Firestone Announces Findings of Root Cause Analysis
19 December 2000Firestone Announces Findings of Root Cause Analysis: Increased Rate of Tread Separation Claims Caused by Combination of Factors
- August Recall Was More than Adequate, Company Says - NASHVILLE, Tenn., Dec. 19 Firestone announced today the findings of the company's intensive four-month analysis into why a certain group of its tires, primarily on Ford Explorers, may have been more likely to experience tread separations. The company found that in extreme cases several factors acting in combination could cause an increase in tread separations. A combination of the company's Radial ATX tread design in the P235/75R15 size, the effect of external factors relating to the tires on Ford Explorers, including low tire inflation and the loading of the vehicle, and certain manufacturing factors relating to Firestone's Decatur, Ill. plant were identified as principal reasons why a small percentage of P235/75R15 ATX and Wilderness AT tires produced at the Decatur plant experienced an increased rate of tread separation claims. "Our team's findings confirm what the initial statistical claims information demonstrated from the outset -- that a small number of tires generated higher rates of tread separation claims when used on Ford Explorers and that our recall initiated in August was more than adequate to protect the public," said John Lampe, Bridgestone/Firestone President, Chairman and CEO. "Our engineers and analysts found through their review of the data a higher claims rate for those tires we recalled, but this same data reinforced our belief that other Firestone tires perform at world class levels. This superior performance includes Firestone's line of 15" and 16" Wilderness AT tires produced at Joliette, Quebec, Canada, and Wilson, North Carolina, as well as the 16" Wilderness AT tires produced at Decatur. "We committed to our customers and the motoring public to find out why some Firestone tires were more likely to experience tread separations on Ford Explorers. That has been our principal focus since we announced our recall effort in August," said Lampe. "Today, I can say I am satisfied that our root cause team has conducted an exhaustive and comprehensive study and has examined all factors that could have caused the tire problems." Firestone began its review and analysis nearly four months ago, bringing together a joint team of engineers, technical experts and statisticians. The review involved testing tires in the company's technical and research centers, analyzing recalled tires including failed tires, and using finite element modeling. In addition, the company analyzed reported claims information, tire performance information and manufacturing information from all of its plants in the U.S. and used two outside laboratories to provide an independent source of analysis. The team worked together with and separately from Ford Motor Company specialists, sharing information and comparing results with them. Additionally, the company brought in an independent expert, Dr. Sanjay Govindjee, to conduct his own review, the results of which are expected later this month. Firestone has been working closely with the National Highway Traffic and Safety Administration (NHTSA) and Ford Motor Company as it conducted its investigation and provided them with the company's findings last week. Firestone believes that once NHTSA and Ford review the materials they will agree with the company's findings. Lampe added that Firestone is certain that tread separations alone are not the exclusive cause of serious accidents, including those that involve rollovers. The investigation Firestone conducted did not deal with why a vehicle may roll over in the event of a tire failure. Firestone's Analysis In order to find the cause or causes of why some tires had higher claims rates, Firestone reviewed data from millions of tires produced from 1991 to 2000. The company examined more than 2,500 recalled tires, cutting apart more than 850 tires to analyze tire components. In addition, the root cause team conducted the same investigation on new production Wilderness AT tires. The cut tire inspections included looking at 40 measurement points per tire and analyzing the following tire components: * #2 Belt Width * Belt Step * Inter Belt Gauge * Wedge Gauge/Location * Buttress Gauge * Base Gauge * W7 Gauge (Distance from the bottom of shoulder pocket to the belt edge) Extensive testing was undertaken at the company's technical and research centers in Akron, Ohio, and the company performed real world field tests on the tires at its technical proving grounds in Fort Stockton, Texas, to determine what unique factors in the recalled tires could result in a higher rate of tread separation claims. These tests focused on three key areas: physical properties of the tires; materials and manufacturing processes; and vehicle and external conditions that can affect tire performance. Specifically, the team analyzed design specifications changes between the P235/75R15 size tires in the ATX line and the Wilderness AT tires of the same size and manufacturing practices among the plants that could lead to variations in interbelt materials. External conditions such as vehicle load and pressure differences, ambient temperature, low inflation and high speed were also studied. The results were then compared with the statistical claims data for the recalled ATX, ATXII and Wilderness AT tires and with other tires produced by the company to determine how the technical analysis compared