While uneven wear is a concern, more critical is the negative effect of underinflation on tire durability. It can produce excess fatigue in the tire and excess deflection, which leads to heat generation. The excess heat can cause failure in several areas, including the bead area and sidewall. Commonly, it will result in "zipper" ruptures.
"Underinflation is one of the main reasons for tire issues," indicates Dan Steltmann, vice president of research and development, Titan Tires. "For on-road tires, speed is an issue. But with off-road tires, heat generation is more of a problem because the thickness of the tread is greater for abrasive off-road conditions. When you have that much rubber in a tire, heat can be harder to dissipate."
"If you overinflate the tire by 5%, the tire will experience irregular wear in the center," adds Jones. "It will be more susceptible to road hazards and sidewall damage. At 10% below the recommended inflation pressure, the casing has a tendency to flex more in the sidewall. If it's severely underinflated, you will run on the shoulder of the tire and get irregular and rapid wear on the shoulder. Excess flexing in the sidewall can result in a zipper rupture."
Jones equates the cords in the tire to a clothes hanger. "If you bend it beyond its yield position, it will break. The same thing happens with the cords in the sidewall of the tire," he states. "They flex beyond their bearing capacity. One wire will break, then those on each side will break. Eventually, you will have what appears to be a 'zipper.' "
A bias tire is particularly susceptible to failures caused by overinflation since, by design, it resists flexing, says Birkenholz. While it's not advisable to run any tire underinflated, a radial is a bit less susceptible to these types of failure. "Because of its construction, a radial tire likes to flex in the sidewall," he points out. "It likes to bulge, so you don't create that friction."
Develop a checklist
Because maintaining proper inflation is so critical to tire life and wear, tire manufacturers advise fleet owners and managers - regardless of fleet size - to implement a tire maintenance program. Following are some checklist items to include:
Pre-trip inspection - A pre-trip inspection, which includes vehicle and tire inspections, is required by the DOT for equipment operated on-highway. However, it makes sense to perform a similar inspection on equipment and tires operating off-road, as well.
Fleet inspection check - Appoint someone to inspect the fleet periodically when vehicles are domiciled. Fleet managers can perform the checks themselves, outsource them or do a combination of both.
"However it's done, make sure the dealer understands what your tire maintenance system mandates," says Jones. "Even if you outsource it to your dealer, don't take a hands-off position. You still need to be actively involved."
Look for nails, cuts, scrapes and deep gouges. "Also check valve caps," suggests Steltmann. "For starters, make sure you have them. They are usually well hidden, but they can get damaged or packed with mud, which can lead to a tire failure."
Monitor inflation pressures - Ideally, someone should check inflation pressures on a weekly basis. To determine proper pressure, refer to load and inflation tables, which are available on manufacturer web sites or in tire handbooks.
Michelin also offers stickers that identify proper inflation pressure for individual tires. These can be affixed to the vehicle for handy reference.
Conduct a scrap analysis - Determine why tires are coming out of service. It could be because you're running a tire that isn't the best option for the application. Many manufacturers offer a selection of tires specifically designed for on- or off-road or a combination of both.
Use an accurate tire gauge - Options include digital, dial and stick gauges. Verify your gauge periodically to ensure you're getting accurate readings.
While stick gauges are the most popular, they're mechanical, so they can wear out. "Digital gauges are a good option, because if they fail, they fail completely," says Lutz. "They won't give a false reading."
Maintain a written history - By keeping a written history of air pressure checks, you can determine if any trends develop. "If you notice that you have to keep adding air to the same tire, you can identify problems," says Lutz.