EPA-AA-SDSB-80-14
Technical Report
Summary and Analysis of Comments
Received in Response to Draft EPA Procedures
for the Measurement of Tire Rolling Resistance and
Subsequent Grading and Labeling of Tires
by
Glenn D. Thompson
July 1980
NOTICE
Technical Reports do not necessarily represent final EPA decisions
or positions. They are intended to present technical analysis of
issues using' data which are currently available. The purpose in
the release of such reports is to facilitate the exchange of
technical information and to inform the public of technical devel-
opments which may form the basis for a final EPA decision, position
or regulatory action.
Standards Development and Support Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air, Noise and Radiation
U.S. Environmental Protection Agency
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INTRODUCTION
The area of tire rolling resistance, particularly as it affects the
exhaust emissions and fuel economy of a vehicle, has been a concern
to EPA for several years. For example, EPA Advisory Circular No.
55A, dated February 8, 1978 stated that:
"Due to the potential for abuse of the option to alternatively
determine the dynamometer power absorber setting and, in
particular, the large effect of such potential abuse on the
fuel economy test values, EPA is concerned that only validly
determined and fully representative alternative dynamometer
power absorber settings be used in certification and fuel
economy testing. To this end, EPA has requested through this
advisory circular data on the tires used on the coastdown
vehicle, the tires used on all other vehicles which the
original coastdown vehicle represents, and the tires which
will be used in production."
"EPA's request for data on production tires, which will be
used to specify the tires to be installed on test vehicles, is
intended as an interim measure to ensure tire representative-
"At some later date, when a standardized, acceptable test
procedure for measuring tire rolling resistance is available,
this tire performance information will be requested rather
than the categorical tire information requested through this
advisory circular. Tire rolling resistance information will
provide EPA with a better tool for specifying tires to ensure
the representativeness of emissions and fuel economy data."
As the initial step in developing a standardized acceptable
test procedure for measuring tire rolling resistance EPA prepared a
technical report, Determination of Tire Energy Dissipation-Analysis
and Recommended Practices, in April 1978. This report and its
draft recommended practice for determining tire energy dissipation
was circulated to the automotive and tire industries with a request
for comments. The comments received were compiled and analyzed in
a subsequent EPA technical report, Summary and Analysis of Comments
Received in Response to the EPA Report, Determination of Tire
Energy Dissipation, Analysis and Recommended Practices, which was
released in January 1979.
As a result of this first analysis of comments the proposed
recommended practice for determination of tire energy dissipation
was substantially revised. This revised draft test procedure,
together with a draft proposal for grading and labeling of tires
for rolling resistance was again distributed with a request for.
comments to the automobile and tire.industries in April 1980.
This report presents, summarizes and analyzes the comments
received in response to the April 1980 distribution of the Draft
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EPA Recommended Practice for Determination of Tire Rolling Re-
sistance Coefficients and the Draft EPA Recommended Practice for
Grading and Labeling of Tires for Fuel Efficiency.
SUMMARY AND ANALYSIS OF COMMENTS
Comments on the EPA Draft Recommended Practices were received
from the following parties:
MTS Systems Corporation, Gerald R. Potts
SAE Rolling Resistance Subcommittee, Tom Baker, Chairman
Marion G. Pottinger and Nicholas M. Trivisonno
Rubber Manufacturers Association, Tom Cole
Motor Vehicle Manufacturers Association, Harry Weaver
ASTM Committee F-9 on Tires, W. Bergman
General Motors, T. Fisher
Ford Motor Co., Hellen Petrauskas
This section of the report presents .and analyzes the signifi-
cant technical aspects of the comments received from these re-
spondents. The headings and subheading of the following sections
of this report correspond to those of the original draft recom-
mended practices. These recommended practices are provided as
Attachments I and II of this report. The complete record of the
comments which were received are also provided as Attachment III.
