E. Linacre

11/'98

In some regions, such as the Great Plains of North America, the passage of a cold front may produce a dramatic change in surface air temperatures, for instance from +12° C to -32° C in the course of 6 hours. If a car is parked outdoors in these conditions, the reduced temperature will lead to a proportional lowering of pressure in the car's tyres (ideal gas law, Note 1.G). The pressure is proportional to the absolute temperature, given in degrees Kelvin. For instance, in the above example, the tyre pressure would drop from a normal 207 kPa (30 psi) to 175kPa, ie (273 - 32) x 207/(273 + 12). This will make the tyre appear slightly flat at 25 psi.

Conversely, frequent braking, or driving fast over a rough road, or carrying a heavy load may raise the temperature of air within the tyre 40° C or more above the ambient air temperature. In this case the same tyre’s pressure would be raised to about 34 psi [236 kPa, ie (273 + 52) x 207/(273 + 12)]. This contributes to catastrophic sudden tyre ruptures.