Your tires come with a diagnostic manual built into them. By “reading” tread wear, you can identify a number of problems and give yourself a smoother, safer and less expensive ride.
When you read your tire treads looking for problems, you’re looking for “irregular wear.” The Bridgestone/Firestone videotape “Saving Through Reducing Irregular Wear” makes it clear just what irregular wear is. Typical of over-the-road operations where a tire tread can last a long time, irregular wear is always a situation in which one part of the tread wears faster than another. The rub, so to speak, is that what wears out first always determines the removal mileage. So, the goal of good tire maintenance is to make the entire tread surface wear out at exactly the same time.
Some kinds of irregular wear – the spotty stuff that occurs because of uneven seating of the rim on the wheel, imbalance, bad shocks, or panic stops that lock the wheels – is like cancer. Once it starts, it grows, because the wear itself makes the tire hop up and down. That makes the wear continue in the same uneven pattern.
Other types of irregular wear occur either because the weight of the truck is not properly distributed on the tread, or because the wheels aren’t all headed down the road in the same direction. The latter situation, known as misalignment, will cause one or more tires to roll at an angle, producing what are called side forces. The tires are literally scrubbing sideways as they roll down the road. A 2-inch misalignment over a 181-inch. wheelbase means the steer tires have to pull the vehicle 58 feet to one side each mile. This is equivalent to scrubbing 1,100 miles sideways in 100,000 miles!
Doug Jones, support manager, North America, at Michelin N.A., Inc., recommends you think of the vehicle as a whole when doing alignment and that you follow the Technology and Maintenance Council’s Recommended Practice 642 covering Total Vehicle Alignment. The Bridgestone/Firestone tape makes it clear why this is so important. We usually think of alignment as a front end or front axle problem because the front axle does the steering, and, for that reason, has the most wearing parts. In fact, drive axles that are misaligned with each other and with the front axle present the most common misalignment problem on trucks. The second most common problem is trailer axle misalignment.
To do the alignment right, a tractor needs to be put on a large alignment rack, the most sophisticated of which use lasers, so that all the axles can be properly put in line. If you regularly pull the same trailer, its axles should be set at the same time.
Learn how to read tread
Both Jones and Al Cohn, manager of training and technology at Goodyear, agree that your fingertips can help your eyes when it comes to reading tire tread condition. Cohn calls this skill “fingertip diagnostics.”
When you rub your fingers across the surface of a tire that is scrubbing down the road, the tread will be rough in one direction and smooth in the other. Bridgestone/Firestone says that this symptom almost always means side force is at work.
Diagonal wear consists of localized flat spots worn diagonally across the tread, often repeating several times around the tire’s circumference. Diagonal wear can develop from a brake skid, spot wear, shoulder wear, or other conditions, but takes a lot of miles to develop. It can also begin with loose wheel bearings and is aggravated by misalignment.
Toe problems: Imagine a situation where the two front wheels have toe-in, meaning they are turned so that their forward edges are closer together than the back ones. In this case, the tread will feel smooth when moving inward toward the center line of the truck, while it will feel rough or reveal “feather edges” when moving outward. When tires are toed out so that their rear edges are closer together than the forward ones, the tread will feel smooth when moving outward, and rough moving toward the centerline of the chassis. Incorrect toe-in settings in the front suspension, or problems with Ackerman angles (the different angles that wheels must rotate to when making a turn) can cause toe-in or toe-out wear symptoms.
Thrust angle: On some trucks the drive axles are nearly parallel with one another but not aligned properly with the chassis and front axle. This is called a misadjusted thrust angle, and it will send the truck down the road at an angle. To detect a misaligned thrust angle, “You’ll need to look at two tires,” says Cohn. You’ll feel tread wear similar to what you get with toe problems except that the wear pattern on one side will feel like a toe-out wear pattern, while the other will feel like a worn toe-in pattern. You’ll eventually be able to see fast wear on the outside on one side, and fast wear on the inside on the other.
Scrub angle: There are cases where the drive axles are at an angle to one another but don’t send the truck down the road at the wrong angle. They are both skewed, but the angles average out close to the correct alignment. In these cases the scrub angle is said to be misadjusted, and you’ll likely see just a diagonal wear pattern on the drive axle tire treads. Axles that shift around because of loose or worn axle components can also cause these tread wear patterns.
Dog tracking: You are said to have a dog tracking problem when either drive or trailer axles are out-of-line, creating a situation where the trailer does not follow directly behind the tractor. If the dog tracking is bad enough, the driver will be able to see more of the trailer in the mirror on one side than on the other.
