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While not trying to reinvent the wheel, many wheel manufacturers are stepping up their game to improve construction and strength of the performance wheel. Ever since the ‘40s when hot rodders reversed the center section of a factory steel wheel to change the offset and widen the vehicle’s track to enhance handling or clear modified frame rails, manufacturers have introduced new designs, materials and manufacturing methods to produce a wide variety of rolling product for the performance and racing markets.
Consumers with a clear understanding of how wheels are manufactured will have the edge when selecting wheels for demanding performance applications. Wheels shouldn’t be purchased solely on style or trendy fashion cues. Cheap knock-offs of popular wheels can affect the safety and performance of any vehicle.
With regards to wheels available in the performance aftermarket, aluminum is by far the most popular material. Steel wheels still have a loyal share of the overall automotive wheel market, mostly as original equipment and on trucks because of low cost and overall strength.
Magnesium was popular with racers in the ‘60s; however, corrosion problems and durability concerns slowed development. Recently, advancements in magnesium surface treatments and new forging technologies have prompted renewed interest, especially in Europe and Japan, although the market remains limited.
Exotic materials such as carbon fiber and titanium have been used for custom applications, but these materials are far from being considered for widespread wheel production. Advancements in 3D metal printing may also introduce new alloys in wheel design; however, there are still tremendous technical difficulties that need to be solved before these options are within the reach of the average performance enthusiast.
“Aluminum ranks higher in the trifecta—that is, low weight, high strength and expense—when properly made than any other material,” notes Daniel Finke, engineer and founder of Texas-based wheel manufacturer Finspeed.
Aftermarket aluminum wheels are available in a wide range of styles and price points not because of choice between various alloys but rather a choice between different manufacturing methods. In other words, not all aluminum wheels are made the same way. Performance, finish and durability are all qualities affected by the manufacturing approach, which can be very sophisticated and controlled, or it can be crude and basic without consideration of performance standards.
There are three manufacturing techniques generally used in the aftermarket wheel industry: casting (for which there are two different methods), forging and rim rolling (also known as flow forming). In addition, wheels can be found in one-, two- or three-piece designs. Each construction method has pros and cons that balance the wheel’s strength, performance and price point.
The most common type of aluminum wheel is a one-piece casting. Cast wheels are formed when molten aluminum is introduced into a mold and the aluminum cools to form the desired shape and size. Casting is the simplest method of wheel construction, yet there are numerous ways of getting the molten aluminum into the mold. Gravity casting and low-pressure casting are the most popular ways that wheel enthusiasts will encounter. However, it’s important to note some of the other lesser known methods.
The gravity process uses the earth’s natural gravitational pull to fill the mold with molten aluminum. It’s the least expensive of all wheel manufacturing methods and allows for some creative designs when weight isn’t a performance factor. The downside is that after solidifying, the metal may be porous because the molten material isn’t compressed as densely as other processes. That means the wheel design may have to be heavier to achieve the desired strength.
With the low-pressure casting method, molten aluminum is introduced into mold from the bottom using positive pressure forces. This method results in a denser material that is stronger with less porosity. When the OEMs specify a cast wheel, this is the process used most often.
Some of the other methods of wheel casting include:
A forged one-piece wheel represents the lightest yet strongest construction method available in the wheel industry. The process starts with a piece of solid billet aluminum, which is then shaped between numerous forging dies under extreme heat and pressure. This method compacts the metal, controls the grain structure and results in an extremely dense material, allowing excess material to be machined away for light weight and low-profile designs.
Forging requires expensive tooling and machinery, so the end cost to the consumer is quite high. Another pricey method of wheel production is the billet wheel. Here, a solid block of billet aluminum slightly larger than the overall wheel dimensions is CNC-machined for hours until a finished wheel is produced. Billet wheels are used mostly on high-end show cars where unique one-off designs can be produced without the upfront cost of tooling for a forged one-piece wheel. On the downside, billet wheels leave considerable waste.
“Cutting from a solid billet would be at least four times the cost of cutting from a forging,” notes Finke.”
Choosing between casting and forging previously left wheel manufacturers with few options to produce lighter but stronger one-piece wheels at a more affordable price point. However, combining the principles of casting and forging has led to a process called rim rolling, and this procedure is helping bridge that gap.
Flow Forming—sometimes called rim rolling or hot spinning—starts with a low-pressure casting that is spun on a special rolling machine. The outer portion of the casting is heated, then the casting is spun. Pressure from rollers stretch the metal across a parent drum to form the rim or barrel area to its desired width and shape. The resulting material is thinner, an subsequently denser. Overall, a flow-formed wheel is about 25 percent lighter than an identical cast version.
The discussion up to now has focused on one-piece wheels; however, two- and three-piece wheels have carved out significant niches in the aftermarket. Multi-piece wheels allow for numerous options, such as using different materials in the same wheel and offering a wider variety of designs, sizes and offsets. Perhaps the most popular benefit of a multi-piece wheel is the option of executing different finishes on each of the pieces. For example, a true chrome center section can be mated to a satin-black rim.
A two-piece wheel is made up of a center section that is either bolted to or welded to a rim, also called the barrel of the wheel. The two pieces are usually the same material, but the center section could also be aluminum that is secured to a steel rim. Aftermarket performance two-piece wheels are often constructed of a billet aluminum center section that is machined to the desired style and size, and then attached to an aluminum rim that was spun from a flat aluminum disc.
The three-piece is just a more modular approach to wheel construction. The rim is made up of inner and outer sections that are constructed by spinning from a flat aluminum disc. The center most often is a forged billet piece that is machined to the desired spoke design. Upwards of 40 nuts and bolts are then used to attach the three pieces together. The result is a strong, relatively lightweight wheel that can be used in performance applications if properly designed.
Three-piece wheels are expensive to manufacture because they must be hand assembled, yet they’re also popular because designs and finishing options are unlimited. The vehicle can literally have a different look every year by changing the center sections.
“One of the benefits of a multi-piece wheel for performance applications is that if you damage a component, you only have to replace that component and not the entire wheel,” says Finke. “The disadvantages are that multi-piece wheels have a high moment of inertia (MOI) compared to a similar-sized one-piece forging. That’s due to the number of required fasteners, even when exotic materials like titanium are used. Three-piece wheels are also prone to losing pressure in performance situations, which has led some sanctioning bodies to ban their use.”
Being aware of the different construction processes is critical to selecting the right aluminum wheel for each application. All of the choices are a balance between strength, appearance and costs. Unlike the days of the chrome reverse rims, there are hundreds of options available to performance enthusiasts—though, not all are good options. The right wheel, however, will add value to the vehicle, improve performance and personalize the vehicle to suit the owner’s identity.