ROCHESTER HILLS, Mich. (June 19, 2006) — With the economy still strong and trucks racing up and down the highways, fleets are still struggling to attract and retain enough drivers to haul the freight that's out there.
Driver retention is a serious problem, with truckload carriers reporting a 129-percent turnover rate.
One of the lures that fleets are using to attract and retain drivers is the trucks they specify. More amenities than ever before can be found in the roomy and luxurious interiors of these rolling palaces. Besides wrap-around dashes, automated manual transmissions, noise-reduced cabs and smooth-riding suspensions, today's trucks also can come equipped with refrigerators, microwaves, portable johns, televisions, surround-sound stereos and extra large beds. If things keep going this way, we'll probably see in-cab lap pools and bowling alleys in the not too distant future!
Not all fleets are equipping their trucks with these features, but they are spec'ing attractive and comfortable cab environments, aluminum wheels and other things that will make drivers proud to drive their trucks and hesitant to walk away from them to drive for someone else.
While fleets are spending the big bucks right now on trucks, they also are looking at maintaining their smooth ride. After all, what's the point of making a large investment in a beautiful vehicle only to have it bounce down the highway and rattle the teeth out of the driver after a new set of tires is installed? No driver will want to stay in a chariot like that no matter how pretty it is. As a result more fleets are paying attention to factors that affect ride, like balance and runout.
Correcting out-of-balance
The cause of an out-of-balance condition is the unequal distribution of weight around the axis of rotation. To correct this, tire and wheel assemblies can be balanced in two major ways: externally with weights appropriately distributed around the rim flanges and internally with balancing materials. The oldest method uses lead weights to offset an unequal distribution of weight within the tire/wheel assembly. There are two ways that this can be done: statically and dynamically.
Static imbalance causes the assembly to hop or bounce. To balance a tire/wheel assembly statically, it is not necessary to rotate the wheel. This can be done on a simple bubble balancer or a spin balancer. The procedure for balancing an assembly in this manner is to split the required amount of balance weight and attach half of the weight on the inside rim flange and half on the outside rim flange exactly opposite.
When the assembly is in static balance, it will remain at rest when stopped at any point in a rotation. However, once you have statically balanced an assembly, it can still be dynamically out of balance.
Dynamic balance means balance in motion. Therefore the tire/wheel assembly must be rotated to determine where balance weight is needed. As the assembly rotates, weight not only must be distributed equally around the perimeter but also distributed equally on both sides of the tire centerline. To do this, the tire/wheel assembly must be placed on a dynamic spin balancer. The balancer determines the size and location of the balance weights that are not split side to side but are each precisely specified for the appropriate location on both rim flanges of the assembly. Once an assembly is dynamically balanced, it is also statically balanced.
Dynamic imbalance will cause the wheel to wobble. The wider the tire, the more wobbling is noticed during the ride. Therefore, wide-base tires are affected much more by dynamic imbalance than conventional truck tires. So keep this in mind if you have fleets changing over to this type of tire.
Some truck tire makers put a balance mark noted by a dot of paint at the light spot of their tires. Since the valve is usually the heavy point of the wheel, this balance dot should be placed at the valve stem. If the tire/wheel assembly out of balance requires in excess of 10 ounces of weight be installed, rotate the tire 180 degrees on the rim and recheck the balance. Over-the-highway steer tires 20 inches and larger should never have more than 15 ounces of weight installed on their rims, and drive tires should never have more than 19 ounces installed on their rims. Maximum weights for wide-base tires are 22 ounces for steers and 26 ounces for drives.
Choosing methods
One of the major advantages of wheel weight balancing is that nearly everyone is familiar with it because it's been around almost since the invention of the wheel itself. A minor problem with it is the galvanic corrosion that occurs between the different metals used in the balance weight and the wheel. You don't see this too often on painted steel wheels, but it can create an appearance issue on uncoated aluminum wheels. However, coated lead weights will eliminate this problem.
One problem that wheel weights cannot address is brake flat spotting and other irregular wear conditions that can change the original balance of the tire when it was new and first mounted. Internal balancing materials, however, can move and adjust for changes in balance conditions as the assembly rolls down the highway. These materials are available in both liquid and dry form.
However, when selecting and using these materials there are several cautions that should be kept in mind.
They must be chemically compatible with the tire and the wheel so that the warranties will not be affected. Users and dealers almost always remember to ensure that these materials won't harm the tire, but they forget about the wheel. Many fleets have used liquid balancers only to find that their wheels had turned into balls of rust when the tires were removed. Therefore, liquid balancers must contain rust inhibitors that will protect the wheel and won't attack the tire.
Dry materials must not absorb water or moisture. Inflation air is often full of moisture. You want to be sure that the balance material used will not turn into concrete sea-monkeys by just adding water. If you are servicing fleets that use these materials, be sure to have an air dryer attached to your compressor and drain your air lines regularly, especially if you are in an area that experiences high humidity.
When mounting tires, care must also be taken to ensure the balance material is not lodged between the tire and the bead seating area of the rim or air loss will occur. Also, mounting lubricant should be carefully applied so that it doesn't drip inside the tire. By quickly seating the tire beads after applying the lubricant, drips can be avoided.
