Jeep Wrangler YJ 1987-1995. Don Alexander
is better, an all-terrain or a mud-terrain tire? Which side of the fence are you on? While there has never been a clear winner in this argument, there are many factors to take into account: highway versus off-road miles, weather conditions, terrain, and road surface are just a few. But with the launch of a new off-road tire category, called by some the rugged terrain (RT) or extreme all-terrain, the line between all-terrain and mud-terrain is blurred. These new-generation all-terrain or RT tires offer more aggressive tread patterns and sidewalls while retaining a smaller void ratio for reduced noise on the highway and harder rubber compounds for better tire wear.
Tread Design Differences
The most obvious difference between the all-terrain and mud-terrain tire lies in the tread design. A mud-terrain tire has more aggressive tread blocks and a larger void ratio. The void ratio is the percentage of the total tread block area versus the area of the total tire tread. The tread blocks on a mud-terrain tend to be larger and thicker than an all-terrain tire.
The thin slits in the tread, called sipes, allow the tread to flex and the edges to better grip hard surfaces such as rocks. Sipes are used in both all-terrain and mud-terrain tires. Generally, an all-terrain tire will have more sipes, which improves grip on hard surfaces. The new, more-aggressive all-terrain tires create as much or possibly slightly more grip than the same size mud-terrain tire.
While a good mud-terrain tread design can keep rocks from lodging between tread blocks, the design must also eject mud and snow. Some designs do this more efficiently than others. In general, the larger void ratio of the mud-terrain is better in soft surface conditions, but the tread blocks need to flex to eject snow and dense mud.
Tread design plays a role but so does tire pressure. Lower tire pressure helps when the voids become filled with debris, ice, snow, or heavy mud. And some all-terrain tread designs are better in snow but lack the ability to keep small stones out of the tread voids.
Sidewall Design
One of the most important elements of tire design is the structure of the sidewall. When four-wheeling on rocks, in ruts, or on side slopes, it can be critical for the sidewall tread to have good grip. A lack of grip means the tire can slide sideways off the edge of a rock, the slope on a side hill, or within a series of ruts. Slipping can alter your desired path or cause you to bang a rock slider, skid plate, or bumper on a rock that you thought you could avoid.
The tire sidewall needs to be able to flex to conform to the road surface and to have a design that allows rocks and snow to be ejected from the tread pattern. Most off-road tires 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