Effects of improper inflation pressure on treadwear

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.

Many people overlook the effect that mismatched pressures in dual tires have on treadwear.

It's a fact that an inflation mismatch of greater than 5 psi will result in two tires of a dual assembly being significantly different in circumference. (A 5 psi difference creates a 5/16th-inch difference in tire circumference. In a single mile this 5/16th-inch difference causes the smaller tire to be scuffed 13 feet. In 100,000 miles, the tire is scuffed 246 miles.)

Because they are bolted together, duals have to cover the same amount of road in a single revolution. As a result of the larger diameter tire dragging the smaller one (with less inflation pressure), both tires can develop fast or irregular wear; usually the smaller one develops the worst wear.

Mismatched inflation pressures are usually the cause of heel/toe wear and alternate lug wear on drive tires and multiple flat spot wear and erratic depression wear on trailer tires.

In addition, improper inflation pressure is also the cause of center wear on wide-base tires on drive axles and shoulder step/chamfer wear on steer, drive and trailer tires.

So how can inflation-related irregular wear be prevented? Here are a few tips:

1. Inflation pressure should be checked on a regular basis. Most tire companies recommend checking tire pressures weekly, but this is not always possible for many types of fleet operations. Therefore all fleets should have a structured tire maintenance program that specifies when pressures should be checked. This might be when power equipment is scheduled for preventive maintenance inspections (PMs), which could be every 25,000 miles, for example. Since trailer PMs that are based on brake or other component wear could be years apart, selecting an arbitrary time interval would be more appropriate, such as weekly, monthly, every 90 days, etc.

2. All vehicles should have an inflation update decal placed in a clearly visible spot on the vehicle that allows the technician to write the next inflation check due date on it and erase the old one.

3. Accurate tire gauges should be used to check pressures unless a tire pressure monitoring system can provide accurate data. Ball peen hammers, pointy-toed boots and visually observing the deflection of the sidewall are not calibrated gauges. Service gauges also should be checked against a master gauge weekly.

4. Every fleet should establish inflation pressure settings for the tires on its vehicles. These settings should be put in writing and made available to anyone who checks tire pressures.

5. Maintenance facilities and service trucks should have regulators on air lines that ensure the proper pressure is always delivered to tires. "Spider" or "octopus" air lines connected to a regulator should be used to inflate dual tires on one or two axles at a time to ensure that all the tires are inflated to the same pressure and eliminate mismatched dual-tire pressures.

6. Naturally, valve caps always should be installed on tire/wheel assemblies and dual tires should always be mounted so that the valve stems are 180 degrees apart so that the inside tire's valve stem is accessible.

Many trailers are now equipped with automatic tire-inflation systems that are designed to supply tires with air when needed as they run down the road.

Many fleets have found that these systems are not something they can spec on their trailers and then forget about since all their tire maintenance is automatically addressed. Automatic tire-inflation systems do not work when trailers are parked or dropped from tractors so tires can go flat at these times.

They also can develop leaks that can cause flat tires if they are not maintained properly. Therefore it is essential to inspect tires periodically to ensure they are serviceable, have adequate tread depth, are properly inflated and that the systems are turned on, have no leaks and are working correctly.

Fast and irregular treadwear caused by improper inflation pressures can rob fleets of tire mileage and fuel economy.

In this day and age of readily available technology (tire pressure monitoring systems and automatic tire-inflation systems) that can help fleets improve their tire-inflation maintenance, there's no good reason why these wear patterns should still plague the industry.

Peggy can be reached via e-mail at [email protected].