|A tire is a pneumatic system which supports a vehicle's
load. It does this by using a compressed gas (usually air) inside to create tension in the
carcass plies. It is important to realize that a tire carcass has a high tension strength,
but has little or no compression strength. It is the air pressure that creates tension in
the carcass and allows the tire to function as a load-carrying device. That's why
inflation is so important. In an unloaded tire, the cords pull equally on the bead wire
all around the tire. When a tire is loaded, the tension in the cords between the rim and
the ground is relieved by pressure from the ground. The tension in other cords is not
changed. Therefore, the cords opposite the ground pull upwards on the bead. This is the
mechanism that transmits the pressure from the ground to the rim.
However, a tire's job is more than to hold a load. It must transmit handling (acceleration, braking, cornering) to the road. Cornering forces are transmitted to the rim in a similar manner to load. Acceleration and braking forces rely on the friction between the rim and the bead. Inflation pressure also supplies the clamping force which creates this friction.
A tire also acts as a spring between the rim and the road. This spring characteristic is very important to the vehicle's ride. Too high an inflation pressure causes the tire to transmit shock loads to the suspension and reduces a tire's ability to withstand road impacts. Too low an inflation pressure reduces a tire's ability to support the vehicle's load and transmit cornering, braking, and acceleration forces. Finding the optimum inflation pressure requires extensive engineering efforts on the part of tire and vehicle manufacturers.
Underinflation can cause many tire related problems. Since a tire's load capacity is largely determined by its inflation pressure, underinflation results in an overloaded tire. An underinflated tire operates at high deflection resulting in decreased fuel economy, sluggish handling and excessive shoulder wear. High deflection also causes excessive heat buildup leading to catastrophic tire failure.