Wheels might seem like the most mindless part of your car to modify, but if you care about making whatever it is you drive any better, you've got all sorts of things to think about beyond how they look. Besides whether or not a wheel might bolt into place and fit, you've got to consider its weight and rigidity, shape and design, manufacturing process, and what sort of material it's made out of. It's all of those things that can mean the difference between you hard-parking your Civic at your next boba shop meet or chilling in a ditch on the side of the road because of some slipshod knock-offs that all of a sudden came apart.
The time to stop thinking of wheels as nothing more than some sort of aesthetic improvement is now. Switch to something lighter, wider, or of a different diameter and you just might've made one of the biggest improvements you could hope for. Get rid of enough weight and all of a sudden your car just got a whole lot faster. Size things up properly and get ready for increased mechanical grip at the tires.
Materials and Manufacturing
With the exception of heavy steel slugs you've got no business caring about, most wheels are made out of aluminum or some mixture thereof. Aluminum's relatively inexpensive and, when compared to steel, exhibits a higher strength-to-weight ratio. In other words, aluminum wheels can be strong and light.
Just how strong and how light they are, though, depends on how they're made. There are only a handful of ways wheels can be manufactured, the most popular of which are a couple of different types of casting processes, flow-forming, and the most expensive of all, which accounts for the sort of wheels that keep you up at night, forging.
The results of whatever material your wheels are made out of and how exactly they're manufactured will determine how well and how safely your car will perform. In the world of wheels, light weight and high rigidity wins; skimp on either and you've just passed up one of the simplest ways of making your car better.
Speaking of bunk wheels, you might think you're outsmarting science by wrapping a set of high-dollar R-comp tires around your knock-off wheels, but know that the better a tire does its job, the harder the wheel's got to work, which can result in premature failure and all sorts of collateral damage that will make the $500 you saved by skimping on rims not so big of a deal anymore.
Wheels can be cast in two ways: by means of gravity or through a low-pressure application. Gravity casting is the simplest and least expensive process. Here, molten aluminum gets dumped into a mold where it's spread, set, hardened and turned into a wheel. No forces other than the Earth's pull pack the aluminum into place, which means gravity-cast wheels aren't the strongest.
Low-pressure casting applies a vacuum onto the molten aluminum, packing its molecules into place, reducing impurities and resulting in a denser, stronger wheel. If whatever car you've got was originally sold with aluminum rims, chances are they've been low-pressure cast.
As it turns out, aside from being heavy, those OEM wheels aren't all that bad. Government regulations require original-equipment rims to be a whole lot stronger than they really need to be—at least twice as strong as recommendations for aftermarket wheels. Speaking of aftermarket wheels, here, regulations aren't enforced outside of places like Germany or Japan, which means you sourcing your next set from a reputable company with some sort of racing pedigree just got a whole lot more important.
According to Danh Vo of SSR Wheels, there are other advantages to cast wheels, like their ability to yield more intricate and detailed designs when compared to other manufacturing methods that result in more angular shapes. And thanks to computer-controlled manufacturing processes that companies like RAYS implements, for example, cast wheels can still be quite strong. According to Eddie Lee of Mackin Industries, the process, which features special water jackets that precisely control casting temperatures, reduces the sort of metal porosity that's inherent with casting and results in the kind of failures you've come to expect from cheap, knock-off wheel makers.
Also known as spun-rim manufacturing, flow-forming starts off like low-pressure casting but finishes with a whole lot of heat and a series of steel rollers that finalize the wheel's shape. What you end up with is something almost as strong as a wheel that's been forged but for a little less money.
In terms of strength and durability, forging can't be beat. Forged wheels begin life as hunks of billet aluminum that are heated up and compressed using all sorts of expensive machinery until they resemble a wheel. Billet aluminum is about as dense as it gets, which means more material can be removed, resulting in something extremely light but without sacrificing strength. The results can lead to a wheel three times stronger than something cast yet weighing as much as 25% less. The forging process also allows for less material to be used, which means larger brake components that typically wouldn't clear now do thanks to a potentially lower-profile design.
Finally, there's roll-forging, which starts with a rough-cast wheel and is pressed into its final shape while rolling. The process allows roll-forged wheels to use thinner materials, which can lead to even less weight.
Multi-Piece vs. One-Piece
Every wheel consists of a hub, some spokes, and a rim. Multi-piece wheels are made up of two or three sections that when assembled make up a complete wheel. In some cases, multi-piece wheels can be made up of components that each underwent different manufacturing processes. For example, a two- or three-piece wheel's center section can be cast or forged while its rim is spun. The advantage here is versatility. Three-piece configurations allow manufacturers to offer a whole lot more sizes that would typically be cost-prohibitive with casting. Because of the way multi-piece wheels are bolted together, they're often heavier than similar one-piece versions and sacrifice some rigidity.
