Ford Defends SUV, Hits Tires

DEARBORN, Mich.--Engineers and scientists at Ford Motor Co. continue to meet with both the National Highway Traffic Safety Administration and Firestone on a regular basis, and the company reports that “our ongoing statistical analysis and test data seem in agreement with many of the preliminary conclusions of Firestone, as we understand them.”

While Ford’s investigation continues into the cause of the Firestone tire tread separations, the preliminary root-cause hypothesis from the automaker is: The design of the tire generates high stresses and heat in the wedge and belt area.

“Manufacturing processes at Firestone’s Decatur Plant reduce the cohesion level of the rubber in that same area of the tire. This reduced strength permits cracks to propagate between the steel belts. We believe it is a combination of manufacturing factors and the reaction of the tire design to field operating conditions including hot weather and very low tire pressure, that have caused the increased failure rate of these tires,” Ford said in a statement. It did not discuss the Explorer’s suspension--which has been cited as a possible cause of rollover crashes.

Ford reported:

• Our testing on Ford and other makes of vehicles and on tire test rigs show that P235/75R15 ATX and Wilderness AT tires do run hotter than similar competitive designs, although we do not know what aspects of the design cause this.

• Our test data also show that rubber cohesion is lower in the belt area of Decatur-built tires. The test data, however, do not tell us what aspect of the manufacturing process may cause this.

• We have not tested or analyzed tires that have had a puncture repaired, but it seems reasonable that a tire that is already sensitive to heat because of design and manufacturing aspects could be progressively damaged if it was run hot because of lower inflation pressures prior to or after a repair.

• The weight of the Explorer is well within the load rating of the tires. The curb weight of the heaviest four-door 4x4 model of Explorer increased only 154 pounds from 1991 to 1997. Our experience with more than 2.9 million competitive make tires that performed almost flawlessly in the field leads us to conclude that the vehicle is not a significant contributor to tread separation.

• Firestone’s data show that the somewhat higher occurrence of claims for tread separations on a vehicle’s left rear tire happens on a majority of light trucks and SUVs from most manufacturers. The difference in weight, rear left to right, on the Explorer is less than 100 pounds. Our analysis indicates this left-to-right rear weight distribution on the Explorer is not significantly different than on similar competitive vehicles.

• Independent real world accident data continue to show that the Explorer is among the safest vehicles in its class for all serious injury accidents, as well as for rollover accidents.