EPA Recommended Practice for Determination
of Tire Rolling Resistance Coefficients
I. Introduction
Comments/Analysis
None.
II. Test Equipment
A. Tire Dynamome.te.r
Comments .
SAE, ASTM, RMA, MVMA, and Messers Pottinger and Trivisonno all
commented that the requirement that the test machine determine tire
energy dissipation by measurement of spindle reaction force was
overly restrictive and that torque or energy methods should also be
allowed. Messers Pottinger and Trivisonno specifically noted that
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few machines in existence today were equipped to measure spindle
force.
MTS commented that the recommended use of a 67.23 inch dia-
meter tire test machine was unduly restrictive and suggested that
the following statement be added to the EPA Recommended Practice.
"The Development of Preferable Flat Surface Laboratory Ap-
paratus is progressing a pace which may soon bring it into
use. The recommended practice is written in such a way that
it can be used without modification when such equipment
becomes commonly available."
Analysis
There are several reasons why spindle force can be the pre-
ferred approach. The primary reason is that this approach has the
minimum parasitic losses which are included, and must be corrected
for, in the measured parameter. Spindle force measurements only
include the spindle bearing losses in addition to the tire energy
dissipation. The torque method also includes the test wheel
bearing losses, and the energy method includes all of the parsitic
losses of the system including the drive motor.
Spindle force also has the advantage that it may be easier
to retrofit an existing machine for spindle force measurement than
for torque measurement since the torque measurement modification
requires instrumentation of the test wheel drive mechanism.
Finally, it is possible to instrument multiple spindle force
stations on a single test wheel and to thereby test multiple tires
simultaneously.
The disadvantage of the spindle force method is that the load
cell used to measure the force must be capable of accurately
resolving the transverse tire dissipation forces in the presence of
the large tire loading force normal to the test wheel. This
resolution problem subsequently requires greater care in the test
machine alignment and in the test machine operation than is re-
quired by the other methods.
There is no strong technical reason why other methods of
measuring tire rolling resistance should not be allowed. There-
fore, it is recommended that the EPA recommended practice be
revised to state that measurements be made by spindle force trans-
ducers or by any other approach which yields equivalent results.
While measurement of tire rolling resistance on a 67" drum by
instrumentation systems other than spindle force transucers are
directly compatible with the current EPA Recommended Practice, the
use of a flat surface machine is not. This incompatibility,
occurs because the rolling resistance of the tire on a curved
surface is higher than that on a flat surface. While several
correction approaches have been proposed, no correction approach
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was included in the EPA Recommended Practice because of the concern
over the universal applicability of the available correction
equations.
In the earlier version of the EPA Recommended Practice a flat
surface test machine was proposed and virtually all commentors
opposed this recommendation because of the limited availability of
this type of test machine. Consequently at this time there does
not appear to be any apparent alternative to recommending the 67"
diameter test wheel. When flat surface machines are in greater
service more data will be available to evaluate the accuracy of the
curved-to-flat surface correction factors or to develop improved
versions. At that time the EPA Recommended Practice can be
revised to prefer the use of a flat surface machine with correction
factors provided to allow continued use of a circular test surface.
1. Test Machine Alignment
Comments
The MTS corporation and Messers Pottinger and Trivisonno
commented that the specified machine alignment tolerances were
excessive for a spindle force machine. Messers Pottinger and
Trivisonno recommended that the alignment accuracy should be +0.01°
while Mr. Potts of MTS recommended alignment accuracy of +0.03° and
that efforts be made to measure or eliminate the remaining cross
coupling error.
In addition to the longitudinal alignment accuracy require-
ments Messers Pottinger and Trivisonno suggested that the slip
angle tolerance be 0.01° and that:
"For consistency we would make the wheel plane to test surface
angle, 90° to within O.OlV
Analysis
The analysis of the alignment problem presented by MTS
was particularly convincing, consequently the allowable align-
ment tolerances should be reduced, particularly for longitudinal
alignment. The comments on slip angle tolerance indicates that
this parameter is not as critical as the longitudinal align-
ment. Consequently, based on the comments it is concluded that
an alignment accuracy of 0.03" is sufficient for both longitudinal
alignment and slip angle if care is taken to measure or eliminate
any remaining cross coupling errors. The alignment accuracy is
recommended. The more stringent alignment of 0.01° can, of course,
be adopted by any test laboratory.
