Force variation refers to a radial or lateral movement of the tire and wheel assembly which is similar to runout, force variation has to do with variations of the tire construction. These variations in tire construction may actually cause vehicle vibration, even though the tire and wheel assembly runout and balance may be within specifications.

Radial force variation refers to the difference of tire sidewall stiffness as the tire rotates and contacts the road. Tire sidewalls have different stiffness due to different plies of the tire, but these different stiffness do not pose a problem unless the force variation is great. Stiff spots (1) in a tire sidewall can deflect a tire and wheel assembly upward as the assembly contacts the road.

Lateral force variation refers to the difference in the stiffness or conformity of the belts within a tire as the tire rotates and contacts the road. Tire belts may have some stiffness or conformity differences, but these differences do not cause a problem unless the force variation is excessive. These differences in tire belts can deflect the vehicle sideways or laterally. A shifted belt inside a tire may cause lateral force variation.

In most cases where excessive lateral force variation exists, the vehicle will display a wobble or waddle at low speeds, 8-40 km/h (5-25 mph) - on a smooth surface.

Perform the following test to determine if force variation is present in the vehicle.

1. Substitute a set of KNOWN GOOD, pre-tested tire and wheel assemblies of the same size and type for the suspected original assemblies. See Tire and Wheel Removal and Installation.
2. Road test the vehicle to determine if the vibration is still present. Refer to Vibration Analysis - Road Testing.
3. If the vibration is still present while using the KNOWN GOOD set of tire and wheel assemblies, then force variation is not the cause of the vibration.
4. If the vibration is eliminated when using the KNOWN GOOD set of tire and wheel assemblies, install the original tire and wheel assemblies with the matchmarks made prior to removal. See Tire and Wheel Removal and Installation. Road test the vehicle to determine if the vibration has returned. Refer to Vibration Analysis - Road Testing.
5. Continuing the process of installing the original tire and wheel assemblies one at a time, then road testing the vehicle until the set of tire and wheel assembly which is causing the vibration has been identified.
6. Replace the tire on the vibration-causing tire and wheel assembly then balance the assembly. Refer to Tire and Wheel Assembly Balancing - Off Vehicle.