Wheels and Tires: From Form to Function
Examining Wheel and Tire Design, Technical Aspects, and Trends
While you've read stories in past issues of OFF-ROAD that focused on the fundamentals of wheels and tires, this isn't one of them. If you need to brush up on backspacing, offset, sizing, fitment, and related tire and wheel basics, we suggest you check out "Wheels and Tires: A Technical Perspective," in the May '01 issue of OFF-ROAD. For this story, we've elected to focus on the fine points of wheels and tires, such as tread analysis, a look at wheel strength, the latest functional trends, and some of the secrets used to make wheels and tires perform to their maximum capacity.
The Wheel: An Introduction
Custom off-road wheels are truly a unique component on an off-road machine, since an alloy hoop can deliver a one-two punch of form and function better than any other mechanical part. And even though it may seem like there is little new in aftermarket wheels, dedicated wheel manufacturers keep pushing the envelope, producing stronger wheels at an increasingly lower cost. Furthermore, trick forged and bead-lock wheels previously found only on bucks-up race trucks and purpose-built 4x4s are becoming increasingly common on daily drivers and weekend warriors alike. And style? Yikes! The aftermarket has a slew of styles to fit any 'wheeler's taste, from the basic to the outlandish.
While we can't tell you which wheel will look cool on your truck, we can provide tips and tech regarding wheel strength and construction, and that's what follows.
Wheel Strength: The Strong Survive
The overall strength of a wheel is dependent on several factors, but the method by which the wheel is constructed is the most crucial to a wheel's strength. Construction refers to two basic segments: the way a wheel is formed and the way a wheel is assembled.
Most aluminum alloy wheels -- both OE and aftermarket -- are of one-piece or two-piece construction. All steel wheels are two-piece in design. Although there are three-piece wheels available on the aftermarket, they are not commonly seen on off-road machines. A one-piece wheel is just that -- a single piece of aluminum shaped (either by casting or forging) into a wheel. A two-piece wheel uses an inner section welded to an outer rim. On alloy wheels, the inner section can be cast, billet (cut from a large piece of aluminum), or forged. Steel wheels use an inner section that's stamped on a press and then is welded to a formed steel rim.
The casting process used on aluminum wheels is typically one of two types: low-pressure/gravity casting or counter-pressure casting. Low-pressure casting takes place when molten aluminum is poured directly into a mold shaped like a wheel and allowed to cool. When the mold is opened, the wheel is removed, machined, and powdercoated, polished, or chromed, detailed, and it's finished.
Counter-pressure casting draws molten aluminum into a mold under high vacuum pressure, which makes for a stronger wheel because the pressurized aluminum is less porous and of a higher density (tighter molecules) compared to a gravity-cast wheel.
The strongest overall aluminum wheel is a forged version. During forging, a billet (piece) of aluminum is forced (forged) into the shape of a wheel by a huge press using tremendous pressure. Forged wheels can be one-piece or two-piece with a centersection welded to an outer rim.
The Hub-Centric Concept: Well-Centered Wheels
An increasing number of new wheels feature a design element known as hub-centric. On a non-hub-centric wheel, the lug nuts perform double-duty as they pull the wheel into contact with the wheel hub and also provide support to the wheel in the vertical plane. In other words, the lug nuts support the entire weight of the vehicle. With a hub-centric wheel, the large center hole is precisely sized and has a chamfer where it meets the wheel hub. When the wheel is installed and the lug nuts are tightened, the raised centersection of the hub fits tightly into the chamfered hole of the wheel. With a hub-centric design, the lug nuts don't support any of the vehicle's weight; the wheel is supported in the horizontal plane by the precision fit between the hub and the wheel's center bore.
Bead Locks: Positive Tire Retention
If there's one must-have accessory for 2002, it's a bead lock-equipped wheel. The theory behind a bead lock lies in its mechanical retention of a tire. On a non-bead-lock wheel, the tire's bead is retained on the wheel by the wheel's bead seat (the area between the outer wheel lip and the raised inner hump) by rubber-wrapped steel cables within the tire's bead and by the tire's internal air pressure. During most on- and off-road driving, this setup suitably retains the tire.
However, when a tire is aired down (less than 15 psi) for max on-dirt traction, the tire may move laterally on the wheel's bead seat, resulting in a loss of air pressure. When the tire loses pressure, the tire bead can slip entirely off the wheel.
