Man has long argued as to what was the most important discovery in the technological history of the world. Was it fire? The integrated circuit? Clear cola? Or perhaps the humble wheel?
The wheel is the least pretentious of the simple machines, and probably the one that has gotten us the furthest. Wheels have progressed from basic tools to status symbols these days, but their function is just as basic—and just as important.
While all cars use wheels, their design and construction can vary greatly, from inexpensive stamped steel to high-tech forged alloy examples. Most people base their wheel selection on looks and price, but the process of picking the right wheels for your car should go much deeper.
For the most part, wheels are made of either steel or aluminum alloy. Steel wheels are cheap and easy to produce, which explains why many passenger cars come with them as original equipment. They may look a little plain, but a set of inexpensive hubcaps can quickly change that.
Aluminum alloy wheels, sometimes referred to as just “alloy” or even “mag” wheels, have some serious advantages over basic steel ones. For starters, aluminum alloy wheels, by nature, better dissipate heat and provide more rigidity under high-load cornering maneuvers.
Alloy wheels are usually lighter than steel wheels, which reduces unsprung weight (all weight that is not supported by the suspension). Any reduction in wheel weight reduces the amount of inertial weight at the rotational axis. The less weight there is spinning around, the easier it is for the engine to turn the wheel and the easier it is for the suspension to control wheel and tire movements.
Even small weight reductions of two to three pounds can make a significant difference. As an example, imagine spinning a two-pound weight on a string over your head. Now imagine the same thing with a 10-pound weight. The 10-pound weight will require much more effort to spin, and will be harder to control. The same principle holds true for road wheels and unsprung weight.
Another benefit of reduced wheel weight is that steering response will improve as there is a lighter load on the steering rack. And lighter wheels reduce the rotating mass of the vehicle, providing quicker acceleration and shorter stopping distances.
If you look at the wheels used in our plus-sizing tire test, the 14x6-inch original-equipment steel wheels weigh more than the significantly larger 16x7-inch TRMotorsports Typhoon alloy wheels.
While it may not make your car faster through your favorite series of corners, alloy wheels also tend to look pretty cool.
Nowadays, there are basically three ways in which alloy automotive wheels are constructed. The three types of alloy wheels can be referred to simply by their common names: cast, billet and forged. Let’s take a look at these manufacturing processes, and how they make the end product better or worse.
Casting is a relatively inexpensive way to produce a high-quality, fairly strong alloy wheel; many aftermarket alloy wheels designed for street use are made this way.
In common gravity casting, the wheel maker begins with a prototype “plug” that is used as the positive to produce the mold. This plug is usually made by machining a piece of material (often plastic or other phenolic material) on CNC machining equipment to produce a highly precise model.
The “negative” is then made from the positive by pressing casting sand around it. The sand is actually a composite slurry that, when compressed under high pressure, becomes quite hard. Think of what happens when you walk down the beach: Your foot compacts the sand and makes a very accurate, very stable negative impression. It’s the same principle, just with high-tech sand.
Next, molten aluminum alloy is poured into the sand mold and allowed to cool. When the sand is broken away, you’re left with a wheel that only needs minor finishing (like drilling and possibly trimming of some excess metal) to be considering complete.
Negative pressure casting is a similar process, but instead of pouring the molten material into the mold, the molten alloy is drawn up into the mold using a high-pressure vacuum. This eliminates much of the trapped gas found in the gravity casting process, producing a stronger wheel that is much less porous than a gravity-cast one.
Billet wheels are machined from a solid chunk, or “billet,” of material. First, a telephone pole-sized piece of aluminum alloy is produced (or bought from a vendor). Since this piece of stock is generally extruded, the grain runs through the stock, much like the fibers within a single strand of wire. The stock aluminum is then sliced up into sections which are machined down into either complete wheels or just wheel centers.
Since they retain the grain structure of the extruded stock material, billet wheels are extremely strong. This grain structure, which is not present in a cast wheel, gives the final product a backbone—makes the wheel even stronger without adding weight.
Of course, billet wheels are also extremely expensive to produce because much of the original material is wasted. A lot of time is also spent machining the original stock down to a finished wheel, which only adds to the cost of the final product.
Actually, most “billet” wheels are actually billet centers bolted into stamped or spun rim halves. Entire wheels forged from a single billet are so rare as to be almost nonexistent, and are usually seen only on show cars. Billet centers on multi-piece wheels, however, are common.
Unlike casting or machining (billet), forging uses intense heat and pressure to transform a slug of alloy material into the final shape of a wheel. Under this heat and pressure, the original grain structure of the stock material is forced from the center of the wheel towards the outer edge. This grain structure is even stronger than the one found in a billet wheel because it runs along the spokes and serves to further strengthen the forged wheel’s spokes, while the grain in a billet wheel simply runs through the spokes. Thanks to this process, a forged wheel can be up to 300 percent stronger than a cast wheel. Additionally, since forged aluminum is stronger than cast aluminum, less material is needed to produce the wheel, resulting in a lighter product.
When shopping for a forged wheel, you may want to ask how close to net the forging is—the closer the forging is to the final product, generally the stronger the wheel.
Because of the basic limitations inherent in forging, most forged wheels are two- or three-piece units. In two-piece construction, a center is forged and then welded or bolted into a spun or stamped outer rim. In a three-piece wheel, the center is bolted to an inner and an outer rim half. Three-piece wheels have the advantage of being easily customizable for a variety of widths and offsets. Crash damage in the form of bent rim outers can also be repaired.
Honestly, most any type of wheel (except wire) is strong enough for most types of racing. A good quality, pressure cast wheel, if made with the right material (T-6 aluminum), is plenty strong enough for a road racing car, and certainly for an autocross car.
The payoff in forged wheels comes in weight and durability. These racing wheels certainly cost more, but are generally stronger and lighter than an equally-sized cast wheel. Plus, their multi-piece construction allows for custom offsets and the opportunity to be repaired as needed.
And while we’re on the subject of repairs, we should mention wheel failure. Few things can be more catastrophic than a wheel coming apart on a race car. Wheels used for racing can be checked for cracks using inexpensive dye-penetrant kits available from racing suppliers or aircraft tool supply outlets. This treatment should be done seasonally at a minimum, and certainly after any incident that you feel could have damaged a wheel. Multi-piece wheels should also have their fasteners checked and re-torqued periodically.
All right, back to picking what’s best for you. Let your type of racing and vehicle be the judge. If you have a heavy GT1 Camaro, cast wheels are probably out of the question. Likewise, if you race a Miata, you may not see the gain that you had hoped for with expensive forged wheels. A quality wheel, properly maintained, will provide you with years of competent service on your race car. What’s that worth?
When you’re talking wheels, some generalizations between the different types can be made:
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