2. Test Machine Control Accuracy
Comments/Analys is
»
None.
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3. Test Machine Instrumentation Accuracy
Comments .
Messers Pottinger and Trivisonno commented:
"The spindle force accuracy of 0.1 Ibf will require the use of
very light-duty load cells preferably 100 Ibs. or less. This
requires very careful design and operating procedures to avoid
incessant breakage."
Analysis
The precision of 0.1 Ibf is considered quite desirable and is
therefore specified in both the EPA and SAE recommended practices
for measurement of tire rolling resistance. This comment therefore
considered, primarily as a cautionary note, and is so appreci-
ated.
B. Test Cell Requirements
1. Thermal Control
Comments
RMA and Messers Pottinger and Trivisonno commented that
100°F test cells are commonly availably for the DOT Federal
Motor Vehicle Safety Standard 109 and that it would be perfer-
able if such cells could be used for rolling resistance measure-
ments. ASTM, however, commented that the allowable temperature
range specified in the EPA recommended practice +5°F could signi-
ficantly affect the measured rolling resistance of tires.
Analysis
The average temperature environment of an in-use tire is
closer to 75°F than it is to 100°F. Since the goal of the recom-
mended practice is to promote fuel conservation through improved
tires it is logical to choose a test cell, temperature which
approaches average in-use environment temperature, yet is easily
attainable. The proposed test temperature, 75°F is such a com-
promise. The selection of such a common laboratory temperature
should not impose a significant burden.
This is also the test cell temperature specified by the SAE
Rolling Resistance Measurement Procedure for Passenger Car Tires -
SAE J1269. It is therefore concluded that the nominal test cell
temperature should remain at 75°F.
The ASTM comment that temperature significantly affects tire
rolling resistance is in concurrence with EPA experience in testing
light-duty vehicles. Furthermore, our experience indicates that it
is difficult *to accurately correct for temperature effects over
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wide temperature ranges. Consequently we do not concur that the
allowable test temperature range should be expanded to include
100°F.
The ASTM suggestion of adopting a temperature correction
factor to adjust to a nominal 75°F within the current 70° to
80'F range is desirable. It is recommended that a temperature
correction, of a form similar to that used in EPA Advisory Circular
AC 55/C, be adopted. However, it is recommended that coefficient
of the temperature correction not be specified at this time, but
that data be requested from the commenters to provide a stronger
data base for the determination of the value of this coefficient.
2. Temperature Measurement Precision
Comments/Analysis
None.
III. Test Procedure
A. Tire Mounting
1. Rims . '
Comments .
The RMA commented: - ;
"It is not certain what the effect of wide versus narrow rim
width is oh rolling resistance. Consequently, it is recom-
mended that test rims be those specified by the Tire & Rim
Association, Inc. as "design rim width," _+pne-half inch. For
tire sizes not standardized by the Tire & Rim Association,
Inc., reference should be made to the standardizing organiza-
tions listed in Federal Motor Vehicle Safety Standard 109."
Analysis
The RMA comments . appear reasonable and probably should be
adopted.
2. Inflation Pressure
Comments
GM and the MVMA commented that all Alpha Numeric tires should
be tested at the same inflation pressure rather than varying the
inflation pressure with the tire load range. The same comment was
made regarding "P" type tires.
On the subject of the recommended inflation pressures, ASTM,
RMA, SAE and MVMA all recommended higher test inflation pressures,
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generally 32 and 35 psi for Alpha Numeric and "P" type tires
respectively.