to the company's claims experience. The investigation and finite element modeling analysis identified a number of individual aspects in the design and manufacturing process and individual vehicle and external factors that could contribute to a higher percentage of tread separations. The investigation found that each of these individual factors would not alone lead to the increased tread separation claims rates. Instead, all of the following factors acting in combination appeared to explain the increased rates: * The shoulder pocket design of P235/75R15 ATX tires could lead to cracking at the shoulder pocket bottom. This could become the starting point of a failure, which when combined with the other factors result in a reduction of resistance against belt detachment. * Low inflation pressure in the recalled ATX, ATXII and Wilderness AT tires increased the running temperature of tires and would contribute to a decreased belt adhesion level. * Vehicle load levels and low standard tire pressure initially specified for the primary vehicle for which the tires were designed led to a decreased margin of safety for tire performance. * ATX P235/75R15 tires and Wilderness AT tires of the same size produced at Decatur exhibited different belt adhesion characteristics, including lower initial adhesion, than those same size and line tires produced at other BFS plants. "These factors the team has identified, when working in concert, are the major causes for why some P235/75R15 ATX, ATXII and Decatur produced Wilderness AT tires experienced an increased rate of tread separation," said Lampe. He emphasized that the root cause findings do not affect all of the recalled tires. "Tire industry experience demonstrates that many tread separations can be caused by various forms of tire damage encountered in daily use, such as punctures, improper repairs, severe impacts or being misapplied. Many of the reported incidents are attributable to these factors, and not those identified in the company's root cause investigation. This was one of the difficulties the team encountered in conducting our analysis." Recommendations, Enhancements Made Regarding P235/75R15 Tires "As soon as we began to understand our findings, we announced recommendations regarding the recalled tires and implemented enhancements to our manufacturing processes based on our ongoing root cause review," Lampe said. "Although we have reported the results of our analysis today, as soon as we began to better understand the factors that might have been involved, we made recommendations regarding the use of the tires. We also instituted manufacturing enhancements as part of our commitment to continuous improvement." At the time of the recall, Firestone recommended that the inflation pressure on Firestone P235/75R15 tires mounted on Ford Explorers be increased. Based upon Firestone's recommendation, Ford implemented a change in the recommended inflation pressure from 26 pounds per square inch (psi) to 30 psi in September. The P235/75R15 ATX and ATXII tires are no longer in production. The tread of the Wilderness AT line of tires, which replaced the ATX II as original equipment on the Ford Explorer in 1996, is significantly different from the ATX and ATXII tires. The Wilderness AT was introduced with a new tread design as part of that year's redesigned Ford Explorer. Although the skim stock, a rubber coating surrounding the steel belts, from Decatur was within specifications, Firestone is implementing changes to the processing of skim stock at Decatur so that its physical properties will be equivalent to that of the other Firestone plants before it begins using that material again. The company is changing how it will collect, analyze, and manage the data it receives regarding tire claims and adjustments. This will allow the company to assimilate data and respond as soon as practicable if any anomalies in tire performance are observed. "I said we would work diligently to examine all known facts that were relevant to the recalled tires and then immediately make all changes necessary to ensure public safety," said Lampe. "I believe this team has done just that. While no company can ever make a tire that is indestructible, I can say with overwhelming confidence that the review conducted by this group and the resulting enhancements Firestone has made has refocused our company on manufacturing the highest quality tires found anywhere in the world." In addition Lampe said, the company is implementing the findings of a special internal Comprehensive Quality Review team, comprised of technical, engineering and safety professionals. This team reviewed the overall tire manufacturing and data collection systems within BFS, focusing on production operations, tire enhancements and quality assurance. The team members have completed their review and Bridgestone/Firestone is putting their recommendations into practice. "We are a proud company with a long history and a tradition of customer service and satisfaction. The fact that some of our customers have questioned our commitment to them and our commitment to their safety has been a soul- wrenching experience," Lampe concluded. "By identifying the sources of the increased claims rate and confirming that those factors are unique to the recalled tires, by instituting the enhancements identified during this investigation along with improvements in how we collect and analyze tire performance and claims data, I am convinced that Firestone will continue to regain our customers' confidence and trust in our products."