Thermal Equilibrium

As a vehicle moves, its tires naturally heat up due to friction from the road and the flexing of its side-walls. Weight, vehicle speed and a tire’s starting inflation pressure all have an impact on how much, and how quickly, heat is generated.

As the tire generates heat, its pressure increases, causing a reduction in side-wall flexing. Less side-wall flexing and road resistance, combined with air rushing past the tire as the vehicle moves, effectively counteract the conditions that cause the tire to heat up. As a result, the temperature increase tapers off until the tire reaches a point of balance called “thermal equilibrium.”

Tire thermal equilibrium is the point where the heat being generated is equal to the heat being dissipated. Tires are designed with the principles of temperature and pressure in mind in order for them to achieve thermal equilibrium. Once a properly inflated tire reaches thermal equilibrium, it will operate at its peak; providing the best performance, handling, tire life and fuel economy.

SmarTire Trailer-Link™TPMS Temperature Compensation

Since a tire’s contained air pressure naturally increases as a vehicle moves, it can be difficult to tell if a hot tire is under-inflated. Without some form of temperature compensation, a hot tire that is under-inflated might appear to be fine because its contained air pressure is at, or above, its Cold Inflation Pressure (CIP).

For example, a tire correctly inflated to a CIP of 105 PSI at 65°F will reach thermal equilibrium when its temperature increases to 152°F and its pressure increases to 125 PSI. A tire starting at 95 PSI at 65°F (10 PSI under inflated) would have to reach 202°F for it to reach thermal equilibrium (125 PSI). The tire will then be running 50°F hotter than it should be, causing more tire wear and the potential for a catastrophic failure or tire fire.

When checked using a handheld gauge or a tire monitoring system that does not measure operating temperature, this 10 PSI under-inflated tire can appear to be normal. When equipped with tire sensors that mount inside the tire, SmarTire Trailer-Link TPMS measures both tire pressure and temperature in order to provide “Temperature Compensated” pressure deviation values and alerts. By measuring the operating temperature of a tire and comparing it to the CIP value programmed into the system, the SmarTire Trailer-Link system will know what a tire’s pressure is supposed to be in relation to its operating temperature.

The system is able to warn the driver of an under-inflated tire even if that tire’s actual contained air pressure is at — or above — its CIP.