The EPA recommended practice for measurement of tire rolling
resistance included test inflation pressures for "T" type tires.
RMA, Ford, MVMA and GM all commented that these tires are tem-
porary usage tires and should be excluded from the rolling resis-
tance measurement and grading procedures.
Analysis
From the standpoint of ranking tires it is unlikely that
significant ranking changes would occur in the 6 or 8 psi range
allowed for extra load tires. Therefore, in the interest of
simplicity, a single inflation pressure is recommended.
The test inflation pressures prescribed in the draft recom-
mended practice were intended to represent typical in-use inflation
pressures. It is a major concern that the higher inflation pres-
sures often recommended for current vehicles may not be maintained
in actual consumer use. Still, there is a potential for fuel
conservation by encouraging higher test inflation pressures.
Consequently, it may be appropriate to use higher .test inflation
pressures and to stress in EPA information programs and in the
labeling program that rolling resistance values were obtained at 32
or 35 psi, that these pressures are recommended and are safe for
the tire operation. Such an approach would have little probability
of misleading a consumer in tire selection and would provide some
encouragement for increased inflation pressure. Consequently,
adaption of these higher test pressures is recommended. The
increase in the test inflation pressure will, of course, require a
revision of the categories of the Recommended Practice for Tire
Labeling and Grading.
To the extent that T type tires are only used as temporary
"spares" it is .reasonable to exclude them from the current proce-
dure. Therefore, at the present time it is recommended that they
be excluded. If such tires are used for common service or if such
a tire designation is later used for common service tires then the
determination of the energy dissipation of these tires should be
considered.
B. Tire Break-In
Comments
The EPA recommended practice requires a one-hour tire break-
in. SAE, RMA, MVMA, and GM all commented that break-in was not
required for radial tires and, therefore break-in should only be
required for non-radial tires. No specific data were provided by
these respondents.
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Analysis
The available data does indicate that tire break-in is
more important for non-radial than for radial tires. However,
such data are usually obtained under regulated pressure conditions
rather than the capped method specified in the EPA Recommended
Practice. Elimination of the break-in for radial tires tends to
complicate the test procedure by introducing different test re-
quirements for different tire types. It would probably also induce
some increase in the variability of the results between the initial
and subsequent tests of a tire. For these reasons it is recom-
mended that tire break-in remain as part of the EPA Recommended
Test Procedure for all tires. However, it should be noted that the
recommendation which was made in the test equipment section, to
allow alternate approaches which yield equivalent results, would
also allow deletion of the tire break-in if break-in effects are
negligible. Also, deletion of the break-in by a tire manufacturer
could logically only result in a conservative rolling resistance
grade for the tire. Therefore, there should be no objection if a
tire manufacturer wishes to delete the break-in.
C. Thermal Conditioning
Comments/Analysis
None.
D. The Rolling Resistance Measurement
1. Installation on the Test Machine
Comments
This section of the EPA Recommended Practice states that the
tire should be installed on the test machine, the normal load
should be applied and then a final pressure check should be made.
ASTM, RMA, MVMA, and SAE all commented that the order of the
pressure checkmachine installation sequence was not important, but
that it was currently common practice to measure the inflation
pressure of the tire prior to installation loading because this was
more convenient for the test machine operator.
With respect to the final pressure inflation measurement, RMA,
GM, and Messers Pottinger and Trivisonno all commented that the
accuracy specified in the EPA Recommended Practice was excessively
stringent. GM and Messers Pottinger and Trivisonno recommended a
pressure gauge accuracy of 0.25 psi while RMA suggested 0.5 psi.
Analysis
The sequence specified in Draft EPA Recommended Practice is
the typical sequence that would occur on an in-use vehicle. There
is, however,* little reason to believe that the order of the
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sequence would have a measurable effect on the tire rolling re-
sistance coefficient. Therefore, it is recommended that the
sequence specified be revised to coincide with current common
practice.
