What is GSP9700 Road Force Measurement™?
Loaded Truth on Tire Force and Runout Measurement to Solve Vibrations
GSP9700 Road Force Measurement (a roller turning against a tire under high load) accurately locates and quantifies the forces in the wheel/tire that cause non-balance related vibration and radial tire-pull complaints. The combined effects of geometric (eccentricity) and constructional (hidden internal) related issues can be measured only if a load is placed against the tire during measurement. It is the most effective way to quantify the effect of all items in a tire and wheel contributing to non-uniformity of the assembly. Loaded (force) measurement has been the accepted industrial standard used by tire and vehicle manufacturers for over 20 years.
Tire Related Forces Are Revealed Only When Testing Under Load
Road Force Measurement is displayed in pound force (Lbs.), kilogram force (Kg.) or Newton (N). There are two major components analyzed during Road Force Measurement that contribute to the total non-uniformity of a tire and wheel assembly: (1) Loaded Runout Measurement and (2) Tire Stiffness Measurement.
Road Force Measurement uniquely combines loaded runout (eccentricity) measurement of the tire-wheel with the effects of the tire’s overall stiffness. Eccentricity, when stated in terms of runout alone, does not quantify the actual force because the force is dependent on the stiffness of the tire being measured. Runout does not necessarily create the same force from tire to tire. For example, two tires with the same runout will differ in their ability to create vibration when a stiff tire sidewall is compared to a similar tire with soft sidewall. The stiffness of the tire will reduce or increase the eccentricity of the assembly when rolling under load.
Runout Measurement is a distance measurement, not a force. Runout is traditionally measured in thousandths of inches (0.000”) or hundredths of millimeters (0.00mm). Runout can be measured in an unloaded/free state or loaded “road test” condition. Runout measurement of a tire is a compromise compared to calculating forces and is not as effective when diagnosing vibration problems. Runout measurement in a loaded “road test” condition is faster than unloaded runout measurement, however it is not as effective as calculating the Road Force since the stiffness effect of the tire are not taken into consideration.
Loaded Runout Measurement on
a tire is accomplished with a loaded roller placed
against the footprint. The load roller performs a ‘simulated’ road
test but differs from Road Force Measurement because
tire stiffness is not measured. Loaded runout is
the most effective method of runout measurement that
samples the entire tire footprint. Measuring the
tire footprint with a loaded roller is unlike any
form of tread sampling with unloaded runout. The
tire under load is tested in a similar manner as
it is driven on the road. A loaded runout test is
faster than single area sampling and automatically
averages the runout of the tire footprint. A tire
is compressible and therefore loaded runout measurement
is a preferred method over unloaded runout.
* Unloaded – Tire runout measurement is problematic and limited in use since the tire has spring rate and is compressible. Non-contact runout is limited and not as repeatable since typically small areas of the tire face are measured. In most cases, non-contact, unloaded runout is used on tires for visual related issues and is not the best measurement choice to estimate vibration-causing eccentricities. A wheel is not compressible in the same manner as a tire and therefore unloaded runout is acceptable. The best wheel measurement for vibration-causing eccentricities provides two measurements taken at or near the bead seat area. The two measurements are vector averaged to find the true low-point average of the tire bead seat areas of the rim.
The GSP9700 measures loaded runout and
tire stiffness simultaneously...so you
know the magnitude of the vibration force
that occurs when the assembly is mounted
on a vehicle.