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Underinflation doesn't always reap same effect

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HILTON HEAD, S.C. (April 12, 2004) — Underinflation, as everyone knows, has dire effects on truck tires.

But it's not always the effect people expect, according to speakers at the Clemson Tire Industry Conference, held March 10-12 at Hilton Head.

The normal assumption with dual truck tires, for example, is that if one is severely underinflated or goes flat, it disintegrates during operation and leaves a so-called “alligator” on the road, said Cesar Zarak, tire research engineer at Michelin Americas Research Corp., a unit of Michelin North America Inc.

“I've seen one of the rear axle treads flying out from a truck while driving on the highway, the tread hitting other cars,” Mr. Zarak said. “I wanted to try to understand the phenomenon of how the tire comes apart while running next to another tire.”

Using new 275/80R22.5 drive tires, Michelin ran accelerated and normal endurance test procedures, running one tire in a dual package at the prescribed pressure of 100 psi and the other tire at inflations ranging from 80 to 0 psi.

What Mr. Zarak discovered, to his surprise, was that the tire at full inflation was the one that came apart—while in dual operation. The explanation, as far as Michelin saw, was that the fully inflated tire was forced to carry a disproportionate share of the vehicle load.

“Although the tire that failed was the one fully inflated, it is possible that the low-pressure tire might fail upon subsequent inflation,” he said. “When tires operate at 80-percent inflation of that which is needed to carry the load, they should be inspected before going back to service.”

Trying to get a handle on the effect of tire underinflation on commercial vehicle safety and operating costs, the Federal Motor Carrier Safety Administration hired Booz Allen & Hamilton in 2001 to conduct a study. The American Trucking Associations' Technology & Maintenance Council conducted a survey of truck tire pressures to aid the study, said Peggy Fisher, president of Fleet Tire Consulting and a Tire Business columnist.

“Booz Allen said it would hire young people to conduct the surveys,” Ms. Fisher said. “We said, 'Don't do that; they'll get uneven information. Let us do it.'

“So we took a team of tire manufacturers' field service engineers to the Truck Appreciation Days in Iowa and Nevada in July and August 2001. The cold pressures were taken by people who knew what they were doing.”

A well-maintained truck fleet, Ms. Fisher said, should have 85 percent of its tires within 5 psi of the recommended pressure. But the field engineers found only 44 percent of fleet tires were within those inflations.

“Forty-four percent isn't real good,” she said. The other statistics were worse:

c One truck in five had at least one tire underinflated by at least 20 psi; and

c 20 percent of all tractor dual tires and 25 percent of all trailer duals differed in pressure by more than 5 psi.

The study determined the typical tractor-trailer suffers about 2.2 road calls annually—half of those due to tire failures. The average lost time per breakdown is 2 hours, with average direct billed costs of $265 and direct losses due to delays of $191. That does not include the cost of a new tire or “any loss in revenues due to late shipments or loss of customer goodwill,” Ms. Fisher said. Costs vary, however, depending on the type of fleet, she added.

One aspect of the Booz Allen study was whether tire pressure monitoring systems would be cost-effective for a truck fleet. The answer? “If systems are too expensive, they may not be,” Ms. Fisher said.

The two questions yet to be answered about tire pressure monitoring systems, she added, are to what extent do such systems reduce costs caused by underinflation and whether the National Highway Traffic Safety Administration (NHTSA) will mandate tire pressure monitoring systems for trucks.

Many aircraft and some large off-the-road tires are inflated with nitrogen instead of air, noted Guy Walenga, engineering manager, North American Commercial Products for Bridgestone/Firestone (BFS). Nitrogen is required in tires equipping aircraft weighing more than 75,000 pounds at takeoff.

So, Mr. Walenga asked, would nitrogen be beneficial for truck tires?

“In every case, nitrogen allows the tire to maintain more of its original properties than air,” he said. Among other things, nitrogen offers less oxidation of tire components, less pressure buildup in service and longer tread-life.

“With nitrogen, the casing may be more durable and more retreadable,” he said.

On the other hand, nitrogen has some qualities that are negligible or even problematic with truck tires, Mr. Walenga noted. “Mining and aircraft tires use nitrogen to reduce fire incidence,” he said. “This is not a big factor in truck tires under normal circumstances.”

Portability is also a problem with nitrogen, particularly in road calls, Mr. Walenga noted. “If you have a blowout with a nitrogen tire, you have to refill it with air,” he said. “Then you have to take out the air eventually and refill the tire with nitrogen. But how many drivers would think to tell the maintenance crew?”

With the greater expense of nitrogen, it might not be cost-effective with truck tires, and BFS needs to do more testing on the subject, Mr. Walenga said.

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