One of the goals when specifying the proper inflation pressure is to create the optimal footprint, which is the portion of the tread that's in contact with the ground when the vehicle is fully loaded. When a tire is properly inflated, its footprint should resemble a rugby ball. Under ideal conditions, the widest part is in the middle and it gradually narrows as it approaches the shoulders. Operation with the correct footprint leads to a number of benefits, including smoother tread wear, minimal rolling resistance and improved casing durability.
But selecting the correct inflation pressure involves a lot more than inflating the tire to the pressure listed on the sidewall. In many instances, the values listed on the sidewall result in an overinflation condition because the maximum capability of the tire is far more than is necessary for the weight on the axle. It's going to require some work to find the best inflation pressure for a particular fleet, but the payoff is lower tire costs. The Tire and Rim Assn. publishes Load and Inflation tables that list the exact weight carrying capacity with the corresponding inflation pressure for different tire sizes.
Until recently, the basic principles of improper tire inflation pressure on treadwear were easy to understand. When the tire is overinflated, the center of the tread will be forced out so the tire begins to look like an inner tube. The footprint will look more like a football — very narrow at the edges and wide in the center. Since the load is not properly distributed, the rubber in the center will wear out faster.
When underinflated, the sidewalls have a more pronounced bulge, which means more of the weight is being carried at the tread shoulders. You also create additional heat at the belt edges as a result of the excessive flexing, and the footprint changes to something that resembles a wide bar with a slight bulge. An infrared image will also reveal higher temperatures around the belt edges. In this case, the tread area in the shoulders will be worn away faster than the tread in the center.
The introduction of ultra-low-profile wide-base tires has reversed the relationship between improper inflation pressure and treadwear, leaving many fleets puzzled. I'd like to try to explain why overinflation on these tires wears the shoulders faster and underinflation wears the center faster.
First of all, the footprint on this ultra-low-profile tire is very wide and the belt package is incredibly stiff. There's little to no flex in the center, so excessive inflation pressure will not cause it to bow. The extra pressure actually forces the belt edges out, so it's almost like riding on an invisible rail. This causes the shoulders to wear faster since they are carrying more of the load.
In an underinflated condition, there isn't enough pressure to force the shoulders in proper contact with the pavement so the footprint shows less contact at the edges. This naturally wears the center of the tire out faster because the shoulders are not carrying enough of the weight. The tire is basically riding on the center of the belt because it is so stiff and rigid.
Regardless of the type of tire, however, improper inflation pressure is not good for the bottom line. Tires are an expense that can be controlled, but they require constant maintenance and attention. And when steps are taken to ensure the correct footprint on a regular basis, fleets get the most return for their tire dollar.