A tire's chronological age alone cannot determine its service life, according to a study of more than 14,000 scrap tires performed by the Rubber Manufacturers Association (RMA).
Treadwear is far and away the true indicator of service life, said RMA General Counsel Laurie Baulig at a May 23 teleconference announcing the study results. Road hazard damage and improper repairs also are key factors, according to the RMA's findings after scrutinizing tires at scrap tire processing facilities in seven states.
However, an advocate of a six-year ``expiration date'' for tires dismissed both the study and the RMA's approach to it.
``Is it a study that's really about tire age?'' said Sean Kane, president of Safety Research and Strategies Inc., a Rehoboth, Mass.-based legal research firm. ``It's pretty obvious to most people that it isn't.''
The RMA has submitted the study data to the National Highway Traffic Safety Administration (NHTSA), which the RMA petitioned in June 2005 to issue a consumer advisory regarding the factors that affect a tire's service life.
NHTSA is developing an aged tire endurance test to add to its tire safety and performance standard, and the RMA along with other interested groups is cooperating with that effort. According to Ms. Baulig, the association saw the new survey as an important first step in the process.
``We believe that a good starting point for a discussion about chronological age and tires was to examine tires that had been removed from service,'' Ms. Baulig said at the teleconference.
The RMA sent 14 two-person teams of technicians-each team working 10 hours a day for two or three days, according to Ms. Baulig-to examine scrap tires delivered to the largest scrap tire processing facilities in California, Florida, Arizona, Massachusetts, Illinois, Oregon and Pennsylvania. The sites were chosen from the major geographic regions in the U.S. to obtain a representative cross-section of tires.
The technicians were instructed to collect the production date code from each tire, as well as whether the tread depth was above, below or at the tread indicator; whether each tire bore any sign of road damage; and whether each tire had been repaired.
Of the 14,271 tires examined, the RMA found that three-fifths of them were four years old or less, though a few-0.5 percent-had lasted as long as 15 years. Fifty-nine percent of the tires had low or uneven treads, and 29 percent had been damaged in the bead, tread or sidewall. Seventeen percent of the tires had been repaired, and 87.5 percent of those repairs were improper, made as patch only or plug only instead of patch and plug, Ms. Baulig said.
The timeline for the service removal dates was a smooth curve with no spikes, Ms. Baulig said. ``This is what industry engineers call a `natural decay' curve,'' she said. ``This shows there is no particular `magic date' by which a tire should be taken out of service.''
Mr. Kane-whose organization petitioned NHTSA in November 2003 for a tire expiration date of six years after the date of manufacture-said the RMA study is flawed in its basic conception and asks the wrong questions.
``They're looking at scrap tires, and those tires, predictably, are worn out,'' he said. ``Why would we want to look at tires that are worn out? The tires we'd want to look at are those that have had catastrophic tread separations, which may total about two dozen out of the 14,000 they looked at.
``Or else you can look at the claims data, which of course the RMA is trying to keep confidential,'' he added. ``With this study, what they're doing is buying some time.''
In response to Mr. Kane's accusation, an RMA spokesman said the study was a comprehensive survey of tires taken out of service.
``If Sean Kane's assertion were correct, and there is an age at which tires generally begin to fail, we would have seen it,'' the spokesman said. ``As for the tires that suffered tread separations, he can't project that age was the sole or primary cause.''
In 2004, NHTSA promised to make certain tire and auto ``early warning'' data public by Oct. 1 of that year. However, it withdrew that promise after both the RMA and Public Citizen sued the agency.
The RMA argued that data should be kept confidential except in the case of an official defect investigation, while Public Citizen objected to the agency's placing any restrictions whatsoever on which data should be released to the public. The case is still ongoing before the U.S. District Court for the District of Columbia.
A spokesman for Ford Motor Co. said the company was still analyzing the RMA tire study at presstime, and a spokesman for Bridgestone/Firestone deferred all questions about the study to the RMA.
Ford is one of several auto makers that recommend in their owners' manuals that motorists change their tires after six years of service.
John M. Baldwin, Ford polymer technical leader who has made an extensive study of tire aging, told the audience at the Clemson Tire Industry Conference last March that Ford will petition NHTSA for a consumer advisory to motorists to change their tires after six years.
Bridgestone/Firestone, Michelin North America Inc. and Continental Tire North America Inc., all recommend 10-year tire service limits to their customers.