Maintaining the proper inflation pressure in tires is the single most important thing you can do to get the most mileage out of a tire.
Everybody says that, but like the weather, everybody talks about it but nobody does anything about it, or even explains exactly what happens to the tread when inflation pressure isn't maintained properly.
As you know, air carries the load, so maintaining a minimum cold tire pressure is critical to the health of the casing to prevent the body ply cords in the sidewall from fatiguing and breaking. But exactly how does pressure affect the tread?
Changes in inflation pressure affect the tire's footprint, the area that contacts the road.
When a tire is underinflated, the contact patch or footprint flattens out and becomes larger. For example, a typical 295/75R22.5 dual tire has about 18-percent more rubber on the road at 70 psi than at 100 psi for a given load.
This story appeared in the Sept. 11 print edition of Tire Business.
The opposite happens when tires are overinflated; the tire footprint shrinks in size, the shoulders of the tire tend to come up off the road a bit compared with when it is properly inflated for the load being carried.
This is vital to understand when you consider that medium truck tires rotate about 500 times over the course of a mile. In 100,000 miles, each part of the tread gets pressed against the pavement about 50 million times, so if a tire is over- or underinflated, its footprint will change from optimum contact with the road and parts of the tread will experience excessive movement, which creates and/or accelerates irregular wear.
Since irregular wear results from uneven abrasion of the tread against the pavement, tires should be maintained at a consistent shape throughout their life in order to provide slow and even wear.
When they run underinflated, the slippage of the tread in contact with the ground becomes greater, which accelerates tire wear and causes irregular wear.
Overinflation will cause irregular wear, too. Let's look at some of these conditions.
Underinflation is a common cause of cupping/scallop/wavy wear on steer tires.
Overinflation usually is the cause of rib depression/punch wear on steer tires and rapid shoulder wear on both shoulders of drive and trailer tires. Due to the smaller footprint created by overinflation, the shoulders are lightly loaded and wear faster.