Loaded Runout Measurement is performed on a tire with a loaded roller placed
against the footprint of the tire. 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 it is driven on the road. A loaded runout test is a 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 Runout Measurement is a good choice when measuring a non-
compressible object such as a wheel. The best way to measure a wheel is to average
the runout of bead seats. Unloaded runout of a tire is the least preferred method to
quantify vibration issues. Unloaded runout involves measuring a small area of the tire
such as the center rib. Usually the center area of the tire is used because unloaded
runout measurement has been proven to be different at various spots across the
tread. The problem with center tire measurement is that measuring in the center
tread area has the least influence on the actual forces transmitted to the vehicle. It’s
logical that unloaded runout at the center tread area completely flattens against the
road as the tire rolls. This is why unloaded runout is the least preferred method to
quantify vibration problems. The wider the aspect ratio the less any runout in this
area will be felt...it's the sidewall stiffness variation that transmits to the chassis.
Furthermore, tire sidewall variations and hidden constructional issues in the tire are
not taken into consideration when unloaded runout is measured. Unloaded runout
measurement can not indicate how much force is being created as a result of the
runout since the tire stiffness can not be measured. Tire shoulders and other areas
not measured may amplify or diminish the runout (and forces) when measured under
load.
Summarization:
Force
The forces generated by a tire and wheel must be measured by placing a tire under
load. Tire stiffness also plays a role in force calculations since this amplifies or
diminishes the vibration effects of the runout measured. Road Force Measurement®
uses loaded runout and tire stiffness measurements to calculate the actual force
created by the loaded runout alone. 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 sized tire with a soft sidewall.
Runout
* Loaded – The entire footprint of a tire face is sampled faster and more accurately
with a loaded roller. The shoulder areas in many cases have more effect on the
vibration possible than unloaded eccentricity measured at a center location of the tire.
* Unloaded - For tire measurement this is problematic and limited in use since the tire
has spring rate. It is limited in quantifying the vibration magnitude since typically a
small area of the tire face is measured. For wheel measurement it is acceptable
providing two measurements are taken at or near the bead seat area and then vector
averaged to find the true low-point of the rim.
Important:
Runout measurement alone diminishes the ability to solve tire related vibration
complaints. Unloaded tire runout ignores (a) constructional forces hidden inside the
tire, (b) sidewall stiffness and ‘spring rate’ contributions and (c) it samples isolated
areas of the tire instead of the entire footprint. Locating non-balance related vibration
issues might very well diminish to a “game of chance”. As a result the best possible
ride quality is diminished and repair time will increase in frequency.
138
Theory of Operation GSP9720JLR Wheel Balancer Operation Instructions
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