Radial tires now approach 80% of the North American commercial tire business due to superior treadwear and fuel economy. One of the last areas for radial tire market penetration is the city bus market.
A radial tire is less tolerant of high brake heat temperatures because:
There is less rubber and reinforcement bulk than a bias tire to act as an insulator in the bead area.
The steel ply is a good heat conductor.
High sulfur compounds are required for good wire adhesion and these compounds are susceptible to heat degradation.
Radial tires are developed to withstand a maximum continuous running temperature of 90 degrees C. Exceeding this temperature for short periods causes no problem; however, longer exposure may cause a loss of material component strength. Therefore, it is important to know the heat flow characteristics between a brake drum and the tire.
In this paper, various drum-bead heat studies will be discussed. From four city bus studies, the heat flow rate between a 16.5-inch brake drum and a 22.5-inch wheel will be presented. Other studies compare a 22.5-inch and a 24.5-inch wheel on the same brake drum, steel versus aluminum wheels, and wheel and drum cool-down rates.