In tire and wheel mounting, the line between right and wrong is a thin one. A few thousandths of an inch on either side of correct can throw tires and wheels into a mean wobble or cause them to bounce up and down like pistons.
Working backward from where the tire meets the road, we can identify at least a dozen conditions that will cause radial or lateral runout or bad wheel balance. In almost all cases, the condition can be traced back to improper installation. Bent or out-of-round wheels do make it into service, but they shouldn't.
"Worn and/or damaged components are factors," says Tim Miller, marketing communication manager at Goodyear. "But someone has to install them. All wheel components should be inspected before being put back into service, even if there's no reason to suspect a problem."
Tire and wheel installers have to get the assembly on the truck with the least possible deviation from straight and true, and then confirm proper mounting using a runout gauge or a suitable stand-in - which in a pinch could be any object placed in close proximity to the tire as it is rotated on the jack so the tech can visually check the trueness of the assembly.
The American Trucking Associations' Technology & Maintenance Council Recommended Practice 214C describes runout like this:
"Radial runout appears as a changing radius of the rotating assembly.
For a tire or wheel, its effect is to raise and lower the vehicle as it rolls along, giving the perception of a vertical hop or bounce.
"Lateral runout appears as side-to-side movement of the rotating assembly. For a tire or wheel, its effect is to lead a vehicle alternately left and right as it rolls along, creating the perception of a shimmy or wobble."
TMC's RP 214C lists the following runout tolerances for a rotating tire/wheel assembly in over-the-road applications:
- Lateral: Steer 0.095 inch, Drive/ trailer 0.125 inch
- Radial: Steer 0.095 inch, Drive/ trailer 0.125 inch
Before putting a used wheel back in service, TMC suggests the true-ness of the wheel should be checked, too. The following tolerances apply to wheels only (without tires). If the wheel is outside the tolerance, it should be removed from service:
Steel disc wheels: Radial runout 0.07 inch, Lateral runout 0.07 inch
Aluminum disc wheels: Radial runout 0.03 inch, Lateral runout 0.03 inch.
Demountable rims: Radial runout 0.07 inch, Lateral runout 0.07 in.
The first step is to ensure the tire is concentrically mounted on the rim.
Tires have a bead seating ring to make visual inspection simple. With the bead seated against the rim, the distance from the seating ring to the rim should be measured at four points 90 degrees apart around the rim. The gap between the ring and the rim should be the same at all four points.
"This is an easy one to get right, if you mount the tire on the ground, horizontally, and keep the bead lubricated," says Todd Labbe, the metro manager of a Wingfoot Commercial Tire outlet in Brunswick, Ohio.
"Where installers go wrong is not using enough lubricant on the bead so the tire won't slip around easily on the rim. Sometimes a guy will mount a tire vertically, up against a wall or even still on the truck. Gravity will pull the tire down on the rim and there's no way you'll get it mounted concentrically."
It's equally critical that the wheels mate properly against the brake drum and against each other in the case of a dual. It's also important to ensure the wheels are concentrically mounted on the studs.
"The mating surfaces must be scraped clean with a wire brush prior to mounting so that no foreign material can compromise the mounting torque," Labbe says. "If there's any dirt, rust or even blistered paint on a steel rim, it will prevent the wheel from sitting flat against the drum. When you apply the proper torque to the wheel studs, it will crush whatever may be on the surface. If it slips out after torquing, the torque measurement will be lost and the studs could very easily work loose."
Any foreign material between the rims and the drum also could create lateral runout, giving the wheel a wobble.
Centering on the hub
On the other axis, the wheel has to sit correctly on the hub pilots to ensure it is rotating uniformly around the axle. For a variety of reasons - manufacturing tolerances, wear, etc. - stud- and hub-piloted wheels may not center precisely on the hub. A gap between the hub pilots and the rim no thicker than a business card can result in an off-center wheel that produces an "egg-shaped" rotation. This uneven motion results in wheel vibration and can produce irregular tire wear.
Check for worn drum and hub pilots during service. Both iron and aluminum hub pilots can gull and wear if not properly serviced, Meritor advises. Large pilot gaps between the drum and hub cause excessive wheel end assembly runout.
Several methods exist to correct this problem, including fastening nuts with sleeves that fill the gap between the stud and the hole, and tools that will center the hole over the stud before tightening the fasteners, taking the pilots out of the equation altogether.
Lining up the holes and studs
The WheelTite Sleeve Nut from Okabe Inc. and Skirt Nut (distributed by Esco) offer a similar approach, using a chamfered sleeve that fits into the stud hole on the wheel to center the holes over the studs as they are torqued down.
The result in these cases is the wheel is truly centered on the studs, and may not even be in contact with the hub tangs. As an added benefit, because there's no void between the stud and the hole, wheel clocking is eliminated, says Mike Jordan, director of strategic planning at Esco.
"If there's movement between the stud and the wheel, the flange nut can turn, causing the nut to loosen," he says. "If you eliminate the movement, you reduce the risk of losing a wheel."
Finally, don't torque down the wheel nuts on the truck without verifying the wheel is running true. Use a runout gauge, preferably, or a suitable substitute to measure runout. If runout is still present, the drum can be rotated 180 degrees from its original installation position on the hub to improve its assembly runout condition. If this does not improve runout, there is likely damage to the drum or hub and the parts should be closely inspected for damage.
Improper bearing adjustment can also affect runout, but that's an entirely separate story. Wheel installers should be checking for runout before they release the truck, notes Jeff Lecklider, president of Gem City Tire in Dayton, Ohio.
"There are the OSHA guidelines for tire and wheel installation, and TMC guidelines, too, but not all the shops follow those," he says. "You'd do well to audit your service providers to be sure they aren't cutting these final corners on installation."
When wheel-end problems cause tire wear
Like any form of tire wear, mechanical deficiencies and irregularities always leave telltale clues. An expert set of eyes can identify wear patterns associated with wheel-end problems.
Most wheel-end problems will cause irregular rotation of the wheel on a radial (up or down) or lateral (side-to-side) plane.
Imperfect rotation will cause rubber to scrub and wear off the tire in predictable ways. If you experience any of these sorts of wear on steer and drive tires, check for improper mounting.
Cupping and scalloping
Localized area of cupped wear creating a scalloped appearance
Usually indicates moderate to severe out-of-balance assembly or improper wheel/rim mounting. May also be caused by some other assembly non-uniformity such as non-concentric mounting. Can also be caused by shock absorber or suspension defects.
Difference in tread depth from one side of the tire to the opposite side
Caused by excessive radial runout or other non-uniformity in the rotating assembly. Problem areas could include the stud circle, wheel, rim and tire mounting, or improper seating of the bead on the rim.
Inner rib depression
One or more of the inner ribs worn more than adjacent ribs
Indicates non-concentric running of the tire or "looseness" in the tire's contact with the road. Improper bead seating, bad bearing adjustment or an out-of-balance condition are usually to blame.
Rib punch wear
Random erratic wear around the tire circumference
Caused by improper bead seating or bearing adjustment out of balance, which can lead to inner rib depression.
From the October 2012 issue of Heavy Duty Trucking magazine.