Forged vs. Cast vs. Rotary Forged Wheels: What’s the Difference?

If you're looking for aftermarket wheels, there are several things that you should consider. As wheels are made using different methods and alloys, they have distinct advantages and disadvantages over one another. Some wheels are inexpensive to purchase, some offer premium quality and some focus on style solely. Modern car wheels are getting more durable and aesthetically pleasing as technology and manufacturing improves. As we continue our discussion of wheels, we'll take a peek at the difference between cast wheels, rotary formed wheels, and forged wheels.

What are cast wheels?

The most common type of aftermarket wheels are the cast wheels. They are made by pouring molten aluminum into a mold to form the shape by either gravity or low-pressure. Cast wheels are the most common type of wheels that account for 90% of OE (Originally Equipped) wheels that are found on most cars. There are two popular casting methods used by manufacturers: gravity casting and low-pressure casting.

Casting Process
  • Gravity Casting: Gravity casting involves utilizing the earth's gravity to fill the mold with molten aluminum. This method is a cost-effective solution for producing a large quantity of identical wheels in a short amount of time. The process solely relies on gravity to do the work, where the aluminum is not as densely packed in the mold as in the other methods would offer. To compensate for the fact there may be voids formed during the cooling process, gravity cast wheels often use extra material on design to account for porosity thus making the wheels heavier.

  • Low-pressure casting: Low-pressure casting is the most popular casting method used by manufacturers nowadays. It involves injecting molten aluminum quickly into the mold by applying positive pressure. It results in a denser and consistent wheel with better mechanical properties than a gravity cast wheel. This method has a slightly higher production cost over gravity casting, but offers a better value for the product with a higher quality.

What are rotary formed wheels?

Rotary Formed Wheels Barrel Being Pressed Out

It has many different names from different manufactures: spung forged, rotary forged, flow forged, hybrid forged etc. Rotary forged wheels combine the manufacturing techniques of both casting and forging. It begins with low-pressure casting to form the wheel face with a shortened and thickened barrel on the back side. Then a rotary machine will spin, heat up  and use rollers to press the thickened part of the barrel into its final width and shape. Through a combination of heat, pressure, and spinning, the process aligns the grain structure and molecular properties of the aluminum alloy, resulting in greater strength on the barrel of the wheel. This hybrid manufacturing technique creates the wheel with a cast spoke face and a forged-like barrel. Many manufacturers now have flow formed wheels lineup available as an intermediate option bridging the gap between cast and fully forged wheels. Rotary formed wheels are suitable for most drivers who are looking to enhance the  look and performance of their car at a lower price point.

What are forged wheels?

Forging in a Vertical CNC Milling Centre

Forged wheels are the pinnacle in aftermarket wheels. 

Forged wheels typically use aerospace-grade 6061 – T6 forged Aluminum alloys as raw material which has the best mechanical property for engineers to work with. To manufacture this raw material, a large piece of aluminum cylinder is heated up under controlled high temperature, then put into a hydraulic press and pressed into a much smaller block under 8,000 to 10,000 tons of pressure. The resulting part is commonly called a “forging”. These forgings are shipped to manufacturers where wheel designs will be realized using CNC (Computer Numerically Controlled) machines. The forgings will go through a series of CNC operations to create the profile, width, offset, and spokes. Then skilled workers carefully hand apply all finishes to create the finished piece.

The forging manufacturing process eliminates any porosity, shrinkage, and other micro-processes presented in the raw material. The resulting aluminum material has a high degree of uniformity in the grain structure which allows the material to have a very high weight-to-strength ratio. As a result, forged wheels can be much more durable with much less material thus far lighter than cast wheels. 

Since most forged wheels are individually made, there's a wide array of customization options such as diameter, width, offset, PCD (Pitch Circle Diameter/Bolt Pattern), brake clearance, and the finish.You can virtually customize every specification of the wheels to make them fit your car and your likings perfectly. The costs of material, labour, tooling, development, and manufacturing processes make forged wheels very exclusive and come at a very high price point. However, if you only want the best in aftermarket wheels, forged wheels are for you.