If you have to demount tire/wheel assemblies with these materials in them, ensure that the balance material is disposed of properly. Many compounds cannot be simply flushed down the drain. To avoid a visit from your friendly Environmental Protection Agency official, check the product's MSDS (material safety data sheets) for disposal instructions.
For those of you who want to test on your spin balancer how good these materials are in balancing tires, you should know that they probably will not zero out because they rely on load and the road contact area to be properly distributed throughout the tire.
Another method to balance tires is the balance ring. These types of devices have a material enclosed in a ring that rotates as the vehicle moves. They are mounted between the brake drum/hub and the wheel. The material in the ring (metal balls or liquid of some kind) moves and constantly balances the axle end. When installing wheel assemblies on a vehicle, it is vital to make sure that the balance ring does not interfere with the brake drum or wheel and that there is no damage on the balance ring face. If the metal is deformed around the stud holes or burrs are evident—which can happen after tires are changed several times—loose wheels will result.
As a general rule, do not advise your truck tire customer to add any products to the inside of the tire that can negatively affect the liner of the tire or the wheel. This includes golf balls. They will deform, grow sharp edges and cut up the tire innerliner. (I don't care if they are Titleists or Bridgestone Precepts. They won't “go the distance” inside a tire.)
You probably already know that most fleets balancing tires do the steer tires first. That's because the front tire/wheel assemblies generally are more responsible for vehicle vibrations felt at the steering wheel and/or floor pan under the driver's feet.
Proper balance of steer tires also helps to minimize objectionable irregular tire wear. Drive tires usually are balanced to eliminate vibrations in the driver's seat. Because a driver can't feel what's happening 40 feet behind him in the trailer, most fleets don't bother balancing trailer tires unless they are having some exceptional irregular wear problems.
However, most tire and vehicle ride engineers recommend balancing all wheel positions since they believe it will improve tire wear and reduce in-cab vibrations. The benefits of balancing are seen more readily on trucks with air ride suspension systems that are used in high-speed operations. Fleets have to weigh the cost of balancing tires with the possible loss in tread wear they'll get and the rough ride their drivers will experience.
Walk, don't runout
The other factor that can set up axle-end vibrations is runout, which encompasses two elements. The first is lateral runout—the side-to-side movement when an assembly is rotated, resembling a “shimmy” or “wobble.” The second is radial runout, which is the “hop” or “bounce” that commonly is referred to as “out-of-round.”
In the real world, the runout of a tire/wheel assembly is made up of a combination of the tire runout, the wheel runout, the hub runout, the variation that can exist in mounting the tire on the wheel or rim, and the variation that may be attained when attaching the wheel to the hub.
If you encounter a problem that appears to be a runout issue, use a tire runout gauge to check for lateral and radial runout. Tire lateral runout should be checked on a smooth surface along the tire's upper sidewall. Radial runout should be checked on a smooth rib in the center of the tread.
If radial runout exceeds 0.095 of an inch on a disc wheel assembly, deflate the tire and unseat the beads. Lubricate the rim and rotate the tire 180 degrees on the rim. Then re-inflate the tire and recheck the rim.
Lateral runout is rarely the cause of ride problems. However, if lateral runout exceeds 0.095 of an inch, remove the disc wheel, clean the mating surfaces, reinstall the wheel and recheck the lateral runout.
Mounting a tire on the rim properly is vital to attaining proper tire balance and runout. Thorough lubrication of the tire bead and rim flange area is necessary to ensure the tire is mounted concentrically. Otherwise, one bead or part of a bead can get hung up on the dry rim. Then lateral runout, radial runout and imbalance can result. Always check the distance between the rim flange and guide rib or “GG ring”—that's the cosmetic ring on the tire above the rim flange—in four places equally spaced around each side of the tire. The measurements should not exceed 2/32nds of an inch.
Radial force variation
Nowadays, you hear many people talk about radial force variation. It seems that everybody talks about it, but nobody understands it.
Radial force variation is very similar to radial runout. It is a result of tire non-uniformity, which can occur during the manufacturing process. One area of the tire may be thicker than the rest due to variations in tire component thickness, placement and overlapping. Radial force variation applies more force against the road at the tire's thicker spot as the tire runs, resulting in one sidewall flexing differently than the other. The amount of radial force can change with speed and load.
The results are tire/wheel assembly vibration and irregular treadwear. Radial runout can be an indicator of radial force variation. However, exact measurements of radial force variation can be made only in a laboratory on a test wheel or with scientific instrumentation on a vehicle. So, don't bother trying to test or measure for it.
Truck driving is a hard life that requires long periods of time away from home and family in addition to facing the challenges of highway traffic that gets more congested every day. Fleets are increasing wages to attract and keep drivers, but driver welfare and comfort will continue to be main concerns to reduce fatigue and keep drivers happy.
A smooth ride is integral to this effort, and balance and runout problems are receiving a higher priority from fleet maintenance personnel. I wonder how they'll do in the future when faced with chlorinating the lap pools and repairing the pinsetters.