Big Bite

When people talk about how to improve truck productivity, they think big. Heavier gross weights, longer trailers, more volume, volume, volume. But these days less is more.

Frugality rules the day. Why use two of something when one can do the job just as well? That would seem to run counter to the industry’s push for more capacity on a single vehicle, but it’s not necessarily so.

Tire manufacturers are answering the call for greater efficiency by introducing wide-based tires designed to replace a set of duals. The leader in the market is Michelin, which wheeled out its X-One family of drive axle and trailer tires almost two years ago. Bridgestone, Goodyear, and Continental are in various stages of development with similar tires.

The benefits of these new singles are just what over-the-road trucking operations, distribution fleets, and specialty haulers are looking for. Using aluminum wheels, the tires will cut the tare weight of a tandem tractor and tandem trailer combination by 1,000 pounds or more, depending on the make of tire and wheel. They cover the pavement more evenly than a dual pair, providing better traction and roll stability, and wear at least as well as duals under similar conditions, since there’s no issue with mismatched tires or air pressures. In contrast to earlier experiences with wide tires, drivers have said single tires outperform a dual pair on overall ride and traction, even in winter.

Unlike the super-single tires you see on the front axles of bulk-hauling trucks, these tires are sized to match the rolling radius of a low-profile 275/80R22.5 tire (Michelin offers a 455/55R22.5 size in Canada). As a replacement for duals, they require no changes to gearing or vehicle height. Two singles cost about the same as a pair of dual tires, so there’s no premium on the price to switch.

“We see orders where guys are setting up four trailers, or two tractors. Some of those are retrofits, but the original equipment market is coming along, too, which is encouraging,” says Larry Taylor, vice-president of sales and marketing for Accuride, which produces a 14-inch aluminum wheel. “If people didn’t see potential in it, they wouldn’t even try it.”

But the obvious benefits-lower tare weight, better handling, reduced maintenance-have to be weighed against regulatory penalties on axles with single tires. Pavement engineers say single tires cause severe ruts in the road surface, citing research dating back to the mid 1980s. If you draw the analogy to a coat hanger that you bend back and forth until it breaks, that’s what happens to the road surface when wheels pass over it. A set of dual wheels creates heavy strain in a longitudinal direction, or in the direction of travel. With single tires, the longitudinal strain is no different than a dual assembly under the same load, but the lateral or side-to-side stresses are typically four times higher than duals. It’s these lateral stresses that cause ruts in the pavement.

In 1988, the provinces agreed to a set of minimum standards in the Memorandum of Understanding on Interprovincial Vehicle Weights and Dimensions. The MOU says the maximum weight on a single axle is 9,100 kg, but the weight on any single tire cannot exceed 3,000 kg. When you put two single tires on the axle, your limit is 6,000 kg, a restriction the four Western provinces and four Atlantic provinces apply, and Ontario enforces only on 53-foot trailers. In Quebec, where the maximum load is 9,000 kg per axle, the use of single tires carries a 1,000-kg-per-axle penalty. The policies have made the X-One impractical everywhere but Ontario and Quebec.

“If a fleet is running with 80,000-pound gross weights, the penalty is inconsequential. Otherwise, it’s prohibitive,” says Ralph Beaveridge, marketing manager for truck tires at Michelin North America’s office in Montreal.

At issue is whether wide-base, low-profile single tires have the same effect on roads as other types of single tires. Michelin is collaborating with the Quebec Ministry of Transportation and Laval University in Montreal to study the stresses and strains in the top two centimeters of the road surface, where rutting tends to occur.

Michelin hopes the test will show that the tires are damage neutral compared with dual tires.

“The rules pre-date this specific design,” says Beaveridge, who has lobbied provincial governments for changes to axle-load limits when wide-based, low-profile tires are used. “This design is different from what governments had in mind when they signed the MOU.”

Government officials are skeptical. John Pearson, an engineer and secretary to the Council of Ministers, which co-ordinates provincial transport policies, says if the Laval tests show that the impact characteristics of the design are different, the rules may be relaxed provided that the type of tire can be clearly identified-stamped so an enforcement officer can distinguish one tire from the next.

But so far, the historic problem caused by single tires-rutting-has not been addressed, Pearson says. “I’m interested to see any new data that’s available, but right now it doesn’t look like there’s anything fundamentally different in terms of impact that would cause the regs to be amended, in spite of the different design of the tire.”

Beaveridge hopes the results of the Laval tests, available this summer, will change some minds.