The GM comments estimated that the effect of an inflation
error of 0.5 psi would be between 0.10 and 0.15 Ibs. in tire
rolling resistance. In general the Recommended Practice attempts
to maintain an accuracy of 0.1 Ib. whenever feasible. Consequently
an inflation pressure accuracy of 0.25 psi would appear to'be
adequate while an inflation pressure accuracy of 0.50 psi is
marginal. Consequently, it is recommended that the EPA Draft
Recommended Practice be revised to specify an inflation pressure
accuracy of 0.25 psi.
2. Tire Warm-Up
Comment s/Analys i s
None.
3. Rolling Resistance Measurements
Comments/Analysis
None. .
A. Measurement of Parasitic Losses
Comments
A "skim" reading method in which the tire lightly contacts the
test wheel was specified in the EPA Draft Recommended Practice as
the method of determining the parasitic machine losses. Most
comraenters expressed reservations about this approach and expressed
preference for a "machine" reading in which the tire is completely
unloaded and only the residual machine signal is noted.
GM, however, expressed perference for an approach in which the
rolling resistance of the tire is determined for a very light load
and the rolling resistance coefficient then be determined by the
measured change in the rolling resistance divided by the change in
the load.
Analysis
The "machine" reading approach preferred by most commenters
is the simplest approach. However, this approach does include
the tire spindle bearing losses in the rolling resistance.
To the extent that spindle bearing losses are different on dif-
ferent test machines this will result in some variability of the
results between different test machines.
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The approach recommended by GM has the advantage of removing
the variations which might occur in spindle bearing losses.
However, the GM approach does have the disadvantage of also
subtracting the tire aerodynamic losses.
The ideal solution would be to remove the spindle bearing
losses while retaining the tire aerodynamic losses. In this
manner the measured tire rolling resistance.would be closest
to the load which a tire would impose on a vehicle on the road.
However, the aerodynamic drag from the tire when on the test
machine may be different from the drag when installed in a wheel
well cavity. Consequently, it is concluded that it is more
important to remove the variability induced by spindle bearing
losses than it is to preserve the aerodynamic component of the tire
losses. Therefore, it is recommended that the GM approach be
adopted.
It should be noted that, use of a "machine reading" would
result in a more conservative measurement of the tire rolling
resistance and therefore, if adopted by a tire manufacturer
this would not be objectionable as discussed in the previous
section.
IV. Data Analysis
Comments
Minor editorial comments only. ;
Analysis
Corrections and revisions will be adopted as necessary.
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EPA Recommended Practice for Grading and
Labeling of Tires for Fuel Efficiency
I. Introduction
Comments
The introduction to this EPA Recommended Practice stated that
tires had a significant effect on vehicle fuel consumption and that
the measured rolling resistance coefficient of a tire was a good
predictor of the fuel consumption effect of the tire. Several
commenters, particularly Ford and ASTM questioned if measurement of
the energy dissipation of a free rolling tire was adequate to
predict in-use fuel efficiencies of tires.
In addition, these comraenters expressed concern over the
ability of consumers to conveniently utilize the tire grade in-
formation or to convert this information into predictions of the
relative vehicle fuel consumption with different tire grades.
Analysis
The accuracy of the current proposed test procedure was also
an original concern to EPA and was the reason that the EPA recom-
mended practice of April 1978 included determination of tire energy
dissipation over a transient cycle. Numerous comments and data
submitted in response to this recommended practice indicated that
the current simplified procedure is adequate to predict the fuel
efficiency of tires. Data obtained by EPA also support this con-
clusion. Consequently, unless data supporting the need for more
complex test procedures are developed, the current recommended
practice is considered adequate.
With respect to the use of the tire grade information and the
ability to convert this information into fuel efficiency, the use
of letter grades was recommended specifically to simplify inter-
pretation of the grading system. In addition, EPA will provide an
information pamphlet to allow estimation of the relative effects on
vehicle fuel efficiency of various tire grades.
