Crash Course on Wheel Types (Manufacturing methods)

I posted this on e90post, I think it helps to have it here too.

Cast, forged, flow formed, Multi-piece
By Setian, A.K.A. e90post member: DTMrack.com

It’s easy to get confused when shopping for new wheels. There are so many different styles, fitments. Do I buy forged, three piece, flow formed, cast? If it is cast will it be gravity or low pressure.

For some of us, all those words have clear meaning. But there are a lot of prospective enthusiasts who struggle with the lingo. This is for all of you =)

Manufacturing

Unlike us, all wheels were not created equal. Inside their aluminum metallic surface lays a world of micro-structural difference. This world is a result of the manufacturing process of the wheel. There are essentially two ways to manufacture a wheel: forged, and cast. The latter has a couple of subcategories.

Forged:

Forged is definitely the king of manufacturing when it comes to wheels. Top-notch quality. The forging process uses immense amounts of compressive force to shape the metal.

Let me give you an example to make this easy to understand. Have you ever wrinkled aluminum foil and then tightened it as much as possible by hitting it with a spoon to shape a ball? If you have, this is essentially what forging is. Once that little ball is as small as possible it is extremely hard. This is virtually what happens during forging, to an infinite power. The piece is pressed between a Ram and an Anvil. (see diagram above)

Forging can produce a wheel that is stronger than an equivalent cast or machined part. During the process the internal grain is deformed and rearranged to the shape of the part. This deformation due to compressive force improves the strength uniformly throughout the entire structure of the wheel. As a result you have a stronger, lighter wheel.

There are different types of forging processes that offer subtle differences, but I will write about them in another article, specifically targeting Forged Wheels.

Cast: Casting is the most popular way wheels are manufactured today. And all cast wheels are also not created equal. Casting is basically taking metal, heating it to achieve liquid form, and then pouring it into a mould with a specific shape. This process is very cost-effective as it takes very little time to produce one wheel. Hence it’s popularity. However, there are three ways, essentially, to cast a wheel: flow formed, low pressure cast, and gravity cast.

Gravity Casting-

Gravity casting is the simplest form of casting. As it’s title suggests, this form of production allows gravity to do the work. To start, the molten metal is poured into the mould. Once the mould is full it is cooled down and the wheel is then retrieved. This is an excellent alternative if you are looking to purchase wheels on a budget or with cosmetics as a priority. Because of its low cost, it is the most common form of production for aftermarket companies.

Low Pressure Casting-

Low pressure casting is a step up in quality from gravity casting. The process is the same but the molten metal is injected into the mould. By applying pressure, the molten (liquid) aluminum enters the mold quicker. This allows the mechanical structure of the wheel to be denser because of the pressure, denser is better. The end properties result in a lighter and stronger wheel than those that are gravity cast. Most wheels produced by O.E.M. companies, such as BMW, are created using this manufacturing system due to its advantages in cost and higher quality.

Flow-forming-

Flow forming is the superior over its two other cast brothers. It is done by taking a piece that is cast using low-pressure casting, the piece is then spun. The outer area of the cast wheel is heated and pressed by the use of steel rollers to achieve its final shape. By combining heat, pressure and circular rotation of the piece, flow forming rearranges the mechanical characteristics of the wheel, creating a kind of hybrid wheel with properties similar to those of forged wheels.

What about multi-piece wheels?- Three piece wheels were originally designed for track use. The purpose behind the wheels was to have the capability of interchanging the barrel of the wheel incase it bent or cracked while racing. Nowadays, you are most likely to run into the ‘three piece look’ then the real thing. Some companies even weld their wheels because their products have air leaks which defies the purpose of having them entirely. Multi-piece wheels can be a combination of forged barrels and cast faces, vice versa, or any combination yielded by the two. If you are set on purchasing multi-piece wheels make sure the company is a legit multi-piece manufacturer, you WILL regret it if you don’t.

So Why does this all matter? For starters, the way the wheel was manufactured will most likely determine how much your wheel will weigh. For motor-sport enthusiasts like us, this is a huge deal. In the vehicle, there are essentially two ways of measuring weight: sprung weight, and unsprung weight. The suspension in your vehicle divides the mass distribution in your car to sprung mass and unsprung weight. The section of your car above the suspension is the sprung mass, this means the frame of the car, engine, etc…even you. The unsprung weight is whatever is below the suspension, such as the wheels, rotors, calipers, etc… A pound of unsprung weight can translate into as much as 4-20lbs of sprung weight depending on your vehicle. The less weight, the better. The ratio between sprung weight/unsprung weight affects the smoothness and handling of your car. The general rule of thumb is that the higher the ratio, the better.

There is one VERY important thing to consider. Most wheels are engineered to withstand a specific weight load. This means that if you are choosing between a set of forged wheels, and a set of cast wheels, and they were both have the same weight load capacity, they will likely share the same damage resistance when it comes to force impact. In other words, they may be equally strong. So in this case, don’t buy the forged wheels because you think they will be stronger. The difference will be that IF you hit a pothole at high speeds, the forged wheel will most likely bend, where the cast wheel will crack. This goes back to the wheel structure yielded by the manufacturing method.

I hope this humble crash course in wheel manufacturing methods will help you when shopping around for wheels. Always find out if the company you are purchasing from meets JWL, TUV, and other standards and certifications. And remember to ask questions about the wheels before you purchase them.