Bead locks eliminate the tire's bead movement through the use of clamping pressure. A bead lock is basically two aluminum or metal rings. An inner ring is welded to the wheel in place of the outer bead lip (which is machined off); while the second (outer) ring is attached to the inner ring with bolts, thus positively clamping the tire's bead between the two rings. With the tire thus retained, lateral bead movement is eliminated, and the tire will remain on the rim even with zero air pressure in the tire.
OMF Bead Support Rings
If you've ever experienced a bent wheel, you can appreciate OMF's bead-reinforcement rings. Once these alloy rings are welded in place within a wheel's outer bead lip, damage from impacts with rocks and other harder-than-aluminum objects is eliminated in most cases. The thick reinforcement rings are precision-milled, are drilled for light weight while retaining maximum strength, and are available to fit 15-, 16-, and 17-inch-diameter wheels. The satin finish billet alloy rings can be polished after they're welded in place, if you're after a custom look.
The Tire: Beyond Basics
Every type and style of off-road tire derives its performance from its fundamental design and construction. As you would expect, not every tire is the same. Depending on what type of performance and terrain any particular tire is designed for, there are variations between similar-appearing tires regarding the number of plies, the depth and style of the tread, the basic carcass construction, and whether or not the sidewall is designed to provide traction under certain conditions. That said, here's an overview of what makes a rugged, traction-enhancing off-road tire.
Tire Foundation: Strength and Flexibility
Since a tire's construction incorporates many segments, the best way to understand the various tire elements is with an examination of a tire cut-away. This Mickey Thompson Baja Claw Radial represents state-of-the-art design and construction. The basic carcass is constructed with two steel belts for strength and stability (A). On top of the steel belts are twin stabilizing belts of polyester construction (B). The Baja Claw's tread is where the trick design elements are on display. Note the directional tread (C), which is angled at 23 degrees for enhanced traction and a self-cleaning effect. Also note the spacing of the tread's lugs: The space (pitch) between each lug varies, which reduces tread noise. The tread also features circumferential grooves (D), which channel water away from the tread, reducing hydroplaning. Finally, M/Ts Sidebiter technology places a tread on the sidewall, which provides additional traction when the tire is aired down and overall protection for the sidewall against puncture from contact with obstacles (E).
Built-In Bead Protection by BFG
If you've done much 'wheeling, you probably realize that the bead area of a tire and wheel are always potential trouble areas. Because of the location of the wheel and tire beads, they're subject to damage from encounters with rocks and tough terrain. Earlier, we looked at bead-lock wheels and wheels with bead-reinforcement rings as two useable methods for keeping the tire and wheel intact. Another small but effective shield against damage to a wheel's outer bead lip is BFGoodrich's Rim Protector, used on BFG's Radial Mud-Terrain T/AKO (arrow). Essentially, a thick piece of rubber that extends outward (laterally) away from the wheel and tire bead area, the Rim Protector provides an extra cushion against the damage inflicted by rocks and other solid obstacles. While it's hardly high-tech, the BFG Rim Protector is simple, reliable, and functional.
Tread Talk: Fancy Footprints
Looking at the various tread designs available on the aftermarket is guaranteed to confuse even the best-informed enthusiast. Each manufacturer has its own unique tread style, and each design is supported with plenty of reasons why it's the best for producing traction. Short of testing all the tires on the market, an enthusiast can make a fundamental tread decision based on projected use, available size, and cost. Basically, your choices are between radial or bias-ply construction and between mud-terrain or all-terrain tread. Most of the experts we talked to recommend shoeing your ride with an aggressive tire; the thinking is that you should always be prepared for the worst. When things get muddy, an all-terrain tread won't cut it and you'll get stuck. The opposing view is that all-terrain tires have become better in various types of terrain and are even capable in light-to-moderate amounts of mud. All-terrain tires are a compromise, no doubt, but with a locker or limited-slip in the rear axle and an air locker in the front diff, an all-terrain tire-shod 4x4 can make serious headway across rugged terrain.
BFG's Radial All-Terrain (left) and Radial Mud-Terrain (Moab Edition shown) are possibly the most familiar off-road tires on the planet, and both use well-engineered tread elements for impressive traction. The All-Terrain tread features interlocking tread lugs, which stabilize the center tread section for improved control during acceleration and braking, and an open-shoulder lug that channels water away and sipes across each lug to improve wet-weather performance. The Mud-Terrain tread (right) also uses interlocking tread lugs, but in a much more open, high-void design. As with the All-Terrain tread, the interlocking lugs stabilize the tread during acceleration and deceleration. The Mud-Terrain's computer-designed tread features a pattern that is both self-cleaning and quieter than previous Mud-Terrain tires, thanks to staggered spacing between the shoulder lugs.