Comparison

Weight

Wheel weight is the most popular topic when people talk about aftermarket wheels. Mass of the wheels contribute to unsprung weight on a car. For those who don’t know, unsprung weight refers to car parts weight below vehicle suspension including wheel, tires, and brakes. As the unsprung weight can heavily affect how the vehicle feels and handles. The lighter the unsprung weight, the better the car handles. Weight reduction is a main goal when it comes to performance wheels. 

Aftermarket cast wheels can generally be lighter than OE wheels depending on style and sizes. However to combat the material irregularity in the casting aluminum, manufacturers often have to add redundant material to spokes and barrels to increase its strength, thus becoming heavy. 

Flow formed wheels being the latest technology combining cast and forge manufacturing techniques together, they are much lighter compared to cast wheels. With the face/spokes being the same as a cast wheel, the barrel is rolled out with heat and pressure. Therefore the barrel poses a forged-like strength which in return reduces its thickness and reduces overall wheel weight. 

A fully forged wheel is the lightest among them all, it’s generally 25-30% lighter compared to a cast wheel in the same size. The forged 6061-T6 aluminum has very good mechanical strength, manufacturers are able to use a lot less material to achieve the same strength compared to a cast or rotary formed wheels. Therefore many forged wheels have slimer spoke designs and have weight reduction pockets cut into the back, which further reduces its weight.

Strength

Regardless of what type of manufacturing methods are used to make wheels, all wheels need to undergo physical or computer simulation tests to make sure they meet all applicable regulations and safety standards. However, the difference in material shows if there is an impact to the wheel. Under heavy impact, cast wheels tend to crack and break due to voids presented in the casting aluminum. In most situations it would be fine, but if the wheel snapped, the result can be catastrophic. So when buying cast wheels, it is always recommended to choose a reputable brand because of safety concerns. Rotary wheel barrels are stronger than cast wheels, however due the spokes being the same as cast wheels, a similar cracking and breaking situation would happen under heavy impact. Fully forged wheels are the strongest in theory, however because manufacturers use less material to achieve the same strength as all others, it will take the same amount of force and loading. The difference is that under impact, the forged aluminum will bend out of shape instead of snapping to allow for sufficient time for the driver to stop the car safely.  

Performance

Performance of the wheel affects how a car handles and feels, and it is largely due to the weight of the wheels like mentioned before. “Unsprung Weight/Mass” commonly refers to weight of all car parts that are below suspension such as brakes, wheels and tires. It affects how quickly a car can turn, accelerate and turn. Therefore, a low unsprung weight will improve a vehicle’s handling and performance. Forged wheels being the lightest of them all brings the driver the most significant improvement, followed by rotary formed wheels, then cast wheels. In addition, because of the material strength and rigidity used to produce forged wheels, they provide better lateral support to the vehicle when taking corners under high speeds, as well as providing excellent feedback from the road surface.

Appearance

Because the casting process involves a casting and a de-molding process, in order to produce parts without defects, many manufacturers choose simplified designs to make sure molten aluminum can reach every corner of the mold and can be easily taken out of the mold after cooling down. As a result, cast wheels are often plain and rounded on the design with painted finish to cover any surface imperfection. Rotary formed wheels are similar to the cast wheels, however often they go through a simple machining process to add additional features and elements. Forged wheels often have complex designs thanks to modern precision machining techniques. Starting from a solid chunk of forging, a high-precision CNC machine is capable of carving out the smallest surface detail that the designer envisioned. With hand-applied premium finishes such as brushed or polished finish. Forged wheels are often considered as a precision art piece that can greatly accentuate a car.

Costs

Cast wheels are the least expensive among all due to its simple design and mass production capability. They are popular among all levels of car enthusiasts due to its availability and affordability. Rotary formed wheels typically cost more than cast wheels due to a more complex manufacturing method. However they are still typically about a quarter of the price of fully forged wheels. Therefore it has gained a big popularity in recent years as they are relatively inexpensive and also much lighter compared to cast wheels. Forged wheels are the real premium products. A set of forged wheels are typically in the price range of thousands of dollars due to the labour, material and equipment involved in the manufacturing process. However it offers the most in custom wheel experience as almost every set of wheels are individually made so they can be tailored to your car.


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