The knee-bone connects to the leg-bone. The leg-bone connects to the ankle-bone. The axle-bone connects to the wheel-bone, and the wheel-bone connects to the tire-bone … so the song goes, sort of.
Because the tires, wheels, axles, and the suspension are so intimately related, anything that happens to one will have some effect on everything else. In other words, if you're seeing less than optimum tire wear, something upstream of the axle is probably to blame.
Irregular tire wear is a giveaway that something is amiss with the wheel-end/axle/suspension setup.
"There's a very direct correlation between suspension condition and tire wear. They are highly interdependent on each other," says Pat Martindale, vice president of maintenance at Penske Truck Leasing. "Many times, we'll blame the tire if it goes bad, when in fact the suspension is probably the culprit."
There's no evidence to suggest that certain types of suspensions - air versus mechanical, axle-under-air versus axle-over-air, or trailing arm versus parallelogram - have any particular adverse impact on tire life. No one we spoke to was able to single out a suspension arrangement that was more likely than any other to chew up tires, as long as everything was in good condition and operating at spec.
Doug Jones, customer engineering support manager at Michelin Americas Truck Tires, says if there is a difference in wear rates between the two, it's more perception than factual.
"Mechanical suspensions are usually stiffer than air ride suspensions, so handling and ride characteristics are different," Jones says. "While it is believed that there is a difference in wear rates and wear patterns between mechanical and air ride suspensions, we are not aware of any definitive studies isolating those differences."
Suspension-related tire wear almost always stems from some excess motion somewhere in the system (looseness - or in some cases, binding), or a wheel that is not running true. Tires exhibit characteristic wear related to certain component abnormalities, but the clues can sometimes be misleading.
For example, inner rib depression wear and cupping/scalloping could be caused by improper shock absorber function and/or loose bearings, or they may have separate and distinct causes. Mike Beckett of MD Alignment Services in Des Moines, Iowa, cautions against jumping to conclusions.
"If you're diligent enough to notice tire wear before it gets out of hand, and then take steps to correct it, you won't want to waste time and money chasing gremlins," he says. "If you suspect it may be - in this case - bearings or shocks, check the easy stuff first. The body of a shock absorber gets warm in normal service. Drive the truck around for a while and then feel the body. If it's cold, it's probably finished. Watch for obvious signs like oil on the body of the shock or worn shock bushings. If that checks out, move on to the bearing adjustment."
That's accomplished by simply jacking the wheel off the ground and lifting or pulling the wheel from the top or side to see if there's any play between the wheel and the spindle. This works equally well on all wheel positions as a rough inspection.
As an additional clue, loose wheel bearings on drive and trailer wheels may also cause wear along the inside shoulder of a wide-base single tire and along the inner shoulder of an inside dual tire.
On steering axles, loose kingpins or even play at the output shaft of the steering box can allow the tires to shimmy as they run down the road, causing cupping and scalloping wear. A visual inspection of these parts should reveal a problem if one exists.
On the other hand, loose axle mounts (U-bolts) or worn suspension bushings are equally likely to produce cupping and scallop wear. This can occur on any axle that's not securely mounted.
John Knutson, head of tech support for on-highway suspensions at Hendrickson Truck Suspensions, suggests the best thing you can do for your suspension, and by extension your tires, is to keep all the fasteners tight and properly torqued.
"Proper fastener torque is really important," he notes. "Loose U-bolts can lead to broken springs, and a possible increase in irregular tire wear. You should do periodic torque checks, a visual inspection every three to six months minimum, as well as annual torque checks."
Three types of wear, several culprits. Diagnosing wear problems is by no means a slam-dunk, but a trained eye can cut the diagnostic effort considerably.
Problems with axles and wide-base tires
Tires and suspensions get along pretty well for the most part, but in recent years a couple of issues have cropped up that are worth looking at. The first is the use of wide-base single tires on standard track-width axles.
"Using wide-base singles with a standard axle, you need a 2-inch offset wheel," explains Bill Hicks, director of product development and planning for trailer suspension at SAF Holland. "When you go to a 2-inch outset rim, you move the centerline of the tire farther out from the centerline of the bearings on the spindle. That can induce bearing wear as well as inner-edge wear on the wide-base tire."
According to Kurt Burmeister, ArvinMeritor's general sales manager for North American field operations, switching between dual and wide single tires with 2-inch offset rims could significantly alter bearing loads and the service life of both the bearings and the hub (and cause inner shoulder wear on tires, too).
"If you put a wide-base single on a conventional track housing, you have a tendency to load the outer bearing in the wheel-end system more than it is designed for. You'll reduce the life of that bearing, and in the most severe state, you'll see the bearing degrade, and possibly damaging the spindle itself," he says.
This condition is particularly acute when using tapered spindles and bearings. The alternative is the straight spindle, sometimes called a parallel spindle or P-spindle. It's a bit stouter, and both bearings are the same size, notes Hicks.
"Because of the perceived or actual problems with the tapered spindles when used with 2-in. offset wheels, most if not all of the axle manufacturers require that customers use straight spindles," Hicks says. "In some cases, suspension/axle manufacturers may actually derate the axle by up to 3,000 pounds. For example, a standard 20,000-pound axle may be permitted only 17,000 or 18,000 lb loads when the offset wheels are used - depending on the extent of the offset."
The other issue of concern is the use of lightweight axles with wide-single tires. According to Mike Beckett and others, there have been many reports of axles with thinner tube walls experiencing a degree of flex that can cause inner edge wear on tires.
"Due to the way the thinner-walled axles flex, wide single tires may not be recommended with some light-weight axles," he says. "The 9-mm axle tubes seem more prone to flexing. From what we can tell, the 11-mm tube is okay."
From the February 2011 issue of Heavy Duty Trucking magazine.