All the best;)

Setian

Edit 3.22.11

Video added for better understanding. Thanks to CalinTexas and Lemans_Blue_M

NOW I get it!!

good stuff im sure it will be very useful for many forum members.

thanks for sharing.

Thanks for the info!

If I could +rep you I would. Its been a few years since I had any materials/mechanics classes Thanks for the write up!

Thank you very much for your write up. It's funny that you mention forged wheels bending and cast wheels cracking. My friend bought a set of BBS CH wheels years ago, and one of them cracked. He is planning on JB Welding the wheels back together. I told him that he is insane to do that and then put them on his e39 M5.

On a side note: Where I live the roads are terrible at best in the winter time. I had OZ low pressure cast wheels on my S8 for the winter. I bent 3 of them last winter, so I bought Volk forged wheels for this winter. So far so good. I truly believe they are a stronger wheel. I don't know what the weight load capacity is on either wheel, but I guess this means the Volk wheels have a higher capacity. How do I find out what the capacity is? I never see this number advertised.

Thanks for sharing.

Great explanation - Customers always ask us the difference between forged / cast.

Prit@Euro Autowerx wrote

Great explanation - Customers always ask us the difference between forged / cast.

+1 Now I just refer them here

Thanks! great post.

I was wondering if you can inform us on how to differentiate between forged and cast rim by using the naked eye. l am about to buy set of rims and tires for my e92 M3 and want to make sure that what I am buying is forged not cast wheels , since I don't have enough background on how both should look like. Unfortunately I don't have the brand name for the wheel.

Thanks

Vr00mVr00m wrote

Thanks! great post.

I was wondering if you can inform us on how to differentiate between forged and cast rim by using the naked eye. l am about to buy set of rims and tires for my e92 M3 and want to make sure that what I am buying is forged not cast wheels , since I don't have enough background on how both should look like. Unfortunately I don't have the brand name for the wheel.

Thanks

You need the brand name and model. The vast majority of aftermarket wheels are cast. Having the word "forged" or "forge" in the brand name is not an indicator. Without the brand and model name, you might need to X-ray them to know for sure. You could also post a picture. It's likely that someone here can tell you what you are dealing with from a photo of the wheel.

Vr00mVr00m,

Thanks for the question. There are really only two possible ways of telling if a wheel is forged or cast with 'naked eye inspection'.

The first which is normally the biggest tell of any cast product is the parting line. Every cast product is made with a mold that consists of two or more pieces. In EVERY cast product, the place where the two molds come together is called the parting line.

Once the piece is finished, there will always be a very small seam where the parting line was. Think of your childhood toys and how they always had this little thin line all around. Think of a toy rubber ball that has that small line that goes around it. That's the parting line. That is the biggest tell.

Sometimes, companies will go to great lengths to disguise the parting line. In a wheel, the parting line would not be towards the center of the barrel but closer to the face.

The only other way of knowing is simply by picking the wheel up. A 19" 9x5 forged Aluminum wheel should weigh no more than 20-23 pounds AT THE VERY MOST.

There are, however, manufacturing methods that combine casting and forging to form a hybrid wheel and produce very light pieces.

At DTMrack, we believe this technology will position itself over full forged due to the fact that wheels can be made just as light as most full forged wheels, without the SUPER excessive cost to the customer.

If there are any other questions you might have, please do not hesitate. We are here to answer as much as we can so you can make the best decision when shopping around.

Cheers,

Setian

Thanks "DTMrack.com" and "calintexas" for the fast reply, I have learned a lot. It turned out the wheels I was going to buy are cast. I am happy that I asked you guys first.

DTMrack.com wrote

There are, however, manufacturing methods that combine casting and forging to form a hybrid wheel and produce very light pieces.

At DTMrack, we believe this technology will position itself over full forged due to the fact that wheels can be made just as light as most full forged wheels, without the SUPER excessive cost to the customer.

So... would these wheels bend or crack? And would the same reasoning hold true that if the load rating is the same, it would be just as 'strong' as a forged or cast counterpart?

dnvrdrvr wrote

So... would these wheels bend or crack? And would the same reasoning hold true that if the load rating is the same, it would be just as 'strong' as a forged or cast counterpart?

All wheels can bend, all wheels can crack. The nature of a forged wheel creates a tendency to BEND when there is high impact because of the substructure of the metal. However, they can crack. The nature of a cast wheel creates a tendency to CRACK when there is a high impact because of the substructure of THAT metal. However, they can bend.

For the wheels in question, the barrel is forged, by definition, so it would tend to bend.

Thanks for clarifying - great thread

dnvrdrvr wrote

Thanks for clarifying - great thread

Sure thing!

Anytime you have any other questions feel free to ask!

I know vmr wheels are cast, but does anyone know what method they use?

SpoiledBrat wrote

I know vmr wheels are cast, but does anyone know what method they use?

VMR uses Gravity Cast Manufacturing.

Thanks to Lemans_Blue_M who sent this to me (on another site). The first part of the video shows a cast wheel being flow formed. The second half shows a forging blank being flow formed.

calintexas wrote

Thanks to Lemans_Blue_M who sent this to me (on another site). The first part of the video shows a cast wheel being flow formed. The second half shows a forging blank being flow formed.

Nice! I'll add the video to the OP.

Thanks