II. Tire Grade Classification
Comments
ASTM and Ford both commented that the range of tire grades
should be expanded. It was particularly noted that many current
tires would fall in the highest rolling resistance categories and
that perhaps additional higher rolling resistance categories should
be provided.
Analysis
The proposed categories were selected to promote development
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of improved low rolling resistance tires, and to provide a very
simple system for ease of consumer use. Many current tires are
included in the "D grade". However, it is expected that these
tires.will be improved as available technology permeates the tire
manufacturing field. While it is certainly technically feasible to
provide additional tire grade levels, the advantages of maintaining
a simpler system appear greater than those of expanding the grading
system to include tires which will tend to vanish from production.
III. Determination of the Grade of a Tire Design
Comment s
Messers Pottinger and Trivisonno expressed concern that the
proposed grading sytem could, in some instances, extenuate or
mask true tire rolling resistance variations. Several examples
were provided, which demonstrated the importance of small varia-
tions in measured rolling resistance when the results are near the
boundaries of the categories. A numerical labeling system was
proposed as an alternative approach.
In addition, GM proposed a tire grading system which would
reduce or eliminate the tendency for smaller tires to qualify
for higher rolling resistance grades than larger tires. The GM
proposal essentially requires extrapolation of rolling resistance
data versus test load for diverse size tires of a given design.
The rolling resistance grade was then based on the intercept or
zero load value of the extrapolated line.
Analysis
Whenever a category measurement or grading system is used
there is always the possibility of some inequitable treatment near
the category boundaries. However, a numerical labeling system
would be inconsistent with the current DOT tire quality grading
systems and would be more difficult for consumer use. In this
instance the advantage of simplicity and consistancy provided by
the category labeling sytem are judged to be greater than the
disadvantages of such an approval.
There is merit in the concept of the GM proposal to attempt
to make tire grades independent of tire size. Primarily, it is
generally accepted that it is more difficult to manufacture low
rolling resistance tires in small sizes. Consequently a given
brand or design of tire might, under the current classification
system, be a "grade B" tire in larger sizes yet only be a "grade C"
tire in the smaller sizes. This would be a technically correct
statement of the rolling resistance cofficient of the tire, but
might cause some consumer confusion and diminish use of the system.
Likewise this could prevent a general tire line from being adver-
tized as "rolling resistance grade A tires" and therefore, reduce
dissemination of the rolling resistance grade information in tire
advertising.
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There are, however distinct problems with the GM proposal. If
the "adjustment factor" for tire size is determined and included in
the EPA Recommended Practice then this single "fixed" value may not
be appropriate for all tire technologies. If the adjustment factor
is to be determined for each tire design, then the grade could
become very dependent on the particular tires selected to determine
this factor. In general, the logic of a floating adjustment factor
is circuitous since the data obtained from the tests of diverse
tires would be used to determine an adjustment factor based on the
apriori decision that the tires tested were of equivalent design-
technology, where as the purpose of the testing should be to
determine or verify the rolling resistance technology of the tire.
It is recommended that a grading system which provides some
adjustment for the tendency of smaller tires to have higher rolling
resistance coefficients be developed. This system should be based
on a predetermined or "fixed" adjustment factor. However, since GM
was the only commenter suggesting such an approach, the proposed
system should be distributed for comments before final adoption.
IV. Tire Labeling
Comments .
GM commented that, "A simplified version (of the proposed
label) such as 'fuel economy grade B' in dimensions consistent with
the tire quality grading label should be adequate." Messers
Pottinger and Trivisonno commented that point of sale information
should be used rather than molded labeling.
Analysis
With respect to the location of the information it is con-
sidered essential that the information be available on the tire if
the customer is to be assured that the rolling resistance in-
formation is applicable to the purchased tire. However, an ad-
hesive label attached to the tire might be an acceptable initial
alternative to sidewall molded information.