You know that the lighter your car is the better it'll accelerate, stop, and do just about anything else you ask it to. But what you don't know is that lighter-weight wheels also improve your car's unsprung weight. We're talking about all of those pounds that aren't supported by the car's suspension, such as its brakes, tires, and wheels. Reduce unsprung weight enough and get ready for more communicative steering and better turn-in characteristics when pushing it to the limit. According to Lee, heavier wheels mean slower throttle response, longer braking distances, and an overall less-nimble steering feel.
You already know that offset determines where a wheel sits laterally, but what you're missing is just how crucial it is to whether or not your car'll handle like rubbish. Whoever designed your car's suspension went through all sorts of trouble to make it steer, turn, accelerate, and stop the way they thought it ought to. As it turns out, a wheel's offset has a lot to do with all of that. Alter it enough and you've just told the engineer who developed your car's suspension that the scrub radius he implemented isn't all that important to you. It should be, though. It's the scrub radius that allows for traction differences between your two drive wheels, which can keep you in a straight line and out of that ditch. Modest offset changes won't result in these sort of shenanigans, though, and can often be implemented guilt-free in hopes of fitting wider wheels and tires into place. Vo's advice? Stick to something as close to what the factory thought it ought to be in the first place in order to avoid rubbing, poor handling, and premature wheel hub failure.
Wheel Jargon Explained
Load Rating: This one's simple and reflects the amount of weight each wheel's capable of supporting. Find the number, which is typically expressed in kilograms, multiply it by four, and expect it to be a whole lot bigger than your car's actual weight. Be sure to account for any sort of aero mods that increase downforce—and the amount of weight applied to the wheels—as speeds increase.
Diameter and Width: It doesn't get any simpler than identifying a wheel's diameter and width—which is typically expressed in inches—than by peeping the identification sticker. Look for something along the lines of 16x7.0 or 18x9.5.
Offset: Now that you know why you ought to care about offset, look for the numbers following the "OF" or "ET" (German abbreviation for offset) on the sticker to determine what yours is. To keep all of this as complicated as possible, offset is typically expressed in millimeters, not inches, and may look something like this: OF +38.
Flange Contour: Someplace on the identification sticker you'll find a lone letter or three that, no matter how hard you try, will mean nothing to you but happen to represent the complex contour of the wheel's flange and whether or not a safety hump's been integrated into its shape.
Bolt Pattern: You recognize the series of numbers following the "PCD" but, until now, didn't realize that PCD stands for pitch diameter circle, or your wheel's bolt pattern.
Manufacturer-Specific Nonsense: Most stickers include even more information that's specific to whomever made your wheels, like letters or numbers that indicate their color, finish, or lug count.
Wheel Experts Q&A
SS: What are some shortcomings of OEM wheels?
Eddie Lee, Mackin Industries: OEMs err on the side of caution to mitigate potential lawsuits, so their wheels are generally heavy to ensure extreme durability and are manufactured using casting methods that lower production costs. In recent years OEM wheels have gotten more flush and feature more complicated designs, often taking inspiration from the aftermarket, but they're still generally heavier, thinner, and smaller than what the aftermarket offers.
Danh Vo, SSR Wheels: OEM wheels are typically narrower and smaller when compared to aftermarket wheels. A smaller diameter means you'll have a larger tire sidewall, which can flex more. Heavier weight also means there's more rotating mass than a lighter, aftermarket performance-oriented wheel would have.
SS: What sort of things should consumers consider when comparing aftermarket cast, flow-formed, and forged wheels?
EL: Price, but also consider how important technology is to you. Generally, cast wheels are cheapest, flow-formed are in the middle, and forged are most expensive, but that doesn't mean there aren't bad examples of each type. Also, what wheel makes you happy to see on your car?
DV: Budget, design, weight, and size. Cast wheels are the least expensive but won't be as strong or as light as a flow-formed or forged wheel. A wheel's design is important because it has such a dramatic effect on a car's look; get something that suits your taste. A wheel's weight and size is just as important, not just for aesthetics but for proper handling and overall balance.
SS: How big of an improvement can the right wheels make performance-wise?
EL: They'll make a huge difference. With a lighter, larger-sized wheel-and-tire combo you'll get improved lap times, better acceleration, better pick-up, better braking, nimbler handling, better fuel efficiency, and more grip.
DV: The right size wheels can affect handling and overall balance. Go too big or too wide and you'll add a lot of rotating weight, which will result in a heavy steering feel and slower acceleration.