Spotlight: Tread carefully to determine tire failure

Failure analysis always has been about sleuthing-looking at a component, finding evidence of what led to its demise, and then prescribing steps to help customers avoid a recurrence.

But while some components are easier to read than others, tire analysis can swing to either extreme. The following are examples from the Technology & Maintenance Council’s (TMC’s) Radial Tire Conditions Analysis Guide.

Tireside Reading
Tires usually are easy to read when they wear their scars on the outside. Misalignment, imbalance, improper inflation and driver abuse all leave telltale tread wear or obvious sidewall damage. Except for prolonged underinflation and severe sidewall damage, these more obvious conditions usually can be remedied and the tire then can be returned to service.

For example, the tire in Figure 1 displays rib feathering, where ribs are more worn on one side than the other. The cause is excessive side force, almost always the result of misalignment-usually excessive toe-in or out or – damaged steering or chassis components.

On the other hand, scalloping or cupped tread around the entire circumference of the tire, usually at the shoulders (see Figure 2) results from a tire/wheel out-of-balance condition, improper mounting or other assembly non-uniformity. And, since any tire/wheel assembly has some amount of imbalance that often is held in check by shock absorbers, a bad shock can be the culprit.

When there’s significant imbalance, one part of the assembly is substantially heavier than the rest, causing that part of the tire’s tread to strike the pavement with additional force. This chews away rubber until that part is no longer the heaviest.

Then, a new heavy spot often develops, and the scalloping migrates around the entire circumference. Once the imbalance is fixed, the tire often can be returned to service on a drive axle, according to TMC, where the tread irregularities will be evened out.

Deeper Problems
Tires become much harder to read when they’re internally damaged, or after a major failure. To further complicate matters, such damage often is the result of an improper repair of a lesser problem.

For example, the tire in Figure 3 has a crack extending from a repair (internal view). The problem here is that the original injury was too large for the nail-hole repair that was performed. More of the original injury should have been removed, and a section repair performed.

Since an improper repair often allows moisture intrusion and subsequent belt corrosion and weakening, the result can be separation of the tire components.

Worse than an improperly repaired tire is one that shouldn’t have been repaired at all, and instead should have made its way to the scrap pile. While that’s a recipe for a catastrophic failure, a gray area exists concerning what should and shouldn’t be repaired.

Limits of tire repair can differ from manufacturer to manufacturer, according to Kevin Rohlwing, senior vice president of training for the Tire Industry Association (TIA). As such, he advised obtaining this information for every brand of tire serviced and to keep technicians trained on a regular basis.

The most common catastrophic failure can be seen on most any highway, but it’s still worth mentioning. Road alligators, shards of rubber darkening every motorist’s landscape, repeatedly have been shown to have little to do with whether a tire has been retreaded. The disintegration that causes this blight almost always results from underinflation.

How? The reinforcing belts in a tire are meant to flex a certain amount while rolling under load, which generates a manageable amount of heat. When a tire is significantly underinflated, however, the belts flex to a much greater degree, and generate enough heat to break down the integrity of the tire. If bending a paper clip back and forth can burn one’s fingers, imagine what bending a steel belt can do to a tire. When a rubber fragment has wires sticking out of it, it’s not just tread-it’s casing. And that’s a direct result of tire pressure neglect.

A precursor to total tire disintegration from severe underinflation can be observed in a zipper failure. Figure 4 shows a circumferential break in the mid to upper sidewall, exposing an even line of broken cords.

An insidious, potentially deadly aspect of a zipper failure is that it can happen when the already weakened tire is being inflated. The air pressure becomes too great for the cables to hold, and the entire area ruptures suddenly, releasing a tremendous amount of force, which can cause injury or death.

Rohlwing advised that careful inspection-with the tire deflated-can reduce the risk of a zipper. Warning signs include creases in the inner liner, undulations or bumps in the sidewalls, or a crunching noise heard when the sidewall area is pressed (the same sound often heard just before a zipper explodes).

Again, the best way to prevent zippers-and road alligators-is to make sure tires always are inflated to the right pressure. As Harvey Brodsky, managing director, the Tire Retread Information Bureau, said, “Pump ’em, don’t thump ’em. Put the gauge to the tire. If you think you can really determine whether tires need air by thumping them, you might as well thump the hood of the truck to determine if it needs oil.”

Heat is undeniably a tire’s worst enemy, and on rare occasion it can come from sources other than underinflation. The tire in Figure 5 displays brittle, distorted, cooked rubber that’s limited to the bead area. This is a clue that the heat was transmitted to the tire through the wheel.