To tell whether you have thrust angle and/or dog tracking problems due to drive axle misalignment, or a rig dragged out of alignment because of misaligned trailer axles, you need to evaluate the driving symptoms. If the truck pulls constantly to one side, the problem is in the mounting of the drive axles; if it wanders and you need to crank the wheel in both directions to keep it moving straight ahead, the problem is with the alignment of the trailer axles.
Inflation: “Inflation is the number one tire maintenance issue for fleets,” says Cohn. “A point many fleets forget is that just gauging tires isn’t enough. You have to replace or recalibrate old gauges. Many fleets seem to use gauges for years, even after they’ve been repeatedly dropped on concrete, without even recalibrating them.”
“With underinflation, the center of the tread lifts up off the road,” says Jones, “and you’ll get wear on both shoulders.” But, with overinflation “the center ribs will move outward from the center of the wheel. Then, the shoulders will show good wear, while the center of the tire will show high wear.”
Cohn adds that severe underinflation can produce “low and high lugs all around. The tread will look ugly almost everywhere”. Whereas, he says, with overinflation, “strictly the center rib becomes depressed compared with the others.”
It’s important to always be aware that underinflation, the natural consequence of normal leakage without proper maintenance, produces severe overheating of tires. And remember that the ideal inflation pressure is lower when you’re running empty or at lighter loads.
Camber: Camber also affects tire wear. Positive camber is a condition where either improper manufacturing or severe overload has caused an axle to bend downward in the middle. This causes the tires to tilt, so they are too far apart at the bottom and too close together at the top. You’ll see wear on the inside shoulder on both sides. Cohn says the tires will feel quite a bit like they do with toe-in wear.
Cupping/scallop wear consists of localized dished out areas of fast wear creating a scalloped appearance around the tire. This kind of wear typically begins on the shoulder ribs, but may progress to adjoining ribs. Caused by being out-of balance, improper tire mounting on the wheel, or worn shocks.
Cupping wear: Cupping wear is a situation where you get several bands of high wear at various locations around the tire. “The tire is shimmying relative to its axis,” says Jones. Cupping occurs when the tire is not evenly seated on the rim, which means its diameter is not the same all the way around. It can also occur with imbalance, or if you have bad shock absorbers, loose or worn suspension bushings, or misadjusted air springs. It often starts at one location and then spreads around the tire.
Flat-spotting: This occurs when a driver locks up the wheels and fails to immediately get off the brakes. It starts as a single, flat, scrubbed band of shallower tread running across the tire. But it can spread cupping wear around the tire because of the vibration it creates. Having ABS – and keeping it in good shape – helps eliminate flat spotting. The more channels your system has, and the better the maintenance it gets, the better it works in this respect.
Chris Gamble, president of Balance Masters, a maker of dynamic wheel balancers for truck wheels, offers an explanation for a rare type of cupping wear that can wreck havoc with your braking. Older trucks with very aggressive front axle limiting valves, which limit air pressure to the front brakes to a very low level, sometimes experience this unusual form of cupping wear on the front tires. The problem starts with imbalance. If the driver applies the brakes while the wheel is bouncing at high speed, the tire may develop cup wear waves along its diameter. The tire actually starts to skid a little as each cup wave reaches the road. The result, over a long period of time, can be a scallop wear pattern inside the brake drum because of hot and cold spots. Because brake apply pressure is low, the drum may go hundreds of thousands of miles without being replaced or turned to make it smooth. The result is hesitation in the braking that mystifies the driver and technician alike. This unusual symptom reveals why consistent balancing is worthwhile.
Tire imbalance always grows into a larger problem, and may spread beyond the tire itself.
Bridgestone/Firestone recommends that you carefully select the right tire for the job. Generally, tires with a simple grooved tread will wear more evenly than lug tires and should be used unless you’re sure you need the greater traction of lug type treads. Make sure tires are mounted evenly all around (evenly seated on the bead) and properly balanced. Keep them inflated to proper pressure for the load.
Maintain suspension systems and replace worn parts regularly. Align the vehicle before tire wear occurs, not in response to irregular wear. Jones says this means every time you replace tires or, preferably, on a regular maintenance schedule that is more frequent than that.
Get a total vehicle alignment job done that aligns all the axles, and make sure the settings are in the middle of recommended ranges, not just within maximum tolerances.
Keep records so you will know when tires in a certain wheel position are giving trouble even if you rotate tires to minimize the effects of irregular wear.
There’s a tremendous payoff for those who perform proper alignment and inflation maintenance. This kind of maintenance reduces tread wear and improves casing retreadibility. It also greatly improves fuel economy, while reducing driver fatigue and improving driver satisfaction.
For more information contact:
Michelin North America
Sun Tech Innovations-
Pictures for this article were courtesy of the Truck Technology and Maintenance Council/Radial Tire Conditions Analysis Guide, 1993.