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.
For rock crawling, it is a toss-up. The tread design and ply angles will make a difference for both all-terrain and mud-terrain tires.
Tire Size and Off-Road Performance
Larger-diameter and wider–tread width tires have a larger rubber contact patch on the ground, especially at low tire pressures. This improves traction. Since taller tire sidewalls can be aired down with a greater reduction in sidewall height, ride quality is better. The downside of larger tires is increased brake and steering system wear. Fuel economy also takes a negative hit.
Sipes are the thin cuts in the tread blocks. They allow the tread blocks to flex and provide additional edges to help grip the surface. They are most effective on hard surfaces such as rocks and ice. The tire on the left has considerable siping. Siping is limited on the tread design of the tire on the right. Notice the small ridges in the tread between the tread blocks. These ridges help eject rocks and other debris from between the tread blocks.
Sidewall lettering can provide a wealth of information. This Falken WildPeak AT03 has a load range E rating. This is the equivalent to a 10-ply tire. The load range and sidewall design of this tire make it very durable against punctures.
The Nexen Roadian MTX uses a moderately stiff sidewall. The sidewall has good flex. The tire pressure is 12 psi. Grip is good, allowing the tire to comply with the terrain, which improves traction.
Tire Compliance Over Rocks and Obstacles
If you plan to use your YJ for serious rock crawling or steep hill climbs and descents, especially with big ruts, then tire compliance of the tread over road surface irregularities is extremely important. Tire compliance is the ability of the tire tread and sidewall to conform to the shape of rocks, road surface irregularities, or other obstacles. Both sidewall stiffness and tread design are factors. Tire pressures are key, but tire design plays a factor in how low a pressure you can run, especially without beadlock wheels (see the wheel section for an explanation of beadlock wheels).
Tread Block and Sidewall Design for Off-Road Performance
Tread block design is a critical factor for traction in all off-road conditions. Some tread block designs work more effectively in some conditions, such as hard surfaces and rocks, while others perform better on soft surfaces of sand, mud, and snow. Soft surfaces require more forward grip for braking and acceleration, while hard surfaces need forward grip but also need lateral grip or side bite.
Lateral grip is where sidewall design is really important. Sidewall tread patterns can help hold the vehicle on side slopes and the sides of rocks. This is important in rock crawling and in heavily rutted trail sections where only the sidewall of a tire may be gripping the side of a rock, rut, or bank.
Sidewall grip plays a critical role in off-road driving situations, especially in extreme rock crawling. Without adequate sidewall grip, tires can slide off rocks and side slopes often, causing the vehicle to become stuck or worse. The Nexen Roadian MTX mud-terrain tires feature a unique sidewall tread pattern that is different from side to side. Both designs offer great grip.
Siping
Sipes are thin cuts in the tread of a tire. They allow the tire tread to separate slightly, which improves traction on ice and snow. For off-road use, the siping can improve traction when rock crawling. Depending on the siping pattern, they can also increase lateral grip, helping to hold the tire sideways on rocks, side slopes, and ruts.
Lateral Grooves and Crosscuts
Lateral grooves and crosscuts are similar to siping, but they are larger. This allows tread blocks to flex and grip soft surfaces and uneven terrain more effectively. At lower tire pressures, grooves and crosscuts can flex to help expel snow from the tread voids for better traction.
Forward and Side Grip
Tread and sidewall design play a major role in how a tire grips in low-traction situations. Good forward traction is critical for climbing and descending large rocks and hills. Side traction is crucial for holding on to side slopes, ruts, bumps, and rocks. Tire designs are a compromise. Different designs are better in certain off-road situations.
The Milestar Patagonia M/T tire is available in a wide range of sizes from 31-inch diameter in a 15-inch wheel and a 16x32-inch size that will work on the YJ with a slight lift.
The Milestar Patagonia M/T tire features an aggressive sidewall design and unique tread pattern with a fairly low void ratio for an mud-terrain tire.
Nexen Tire is new to the off-road market with both an all-terrain and the Roadian MTX mud-terrain tire. We spent five days comparison testing the Roadian MTX about a year before the tire was available to the public, and we were very impressed with all aspects of its performance.
The Nexen Roadian MTX features a more traditional mud-terrain tread design with effective siping and stone ejectors. Snow was the only condition we were unable to test