Medium Duty: Best of Both Worlds

Rugged enough to handle anything in their way but also able to provide a smooth ride on pavement—you can indeed have it all.


Some off-highway vehicles spend most of their time mucking around debris-filled construction areas or quarries while others spend most it cruising along the highway on their way to the job site, but you still need a durable, tough tire when it’s time to get dirty. So how do you build something that works for both Larry the landfill operator and Linda the landscaping supervisor?

Clearly, these are not your father’s on/off-highway tires—today’s versions have all the advantages of modern technology and years of research, giving fleets the flexibility to have it all. Yet it wasn’t that way just a decade ago, says Bridgestone Tire director of engineering for commercial products and technologies Guy Walenga, because manufacturers were simply building the wrong tire.

“We looked at maybe 80 percent off-highway, 20 percent on-highway, give or take, and we built and compounded the tire accordingly, so the rubber just didn’t stay on the tire for highway, but off the highway it was a very durable product,” he says. “Over the years, we have continued to refine what on/off-highway is, and we actually find out that in most cases, an on/off-highway tire spends maybe 80 to 90 percent of its time on improved roadways, and the other percent is off-highway—it’s not as much as we thought.

“So it’s really very little time (off-highway),” he says, “but the off-highway environment is very, very brutal, so we’ve redesigned tires to reflect more on-highway time and less off-highway, and that’s a big change. Now we’re trying to get more on-highway mileage, however we can’t give up any of the durability we put in for off-highway performance, that’s the trick. We’ve shifted to make it more ‘on-roadable’ but not reduce any of its off-highway capability.”

COMPOUND SOLUTION

On/off-highway tires look the same as your average truck tire, but there is a lot more going on inside, thanks to evolved compounds—the biggest difference in modern tires, Walenga says. And products are being analyzed, tested and upgraded all the time.

“The on/off-highway tire is kind of a tough fit,” he says. “We want a tire that’s going to run on the highway for some amount of time but be able to go off into some pretty rough territory and not get cut up and chipped up and destroyed in that off-highway service. These tires are brutalized.”

So hats off to the guys in the lab coats, who have been busy putting the new information to use.

“We’ve done a better job of finding rubber compounds that will work on the road, and we can still make them more cut and chip resistant when they go into rocky areas,” Walenga says. “The construction of the body of the tire basically remains the same; it’s still a steel body ply. We may use different cords or filaments to make it more flexible, we space those cords a lot farther apart than we do for a highway tire, so it’s a more flexible design. We have a more flexible belt package, so the steel part of the tires—the cables, their spacing, the belt spacing—that is also being tweaked over the years to make the tire more suitable for off-highway, but still even more suitable for on-highway.”

Flexibility is a good thing when a tire has to move around and over objects, as an on/off-highway often does.

“As it rolls over obstacles, you have to try to envelop them, and a steel-belted radial tire doesn’t envelop anything; it’s too stiff,” Walenga says. “So by changing the cables and the belts, spreading them out a little bit more and in some cases making a split belt—where the belt is literally in two pieces next to each other rather than one solid piece, to allow a little more flexibility in that tread—so it can envelop obstructions rather than just smack ‘em. And this makes the tire more resilient and stronger and better suited for off-highway. But, flexibility in a tread means more rapid wear—now we’re back to running on the highway with a flexible tire, and that’s what’s causing the tires to wear out faster. So, you’ve got to find that middle ground in the casing design and the belt design and still top it off with compounds to resist that scrubbing action that you’re getting from more flexible body ply and belt.”

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