(Courtesy of Caterpillar Inc.)The ton-mph formula was developed to predict tire temperature buildup. The system is a method of rating tires in proportion to the amount of work they can do from a temperature standpoint. It utilizes the product of load x speed to derive an index of tire temperature buildup. Maximum tire level-off temperatures of 225° F for fabric-cord tires and 200° F for steel-wire tires are the limits that Caterpillar recommends. Even at these temperatures, failures may be initiated by overstressing the tires. Some companies rate fabric-cord tires at 232° F or on occasion as high as 244° F. These higher temperature levels are questionable under average field conditions. Heat generation in a specific tire at recommended pressure depends on three factors:The weight the tire is carrying (flex per revolution)The speed the tire is traveling over the ground (flexures over a period of time)The air temperature surrounding the tire (ambient temperature) and road-surface temperatureOnce a tire manufacturer has determined a tire’s temperature characteristics and expressed them in ton-mph, any tire’s maximum work capacity can be calculated by using the above-listed specific job conditions in the simple formula average tire load x average speed for the shift. This provides the onsite ability to predict and avoid costly tire separations.
