Don’t ride the Zipper: keep your tires properly inflated.

Get the point? Underinflation is the most common factor in prematurely ruined tires. Period. For the want of about 20 minutes per week, many owner-ops – and fleets too, it should be noted – seem more than willing to give up $50 to $100 worth of tread and casing life. According to the Technology and Maintenance Council (TMC), a proper tire pressure check on a typical tractor-trailer should take in the range of 20 minutes a week. That’s gauging the tire, topping it up, and screwing the cap back on.

TMC notes that just 10% underinflation will shorten tread life by 9 to 16%.
If we use an average tire price of $400, that underinflation costs you $40 per tire. Now ask yourself if you’d consider 10-psi underinflation “close enough” and just let the tire go? It’s alarming, the tire people tell us, how many tires they find consistently underinflated when they go out into the field and do such surveys.

According to Al Cohn, communications manager for Goodyear Commercial Systems, it’s the air that carries the load, not the tire: the tire is just the containment device.

“If there’s not enough air to properly support the load, the sidewalls will flex more than they were designed for, and that flexing causes excessive heat build up,” he says. “Together, the weakening of the steel cords in the sidewall, and the softening of the rubber caused by the heat can trim 15% off the life expectancy of the tire.”

That one-two punch is all a tire needs to send it to the scrap yard long before its time. The effects of heat and flexing are so dramatic that both TMC and the Rubber Manufacturers’ Association (RMA) recommend that any tire found to be 20% or more underinflated should be immediately removed from service, demounted, and inspected for damage. How many of you might consider 80 psi rather than 100 psi “good enough for now”?

Eighty psi certainly won’t feel much different from 100 psi when you boot it on your pre-trip, so how are you supposed to know? You need to check pressure regularly, advises Cohn. “Use a good gauge – a calibrated gauge. If the gauge reads 10 psi less than it should, you’re going to be running underinflated, even after all your diligent work on Sunday morning.”

Even under the best conditions, a tire will lose about 2% of their inflation pressure over a month. If the valves are seeping, or the bead isn’t well seated, the attrition could be worse. So, you’re going to lose air no matter what you do. You can minimize the losses by topping tires up regularly, and maintain the recommended pressure for your loads and application.

Do you inflate your tires to 100 psi or so just because your pal does? That may be a good ballpark figure, but you can do better. Bridgestone/Firestone strongly suggests using the load and inflation tables (available at any tire dealer or on-line at most commercial tire websites) to calculate the optimum pressure for your application. The difference between 95 and 105 psi can have a huge impact on how the tire wears in service – and how long it lasts.

And matching pressures is especially important on dual assemblies. An inflation mismatch greater than 5 psi means that the two tires in a dual assembly are now significantly different in circumference – up 5/16 in., actually. While that may not sound like much, because they’re bolted together, they have to cover the same amount of road in a single revolution. The larger tire will drag the smaller one a distance of about 13 ft for every mile, or 246 miles for every 100,000 miles on the clock.

Aside from the decrease in life expectancy from overheating, underinflated tires don’t make proper contact with the road, and that causes irregular tread wear – not to mention traction issues. Uneven shoulder wear usually results from underinflation, or center wear on an overinflated tire. But don’t wait until the tread starts disappearing to worry about your tire pressure. Get your calibrated tire gauge out and spend 20 minutes protecting your investment.

Mechanical Wear
You don’t have to search hard to find examples of tires literally vanishing before their owners’ eyes. Scrubbing and stresses on the tread caused by bad wheel alignment of frame geometry are everywhere. Unfortunately, the tires often take the rap.

“Well, my buddy’s Brand X tires ran out to 120,000 miles, but mine only went to 80,000, so I got a bad set of steers.” Heard that one before? You probably couldn’t count the difference the two owners’ trucks, operations, lanes of travel, percentage of time on- and off-road, speed, load weights, etc. So how can you expect similar results in tire life?

Tires exhibiting wear are almost always responding to some external factor, such as wheel alignment, says Michelin’s Ralph Beaveridge. “Toe-in and toe-out settings affect tires dramatically, and while all tires will exhibit the same type of wear resulting from an alignment problem, some weather it better than others, depending on the design characteristics of the tires,” he says.

A few years ago, Michelin did some surveying of fleet take-offs and found that an amazing 78% of the tires sitting in a scrap pile were pulled off before their life expectancy was up. “It’s tragic to see a $500 tire only delivering $100 worth of life,” Beaveridge says. “We deal with these maintenance issues all the time with fleets, but it’s much harder to get to owner-operators on it.”

Fleets, Beaveridge explains, have the advantage of seeing patterns develop that clue the maintenance people in to a problem. It’s not the same with one driver, one truck, and one set of tires that lasts a year or more.

“They just don’t have the network of people to discuss these issues with,” he says. “The shop people are on the phone with us or the service people all the time. How often does an owner-operator visit his tire advisor?”

Tire choice is critical for appropriate life expectancy, Beaveridge notes. There can be much more than a fifty-dollar difference between two given tires. If one is designed for a regional application with heavy start-and-stop use, or more turning and scrubbing, it may not wear properly in a long-haul application. “The two make look similar, even cost nearly the same, but sacrificing 20% or 30% of the life of the tire by putting in the wrong application just doesn’t make sense,” says Beaveridge. “The difference is way more then $50.”

It’s interesting talking to two knowledgeable guys like Michelin’s Ralph Beaveridge and Goodyear’s Al Cohn. Both are passionate about tires, both would bleed blue if you cut them – albeit slightly different shades – and both say exactly the same thing about tire wear, care, and maintenance.

“Choose the right tire for the job, keep them properly inflated, and be very aware of the tires’ condition as they wear,” they say (I paraphrase). “The wear is telling you something is wrong. Pay attention and get the problem fixed before it kills our tires.”What’s A Zipper?
A zipper rupture is a circumferential tear in the mid sidewall of a steel cord radial tire. Weakened steel cables in the tire’s sidewall, caused by running underinflated or flat – defined as, a tire that carries less than 80% of proper inflation. They are almost impossible to detect on a visual inspection, and no easier to detect using X-rays, ultrasound, or other tests. When a zipper lets go, the resulting rupture and air blast can explode with the force of as much as three quarters of a pound of dynamite, leaving a 10- to 36-in. gash in the sidewall that looks like a zipper.