LOUISVILLE, Ky.—Higher speed limits: Truckers love 'em; tires don't. That's because higher speed limits mean higher running temperatures, greater impact damage and more rapid treadwear.
It also means potential underinflation and, if you follow the old Tire & Rim Association guidelines, potential overloading.
The effects of higher speed limits on tires—and the corresponding effects on retreaders and commercial tire dealers—was the topic of a panel discussion during the recent International Tire and Rubber Association conference.
Members of the panel included: Len Hollinger, of Bridgestone/ Firestone Inc.'s engineering department; Jack McCammond, Michelin North America's manager of truck engineering support; Al Cohn, Goodyear's marketing manager for commercial tires; and Jim Osborne, Oliver Tire & Rubber Co.'s vice president of technology and development.
Each worked together to present an industry position that, for the most part, stated: Today's tires are capable of running at sustained speeds of 75 mph, as some states have allowed.
So speed limits went up and, all of a sudden, the tires carrying these trucks are capable of higher speeds as well? Sure, Goodyear's Mr. Cohn said.
``We have to build a safety factor into each tire,'' he explained. ``You can't make a tire that's fine at 55 mph, but blows up at 56 (mph).''
Although the bottom line is that dealers should consult their manufacturers' tire data books, the panel detailed the three tire maker's revised positions on load ratings at higher speeds:
All Bridgestone/Firestone line haul truck tires are capable of 75 mph sustained speed without load reductions or pressure increases. In addition, the company has issued a bulletin identifying which other truck tires are capable of sustained 75 mph speeds;
All Goodyear line haul truck tires also are capable of running at 75 mph without load restrictions or pressure increases, but all other tires must follow the Tire & Rim table;
Most Michelin line haul truck tires are capable of 75 mph sustained speed without load restrictions or pressure increases. Some tires, however, will require a 10 psi pressure increase. The information is being added to the company's truck tire data book.
Still, just because the tires are safe at 75 mph, does not mean that speed is an optimal—or even desirable—operating condition, according to the manufacturer representatives.
A tire running 20 mph faster than 55 mph decreases its tire mileage 15 to 30 percent and increases its operating temperature 25 to 35 degrees. Additional speed also lengthens the tire's centerline, which can cause cupping and faster wear and reduces the tire's resistance to sidewall snags and tread punctures.
Higher operating speeds also pose problems in the retreading process, according to Oliver's Mr. Osborne, who said he suspects retreaders will begin to see higher incidence of ply separations, more sidewall splits, tread chunking and chipping, and rib/punch/depression wear.
The potential for more problems makes the inspection process of even greater importance, Mr. Osborne said.
Retreaders, however, have a unique opportunity to offer their customers a value-added service, he said, by being able to take steps that potentially could make the retreads last longer by radiating their heat more quickly:
Make sure the maximum rubber over the belts is 2/32 of an inch;
Use a slightly rounder buffing radius;
Use lighter tread designs if possible;
Keep undertread to a minimum;
Use rib designs when possible; and
Try to help customers understand the downside of higher speeds.