and driveline components.
Go with the FlowFlow forming starts with a specialized low pressure casting (A) of the wheel. The initial casting features an unfinished and thick barrel section (B, right). The next step involves heating and spinning the wheel between specialized dies as pressure is applied to the barrel (C). This process aligns the grain of the aluminum and strengthens the barrel to produce a wheel that’s superior to a typical gravity cast product at a reduced cost compared to forged (D).
Rim Formation (C)Flow forming the rim involves spinning the barrel of the rim under high heat as the barrel is formed under high pressure between two dies (1). As the die is moved, the barrel is formed (2). The end product (3) is an affordable wheel that is almost as strong as forged.
What’s the Deal with Wheels?Automakers typically equip their vehicle offerings with wheels manufactured from steel or aluminum for cost effectiveness, making these the two most common wheel materials on the road. Some aftermarket wheel makers have taken manufacturing and construction to the next level to achieve reduced weight and greater performance from aluminum. A handful of these manufacturers use magnesium and a few are beginning to employ carbon-fiber. These materials offer even greater weight reduction but come at a much higher cost.
Gravity vs. Pressure vs. FlowThe most common manufacturing method for aluminum, known as gravity casting, involves pouring molten aluminum into a mold to form a wheel. This method produces a product that costs less but usually weighs more, relative to flow formed or forged wheels. The second method is low-pressure cast, where the material is poured more quickly into the mold and offers increased density. Taking low-pressure casting a step further, some makers subject the casting to high pressure to further improve the strength of the wheel, permitting designs with less material and reduced weight. Another technique, known as flow forming, begins as a low-pressure casting. The formation of the barrel to its final width is performed under high heat and high pressure between two rollers. This method produces a wheel that is almost as strong as a forged wheel at a reduced cost.
Final machining removes excess flashing and adds refinement to any wheels prior to delivery.
Forged For StrengthForged wheels are often the lightest and strongest aluminum wheels on the market. The process of forging involves subjecting heated billet aluminum to extreme pressure (6-12,000 tons) between two dies. This process ensures that there are no voids or pockets of air in the aluminum while also aligning the grain structure of the aluminum, creating a more compact and denser material with superior grain flow. The resulting improvements in strength permit wheel designers to further reduce the amount of material needed in the design. The end result is a lighter wheel with increased strength. The process of forging comes at a higher cost however, putting forged wheels on the upper end of the quality and cost scale.
Crushing AluminumThe strongest and most expensive method of aluminum wheel manufacturing involves an aluminum billet, a hot fire, forging dies and up to 12,000 tons of pressure. After the initial forging process, the wheels are heat treated to further strengthen the material before being machined, powder coated and finished for delivery.
Rims and FacesMost wheels are manufactured as a single piece for cost effectiveness. However, some makers offer multi-piece wheels in two and three-piece designs where the face and the rim or rims are modular. The components can either be cast or forged, and the assembly can be either welded or bolted together. The benefit of multi-piece wheels is that the manufacturer can offer wheels with offsets that maximize wheel width and optimize offset for a broad range of vehicle applications. The modular design allows for multiple widths, diameters and offsets to be created by using different face and rim halves. The drawback of a multi-piece design is its higher overall cost due to increased production, hardware and labor costs.
EASY AS ONE, TWO, THREEA one-piece wheel (1) is usually the most cost effective and least labor intensive to make. The drawback is that wheel sizes and offsets tend to be limited to only popular fitments. A two-piece wheel (2) and a three-piece wheel (3) allow manufacturers to offer customized fitments, sizes and offsets with relative ease. These do however involve more manual labor for assembly.