Cause and Effect
Finding Reasons For Irregular Wear Saves Downtime and Money
What causes a tire to develop an irregular wear pattern? Often, the answer depends on axle position and the conditions under which the tire operates.
Most uneven tread wear conditions on drive and trailer axle tires are usually attributed to improper inflation pressure. The effects of underinflation are well documented, and the transportation industry’s reluctance to pay for effective inflation pressure maintenance programs basically ensures it will continue to be a problem for years to come. On the other hand, irregular tread wear patterns on steer axle tires generally get blamed on worn or misaligned front-end components.
Recent studies have shown that cost-per-mile almost doubles when original or retreaded tires are pulled early because the tread is worn unevenly or rapidly. Of course, most fleets blame the tire and immediately demand an adjustment which they usually get if they control enough wheel positions. Rather than argue with a customer, too many tire dealers offer some form of compensation so that future business will not be jeopardized. But that doesn’t solve the real problem.
Tread wear patterns are determined by a variety of factors. Alignment, balance, fifth wheel placement, inflation pressure and wheel width are just a few of the issues that can cause a tire to experience accelerated irregular wear.
From an engineering standpoint, any changes to the tire’s footprint will have a direct impact on performance and wear characteristics. Today’s radial tires are designed to evenly distribute load across the face of the tread when the tire contacts the road surface under the weight of the vehicle. Overinflation typically causes a majority of the load to be carried in the center of the tire’s footprint, while underinflation results in more load concentrated on the shoulders.
Continuous improvements in tire and manufacturing technology have led to a uniform and incredibly durable product. Under ideal conditions in a line haul application, a typical radial truck tire will deliver up to 200,000 miles on the original tread and up to 200,000 additional miles on each subsequent retread.
Unfortunately, this longevity actually works against truck tires because many irregular wear patterns cannot be reversed, and often results in some form of ride disturbance in steer and drive positions. However, if everything is correct from the start, the chances of achieving the lowest lifecycle cost possible increase dramatically.
In order to educate the tire and transportation industries on the causes of irregular tread wear, the American Trucking Associations’ Technology and Maintenance Council (TMC) developed the Radial Tire Conditions Analysis Guide. This useful tool includes detailed photos of the most common types of irregular wear patterns and outlines the probable causes for such wear on tires in steer, drive and trailer positions.
By using the TMC guide to diagnose wear problem, commercial tire dealers can offer solutions without making unnecessary concessions or adjustments.
Inflation, Alignment Problems
The most obvious place to start when diagnosing the cause of an irregular tread wear pattern is inflation. When steer tires exhibit an erratic wear pattern known as "rib depression" or "punch wear", one possible cause is improper inflation.
Drive tires with heel-and-toe or alternate lug wear can be attributed to different inflation pressures in dual positions. Trailer tires are also susceptible to erratic wear patterns when inflation pressures are mismatched or below recommended levels.
If uneven tread wear is severe and relatively consistent around the circumference of the tire, the likely cause can be linked to mechanical problems. Steer, drive and trailer axles must be properly aligned for tires to maintain the correct footprint. Any deficiency in the suspension or steering components will also translate into an irregular wear pattern.
When it comes to steer tires, one of the most common tread wear problems is caused by improper toe adjustment. If the wheels on an axle are closer together in the front than they are in the rear, the result is toe-in. Most tire and vehicle manufacturers recommend a slight toe-in for a loaded vehicle.
If the opposite is true, the vehicle has toe-out when unloaded. When the outside shoulders on steer axle tires are worn more than the inside shoulders, the vehicle may have excessive toe-in, while worn inside shoulders are common when excessive toe-out is present. Feather wear can also be attributed to improper toe angles.
But shoulder wear is not restricted to toe-in and toe-out issues. If the drive axles are not properly aligned, the inside shoulder of one steer tire and the outside shoulder of the other will show advanced wear. Many technicians automatically assume that full shoulder wear on steer axle tires is the result of a front-end alignment problem when it’s actually the drive axles that need alignment. Drive axles should be perpendicular to the centerline of the chassis, and tandem axles on both trucks and trailers should be parallel to each other.
Other Mechanical Issues
Regardless of the position on the vehicle, a worn or loose wheel bearing will result in a severe irregular tread wear pattern. Typically, a series of flat spots appear across the tread face, from shoulder to shoulder, around the circumference of the tire. They can be straight or diagonal, and may be accompanied by cupping or scallop wear.
Rib depression can also be caused by a worn or loose wheel bearing. In many instances, vehicles with high-speed empty hauls aggravate irregular wear patterns caused by bad wheel bearings.
Flat spots caused by frozen or improperly adjusted brakes are usually easy to recognize. They are relatively uniform and extend evenly across the tread face. In dual applications, the flat spots will be in identical positions on both tires. Once again, this problem will not correct itself in service. In fact, as the tire "skips" every time it rotates, it creates another flat spot, which eventually covers the entire tire.
I’ve actually seen new tires with severe brake skid damage repaired by building up the worn area with extruded rubber, curing the tire like a retread, buffing it to make it round again, and skiving out the original tread pattern! It doesn’t look pretty.
Mismounted or out-of-balance tire/wheel assemblies can also cause irregular tread wear patterns. These are characterized by both uniform and erratic depressions and are generally present around the circumference of the tire. In some instances, the problem can be corrected so the tire can be run out. In extreme cases, though, the tire must be retreaded or scrapped.
Too Many Variables
It is literally impossible to describe all of the reasons why a tire develops uneven tread wear. There are so many variables related to alignment, balance, inflation, suspension and even driving routes that tire technicians could not diagnose every problem without some outside assistance.
Anyone can replace an irregularly worn tire and give the customer some form of adjustment. However, if the cause is not considered, the fleet is destined to have continuous tire wear problems that force the dealer to constantly fight with the manufacturer to adjust tires that are simply not adjustable. In this all-too-common scenario, the fleet loses and the dealer loses.
A little education and support can make the dealer’s "sales" job to eliminate the actual cause of the wear a lot easier.
This article was reprinted from the Tire Industry Association’s Commercial Tire Service Today publication.