Bendix Commercial Vehicle Systems SmarTire Trailer-Link Op Manual - Manual (Page 11)

11
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.