134 8. Theory of Operation GSP9700 Series Road Force Measurement System Operation
Tire Radial Force Variation (Uniformity)
To understand the effects of radial force variation, imagine the tire as a collection of
springs between the rim and the tire tread. If the “springs” are not of uniform stiffness,
a varied force is exerted on the axle as the tire rotates and flexes. This creates a
vibration in the vehicle.
IBRATION SOURCE FROM
RADIAL FORCE VARIATION
With the increasing need for low tire uniformity values to minimize vibration
complaints, more tire companies are grinding or cutting tires during the
manufacturing process. As a result, free runout measurement is no longer an
accurate indicator of how the tire assembly will perform when rolling under load. Free
run out measurement should not be used as a guide to determine the tires
contribution to the ride disturbance complaint.
The GSP9700 takes a measurement of the wheel assembly’s radial force variation
and displays the results. If the GSP9700 perceives a problem with the force variation,
it will prompt you to measure rim runout. Refer to “Rim Runout Measurements,” page
54. When the rim runout data is entered, the GSP9700 extracts and displays the first
harmonic force variation contribution of the tire and of the rim. Refer to “Harmonic
Vibrations,” page 131. The GSP9700 allows the first harmonic of the tire to be
matched opposite of the first harmonic of the rim to decrease the first harmonic
vibration. The tire and/or rim may need to be replaced or ForceMatched prior to
balancing. Refer to “ForceMatching,” page 61.
8.3 Radial Force Variation (RFV)
Radial force variation is a term describing the measurement of the tire uniformity
under load, measuring the variation (up and down) of the load acting on the vehicle
spindle (SAE practice J332).
All tires have some non-uniformity in the sidewall and/or footprint due to variables in
the manufacturing process. Tire uniformity measurement values can be affected by
rim width, rim condition, and/or tire mounting variables. Unlike balancing, there is
often a small amount of RFV remaining in the tire/wheel assembly after
ForceMatching and this is generally acceptable.
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