What It Takes To Be A Big Wheel

In some industries, such as earthmoving and mining, companies looking for that big productivity jump pin their hopes on next-generation trucks and off-highway machinery. They want dump trucks, graders, and front-end loaders with larger payloads, better acceleration and top-end speeds while loaded, and longer service lives, not to mention lower purchasing and maintenance costs. A key variable in this equation is the tire. Though similar to tires found on family sedans, they are enormous — the largest measure 12.75 ft in diameter, are 4.6 ft wide, weigh 10,000 lb, and cost up to $50,000 a pop. They come in a wide variety, each tailored to different applications, working environments, and operating conditions. The trick for tire designers is making intelligent trade-offs between various performance characteristics to create tires that squeeze that last ounce of work out of the truck and driver.

TIRE BASICS
Large tires, like consumer tires, come in two basic flavors: steel-belted radial and belted bias. A few vehicles still use inner-tube technology, but they are mostly aging road-maintenance trucks in municipal or state fleets. The next time those organizations buy a new truck, it will likely ride on radials. Radials have several advantages over bias (See "The Radial Advantage"), but bias still command about 55% of the large (over 57-in.-diameter) tire market. Experts expect radial's market share to increase over the next 10 years.

Bias tires use a variety of natural and synthetic rubbers, along with fabric reinforcements, for the 50 or more components in the tire. Layers of rubber, or plies strengthen the tire. Thus a 22ply tire is stronger than an 18-ply tire. The fabric, which reinforces the tire and helps provide cut or puncture resistance, is almost universally nylon. Flex 10 (also known as Kevlar) was tried, but it did not fare well. Bias tires provide good sidewall stiffness and puncture resistance, making them well suited for some applications. For example, a loader taking coal or ore from one spot and placing it in a dump truck does not travel long distances. It can use bias tires because traction, tread wear, and rolling resistance are not major issues. And many graders, which often have to push sideways, use bias tires because their stiff sidewalls give them the strength to handle offcenter loads.

Radial tires rely on steel belts for strength and structural integrity. Belts are wound from cables made up of 11 braided steel filaments and four to six belts run inside the circumference of the tire. Radials for cars, on the other hand, might use only three layers and the cables are braided from four filaments. At one time, tire makers used nylon or Flex 10 belts, but the steel industry responded by dropping the price of filament steel and now it's the overwhelming material of choice for radials.

A variation of the radial and bias tires are rubber-filled tires used in underground mines or where the vehicle is slow moving but the chance of a puncture or air loss is high. Goodyear, for example, injects a rubber compound called Perma Foam, and heats the tire to "cook" it. The foam expands into the voids, displacing all the air, then hardens. "It makes the tire bulletproof, but still gives the driver the comfort of an air-filled tire," says Tom Ford, Goodyear general manager of global tires. "You have to cut the tire apart to get it off the wheel."

Bias and radials are made in roughly the same way. Various components are laid down layer by layer on the outside of a drum. Specific proprietary rubber compounds make up the different parts. The sidewalls, for example, use tough, heat-resistant rubber because that area absorbs most of the loads and generates the most heat. Tread rubber, on the other hand, must rebuff cuts, hacks, and punctures, while the liner needs to be airtight. Once all components are in place, the tire is heated and cured in a mold. The mold also presses a tread pattern and sidewall profile into the tire.

The biggest challenge in turning out large tires, according to Bob Ochsenhirt, sales engineering manager for Goodyear Tire & Rubber Co., Akron, Ohio, is their sheer size. "The tires are just so large. You can walk around inside the rim of some of them. So the equipment needed to handle them is capital intensive. Lead times for molds, for example, are double that for smaller tires."

KEEPING A COOLER, LOWER PROFILE
The size of these off-highway tires affects more than just manufacturing. Once they get past a certain size, delivery and transportation become a challenge. "The tire has to be shipped whole and is ultimately mounted on the truck or vehicle at the work site," says George Couris, senior manager for Michelin Mining Tire Div. "As they get larger, you need special transport permits from highway officials. And if tires are too large, it becomes almost cost-prohibitive to ship them at all."

That's one reason tire makers are developing low-profile tires for offhighway vehicles. Engineers have reduced the sidewall and widened the tire's footprint. "It gives the tire a wider tread, so traction improves, and the footprint is larger so the tire puts less pressure on each square inch of ground it travels over. This reduces the chance of punctures," says Ed Demor, Goodyear's line team leader for off-highway tires. "Smaller-diameter, low-profile tires also lower a vehicle's center of gravity, which makes a truck easier to handle and less prone to tipping."

Low-profile tires also use lower inflation pressures. Michelin's new XK-B tire, for example, is rated for 87 psi (6 bar), while the tire it replaces needed 103 psi (7 bar). "The tire can handle a load of 93 metric tons with that lower inflation pressure. Overall, the lower pressure should increase tire life by about 20%," says Couris.

A major problem in offhighway tires is they generate heat. The loads create internal friction and heat, particularly in the sidewalls. If the tire gets too hot, the rubber compounds irreversibly break down and make the tire prone to punctures and impact breaks. At the extreme, heat leads to spontaneous combustion. "We want the tire temperature to be as far below 200F as possible," says Goodyear's Ochsenhirt. "Unfortunately, off-theroad tires are so thick, they can't dissipate heat as rapidly as consumer tires. Radials, which have thinner sidewalls than bias tires, do run cooler. That's one reason they're used in applications where vehicles are moving a lot."

Goodyear engineers also point out that various schemes have been tried to keep tires cool. "They've drilled holes and filled them with fluids, but that created stress risers and other problems," notes Steve Bausch, marketing manager of off-road tires for Goodyear. "Adding cooling fins on the outside also didn't pan out. Earthmovers don't move fast enough to take advantage of that extra surface area to dissipate heat." The tried-and-true method of controlling heat is to formulate the rubber compounds for the sidewalls and other high-stress areas to withstand more heat. But the best method is to maintain proper inflation, keep the roads the vehicles travel in good condition, and train drivers in proper technique. "There's more than one way to take a corner or climb a hill carrying a couple hundred tons of rock, and most of them overheat the tire or otherwise shorten its life," says Demor.

One method used to prevent tire fires is to inflate the tires with nitrogen. It knocks the oxygen leg out of the three-legged combustion triangle. (The other two legs, as any fire-prevention program will tell you, are heat and fuel.) Nitrogen also prevents oxidation or rust on the rim, and because it is made up of larger molecules than ambient air, nitrogen won't leak out as fast. It is a costly step to take, but some mines use it and Caterpillar fills all the tires of vehicles rolling off its production line with nitrogen.

Engineers don't expect any radical change in tire design in the next 10 years. Tire companies will keep on building tires that can handle larger and larger loads, as well as higher speeds, as their customers demand productivity increases. And they will keep on using steel and rubber as radials take over more of the market. Companies will continue refining and reformulating the natural and synthetic rubber compounds they use, and don't envision other materials making great inroads.