BW8002 Page 10
THE PRESSURE TEMPERATURE RELATIONSHIP
WHY IS IT IMPORTANT? Air naturally expands when heated and contracts
when cooled. Inside a contained vessel such as a tire, this expansion and
contraction causes a change in contained air pressure. As a tire heats up,
its pressure will naturally increase and as it cools down, its pressure
will naturally decrease.
Tire manufacturers specify that tire pressures should be checked and
adjusted when a tire is “cold”, but most people may not know why, or even
what a “cold tire” is. The temperature of a tire has a significant impact on its
inflation pressure.
According to tire manufacturers, a tire is considered to be “cold” when its
temperature is 65°F (18°C). The recommended inflation values provided by
vehicle manufacturers, fleet maintenance personnel or published load
inflation tables are called ‘Cold Inflation Pressures’ (CIP) because they
represent the correct amount of pressure a tire should be inflated to when it
is “cold”. The reason that tires have cold inflation pressures set at specific
temperatures is because a tire’s pressure will change relative to its
temperature.
Tire manufacturers never recommend inflating a tire to less than the
specified cold inflation pressure. In extreme cases, the beads of a
commercial tire can unseat if its pressure gets too low resulting in a
catastrophic tire failure.
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