Why tires fail

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

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

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

Circumferential break in 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.

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

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

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

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

Failure analysis has always been about sleuthing – about looking at a component, finding evidence of what led to its demise, then taking steps to avoid a repeat performance. But while some components are easier to read than others, tires can be among both the easiest and hardest. The following are examples from the Technology & Maintenance Council’s newly revised Radial Tire Conditions Analysis Guide, due to be published next month.

Out of line, out of balance
Tires are usually 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 can usually be remedied and the tire returned to service.

For example, the tire in Photo 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 Photo 2) results from a tire/wheel out-of-balance condition, improper mounting or other assembly non-uniformity. And, because any tire/wheel assembly has some amount of imbalance that, on many suspensions, 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 dishing or scalloping migrates around the entire circumference.

Once the imbalance is fixed, the tire can often be returned to service on a drive axle, according to TMC, where the tread irregularities will be evened out.

Repairs gone bad
Tires become much harder to read when they’re internally damaged, or after a major failure. To further complicate matters, such damage is often the result of an improper repair of a lesser problem. For example, the tire in Photo 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.

Because an improper repair often allows moisture intrusion and subsequent belt corrosion and weakening, the result can be separation of tire components. This tire’s owner would be advised to consult his retreader/repairer for warranty adjustment and, if possible, a new repair.

Worse than an improperly repaired tire is one that should have been scrapped rather than repaired. Whether or not to repair a tire remains a gray area. Limits of tire repair can differ from manufacturer to manufacturer, according to Kevin Rohlwing, senior vice president, education and technical services, the Tire Industry Association. If your maintenance program includes tire repair, “this information must be obtained from every tire manufacturer and retreader that supplies your fleet, and technicians should receive training on a regular basis.”

The scourge of underinflation
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, have repeatedly been shown to have little to do with whether a tire has been retreaded. The disintegration that causes this blight almost always comes 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 your fingers, imagine what bending a steel belt can do to a tire. If you see a rubber fragment on the road with wires sticking out of it, that’s not just tread. It’s casing, and it’s a direct result of tire pressure neglect.

A precursor to total tire disintegration from severe underinflation can be observed in a zipper failure. Photo 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 by well-meaning maintenance personnel. 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.

TIA’s Rohlwing advises 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 sickening sound often heard just before a zipper explodes).

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

A host of possibilities
Heat is undeniably a tire’s worst enemy, and on rare occasions it can come from sources other than underinflation. The tire in Photo 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 abused in the terminal or shop. The torn bead in Photo 6 could have been caused by a forklift, or by poor mounting/dismounting procedures, including inadequate lubrication. As bad as this looks, according to TMC, if there’s no rust or damage to the wire, the bead can be repaired and the tire returned to service. However, if the wire is rusted, gouged, kinked, broken, loose or separated, the tire supplier or retreader will have to make the call on repair versus scrap.

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, like the tire in Photo 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 may still be present. Scrap the tire.

Occasionally, one type of damage can mimic another. What appears to be scalloped shoulders on the steer tires in Photo 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 that thrust angle is the likely culprit. Thrust angle is actually 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, misalignment exists.

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 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 trailer, or they can be retreaded.

Analyzing tire failures isn’t as easy as it may appear. And since tires represent one of your largest maintenance expenses, it pays to investigate rather than jump to conclusions. A simple maintenance or repair mistake left uncorrected, could cost you dearly in the long run.

Doing it by the book
The importance of proper tire maintenance makes TMC’s Radial Tire Conditions Analysis Guide a great resource for any shop. The updated guide will include the examples cited here, and more than 100 others.

“The new book is the result of a coordinated effort by a broad spectrum of fleet users and tire industry professionals,” says TMC Technical Director Robert Braswell. “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 this site.