Dual matching nightmares
One question that comes up during a lot of tire service training classes is related to the different loads between single and dual tires at the same inflation pressure. For some people, it’s a mystery that ranks somewhere between the Loch Ness Monster and Bigfoot. The simple answer is that since roads are not perfectly flat, one of the two dual tires will always be supporting more of the load than the other. That’s why a single tire can carry more weight per tire than each tire in a dual assembly.
The goal for a dual tire fitment is still to evenly distribute the weight between both tires. In order to achieve that goal, the tires must be inflated to the same pressure and have the same diameter. As far as pressure is concerned, when one tire is underinflated for the load, excessive flexing in the sidewall often results in steel cord fatigue (zipper ruptures) and higher operating temperatures. This also causes the properly inflated tire to work harder and suffer the effects of higher operating temperatures.
But the biggest enemy to a dual tire assembly is probably mismatched diameter. In these instances, the taller tire carries most of the load, and the smaller tire is forced to skip and drag to keep up. One tire wears faster (the taller) and the other tire develops irregular wear (the smaller) as it is dragged for 15 ft. or more each mile. Unfortunately, tire sizing is nominal, which means approximate, so there is no guarantee that tires with the same size are the same diameter.
When the size, manufacturer, and tread design are identical, technicians should be able to use the tread depth to determine if dual tires are within the allowable matching tolerance of 1/4 in. As a general rule, the difference in tread depth cannot be more than 4/32nds for matched sizes, manufacturers and tread designs. Unfortunately, this rule does not apply to retreads with the same tread design. Retreaders are telling me that the difference in diameter for the same size casing can be half an inch or more.
There’s also the issue of the 275 and the 285/295 low-profile 22.5 and 24.5-in. truck tires. According to the 2016 Tire and Rim Assn. (TRA) Yearbook, the 275/80R22.5 and the 295/75R22.5 have a difference in diameter of 0.08 in. By all accounts, they are interchangeable; however, the TRA Yearbook also shows that the difference in diameter between the 275/80R24.5 and the 285/75R24.5 is 0.47 in., so they should not be paired in the same position.
The maximum allowable difference in diameter for dual tires is 1/4 in., which means when the tires are standing side by side off the vehicle, the difference in height cannot be more than 1/4 in. When the tires are mounted on the vehicle, the technician measures radius, so the difference in height cannot be more than 1/8 in. Again, when the size, manufacturer, and tread design is the same in a dual position, the 4/32nd rule applies. When any one of those factors are different, the only way to know for sure is to get a tire square or caliper.
Despite the steady market for wide-base singles, dual tires will always be the bread and butter of the trucking industry. They are a proven solution with decades and countless miles of safety and performance. When they are matched across the board (inflation pressure and diameter), they can deliver hundreds of thousands of miles in service. And when they are not, they wear out faster, blow out at a higher rate, and cause nothing but trouble. The cure is simple, but some fleets just can’t stomach the medicine they need to take to get a good night’s sleep.