With respect to the size of the fuel efficiency label there
are no technical reasons why the DOT quality grades and the fuel
efficiency grades need be the same size. The fuel efficiency grade
of tire should be of sufficient importance to warrent clear con-
spicuous labeling. It is questionable if the DOT quality grading
labels, using letter 5/32 in. high can be considered clear and
conspicuous.
In addition to the technical comments discussed in the pre-
vious sections general editorial comments were submitted. Cor-
rections or modifications are recommended as necessary.
RECOMMENDATIONS
The issues which were raised in the comments received are
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summarized in the following tables. These tables also summarize
the recommended actions in response to these issues.
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Table 1
Rolling Resistance Test Procedure
Issue
Other measurement approaches,
in addition to spindle force
should be approved.
Test machine alignment tolerances
should be reduced, particularly
for spindle force machines.
Resistance measurements in
high temperature test cells
(100°F) should be acceptable.
Provide a temperature correction
to correct rolling resistance
measurements obtained in the
range of 70° to 80°F to a nominal
75°F.
The proposed rim specifications
should be more specific.
All alpha numeric tires should
all be tested at 32 psi indepen-
dent of load range.
All "P" tires should be tested
at 35 psi independent of the load
range of the tire.
Delete temporary "T" type tires
from the recommended practice.
Delete the break-in requirement
for radial tires.
The sequence of loading tire
followed by checking that infla-
""^n pressure should be changed
check pressure then load tire.
Recommended Resolution
Provide allowance for the use
of the measurement methods, par-
ticulary the torque approach.
Adopt the proposed reduced
alignment tolerance.
Reject the proposal.
Adopt the proposal, however
it is recommended that the cor-
rection coefficient which is
developed be re-distributed for
additional review and comments.
Adopt the proposed rim speci-
fications.
Adopt the proposal.
Adopt the proposal.
Adopt the proposal.
Include a statement that
other procedures which yield
equivalent results are accept-
able. This will allow dele-
tion of tire break-in of
radial tires at a tire manu-
facturer's option if break-
in does not significantly
affect rolling resistance.
Adopt the proposal.
tion
to
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Table 1 (cont'd)
Rolling Resistance Test Procedure
Issue
The accuracy of the tire infla-
tion pressure gauge should be
reduced to 0.25 psi.
Change current "skim reading"
approach for measurement of the
machine parasitic losses to a
"machine reading" or a modified
"skim reading" approach.
Recommended Resolution
Adopt proposal.
Adopt the proposed modified "skim
reading" method. A tire manufac-
turer could adopt the "machine
reading" approach under the pro-
vision for "alternate methods
which yield equivalent results."
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Table 2
Grading and Labeling
Issue
The rolling resistance of a free
rolling tire may not be adequate
to predict tire fuel efficiency.
The range of tire grades should
be extended.
A continuous numerical grading
system should be adopted instead
of the proposed letter category
system.
Rolling resistance grades should
be dependent on tire load (size)
in~>addition to rolling resistance
coefficients so that there ' is not
a tendency for small tires to
have higher (inferior) grades.
Recommended Resolution
No changes recommended. Sufficient
EPA and general literature exist
to demonstrate that the rolling
resistance of a free rolling tire
is a good predictor of the effect
of the tire on vehicle fuel con-
sumption.
No change in the number of cur-
rent categories, is recommended
however, the resolution of the
final issue somewhat reduces the
need for expanded grades.
Reject the proposal to retain
simplicity.
Adoption of the proposal appears
desirable, however since this pro-
posal was only made by one com-
menter the proposed modficiation
should be distributed for comments.
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ATTACHMENT I
DRAFT
EPA Recommended Practice
for Determination of Tire Rolling
Resistance Coefficients
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DRAFT
EPA Recommended Practice for
Determination of Tire Rolling
Resistance Coefficients
by
Glenn Thompson
March 1980
Standards Development and Support Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air, Noise and Radiation
U.S. Environmental'Protection Agency
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I. Introduction .
