Looking back on the past few years, I’m amazed by how many articles I write about tires. There have been six in little more than three years, about two a year on average. There’s a reason. Not counting fuel, tires are the most expensive maintenance item on your truck.
Tires also affect other operating costs. Improperly maintained tires cause more fuel to be used, cutting mpg significantly. They also can add to shock and vibration that damages cargo and wears out suspension and steering components.
Just last issue, we focused on the number one problem, low tire inflation pressures. Low air has ruined more tires, both new and recapped, than any other single problem. Other articles dealt with selection, tires and fuel economy, and, two years ago, on tire balance and alignment. Since then, Land Line’s circulation has grown significantly. Many of you have asked us to update our tire articles, so here once again are some tire maintenance tips to help your bottom line.
Wheel balance has a major effect on tire life. Balance refers to the masses acting around the tire’s axis of rotation. Any object will try to rotate so there is equal weight all around its axis of rotation. If there is more mass on one side, it wants to create a new axis, but it is kept from doing this by a rigid axle and spindle. The unbalanced tire then rotates with a force that alternately wants to lift the tire, then slam it back onto the pavement. The unbalanced forces wear the tire unevenly, making the problem worse. If you’ve ever noticed an empty trailer with a spring suspension rolling down a smooth highway with its rear wheels bouncing up and down, you’ve seen the effects of unbalanced tires.
There are two types of imbalance, static and dynamic. Static imbalance is when masses are different across the diameter of the wheel. Dynamic imbalance refers to differences about the centerline plane of the tire, perpendicular to the spindle. A tire with two ounces more mass on the upper right of the centerline and another two ounces on the lower left may be in static balance, but when it rotates, it will wobble. The masses want to rotate in one plane, but the structure of the wheel, hub and spindle prevent this. On a steer tire, we feel the result as a vibration in the steering wheel. On other tires, the same uneven forces are wearing the tire and putting additional stresses on bearings, hubs, axles and axle housings.
Static balance is corrected by adding weights to the rim of the tire to offset weight on the opposite side. These weights, while correcting static balance, may make dynamic balance worse. Spin balancing is the accepted way of checking and correcting dynamic imbalance. By spinning the tire, balancing machines detect lateral as well as radial forces. Weights can be adjusted between the inside and outside rims to zero out the forces.
There are devices that balance wheels as the vehicle is rolling. They mount inside the wheel, and contain weights such as steel ball bearings or liquid mercury (a possibly toxic substance in the unlikely event of a wheel breaking in a crash). They work on the principle that a loosely contained set of masses, rotating together, will tend to be self-adjusting to get the entire rotating mass in balance. As tires wear, the devices adjust automatically to compensate. Balance Masters and Centramatic make these devices.
“Equal” is a sand-like compound that, when placed inside a tire, is forced outwards by centrifugal force as the tire spins. Like the contained weight, it follows the laws of physics and it distributes itself more heavily at the lighter spots in the tire. This brings the tire into perfect static and dynamic balance. (Note: Equal must be installed following manufacturer’s directions, using special screened tire valves to prevent slow air leakage.) At a recent Maintenance Council (that’s the Technology & Maintenance Council — TMC) meeting, one fleet manager reported he keeps his tires in perfect balance by putting four golf balls in each tire. Like the granules, they seek the lighter spots and position themselves where needed. We cannot comment on the effectiveness of this method except to say it does make sense according to the laws of physics. At another TMC meeting, the question again came up and while this was not a widely accepted practice, no one using them reported any problems.
Alignment means all tires run in the same direction when going straight, and pivot around a common center when turning. When wheels run true, you get maximum tire life, maximum traction and maximum fuel mileage. When tires are misaligned, they are pulled sideways from their direction of rotation. This sideways force is what wears tires, reduces traction and wastes fuel.
Most operators only align steer tires, although other axles are often out of line. That’s because misalignment is felt most as a pull or wander in the steering wheel, but may not be felt from drive or trailer axles. Those axles should be parallel to each other and perpendicular to the centerline of the truck. Steer axle alignment is more complex, involving caster, camber and toe-in. These are explained in an excellent book on tires, the 104-page “Radial Truck Tire and Retreads Manual” from Goodyear. It is one of the most comprehensive works on the subject I have seen. It is available as part of a mini-CD called “Commercial Truck Tires & Retreads” version 2.0. Your Goodyear truck tire dealer should have a copy, or you can request one at www.goodyear.com.
Trailer axle misalignment can be seen every day out on the highway. Dog-tracking happens because trailer tandems are at an angle to the trailer’s centerline, not pointing to the kingpin. The wheels roll in the direction they are pointed, but are pulled to the side by the tractor. The result is excessive, uneven tire wear and wasted fuel.
Alignment is such an important problem that TMC has a task force to identify its causes, results and corrections. It developed Recommended Practice (RP) 642 on Total Vehicle Alignment. Most of its content is on the Goodyear CD.
If you ever had road service replace a tire, chances are the two tires in a dual set were not properly matched. If they are different sizes, the larger diameter tire pulls the smaller, causing excess scuffing. The larger tire also carries more of the load, accelerating wear and increasing internal heat. When you need a replacement, make sure the dealer measures your good tire and selects one with a diameter within 1/4-inch. The smaller tire should be mounted on the inside, to minimize wear caused by road crowning. This may require dismounting and remounting both tires. Always make sure dual tires are properly matched by size, tread type and measured diameter, especially when buying a set of retreaded tires.
A maintenance program
Tire maintenance starts with inspection. It should be done at least weekly. Feather wear, cupping, diagonal wear, shoulder wear and one-sided wear are all signs of specific alignment problems. They are most easily detected by running your hand over each tire’s surface. It continues with a check of air pressure. This should be recorded on a running log, so you can easily detect any slow leakers before they can cause real trouble. Measure and record tread depth at each oil change or PM interval. Check across three points (inside, middle and outside tread) at no less than two places around the tire’s circumference. This can help spot tire problems and premature wear. Check balance and alignment as part of a winterizing routine, to make sure tires are in top condition during the worst weather.
Those who know my work know it’s hard for me to get through a technical article without mentioning something from the TMC. This is no exception. For the overwhelming majority of on-highway truckers running radial tires, TMC has an excellent reference guide called “Radial Tire Conditions Analysis Guide.” It illustrates what to look for when examining your tires, and it explains why the various wear patterns occur. You can order a copy from TMC at (703) 838-1763.
Follow these tips prepared by experts, and you can cut thousands of dollars from your annual operating costs.