Radial Force Variation

Radial force variation is the difference in the stiffness of a tire (1) as the tire rotates and contacts the road. The tire and wheel assemblies have some variation due to splices in the tire plies. These splices do not cause a problem unless the force variation is excessive. These stiff spots in the tire can deflect the tire and wheel assembly upward as the assembly contacts the road.
If the tire has only one stiff spot, the spot will deflect the spindle once per each revolution of the tire and wheel assembly, thus causing a first-order tire/wheel vibration. If the tire has two stiff spots, the spots cause a second-order vibration. First-order and second-order tire/wheel vibrations are the most common as a result of radial force variation. Third-order, fourth-order, or higher are possible but rarely occur.
Ensure that the tire and wheel assembly runout is at an absolute minimum. This is the most effective way to minimize the possibility of force variation as a factor in tire and wheel assembly vibrations. However, some tire and wheel assemblies exhibit vibration-causing force variation even though they are within runout and balance tolerances. These instances are becoming increasingly rare due to tighter tolerances and higher standards in manufacturing.
If you suspect force variation as a factor in tire and wheel assembly vibration complaints, substitute one or more known good tire and wheel assemblies.
You may buff the tires on a tire matching machine in order to eliminate spindle deflection. This type of equipment, not currently in widespread use,is designed to remove small amounts of rubber from the outer rows of the tread blocks at the location of the stiff spots under load. Do not use any tool that is designed to make the tire perfectly round. These tools will not correct the condition.
You may substitute one or more known good tire and wheel assemblies when a tire manufacturer is not available locally.