In the old days, men were men and wheels were made of steel. While wheels made of steel are still rolling around the Off-Road Nation, the majority are now made from aluminum. The reasons are varied, but the constant is that aluminum is lighter than steel, every bit as strong and can be manipulated in many ways. Since the manufacturing process can utilize aluminum in both its solid and molten state, there are several ways a wheel can be produced. As with anything, some processes are better than others, but when it comes to desert racing or rock crawling, aluminum wheels are a better option than steel.

Most of the aluminum parts you find on your vehicle are actually an alloy, which means it's made up of two or more metals. Some of the more common elements alloyed with aluminum include copper, manganese, silicon, magnesium and zinc. It's the variance of the amounts of these metals that cause the aluminum alloy to exhibit certain characteristics. In the case of wheels, where outside factors can cause them to fail, the alloy is made so the part exhibits both solid strength and corrosion resistance.

This is how the billet comes from the aluminum foundry. Its a solid disc of forged aluminum that weighs in excess of 300 pounds.
 
There are three ways this aluminum alloy comes into play when manufacturing wheels. Billet, casting and forging are all processes used to create wheels and each has distinct differences. So read on to see the what and how of aluminum wheel manufacturing and decide what would work best for your particular needs.

BILLET

Many enthusiasts get confused about aluminum parts labeled billet. Actually, the term billet applies to the original chunk of aluminum from which the parts emerge, not necessarily the parts themselves. Billet aluminum starts as molten aluminum that is between 1,200 and 1,500 degrees Fahrenheit, and the desired alloying elements are added to it. A complex mixture of gasses is forced through the molten aluminum, purifying it and causing the impurities to rise to the surface. The impurities are then skimmed off, and the aluminum is poured into molds, creating the billet.

To get the solid aluminum billet into the desired shape, in this case a wheel, it is machined by a CNC (Computer Numerically Controlled) mill or lathe that follows pre-programmed instructions to remove the unwanted material using high-speed cutting bits. The process is actually fascinating to watch as the CNC machine will pull tools, insert them into the chuck, position them and then make the cuts. It does this over and over, at a high rate of speed, until the machined parts appear before your eyes. Due to the heat generated during the process, the part being machined is constantly bathed with cutting fluid, which cools and lubricates the bit.

To make an item such as a wheel requires a billet weighing as much as 300-400 pounds, yet the finished wheel may only weigh 25 pounds when the process is done. The rest of the aluminum is reduced to chips that are recycled.

The upside to CNC-produced billet parts is that they're quick to produce and cost effective, but they may not be as strong as, say, a forged unit. Due to the original structural makeup of the billet, the grain of the aluminum is aligned in one direction. During the CNC process, that grain is not altered in any way, but pieces of the aluminum are removed, leaving sections where the linear lines of aluminum are cut. The more uninterrupted grains a piece has, the stronger it is.

One way to make a billet piece tougher is to treat it with cryogenics. Since the extreme cold will align the grain better, cryogenics reduces the stress brought on by the billet machining process while adding an increased amount of durability to the piece.

That said, the billet wheel is plenty strong for all but the hardest-driving enthusiast. Unless you're a top-notch driver racing a Trophy-Truck the length of Baja on one set of wheels, chances are you won't have a problem with a set of billet wheels.

The automotive industry is the largest market for aluminum castings, and cast products make up more than half of the aluminum used in cars. Essentially, a cast wheel or part is produced when molten aluminum is poured into a mold and allowed to harden. The cast method is the easiest and therefore most cost-effective way of making an aluminum part. It uses the least material and most closely resembles the finished piece when the process is done. Molds can be made of many types of material, and sand molds have been used for many years. Other types of molds, such as die cast, are literally two-pieces of steel mated together, and the cavity is filled with molten aluminum forced in under pressure. When the aluminum hardens, the mold is split apart, and the piece is ready to be finished.

As stated, the upside to casting wheels out of aluminum is the pieces are made quickly and require less machine and finish work, making them less expensive. But like billet, the grain structure that results from casting aluminum is not as uniform nor as closely aligned, and therefore not as strong.

This is why wheel manufacturers use what is called the "permanent mold" method to produce the strongest unit possible. With this method, the temperatures of both the molten aluminum and the cooling process are closely monitored while casting the wheel, ensuring the grain is as tight and as linear as possible. Also, a cast wheel is designed with more material incorporated into it, making the wheel beefier than even some billet designs. For the majority of off-roaders, the result is a cast wheel that is more than adequate for what it needs to do.

FORGED

Another time-tested process is forging. According to the Aluminum Association, there are three basic types of aluminum alloy forgings, consisting of open-die forgings, closed-die forgings and rolled rings. All forging is normally considered the strongest process of manufacturing aluminum parts; through the pressure of the process, the grain is aligned in such a way so as to add strength. This pressure usually comes in the form of hammering, compressing the material and, with it, the density of the material is increased resulting in superior strength.

Also, forging is strong given the fact that the material is only heated to an elastic or "plasticity state," but not to a molten state. This also acts to keep the grain of the material in check. Once in this plasticity state, the material is either hammered or extruded.

During forging, the material can be hammered into shapes using specially shaped dies with a high PSI hydraulic hammer. And by high PSI, we mean thousands of tons of pressure being applied to the material to force and squeeze it into the desired shape. The hot material can also be fed through progressively smaller apertures until it's compressed to the desired thickness for the part in question (such as a coil spring). The grain that is aligned, through the act of being fed through the apertures, is compressed and uninterrupted¨Cmaking it stronger.

It is this grain alignment and compression that makes forged wheels the strongest on the market. That's the good. The bad has to do with the fact that because they're the strongest, they're also the most expensive. Large, forged, off-road wheels can run upwards of $750 per wheel. And while they may be the hot setup if you are running a Trophy-Truck, for us mere mortals without fat wallets they may be more of a good thing than you actually need. The choice is up to you.

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