Tire Talk - Anatomy of a Wheel -

Tire Talk – Anatomy of a Wheel

Adapted from Scott Blair's articles in Tire Review

Arguably one of the most aesthetic components of any vehicle, wheels reflect the owner’s style, personality and, in many cases, the size of his or her bank account. But wheels encompass much more than just good looks.

Looking beyond the final product all the way back to the beginning, we discover an idea or inspiration dwelling in the mind of the wheel’s creator. That’s where we want to take you. We see and appreciate the final product, but how does it get here? This article looks at the various ways of constructing wheels, from concept to final product.

Cast Aluminum:
Inspiration comes from all around us. Some wheels, like the Torque-Thrust, are rooted deep in the history of custom wheels, relying on a five-spoke design that is simple, clean and timeless. Other styles evoke architectural designs, with arches, long curves and sharp edges. Still others derive their shape from form following function, using direct lines and simple angles to minimize weight, but still maintain load capacity.

Our first look into the design and anatomy lesson is the cast one-piece aluminum wheel. This wheel is constructed by pouring molten metal into a mold, which, when cooled, becomes a raw casting.

To show you how the process works, we’re going to walk through it step-by step with information from MHT Wheels and Ultra Wheels – two major design and manufacturing companies in the U.S.

Once a design concept is conceived, wheels are sketched on paper. After some revisions, the drawing and dozens of others are submitted to a committee. At this stage, the sales staff, the person who created the design and production personnel come together to discuss the feasibility and longevity of the design.

Once the concept drawing is approved, the next step is to develop a 3D rendering with a computer model. High-tech models like these help the overall design concept move more quickly to actual production.

Using special software, the computer model can test strength and rigidity as well as troubleshoot design issues.

When the rendering is complete and any engineering issues have been addressed, the information is transferred to a CNC milling machine. This machine is loaded with a chunk of billet steel. Over the course of a day, the CNC can chew away steel a few thousands of an inch at a time and hold tolerances within 0.0005 inch. Th e result is a segmented investment mold, as shown below.

Notice that there is a top, bottom and sides. Only two sides are shown here, but, in reality, it takes four of these to make up the sides of a wheel. The light color shown is a ceramic coating. Molten aluminum is highly corrosive, so to protect the mold itself, this coating is applied before the mold is put online.

With the mold in place, the casting can start. Ingots of A356 – virgin aluminum – are melted down and mixed with other alloys (the choice of which depends on the properties desired), and then is poured into a vat, which is then slid under the bottom of the mold machine.

From there, the molten metal is injected upward into the mold cavity. This method is called a counter-pressure injection mold system.

After the wheel is cooled and removed from the mold, it goes on to heat treatment. This process hardens the aluminum to prepare it for a multi-step machining process. Bolt holes, center bore, back and front half of the wheel must undergo a machining process, as well.

Here we see an aluminum mod with the rear half cut on the lathe. The wheel will be conveyed to the next machine, which is set up to cut the front half.

With all of the machine work finished, the wheel then goes through first-stage inspection. One part of the inspection process is the leak test. The wheel is sandwiched between two flat panels and submerged in water. Compressed air is forced into the center of the wheel, and if a leak exists, an air bubble will seep through to the outer side. The technician can mark the spot for repair or discard the leaky wheel entirely into the reject pile.

Wheels that complete this inspection stage go to the final finishing stages of painting, polishing or chrome plating. A final inspection checks for finishing flaws, and then it’s off to packaging, warehousing and shipping.

The ending comes when you mount the wheel on your customers’ rides and see their faces light up. That’s a look most wheel designers – and all of the production crew – never get to see, but it is their artistry, imagination and craftsmanship that made it possible.

The Forged Method
Forged wheels are literally forged in a press and made into one-, two- and three-piece wheels. With one-piece cast wheels being shifted more and more to Pacific rim countries, forged wheels are becoming the hot ticket in the U.S. for manufacturers that want to keep or gain marketshare. The biggest advantages of a forged wheel are its strength, lighter weight and outstanding surface finish.

Some forged wheels are made in a press, but there are other, newer processes that actually use a forging press to spin a die while pressing an aluminum billet into shape. This process – rotary forging – causes the molecular structure of the wheel to line up in such a way that it becomes even stronger than a traditional forged wheel.

Depending on the process, billet aluminum can be pressed to a near net shape that requires 20% less finish machining, or it can be pressed into a flat disc with various profiles that can be used to create different designs when milled out. Here, you see a disc being spun into what will eventually be a one-piece forged wheel.

This process produces a raw spun wheel ready to be CNC-machined into the desired design. A lack of seams afforded by forging allows these wheels to be easily painted, polished or chrome plated.

The design of the wheel is created with CAD/CAM software, which produces a 2D and 3D model of the wheel.

After machining, the finished wheel looks exactly like the computer model. But, as is the case with cast aluminum wheels, the variety of widths and offsets possible with one-piece forged wheels are limited.

The Two-Piece
Traditionally, two-piece aluminum wheels have an outer rim, known as a hoop, that is created by taking a straight, flat piece of aluminum and rolling it into a circle. The seam joining the ends is welded, and the hoop is placed onto a machine that presses a roller against the hoop to form the contour of the flanges, bead seats, bell and drop center.

Cast, billet or forged centers can be used to complete a two-piece aluminum wheel. The centers are pressed into the outer hoop using a process known as sweating, in which the outer hoop is heated, causing it to expand. The face is then pressed into the outer hoop at a specified backside setting and then welded into place.

The biggest benefit of a two-piece wheel over a one-piece is the variety of offsets and widths possible. That’s because the centers can be placed virtually anywhere within the hoop. Typically, a two-piece wheel is lighter than a comparably sized one-piece unit, which contributes to better handling and improved gas mileage.

The lighter weight also helps in racing applications because the lighter the wheel, the less torque it takes to turn it. This translates into lower unsprung weight and quicker ETs at the drag strip. And new forging technology is creating spectacular two-piece wheels like never before. Manufacturers are starting to create a forged outer hoop – the benefit of which is added strength – especially in the flanges. Rolled outer hoops don’t hold up well to curb damage or potholes, but a forged one has the strength to go rock climbing, yet still offer the flexibility of custom widths and offsets.

Obviously, the price of new wheels has to reflect all of these advances in technology and production. So, next time you see a two-piece wheel at double the price of a one-piece, you’ll know why.

Did You Know

Not too long ago, a wheel design would have a life of three to five years before being discontinued. Today, a wheel design’s lifespan can sometimes be as short as just 18 months. Like clothing, a trendy wheel design will be a must-have today and a boat anchor tomorrow. The cost of designing, engineering, testing and building the tools and molds all has to be considered against sales potential. Short production runs drive the per-unit cost skyward.

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