The tread pattern is designed to produce varying degrees of traction, cut, flotation, wear and heat resistance. So selection of the proper Off-the-Road tire depends on the job and the conditions. For example, different tread patterns are used to produce maximum traction or flotation on sand, mud and rock. There are five basic tread patterns: rock, traction, block, rib and smooth.
The rock pattern is specially designed to prevent cuts caused by sharp rock. Its large ground contact area provides maximum contact area provides excellent wear resistance. Characteristic grooves running across the direction of the travel mark the most popular tread pattern for Off-the-Road tires.
The traction pattern tire has a directional tread design, which means the direction of mounting on the rim is important. For example, the mounting direction should place the tread facing one way on the drive wheels to produce proper traction, while on the free rolling wheels, the tread should face the opposite direction.
The block pattern tire is mostly characterized by wide tread width and rounded shoulders. Under heavy loads, the block pattern's large ground contact area creates low ground contact pressure, for good flotation properties. Therefore, it is well suited for use on soft, muddy ground. This pattern is also called the "alligator" or "button" pattern.
The rib pattern tire has grooves running parallel to the direction of travel and gives high directional stability. The rib-lug pattern, a variation of the rib pattern, has lugs on the shoulders of the tread. Rib pattern tires are mainly used on free-rolling wheels.
The smooth pattern tire, designed for tire rollers, has no grooves. It is used for compacting and leveling. A smooth pattern tire can also be used on loaders in underground mines because of its high wear and cut resistance. The smooth pattern for loaders, however, has two narrow grooves that are used to measure tread wear.
According to the Tire and Rim Association, Incorporated (TRA), there are three general classifications of tread thickness for Off-the-Road tires: regular, deep and extra-deep. Deep and extra-deep are 1.5 and 2.5 times thicker than regular, respectively. The thicker treads have greater cut and wear resistance. The TRA codes are classified as follows:
Extra-Deep Tread: L-5, L-5S Deep Tread: E-4, L-4 and L-4S Regular Tread: E-2, E-3, G-2, G-3, L-2 and L-3
Although thicker treads give greater wear and cut resistance, they also generate and retain more heat. Accordingly, work conditions for tires with thick treads should be thoroughly evaluated to prevent heat separation and other heat-related damage. Deep and extra-deep tread tires have almost the same overall diameter, which is larger than regular tread tires. When replacing regular tread tires with deep or extra-deep tread tires, the larger overall diameters of the thicker tread tires should be taken into consideration.
Size Identification and Aspect Ratio
The size of an Off-the-Road tire is normally indicated by tire width, rim diameter and ply rating. The nomenclature for this is as follows:
|Tire Width (inches)
|Rim Diameter (inches)
|36PR (narrow base)
|20PR (wide base)
A narrow base tire has an aspect ratio (tire height/tire width) of 96~98% and a wide base tire has an aspect ratio of 80~82%. Widths of narrow and wide base tires of the same diameter are shown below:
For example, the overall diameters of 18.00-23.5 E3 and 23.5-25 E3 are 63.5 inches and 63.8 inches: almost the same. According to TRA guidelines, tires with an aspect ratio of 65~70% are called super low profile tires and provide high flotation and stability and are usually indicated as follows: tire width/aspect ratio-rim diameter (40/65-39). Some low profile tires are also indicted by overall diameter x tire width-rim diameter (42 x 17-20). A suffix, NHS, TG or K may be attached. These are TRA designations used to differentiate between tires requiring certain conditions. NHS: Not for highway service, TG: Tractor-grader tires, not for highway use, and K: Compactor tire for use on 5 drop center or semi-drop center rims having bead sears with nominal minus 0.032 diameter.
Tire Specification Code
It is most critical that Off-the-Road tires are properly matched to the job and road conditions anticipated. Accordingly, Off-the-Road tires are classified by three types: regular tire, cut-resistant tire and heat-resistant tire. The regular type provides general performance for use under standard conditions. Where many obstacles pose cut damage, cut protected types are most suitable. And under good road conditions where higher speeds can be attained, heat-resistant types are recommended. Yokohama follows the above classifications.