How that alloy is made into a wheel makes up the next category. Two different processes are used to make wheels or wheel components: casting and forging. To cast a wheel, molten aluminum is poured into a mold and allowed to cool, which forms the wheel's basic shape. When it comes out of the mold it's machined to remove excess aluminum, cut in the bolt pattern and hub circle, and sized to the mount pad. Then the wheel is finished by polishing, painting, plating, or some combination of the three. With forging, aluminum billets are pressed into dies under extreme pressure, which shape the billet into the wheel. That wheel then goes through a machining/finishing process like the cast wheel.
In general terms, forged wheels are stronger than cast, since forging doesn't alter the aluminum's molecular structure like casting does. Since the forged aluminum is stronger, less of it can be used without sacrificing structural integrity, which can ultimately result in a lighter wheel. Forging is not as design-friendly, however, as it's hard to create curved spokes and intricate wheel-center designs in the forging process. So you'll notice that most forged wheels have straight, flat spokes. Forged wheels generally cost more too, as it is more expensive than casting.
This doesn't mean a cast wheel isn't strong or can't be lightweight. It's just that the casting itself requires more aluminum, to make sure the metal is dense enough throughout the mold's nooks and crannies. Using more metal adds weight, though some companies, like RO_JA, are experimenting with new casting techniques to shave metal from the wheel and achieve forging-like weights. RO_JA's new 17x7 Neo LT 5 wheel, for example, tips the scales at a svelte 15 pounds, whereas cast wheels of that size typically weigh 20 pounds and more. Casting also allows more design freedom, as the molten aluminum will follow the contours of intricate centersections.
Some wheels bring together both worlds, as they combine a forged outer hoop with a cast center. Which brings us to the topic of one- and two-piece wheels. A one-piece wheel is just that: a single unit, with its center and outer rim created together. One-piece wheels can be cast or forged. Two-piece wheels are made from outer rims and center sections that are separately produced, and the two pieces are welded or riveted together to form the wheel. With two-piece wheels, the rims are generally spun or forged, and the centers are cast or forged.
Each wheel has its advantages. One-piece wheels are generally less expensive to make and are produced for high-volume applications (with an offset that will fit many different cars, for example), so they typically cost less than a comparable two-piece wheel. But a two-piece offers greater fitment flexibility, as the centersection can be welded in a number of places within the rim to create a variety of offsets. This is especially helpful when trying to fit a car where the front and rear offsets don't match, like an S2000, or when you need an ultradeep offset to fill a widebody kit's fender well.
Offset is a difficult concept to describe, so maybe this illustration will help. Offset is
Wheel Sizing
Talking about offset brings us to the subject of wheel sizing. There are several dimensions to consider when upgrading your wheels: rim diameter, rim width, bolt pattern, and offset. Rim diameter and width are easy to understand. The bigger the better, right? Well, sort of. There is a finite amount of space in your wheelwell to fit new rims and rubber, and if you've slammed the car there's even less. You don't want to go so big that the tire or wheel makes contact with the fenderwell, steering, or suspension components. Remember, too, from our discussion of plus-sizing that wheels need to get wider as they get taller to maintain the tire's load capacity, and that extra width can interfere with the chassis.
By Drew Hardin
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