Lately, I have seen a lot of people asking questions about wheels, their strength, weight, manufacturing method, which wheels are good, what to buy, etc and I’ve seen some very good responses but I have also seen some misleading opinions so I decided to write this series of articles to make sure you know everything you need to know about wheels.
This information comes from over 5 years of experience in the automotive wheel industry. I’m an Industrial Engineer with a minor in Mechanical Design, I own two wheel companies, have dealt with every aftermarket forged wheel manufacturer in the world, have designed, engineered and manufactured very strong and light wheels in both cast and forged variants and have very close relationships with some of the leaders of the wheels industry in the USA including members of the SAE Aftermarket Wheel Committee and the SEMA Wheel Industry Council.
Article 1
Here is the first article. Hope you like it and could use it to form accurate opinions about your future wheel purchase and can properly and accurately assist others.
What are Alloy Wheels?
Alloy wheels, or most commonly known as “rims” are automobile wheels that are manufactured from aluminum or magnesium alloy metals. They are called alloy wheels since they are not manufactured from pure aluminum or magnesium but using alloys (mix of metals) usually 6000-Series for high end aluminum wheels which contains less than 1% Si and about 1% magnesium.
6060-T6 Aluminum is very good because of its properties. It is very ductile, durable and lightweight, making it very good among wheel manufacturers. The “T6” comes from a Heat Treatment technique called precipitation hardening and an artificially aging process to alter the properties of the alloy
Manufacturing:
Automotive wheels can be manufactured using several process. Setian (DTMRack.com) and crimsone90 made very good articles referring these methods. This articles can be found here (
http://www.m3post.com/forums/showthread.php?t=357430) and here: (
http://www.e90post.com/forums/showthread.php?t=295274).
Casting
Most of you are familiar with this process.
By definition, “Casting is a manufacturing process by which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify.” There are two main techniques to cast aluminum wheels, gravity casting and low-pressure casting.
Gravity casting is a method of manufacturing a cast wheel of an specific metal alloy, typically aluminum, although Magnesium is also used, which involves supplying molten metal to a mould cavity via a feeder through a running system where the molten metal entry point is located above the top of the mould cavity. In this manufacturing process all the metal entering the mould is subject to turbulence. This turbulence can cause severe defects such as oxide inclusions and entrapped gas porosity and in some cases mould erosion and hotspots.
Low-pressure casting in the other hand takes care of the disadvantages of Gravity casting by filling the mould by injecting molten metal at about 25psi through an in-gate just below the top of the mould cavity from a source under the mould through a method which allows complete filling of the mould. The force of gravity acts against the upward flow of the metal and eliminates the possible turbulence caused by the falling liquid metal. The molten metal is stored in a pressured container and by increasing the pressure metal is then forced into the mould.
In low-pressure casting the method in which the metal is forced into the mould is the main difference/advantage over gravity casting and is absolutely controlled. As explained above it results in a low turbulence or turbulence-free mould filling with little to no corrosion.
The molten metal in the furnace is in a closed container under protected atmosphere. Because of this the metal absorbs less hydrogen and any other impurities as well as the oxide formation being greatly reduced. As opposed to gravity casting, the metal surface is not interrupted constantly since it is been forced from under the metal surface. What you get is a very clean quality metal.
Basically low-pressure casting uses positive pressure to move molten metal into the mold quickly thus resulting in a finished product with an Aluminum that is denser than Gravity cast.
The advantages of low pressure die casting process are several:
- higher yield achievable
- reduction of machining costs
- excellent control of process parameters
- good metallurgical quality
- leads to an optimal use of the aluminum
- excellent mechanical and technological properties of the castings.
Forging:
Out of the three, Forging is the manufacturing process that creates the lightest metals. There are several ways to forge alloy wheels, but the most common is press forging.
By definition “Forging is the shaping of metal using localized compressive forces. It is to form or to make by heating and pressing.”
The forging process uses immense amounts of compressive force to shape the metal. During this process a wheel blank is placed between two shaped plates and the press slowly shapes the wheel using continuous forces. 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. Press Forging allows closer tolerances as well. The wheel is typically heated to about 400-Farenheit. The forging process typically uses anything between 4,000 to 10,000 tons of pressure, 6000 being the most common, and reaches up to 16,000 tons to produce forged magnesium wheels. Heat treatment and artificial aging can further improve the wheel mechanical properties.
It is important to understand that forging does not compress the metal. Forging alters the grain structure of the metal and makes it stronger. For example, a 19” aluminum wheel blank that weights 30lbs will still weight 30lbs after the forging process. The major difference is that after the forging process, the 30lbs forged aluminum wheel blank will be much stronger. Since the aluminum is stronger it allows the engineers to manufacture a lighter wheel by removing excess aluminum.
To make it easier to understand, imagine a structure that requires a beam with a diameter of 5-inches. If we manufacture that beam out of forged aluminum, since the mechanical properties have been improved, the beam diameter could now be reduced to around 2-3” in diameter helping to make the structure lighter. You could do the same and keep the diameter of the beam at 5-inches if your purpose is to make it stronger.
In other words, Forging does not make the metal lighter, it makes it stronger. Forged wheels are also formed via flow-forming which is explained below.
Flow-Forming:
Flow Forming is a precision metal forming. It is commonly known as a
forming technique. It is the newest player and is only available by a limited number of manufacturer.
By definition: “Flow Forming is an incremental metal forming technique in which a disk or tube of metal is formed over a mandrel by one or more rollers using tremendous pressure.”
In other words, a disk, or in this case an aluminum wheel blank, is placed between two rollers and the wheel spins while the two rollers form the barrel at high pressures and extends the barrel of the wheel to the desired length. This deformation arranges the grain structure of the metal in the direction of the rotation of the wheel. The blank is typically made using metal casting. This process helps reduce barrel thickness. The blank barrel is usually around 4.0-9.0-inches wide and is extended to 7.0”-11.0-inches depending on the application. Although the piece is cold formed, during the forming process, due to friction, the wheel blank is heated to around 400-Farenheit. Because of the high pressure and high temperature the metal mechanical properties are changed. The mechanical properties of the metal became very similar, if not the same, to that of a forged wheel. No material is lost during the forming process, resulting in less aluminum used to create the finished wheel.
Flow Forming in essence forges the wheel barrel by spinning and forming. Forming can be done hot, warm or cold and is accomplished on mechanical or hydraulic press, or on special forming equipment. Heat treatment and artificial aging can further improve the wheel mechanical properties. Flow forming process can be a cost-effective alternative to conventional press forging.
Because of its properties flow forming is also known as liquid forging.
Please let me know if you have any questions regarding this and I`ll try to reply them as soon as possible.
On my next article, I`ll be covering the Design and Engineering aspect of a wheel. It`ll include information regarding wheel weight, strength and fitment. It's nearly complete and will be posted in a few days.