skim rubber compounds change as they age. Skim rubber is a thin strip of rubber sandwiched between the inner and outer belts and the belt wedge is a thin strip of rubber installed along the outer edge of the tire between the two steel belts and in the area of highest stress on the tire. These changes reduce the ability of the tire to resist crack growth and belt separation. Most of the tread separations occurred on tires that had been in service for at least three years. ATX and Wilderness AT tires manufactured in Decatur started to fail between one and two years after production, but between two and three years for tires from Wilson and between three and four years for tires from Joliette. Tire aging for Wilderness AT tires, regardless of the factory that produced them, was similar to other tires manufactured at Decatur and was worse than comparable tires manufactured by Goodyear. Manufacturing facility The failure rates for ATX and ATX II tires manufactured at the Decatur, Illinois, factory were higher than for the other factories that also manufactured these same tire models, Wilson, North Carolina, and Joliette, Quebec. Tires manufactured in Decatur had "reduced fracture resistance" compared to tires manufactured at Joliette or Wilson. That is, once a crack formed within the tire, the crack would spread more readily than on comparable tires from the Joliette or Wilson factories. Researchers also tested belt adhesion strength by measuring the force that it would take to peel a strip of the second belt from the rest of the tire. ATX and ATX II tires manufactured at Decatur had lower belt adhesion strength than the same tires manufactured at Joliette or Wilson. The only manufacturing difference in the Decatur factory was in "the early material mixing stages which results in the production of materials with differing weight percentages of processing lubricant." However, there was insufficient evidence to conclude that this would weaken a tire or lead to a premature failure. Operating temperature A tire's operating temperature will increase with higher vehicle load, speed, air temperature, and with lower tire inflation pressure. The combination of inflation pressure, speed, and vehicle load could increase the tire temperature as much as 50 degrees C above the ambient air temperature. Higher operating temperatures will increase the rate of oxidative aging. Tires that were used in southern climates were less ductile and stiffer