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4x4 Off Road Truck Tire Basics

Posted in How To: Wheels Tires on November 1, 2008
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We could speak volumes about tires, the technology behind their construction, tread design, and how design affects on- and off-highway performance. We can't possibly detail every aspect of the modern tire in a single article, so we'll just focus on the finer points of fitment and function. We are asked almost daily, "What's the best tire?" Our answer is a question, "What are you going to use the tire for, and what kind of performance do you expect out of it?" Just like a comfortable pair of shoes, tire selection sometimes comes down to personal preference. Sure, our staff has its favorite tires, and we definitely applaud modern tire technology and its higher quality and construction. We know that some tires work better than others in certain types of terrain, but a tire one of us may like, the other may hate.

There are numerous factors to consider when choosing tire size. Extreme off-roading almost always requires a taller tire with an oversized footprint for better traction and ground clearance. It's not rocket science-the more ground clearance between the lowest point of the vehicle and the ground, the more places the rig will be able to travel. The drawback is that the larger the tire, the more suspension and body modifications are needed, and that can get expensive. Let's also not forget that too tall of a vehicle dangerously increases the rig's center of gravity, rendering its off-highway performance ineffectual for real world four-wheeling. In our opinion, for all-around wheeling, if oversized tires can properly be stuffed in a vehicle's wheelwell with the least amount of lift, the vehicle will not only be safer on the trail, but more capable and agile.

While some tires are great for all-around use and work exceptionally well on- and off-highway, others are purpose-built for specific types of terrain. Some off-road tires feature more complex designs than others, but the bottom line when it comes to performance is its direct contact with the road surface. This is called the contact patch. There are generally three key segments of a tire when it comes to overall traction and performance: lugs, void, and sipes. We all know that the tread is the single most critical aspect relating to a tire's overall performance. Large, open areas of a tread are called voids and are used to channel away mud, water, and dirt. Tires with a mud-terrain tread use large lugs for traction and have high-void content. Tires with an all-terrain tread design use small lugs and less void area than a mud-specific tread pattern. The small slits cut into some tires' tread lugs are termed sipes and are designed to route away water from the tread as it becomes trapped between the road surface and the tire's tread.

There are two types of tire construction: radial and bias-ply. The main body (carcass) is constructed using nylon body cords encircled by nylon or stainless steel tread-stabilizing belts (plies). The direction that the carcass cords are arranged is what determines whether a tire is a radial or a bias-ply. The majority of off-road tires are steel-belted radials, which have dominated the market for several years, but it wasn't too long ago when most tires 40 inches and taller were bias-ply. There are still plenty of popular bias-ply tires like the competition-proven Creepy Crawler M8090 from Maxxis, the Mickey Thompson Baja Claw, and tires from Interco, especially the Irok.

Radial Tire Construction
The main reason for the radial's popularity and increased performance lies in its design, which uses body carcass cords that run radially-from bead to bead-in a 90-degree angle to the tread's centerline. The benefit of radial cords is the reduction in deflection at the tire's shoulder and sidewall segments. The curve of the radial cords creates lower rolling resistance, which reduces tread wear. The stabilization belts-normally of steel cord construction-stabilize the tire tread against excessive movement and also greatly add to the radial's puncture resistance.

Tough Sidewalls
We can't talk about tire belting and toughness without mentioning sidewall belting. Obviously, the more belts the tire has on the sidewall the more puncture-resistant it will be. Remember that while four-wheeling most of us air down our tires for a larger footprint (contact patch). The tire will conform better to uneven surfaces, giving it more traction. Keep in mind that the more belts on the sidewall, the stiffer the tire may be, and even when it's aired down, the footprint could remain the same, lessoning its traction capabilities.

Bias-Ply Tire Construction
On a bias-ply tire, the carcass is formed with nylon (fabric) cords that run in a diagonal path from bead to bead. The cords-there are two groups-cross over at a typical angle of 34 degrees to the tread centerline. Consider the fact that the long (compared to radial cords) bias cords allow for flex of the tread area, which brings the negative effect of increased rolling resistance and a resulting decrease in tread life. One benefit of bias-ply tires is their higher load rating compared to radial tires. The bias cords-used without stabilizing belts-are thicker, which makes for a tire carcass with very stiff sidewalls, meaning increased load capacity.

Mud Tires
Another question you might be asking is, which tire works better: an all-terrain or a mud-terrain? Many factors come into play, including personal preference and driver skill. Sure, a typical mud tire works better in the mud, and an all-terrain tire has better street manners, but does one or the other truly offer better performance on general trail rides? Mud-terrain tires will outperform all-terrain tires in thick mud almost every time. The small voids on the all-terrain tread quickly fill up and pack with mud. This causes them to lose traction almost immediately. Mud-terrain tires have the ability to shed mud through their larger voids, helping maintain traction. Some mud tires like the BFG Mud-Terrain KM and the Cooper Discoverer STT have a stone and debris ejector rib in the void between the lugs to help keep them clean.

Street Performance
Traditionally mud-terrain tires have a tendency to create more road noise, but some of the newer, more expensive tires like the Toyo Open Country M/T have a special computer-designed tread pattern that offers great off-highway traction while delivering a quiet highway ride. If you are driving a mud-terrain tire on the street, pay careful attention to air pressure and keep them properly balanced. Once a mud-terrain tire is out of balance and worn unevenly, they'll create a deafening road noise that's pretty cool to most, but can get annoying to some. Most all-terrain tires with smaller voids have a tendency to be less noisy-this also means there's more rubber in contact with the highway, which offers better braking characteristics. All-terrains also offer less rolling resistance, which can lead to better fuel economy. They usually offer longer tread life and deliver more mileage than a mud-terrain before they're worn out.

Sand Wheeling
The vote is still out on mud- versus all-terrain tires in deep soft sand, at least among the staff here at the magazine. The general theory is that when properly aired down, an all-terrain tire will float across soft sand, while a mud-terrain tire will have a tendency to dig itself in. However, we have had exceptional success wheeling in soft sand with mud-terrain tires. The key to successfully negotiating sand is not to stop on inclines or places that look like you might get stuck. Also, don't stop abruptly, and don't start off by flooring it, the tires will dig in and bury themselves.

Rock Traction
There is an exception to every rule, but we have proven that mud-terrain tires perform better and last longer in harsh rocky conditions. This is due to the fact that the larger tread lugs with high voids offer a better bite and claw their way up and over uneven rocky surfaces. Mud tires also last longer in harsh conditions because there's more rubber designed into the tread. Most mud tires also have side lugs, which offer additional traction and protection when the tire is aired down. Nitto Tire, BFGoodrich, Pro Comp Tires, and a few other companies offer specially constructed competition tires that are made with a sticky compound for exceptional traction above and beyond most other tires. They are, however, designed for off-road use only and usually sold only through the tire's race programs.

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