Many factors combine to increase temperatures that tires must endure.

Proper inflation is the lifeblood of tire durability. Modern radials perform admirably in demanding, high-speed, line-haul service when properly maintained. That wasn’t always the case.

In the latter days of bias-ply truck tires and even during early generations of line-haul radials, excessive heat was touted as the archenemy of high-speed truck tire durability.

While that basic engineering relationship hasn’t changed, the industry has developed new rubber compounds, casing designs and lightweight construction techniques that have combined to reduce heat-related tire failures to become nearly a non-issue, absent overloading and excessive speeds.

One noted exception can be found in tires fitted to transit buses in frequent stop/start metro service. The main culprit here, though, is excessive brake heat that can be transferred to the lower bead area of the tire. This contrasts with heat concerns in the tread, shoulder and upper sidewall areas of tires in high-speed, line-haul service.

A number of trends occurring in new over-the-road trucks, while not alarming, are thought provoking. First, the trend from large-rim-diameter, 24.5-inch tires to smaller 22.5-inch and, in some cases, 19.5-inch, low-profiles has resulted in nearly complete conversion to reduced diameter sizes on new trucks compared to older models.

This has been a good thing from the standpoint of fuel economy, improved aerodynamics, size standardization and application of the latest technology, since most development work by tire manufacturers is now concentrated in these sizes. The small-diameter tires do, however, turn more revolutions (from 3% to 4%) per mile and have reduced open area available between the rim and axle-end brake components for cooling airflow. All things considered, these alone are not big changes, but they result in slightly higher temperatures.

Another trend, in the interest of improved aerodynamics, is tighter enclosure of various fenders, fairings and other truck body parts surrounding the tire/wheel assembly. Since smooth airflow tends to reduce aerodynamic drag, care is taken in new truck design to avoid turbulence in panels surrounding and enclosing the tire/wheel assemblies. This can result in heat buildup, which, in racecars, often is addressed by ducting cooling air to the brakes.

As the bodywork on truck tractors continues to evolve in search of lower aerodynamic drag, added focus will be seen on improving airflow under and behind trailers. Skirting designed to reduce turbulence and resulting drag in these areas also will have some effect on airflow available for trailer brake and tire/wheel assembly cooling.

Engine compartment temperatures, also, are on the rise. Emission requirements effective in 2002 resulted in higher underhood temperatures. 2007 engines and revisions scheduled for 2010 appear likely to continue this upward trend. The net effect this will have on temperatures and cooling airflows around tire/wheel assemblies and other components is not clear, but it is reasonable to expect that airflow temperatures exiting the engine compartment will be higher.

There also are expected revisions to FMVSS 121 that will require substantially shortened stopping distances for over-the-highway trucks. Reductions in the range of 20% to 30% from current distances will necessitate fitment of more aggressive, larger brakes, especially on steer axles where single mounted tires are typically deflected more since they operate at a higher percentage of their rated load than drive and trailer axle duals. Larger-sized brakes will not only produce higher levels of heat rejection but also allow less space for cooling air circulation inside the wheel.

Load, inflation and tire deflection affect tire durability. Of these, inflation is the most maintenance sensitive and controllable. The industry is fortunate to have durable, forgiving designs that work well in today’s operating conditions, provided there is inflation maintenance and timely inspection for damage.

The combined effect of these trends will likely raise tire/wheel temperatures somewhat. If this is true, proper tire and wheel maintenance becomes more important than before, and programs to ensure adequate attention deserve a fresh evaluation.