Whereas some belts have the same cover thickness on both sides, most belts are fabricated with the pulley-side cover somewhat thinner (lighter in gauge) than the carrying side of the belt, because of the difference in wear resistance needed. The difference in thickness between the top and bottom covers is referred to as a belt’s aspect ratio. However, the difference in thickness between the two covers cannot be too great, or the belt may cup.
The problem with belts with poorly-designed aspect ratios is that the larger mass of rubber will shrink more than the smaller. Consequently, if a belt has an inordinately large top-to-bottom cover ratio, and the top cover shrinks due to age, exposure to ultraviolet light, or other factors, the belt will cup up, reducing the area of the bottom cover in contact with the idlers. This will make it more difficult to keep the belt running in alignment. This problem is most likely to occur when, in the interest of getting a thick top cover to extend service life, a plant orders belting with a top cover that is too thick for the bottom cover. To provide consistent shrinkage and more consistent tracking, an aspect ratio of 1.5-to-1 is recommended for belts up to 900 millimeters (36 in.), with a 2-to-1 aspect ratio recommended for belts from 1000 to 1600 millimeters (42 to 60 in.). For belts above 1600 millimeters (60 in.) a 3-to-1 aspect ratio is recommended. Belting with a 3-to-1 aspect ratio is suitable for many purposes and is the ratio mostly commonly stocked at belting distributors.
Details about aspect ratio can be found in Chapter 4 of Foundations™ Fourth Edition by Martin Engineering.