we run across rock-strewn mountain passes, or in the urban cul-de-sacs where we stash our trash.
While it's not uncommon for a highway tire to turn 250,000 miles on a virgin casing, some vocational operators consider themselves lucky to see 90 days out of a high-quality tire. They are under constant attack from all sides: hazards such as rocks, nails, debris, curbs, and in the case of sanitation trucks, torque from both the engine and the brakes.
We now have tires built for this demanding environment. That wasn't the case 10 or 15 years ago.
Michelin's Don Baldwin says in the old days, almost everyone had to get by on highway tires, because that was just about the limit of the product line.
"They were taking a pretty severe beating," explains the product marketing manager. "Scrubbing and curbing was beating up tires on transit buses and sanitation trucks, while the off-road trucks were dealing with rough terrain and high load requirements."
But improvements in truck technology, such as bigger engines, aggressive brakes, tighter turning radii, and other factors brought the tire people to the table with robust, application-specific tires.
Tires for sanitation trucks, for example, feature thicker sidewalls, as well as tread compounds that are more resistant to ripping, tearing, scrubbing and curb impact caused by maneuvering in tight locations on inhospitable surfaces with high axle loads. Sidewalls and treads are designed to accommodate the high-torque, fast start-and-stop driving styles of the operators.
The sidewall itself is thicker, and stronger materials are used in casing construction. Unlike highway tires, casing temperatures are not much of an issue in this low-speed environment. Instead, bead temperatures are more of a concern. Aggressive stops increase brake temperatures, which heat up the wheel and eventually the bead of the tire. Torque comes into play at the bead area too, adding additional stress.
But scrubbing is the nastiest element of urban, high-axle-load driving. Tread compounding can add months of life to an urban tire, as can tread design. Bridgestone, for instance, incorporates a solid shoulder in some of its urban severe-service tires to resist curbing damage and to minimize scrubbing at the shoulder. While some believe solid-shouldered tires suffer more irregular wear, Bridgestone says the life cycle of a tire designed for this harsh environment is so short that the tire will scrub away in regular service before irregular wear becomes a concern.
"High-scrub driving, just like a belt sander with coarse sandpaper, is going to scrub off the rubber before irregular wear ever forms," Bridgestone says.
Then there are the hazards of nail, broken glass and other debris. Goodyear says its DuraSeal technology, where a gel-like rubber compound automatically seals small tread punctures, not only reduces flats and downtime, but also extends tire life.
Tires for on-/off-road trucks have a lot in common with sanitation truck tires.
Where the Rubber Meets the Road
If there's any actual road to meet, that is. In the on-/off-road market, the percentage of time the truck spends off-road is a consideration in the tread design and the tread compound used. Building a tire for either on-road or off-road would be a simpler chore, but mixing the two operating conditions demands a blend of features that come into minimal conflict.
Some on-/off-road applications can see as much as 90 percent off-road, while others may see a 50-50 blend. The 50-50 tires need to be capable of resisting road aggression, like a line-haul tire, but at typically lower speeds.
Mixed-service tires also must be resistant to stone drilling and stone damage, common in an off-road environment. First, the tread has to be designed to eject stones that may become wedged between the ribs or tread lugs, and further, the casing has be more resistant to penetration by stones that cannot be ejected.
"Trapped stones can damage a tire so severely it has to be pulled from service, and can prevent casings from being retreaded," Bridgestone says. "The best way to prevent the damage is to prevent stone drilling in the first place."
One operator who knows about stones and the damage they can do is Jim Huffman, vice president of Black Gold Express in Fairbanks, Alaska. His trucks carry dirt and rock in twin-45-foot side dump trailers between a mine site and processing plant nine miles away. He grosses 200,000 pounds. A set of tires might last four months in winter, and half that in summer.
"We consider that outstanding performance," he says. "During spring and fall, we have freeze/thaw conditions that create unstable ground. When we're forced to chain up, the tires suffer," Huffman explains. "In the summer, it's unbelievably muddy."
Traction accounts for the difference in wear rates, and in this type of application, traction is a large concern. In winter, the road is gravel-covered ice. In summer, well, mud is the name of the game. He runs Goodyear G177s on the drives for their deep-lug traction attributes as well as a tread compound that's highly resistant to chipping.
"The better the traction, the longer the tire life," Huffman notes. "We don't have to use tire chains as often, and we're not spinning the wheels and chewing up rubber."
Measured in Months
Whether it's Fairbanks, Alaska, or downtown Chicago, operators who make the harshest demands on their tires remain concerned about longevity. To someone used to getting a year or more from a tire, it might seem like a waste to spend money on a premium tire that has to be pulled in three months.
Michelin's Baldwin acknowledges that customers sometimes question the value there. Why not just go with a cheap tire because it's coming off in such a short period? It's almost like they're disposable.
Not so, he argues.
"We're talking only months in some cases, but there's a big difference between a tire that lasts a month and a half and one that lasts three months," Baldwin explains. "A cheap tire at two-thirds the cost of a premium tire that comes off in half the time isn't good value. And if you manage the tires properly, pull them before they're ruined, you'll get better value from your casings."
The good news in this case is that because the tread wears off so fast, the casings are in near perfect condition at that time, and they're highly suitable for retreading - often many times over.
Retreaders like Bandag, Michelin, and others offer on-/off-road tread designs. Bandag's BDM line, for example, is designed for applications where 70 percent of the operations are on-road, such as mining, logging, construction and waste operations. Those treads combine both over-the-road and severe-service characteristics. Bandag's BDY series, on the other hand, is suitable for applications where 70 percent to 90 percent of the time is spent off the road.
"When planning removal cycles, bear in mind that the shallower you let the tread go, the less puncture-resistant the tire becomes," suggests Guy Walenga, director of engineering, commercial products and technologies for Bridgestone Bandag. "Operators wanting the best value for their casing in a high-cycle environment will give up a little tread life for optimal casing condition."
And Fuel Economy Too?
Tires with deep lug treads and thick sidewalls won't do you any favors in terms of fuel economy, and historically in many severe-service applications, fuel economy hasn't been the first consideration is spec'ing tires. That may change since diesel hit five bucks a gallon last year. Michelin recently rolled out a severe-service version of its X One wide-base single tire, a