Runout compensation - Bosch-Diagnostics OES

41.151_SSD2_Newsl_Felge_GB 11.09.2006 11:45 Uhr Seite 1
Runout compensation
Bosch information
09/2006
In connection with optical wheel alignment systems, which cover the entire rim circumference, the question arises
as to whether rim runout compensation is still necessary.
Bosch commissioned a study to clarify this issue. The results are contained in this document.
What is rim runout
Rim runout is the side runout of a rim perpendicular to the plane of rotation. Rim runout can also be designated
as the maximum form error of an ideally flat disk. Rim runout results from production tolerances and is increased
during service life by mechanical deformation, e.g. by driving onto a curbstone or, in some cases, during a tire
change.
What is the usual magnitude of rim runout
In the course of the study, 100 aluminum rims and 100 steel rims with diameters from 13" to 18" were examined.
Most of the rims were OEM rims (aged from 1 to 10 years) from 17 different vehicle makes, but they also included
retrofitted aluminum rims from well-known suppliers. The results are displayed clearly in the following graphic.
Frequency distribution of measured rim runout
50
40
30
% share
20
10
0
< 0.3 0.3 to 0.6 0.6 to 0.9 0.9 to 1.2 1.2 to 1.5 > 1.5
measured rim runout in mm
Steel rims
Aluminum rims
Steel rims showed considerably higher average rim runout than aluminum rims (0.65 mm compared to 0.40 mm).
What relevance does rim runout compensation have to wheel alignment
To compensate for rim form errors and clamping errors, specialists regard rim runout compensation as absolutely
necessary when using universal quick-clamping holders. The following effect is utilized: The measuring plane of the
adapted reading head, or optical target, executes a wobbling (radial runout) movement when the wheel rotates.
For the axle alignment, the wobble angle between the measuring plane and actual wheel plane of rotation is determined
and taken into consideration during the calculation of the toe angle and camber angle.

41.151_SSD2_Newsl_Felge_GB 11.09.2006 11:45 Uhr Seite 2
Runout compensation
Bosch information
If no rim runout compensation is carried out, this wobble angle is added as an additional error to the actual measurement
error of the axle alignment system.
The above-displayed rim runout initially does not act directly as an angle error. This depends on the rim size and the
distribution of the form errors over the rim circumference. With adapter-free wheel alignment systems, mathematical
methods are used to calculate a measurement plane from a large number of surface points, which compensates
for the smaller form errors spread over the circumference. However, this is still not sufficient to eliminate the basic
rim runout and thus the wobble angle.
In the course of the study, form error measurements were made on each rim at 12 measuring points and this was used to
calculate the related wobble angles.
This following is an example of a 17" aluminum rim with a rim runout of 0.65 mm and a wobble angle of 4.5 angular minutes
(4.5’) viewed from two different perspectives:
x: x-coordinates of the wheel y: y-coordinates of the wheel z: Form error in mm : Wobble angle between compensating and wheel plane of rotation (=4.5’)
Measured form error Compensating plane of form error Wheel plane of rotation
What do the results of the study mean for wheel alignment
Making allowance for varying distribution of form error and different rim sizes, an average deviation of 0.1 to 0.2 mm
would correspond a wobble angle of 1°.
With most of the examined rims, i.e. approx. 75 % of the steel rims and approx. 40 % of the aluminum rims, disregarding
the rim runout would lead to an additional angle error of more than 2’.
The maximum angle error for steel rims was determined to be 12’ and 8’ for aluminum rims.
Summary
The study demonstrates convincingly that rim runout compensation is absolutely necessary, even for adapter-free
or touchless wheel alignment systems, and is thus a basic prerequisite for precise setting of toe and camber.
The study confirms the view held by Bosch that rim runout compensation continues to be necessary in wheel alignment.
This is why automatic multi-point compensation has been integrated in the measurement process for the
KDS New Generation.
Contact
Robert Bosch GmbH
Diagnostic Products
73207 Plochingen
Internet: www.bosch-diagnostics-oes.de
E-Mail: sales.diagnostics-oes@de.bosch.com