Contrary to what many truckers think, irregular tire wear isn’t caused by lousy manufacturing, faulty design, bad luck or gremlins. Instead, it’s simply the result of unusual forces being applied to the surface where the rubber meets the road.
Determining the source of those forces, however, is the tough part. Different problems can initiate similar wear patterns. And tires are often bedeviled by several problems simultaneously. What’s worse, some types of irregular wear, once started, continue to grow even after all unusual forces are eliminated. Only new tires will verify the corrective action’s success.
Fortunately, most of these troubles – and the accompanying frustration – can be avoided with good preventive tire maintenance: balancing, alignment and, most importantly, inflation.
“If you truly get a handle on tire pressure, half of all your irregular wear problems will disappear,” says Guy Walenga, an engineering manager for Bridgestone/Firestone.
Walenga says too many truckers mistakenly rely on a club or hammer to determine their tires’ inflation. “That’s like feeling the side of your engine block to check the oil level,” he says.
Most people understand the importance of proper tire inflation, even if they don’t always practice it, but Walenga says few realize the significance of matching inflation within a dual set. He says an irregular wear pattern can develop when the pressure of joined dual tires differs by as little as 5 psi. This occurs because the larger (more inflated) tire covers a slightly greater distance with each turn. To keep up, the smaller tire must skip, or scrub, the missing rotational length. These spans are short, but they add up quickly. According to an informational booklet published by Bridgestone, a 5/16-inch difference in tire circumference amounts to 13 feet of scrubbing every mile. The resulting wear eventually leaves random bald patches across the face of tires.
Another inflation-related problem that often confuses and aggravates truckers is called circumferential fatigue rupture or, more commonly, zipper rip.
ZIPPER RIP. This is the classic result of a tire that’s been run flat for some distance, then aired up again. These ruptures occur because the sidewall cords of an under-inflated tire are overly flexed and heated while rolling down the road. Soon, the metal is fatigued and unable to contain pressure when the tire is inflated.
Scott Eells, assistant manager at Tire Centers Inc., in Rogers, Minn., says he sees too many tires ruined because drivers believed they could run a flat for 50 to 100 miles as long as it was dualed. “They’re wrong,” he says. “The sidewalls of a flat flex so much that the metal cords get fatigued after a short while. This damage isn’t visible unless you take the tire apart and know what you’re looking for.”
Airing a run-flat tire is always a risk. Eells says emerging zipper rips give little warning before they explode: “You’ll hear something like popping corn, then a giant ka-boom.” Federal workplace regulations require these potential bombs to be put in an approved cage, aired to 20 psi above their normal rating and left caged for 20 minutes. “That’s too big of a procedure to do on every flat,” Eells says, “and longer than most drivers want to wait.” The only options are to use a spare or to buy a new tire.
Flats aren’t the only candidates for zipper rips, though. Tires with slow, persistent leaks can also be affected. “Anything under 50 psi is probably too low,” Eells says. “And when it gets down to 30 psi, you’re asking for problems.”
The only tools needed to keep inflation pressure above the danger zone are a tire gauge and an air compressor, both available at most truck stops. A number of owner-operators tote their own gauges to ensure consistent, accurate measurements. Shop gauges aren’t always trustworthy because they’re often dropped or run over. Even without such abuse, however, gauges can become imprecise over time, says Doug Jones, a Michelin customer support manager. He recommends periodically calibrating all tire gauges using a master gauge. “We’ve gone into some shops and found gauges varying by as much as 10 psi,” he says. “That kind of disparity isn’t very helpful for maximizing tire life.”
Jones says a lot of irregular wear could be avoided if truckers regularly felt their tires. “By rubbing your hands across the face of each tire,” he says, “you can detect when something is starting to go wrong. On drive tires, you’ll be able to pick up early signs of heel and toe wear. On the steers you might find toe or alignment problems.”
SHOULDER WEAR. This pattern is most common on steering tires. Its causes are poor alignment, faulty kingpins, excessive toe or camber. Alignment and toe problems usually result in a feathered wear. Camber and kingpin wear is smooth.
Individually, these forces normally affect just one shoulder of a tire, but they’ll erase both shoulders when combined. Shoulder wear can also be found on spread-axle trailers, where the edges of tires endure significant side scrubbing.
In the latter case, he says, you should check all axles together. “We preach total vehicle alignment. It does little good to have a steer axle properly set if the rest are misaligned. Your tires will still wear in strange ways.”
Surprisingly, most alignment-related wear is caused by drive and trailer axles. This means that owner-operators in drop-and-hook operations are particularly
susceptible to irregular wear because they cannot control the alignment of every trailer they pull. The best defense in these situations is early detection.
Some irregular wear patterns can be corrected if they’re not too far advanced. This normally involves moving the affected tires from a free-rolling axle to a drive position, or from one side of a drive axle to the other. Bridgestone’s Walenga says drive axles act like a giant buffing machine that helps grind tire facings smooth again.
Irregular wear is the leading cause of premature tread loss. Staying ahead of this expensive and needless problem requires persistence, patience and practice. It can be very difficult to decipher, says Roger Stansbie, an engineering manager for Continental Tire. “In a lot of cases, you end up simply going through a process of elimination. There are so many variables on a truck: load, inflation pressure, tire design, alignment, bearing and axle condition. Collectively, they can produce all sorts of funny patterns.”
Stansbie suggests truckers work with a reputable dealer to help track down odd wear and to determine which tires are best suited for their applications. Also, tire manufacturers offer printed materials and videotapes that define various types of wear and describe their likely origins.
“Reading wear patterns,” Stansbie says, “is not so much a science as it is an art.”