At the dawn of long-haul trucking in North America, fleet managers quickly learned that single radial tires simply could not withstand heavy loads. To better distribute weight over tires and axles — and provide a sort of built-in “limp mode” should a single tire fail — the concept of dual drive and trailer tires was adopted, placing two tires side by side on both single- and double-axle configurations.
It’s still an approach widely used, even with the recent advancements in wide-based single tires. It provides a logical life-path for tires purchased new to be used time and again as they are retreaded and moved further and further back on the tractor-trailer until their casings are spent.
However, it is this unique “recycling” ability that can cause headaches, because matching tires with similar amounts of tread wear is critical on dual-tire axles. And it traditionally is one of the most overlooked maintenance issues.
There are two ways dual tires can be mismatched — significant air pressure differences and tire height imbalances.
“There are still dual assemblies being mismatched, but overall, fleets and service technicians have become more aware of the tire performance consequences and are taking strides to eliminate the problem,” says Phil Mosier, Cooper Tire’s manager of commercial tire development.
Drivers are generally unable to recognize mismatched duals through feel or vehicle performance. Visual inspections, by both drivers and technicians, are the best way to catch mismatched tires before serious damage is inflicted on tires. The strongest visual clues that there’s a problem are typically found in the smaller and/or under-inflated tire. The most common sign is irregular or uneven tread wear. The types of irregular wear include alternate lug wear, diagonal wear, multiple flat spotting, and erratic or inner rib depression wear.
That’s because whether you’re dealing with a pressure differential or a height problem, it’s the smaller tire in the set that bears the brunt of the damage. Two tires paired together on a dual assembly must have the same circumference (or height, or diameter) in order to cover the same distance as they roll along. If the diameter or circumference of the two tires are even slightly different, the smaller of the two tires will scrub along the pavement trying to make up the distance traveled by the larger tire. It’s that scrubbing that kills tread life.
But the larger (often newer) tire doesn’t get off scot-free, either. Because of the smaller tire’s limited footprint, the larger tire has to carry a greater share of weight. This causes additional wear to the tread face of that tire and greater stress on its sidewalls. So the smaller tire in a mismatched set of duals is being scrubbed to death, while the larger tire’s lifespan is being shortened because of overloading the sidewall.
For those reasons, fleet consultant Darry Stuart, owner of DWS Fleet Services, says TMC recommends keeping dual tires within ¼ of an inch in height in order to minimize scrubbing and ensure the tires work together on the road.
“I’m not as stringent on that spec as most people,” he admits. “Most people like a closer spec on the tire height. But I’m being honest about the labor costs required to match those tires up away from home. Are you willing to pay $150 to swap a tire out over an extra quarter of an inch when that truck is away from your shop?”
When looking at pressure differences, Cooper’s Mosier notes it is not uncommon to find dual tires with different inflation pressures, but it is far less common in fleets using automatic tire inflation systems. More and more fleets are investing in such systems because they realize the importance of maintaining proper inflation pressures, he says.
“Industry-wide it is acceptable for dual assembly inflation pressures to be within 5 psi of each other. A recent study at three different trucking companies revealed 79% of the dual assemblies fell in the zero- to 5-psi range; 16% were between 6 and 10 psi difference, and the remaining 5% measured at a 11 psi or greater difference. To put the 5 psi difference in tire pressure into perspective, the lowest inflation tire will have a circumference that is 5/16-inches smaller. During every rotation cycle, the smaller-circumference tire must scuff ahead to keep up with the tire with more inflation, just like two tires with mismatched diameters will do.”
Bryan Golden, vice president of fleet maintenance for Southeast Logistics in Tuscaloosa, Alabama, says mismatched-diameter tires are the bigger problem for his vehicles. Golden has a reputation as something of a perfectionist, and says he prefers to keep his duals within 4/32 of each other in terms of height. But, he notes, his best efforts in the shop can vanish in a puff of rubber-scented smoke as soon as a truck is out of his sight.
“We had a truck come in the other day, and when I looked at the trailer tires, one dual tire was a half inch off the ground,” Golden says. “The problem is if there’s a blowout on the road and a tire has to be replaced. We specify 4/32 as our spec for dual tire height in all our vendor profiles. But usually a technician just grabs whatever tire is handy and puts it on the truck or trailer. And we won’t know about it until that truck gets back to the shop and we can take a look at it.”
Stuart agrees that most issues occur when a truck is away from its home base, noting that in all his years as a fleet manager, he can only recall a couple times when someone actually pulled a caliper out to measure tire heights. “The real issue with duals is a new tire next to a worn tire,” he says.
Stuart recommends fleets take extra steps to match tires together and keep problems to a minimum. “If you’ve got one bad tire on a double axle, you can get by with matching it up to the tire next to it, height-wise,” he says. “But if you have four tires bad on a double axle, you’re better off replacing all eight tires on the double axles, then taking the four good tires you have left and match those up with any incoming truck that needs a single dual tire replaced.”
Keeping duals matched correctly can be a headache for fleets, but Golden has no doubt the effort is worth it. “If you have mismatched duals, you’re just rolling money right out the door,” he says. “It’s a problem you have to train your people for, and stay on top of. But if you manage it correctly, the money you save goes right to your bottom line.”