Most off-roaders realize how vital wheels are. Choosing a wheel based primarily on styling might be tempting, but doing some homework can reveal the best supporting cast for your vehicle.
Terminology
Wheelspeak is popular among four-wheelers. Whether reminiscing about old favorites like'80s Western turbine-style wheels or gawking at (mocking?) the latest creations from the dub world, offsets and bolt patterns are part of normal 4x4 conversation. Here is a quick review of standard wheel terms.
Diameter: The straight-across bead-seat-to-bead-seat distance-not the outer-edge distance.
Width: The distance from the front bead seat to the rear bead seat-not the outer-edge distance.
Bolt pattern: The number of bolt holes in addition to the distance from the centerline of one hole to the center line of the one farthest from it.
Backspacing: The distance from the wheel's outer edge to its mounting pad.
Offset: The relationship of the wheel's mounting pad to its centerline. Zero offset is when the pad is centered on the wheel. Positive offset is when the pad is outboard of the centerline (narrower track width). Negative offset is when the mounting pad is inboard of the centerline (wider track).
Load rating: The maximum weight a wheel can safely support. The sum of all four wheels' load ratings should exceed the vehicle's gross vehicle weight rating (GVWR) with some cushion to spare.
Construction
4x4s and steel wheels have a long, glorious history together. Although steel doesn't win many style points, it is generally strong, durable, and repairable. On the trail a deformed steel wheel that loses bead contact can often be hammered back into air-holding position. Rust can be the main drawback to steel wheels, which are normally two pieces: a rim that's welded to a centersection.
Alloy wheels are more upscale than steel, and many wheelers like the unsprung-weight advantage of alloy. According to Center Line, a 1-pound reduction in rotating mass equals an 8-pound static-weight savings. Lighter wheels also improve braking, acceleration, and fuel economy.
Materials and construction methods vary. "Aircraft-quality" A356-T6 aluminum is generally the primary metal, often combined with other nonferrous (nonsteel) metals to create a balance between lightness and load capacity. Exotic metals like ultralight titanium and even composites such as carbon-fiber are used for some racing and high-end sports car wheels. (Magnesium is the origin of mag wheel, a term that has become generic for stylish alloy [aluminum] wheels. True magnesium wheels are big-dollar racing equipment.
Alloy wheels are cast, forged, or CNC-machined from an aluminum billet. Low-pressure casting involves pouring molten metal into a mold. This is the most cost-effective method, but it also introduces pores and impurities into the finished product. Counterpressure casting uses vacuum to minimize the pores, creating a stronger wheel.
 Wheels generally consist of...  Wheels generally consist of a center section and a rim. Note that diameter and width measurements are bead-to-bead, not edge-to-edge. |  Backspacing is actually measured...  Backspacing is actually measured from the outer edge of the wheel's inboard rim to its mounting pad. If the mounting pad is centered widthwise, the wheel has zero offset. Offset is the relationship of the mounting pad to the wheel's centerline. A pad that's outboard of the centerline gives the wheel positive offset; negative offset is when the pad is inboard of the centerline. |  Most truck tires are centered...  Most truck tires are centered by the wheel studs (as opposed to the "hubcentric" centering of classic British sports cars and others). Since the studs and lug nuts center and secure the wheel to the vehicle, proper nut style, star-pattern tightening, and following the wheel manufacturer's recommended fastener torque specs are critical. |
Cast wheels are identifiable by the rough finish of their unmachined areas. They're generally the most affordable alloy wheels. However, casting makes the wheels more brittle than other manufacturing processes do. Also, some manufacturers "test" their cast wheels by computer simulation only, passing the R&D savings onto the customer. If an alloy wheel price sounds too good to be true, check to see if it's actually DOT-certified and covered by a warranty, and read the online enthusiast bulletin boards to see how the manufacturer stands behind the product.
Forging uses high pressure to form solid alloy. The results can be three times as strong and 20 percent lighter than comparable-sized cast-aluminum wheels. Hot forging keeps the metal at a specific temperature while a press shapes it, sometimes realigning the aluminum molecules in a circular pattern to add strength. Cold forging retains more of the alloy's original grain, using only enough heat to make the metal malleable for forming.
Billet wheels generally have two pieces, with only the centersection actually CNC-machined from a single chunk of alloy. Alloy wheels can be one, two, or three pieces. One-piece wheels can be created by casting or forging. Two-piece wheels use bolts, rivets, or welds to join the centersection to the rim. Three-piece "modular" wheels consist of outer and inner rims that are bolted to a center. This often allows greater offset flexibility.
Three-piece wheels typically have rubber sealing gaskets that sometimes get damaged during impact or tire mounting. These wheels can be serviced, and the individual pieces can often be repaired or replaced as opposed to having to buy a whole new wheel.
The Hollander Wheel Interchange...
The Hollander Wheel Interchange Manual is valuable for used-wheel shoppers. It isn't cheap but could be a worthy investment for a 4x4 club whose members wheel-shop at junkyards or on eBay and Craigslist.
Fitment
In recent years, wheel diameters have grown proportionately to baseball players' biceps. The hardcore trail crowd used to balk at anything larger than 15 inches, claiming that "plus-sizing" yielded lower-profile tires lacking the necessary sidewall flex for optimal off-road performance. But as OE wheels have grown and tire technology progresses, larger diameter wheels are becoming more accepted for trail and competition. Their primary functional advantages are caliper clearance for high-performance disc brakes and additional room for knuckle bosses/tie-rod ends.
Spyderlock beadlocks president Clifton Slay elaborates on the big-wheel trend: "I first ran 18s and 20s in 2003 as part of Nitto's push to be the first to market in this trend that was trickling up from the street and show truck market. Beyond the cool Hot Wheels look, there was never a performance gain realized until you surpassed a 38-inch tire for most off-road situations. Wheelers are starting to discover that an 18 or 20 can be an excellent balance of sidewall flex control and ride qualities for 40-inch-plus tire applications. These wheel sizes will continue to gain commonality as people and tire manufactures understand their advantages."
Slay also says that 16-inch wheels are becoming an endangered species. "Tire companies are not building progressive 16-inch tire designs anymore, at least that a wheeler who requires beadlocks would look for," he says. "17 and 15 inches are my most-requested wheel sizes. 17 inches has the most choices in off-road tire styles, as this size is viewed as the most commonly desired for dirt enthusiast and competitors."
Competent tire stores and the mail-order retailers who advertise here are good sources for hits/fits information. When buying used, however, you might have to rely on word of mouth and your own trial fitment. While bolt patterns might be the same, bore centers and backspacings might not make wheels a direct interchange. For more information, refer to the Hollander Wheel Interchange Manual.