do a better job of keeping tread voids cleared when aired down (tire pressure lowered to a lower pressure).
An additional factor affecting off-road tire performance relates to the cord angles in the sidewalls. A small change in cord angles can affect the sidewall stiffness. While this is a factor on all Jeep models, it becomes more critical on lightweight Jeeps such as the Wrangle YJ. If the sidewall is stiffer, the tire must be aired down to a lower pressure to achieve the same spread of the tire contact patch.
The void ratio is the area of tread blocks versus the area of the gaps between the tread blocks (white in the illustration). All-terrain tires use a small void ratio, meaning more rubber is on the ground. Mud-terrain tires use much larger void ratios, meaning less rubber is on the ground but there is more ability for the tire to grip a surface, especially soft surfaces such as mud, sand, and snow. Larger void ratios also create more noise on the highway.
The weight of the vehicle, or more specifically the weight resting on each tire contact patch, is also a factor. Where one tire may achieve the ideal tire contact patch spread at 14 psi on a YJ, another tire may need to be aired down to 10 psi for the same result. A pressure of 10 psi is marginal for a non-beadlock wheel. We have aired down Wrangler JKs weighing more than 6,000 pounds to about 5 psi with beadlocks to achieve ideal pressures for snow, mud, sand, and icy conditions. The sidewalls on those tires were very stiff. To get the same contact patch spread on a 3,500-pound YJ may require airing down to about 3 psi.
Rubber Compound and Wear
All-terrain tires tend to have harder rubber compounds than mud-terrain tires, so the mud tire will wear more quickly with everything else being equal. Even though the all-terrain is harder, it has more rubber on the road or trail surface due to a smaller void ratio. This usually means the all-terrain tire will have slightly better traction on hard surfaces given equal tire sizes. The mud-terrain will grip better on soft surfaces. This is best distinguished when the tire is operating “in” a surface as opposed to “on” a surface. Think “in” sand, mud, snow, and soft, loose dirt versus “on” asphalt, ice, or hard-packed dirt.
Top left: The BFGoodrich All-Terrain T/A KO2 tire has one of the more aggressive sidewall designs. The siping allows flex for better grip. The ridges between the sidewall tread blocks are stepped to help eject rocks and debris from the sidewall. Top right: The Mickey Thompson Baja ATZ P3 all-terrain tire has a sidewall design more like a mud-terrain tire. This sidewall design grips very well on rock edges and the sides of ruts where little or no tread is gripping the surface. Bottom left: The Falken WildPeak AT03 has a very aggressive sidewall for an all-terrain tire. The stepped ridges on the upper part of the sidewall progressively grip the edges of rocks and slopes for better grip. They also dig into soft surfaces for even more bite. Bottom right: This all-terrain tire, the Nexen Roadian AT, has a conservative sidewall design, but the triangular-shaped scallops on the lower portion of the sidewall provide a surprising amount of grip on loose dirt and large, sloped rocks.
Types of Off-Road Surfaces
When off-road, the advantages of the all-terrain versus the mud-terrain tires is less clear. In some conditions, the mud-terrain tire provides a slight advantage. But the differences are minor. The big issue is the type of surface.
Exceptional tire compliance allows for much easier rock crawling by sticking to a slanted rock. This rock has a 60-degree slope. Most of the tread is in contact and the sidewall is contributing some grip as well.
Hard Surfaces
For the most part, the all-terrain tires are better on hard surfaces. While the rubber compound is a little harder, the void ratio is smaller, meaning there is more rubber on the ground for equal-size tires at similar pressures. The advantage diminishes when the surfaces are wet.
For hard dirt or rocky dirt surfaces, the difference between mud- and all-terrain tires is very small. For sand or soft, loose dirt, mud-terrain tires will give better traction. For wet dirt and mud, a mud-terrain tire is best.
Soft Surfaces
Due to deeper, larger tread blocks, the mud-terrain tires perform at their best in soft surfaces. Mud, sand, loose dirt, and snow are conditions giving the mud-terrain tire better performance. In certain types of snow, especially in slushy, sticky snow conditions, the mud-terrain has an advantage only if the tread design (and lower tire pressures) are able to eject the snow from the tread voids. ■
Tire sidewall construction plays a major role in determining how well a tire conforms to the surface. Even though the Nexen Roadian MTX mud-terrain tire features an F load range (meaning the sidewalls are very puncture resistant), the sidewall ply angles allow for a very flexible tire. This 37-inch tire is at 14 psi on a 17-inch beadlock wheel. Most tires would need a pressure of 10 psi or lower to enjoy this much tire deformation on the rocks. At 14 psi, the Nexen does need to be on a beadlock wheel. Keep in mind that most newer mud- and all-terrain tire designs are intended for heavier rigs similar to the Wrangler JK. The lighter YJ would need to run a much lower pressure to allow good tire conformity. On most current mud-terrain tires, pressures well below 10 psi would be needed, which means investing in beadlock wheels.
Tire Size, Diameters, and Ground Clearance
Increasing tire diameter allows more ground clearance and larger contact patches on the ground. Keep in mind that the low points under any Wrangler YJ are the center sections of the axle housings and the leaf spring perches for non-leaf-over spring (stock) suspensions. Spring under lifts raise the chassis/body but not the axle housing. A suspension lift is installed mostly to gain clearance for larger-diameter tires. So going from the stock 29-inch tire in the YJ to a 33-inch-diameter tire will increase ground clearance by 2 inches.
Increasing tire sizes increases tire weight. Off-road tires weigh a lot. The tread is the heaviest part of the tire and it is farthest from the center of rotation, which means all the weight when rotating (called rotational inertia) creates very large loads on ball joints, tie-rod ends, and suspension components. The inertia caused by weight away from the center of rotation increases as rotational speed (RPM) increases, as the distance from the centerline increases, as well as when the tire weight increases.
Worn steering components lead to shimmy and death wobble on the solid axle of a YJ. Increasing tire size requires upgrading key steering components to ensure performance and safe operation. Additionally, as tire size goes up, it is crucial to have tires dynamically balanced (see the wheel/tire balancing section later in this chapter) to reduce the possibility of shimmy or wobble.
Advantages and Disadvantages
On the highway, all-terrain tires offer better handling and steering feel. Mud-terrain tires have more squirm due to larger tread blocks. As for road noise, all-terrain tires are quieter; so are the new generation of extreme all-terrain tires (RT).
If the surface is loose sand, gravel, or small rocks, the mud-terrain tires are best. All-terrain tires are better on harder surfaces. In rain, snow, and ice, all-terrain tires have an advantage with some mud-terrain tires being better in some types of snow.
