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 =)
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 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.
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 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 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
Video added for better understanding. Thanks to CalinTexas and Lemans_Blue_M