Causes here could be frequent hard braking, improperly adjusted, defective or dragging brakes, or insufficient air flow around the brakes. With no hope of repair, this tire is an excellent candidate for the scrap pile.

Sometimes, tires are damaged during a previous service visit or even before they are ever mounted to a wheel. The torn bead in Figure 6 could have been caused by a forklift, or by poor mounting/dismounting procedures, including inadequate lubrication. As bad as this looks, if there’s no rust or damage to the wire, the bead can be repaired and the tire returned to service, according to TMC. However, if the wire is rusted, gouged, kinked, broken, loose or separated, the decision to repair or scrap is less cut and dry.

Still other dangers lurk in the shop. Using the wrong tire lubricant when mounting or dismounting a tire can cause blistering, swelling or sponginess in the bead area, as seen in Figure 7. Wrong lubricants include anything that’s petroleum-based, such as oil or diesel fuel, and antifreeze. Eventually, the bead may appear dry and brittle, although a petroleum odor still may be present. If so, trash the tire.

Occasionally, one type of damage can mimic another. What appears to be scalloped shoulders on the steer tires in Figure 8 was not caused by imbalance. The fact that only the outer shoulder of one tire and only the inner shoulder of the opposite tire are affected is the telltale sign that thrust angle is the likely culprit.

Thrust angle actually is caused by rear-axle misalignment. When the thrustline, which runs perpendicular to the center of the rear axle, departs from the geometric centerline of the truck, you will have misalignment.

The amount of misalignment is expressed in degrees of the angle formed by those two lines. Negative thrust angle, for example, means the rear axle is tracking to the left, and will tend to steer the truck to the right. The driver will have to compensate by steering the vehicle to the left, and the truck will “dog-track” at a slight angle down the road, scrubbing rubber from the tires’ right hand shoulders.

Depending on the severity of the wear, the tires can be rotated to another position, such as the trailer, or they can be retreaded.

Correctly analyzing the causes of tire failure can help you discover more significant vehicle performance issues, as well as if a tire can be repaired or should be scrapped. For customers, the results can be safer, more cost effective operation.


Resources For Expert Tire Analysis, Repair
Deducing the causes of tire failure and taking proper steps to avoid a repeat means taking advantage of the most current, comprehensive information and training available.

TMC’s Radial Tire Conditions Analysis Guide is a must have for any service facility. The guide includes the examples cited here, and over 100 more.

“The book is the result of a coordinated effort by a broad spectrum of fleet users and tire industry professionals,” said Robert Braswell, technical director, TMC. “It represents the consensus of the Council on radial tire conditions it has identified, based on the S.2 Tire and Wheel Study Group’s comprehensive work in this area.”

For more information, or to order a copy, call TMC at 703-838-1763, or call the American Trucking Associations’ customer service number, 800-ATA-LINE. You can also e-mail tmc@trucking.org, or visit http://tmc.truckline.com.

The Tire Industry Association offers numerous training and educational resources for both beginner and seasoned technicians. TIA has provided training and certification to more than 13,000 technicians, and offers a large lineup of reference materials.

For information on available TIA training and resources, contact 800-876-8372, ext. 106, or e-mail training@tireindustry.org.


Feathering
Feathering of tread ribs almost always is the result of misalignment-usually excessive toe-in or out-or damaged steering or chassis components.

Scalloping
Scalloping or cupped tread around the entire circumference usually results from tire/wheel out-of-balance condition, improper mounting or other assembly non-uniformity.

Bad Repair
A crack extending from an internal repair indicates that the original injury was too large for the nail-hole repair that was performed.

Zipper
A circumferential break in the mid to upper sidewall, exposing an even line of broken cords, is a zipper failure. It’s caused by severe underinflation, which fatigues casing cords.

Burned Bead
Brittle, distorted, cooked rubber limited to the bead area indicates heat was transmitted through the wheel. Causes include frequent hard braking, improperly adjusted, defective or dragging brakes or insufficient air flow around brakes.

Torn Bead
A torn bead could have been caused by a forklift, or by poor mounting/dismounting procedures. If there’s no rust on the wire, the bead can be repaired and the tire returned to service.

Petro Damage
Using petroleum-based products or antifreeze as lubricant when mounting or dismounting the tire can cause blistering, swelling or sponginess in the bead area. The tire should be discarded.

Thrust Angle
The apparently scalloped shoulders on steer tires did not result from imbalance. Only the outer shoulder of one tire and the inner shoulder of the opposite tire are affected. The culprit is rear-axle misalignment, or thrust angle.

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