This test procedure determines the tire rolling resistance
coefficient for a free rolling tire at a steady speed. This
procedure conforms to the SAE Recommended Practice, Rolling Re-
sistance Measurement Procedure for Passenger Car Tires - SAE J1269,
generally adopting the recommended conditions of J1269 as the
required standard conditions. The SAE Recommended Practice J1269
and the accompanying SAE information report J1270 may be consulted
for additional information.
II. Test Equipment
The test equipment required is a tire dynamometer which
measures the tire spindle reaction force as the tire is driven by a
large cylindrical test wheel.
A. Tire Dynamometer
The test dynamometer shall be a cylindrical surface machine of
67.23 in (1.7076m) diameter. The test machine shall be capable of
supplying a force on the tire perpendicular to the test surface,
and shall be able to measure the reaction forces acting on the
tire. During this process the machine must be capable of driving
the test surface at constant speed. The width of the test surface
must exceed the width of all test tires, and the test surface shall
be coated with a medium coarseness abrasive (80 grit). As an
example, medium grit 3M Safety-Walk represents a satisfactory
surface.* .
1. Test Machine Alignment
The direction of application of the tire load must be normal
to the test surface within 0.3 deg (5.2 mrad). The wheel plane of
the tire must be normal to the test surface within 0.3 deg (5.2
mrad) and parallel to the direction of motion of the test surface
within 0.1 deg (1.7 mrad).
2. Test Machine Control Accuracy
Exclusive of perturbations induced by the tire and rim non-
uniformities, the test equipment must control the test variables
within the following limits:
* The manufacturer of this product is identified to clarify the
.example and does not imply endorsement of the product.
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-22-
U.S. Customary Units SI Units
Tire Load 5 Ibf 22 N
Surface Speed 1 mph 1 km/h
3. Test Machine Instrumentation Accuracy
The instrumentation used for readout and recording of test
data must be accurate within the following tolerances:
U.S. Customary Units SI Units
Tire Load 2 Ibf 8 N
Surface Speed 0.5 mph 0.8 km/h
Spindle Force 0.1 Ibf 0.4 N
Loaded Radius 0.1 in 0.002 m
B. The Test Cell Requirements
The primary requirement for the test cell is that the ambient
temperature be well controlled. In addition, the support services
of compressed air should be available for tire inflation as should
the necessary gauges to measure tire inflation.
1. Thermal Control
The ambient temperature in the vicinity of the test tire shall
be 75 i 5 °F (23.9 +.2.7 °C).r
2. Temperature Measurement Precision
The instrumentation used to measure the ambient temperature
must be accurate to within 1 °F (0.5 °C).
III. Test Procedure
The test procedure consists of the following steps: tire
mounting; tire break-in; equilibration of the tire to the test
ambient temperature; adjustment of the cold inflation pressure;
tire warm-up and then measurement of the tire rolling resistance.
A. Tire Mounting
1. Rims
The tire shall be mounted on test rims which have an approved
contour and width as specified by the Tire & Rim Associations,
Inc., for the size tire being tested. These rims shall have a
maximum radial runout of 0.035 in (0.88 mm) and a maximum lateral
runout of 0.045 in (1.14 mm).
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-23-
2- Inflation Pressure
The inflation pressure of the tires after mounting shall be
the inflation pressure specified for design purposes by the
Tire & Rim Association, Inc. There pressure are specified in the
following table for passenger car tires:
Alpha Numeric Size Tires
Load Range B Tires 24 psi (165 kPa)
Load Range C Tires 28 psi (193 kPa)
Load Range D Tires . 32 psi (221 kPa)
"P" Type Tires
Standard Load Tires 26.1 psi (180 kPa)
Extra Load Tires 31.9 psi (220 kPa)
"T" Type Tires
60 psi (41K4 kPa)
The tire inflation pressure after mounting shall be correct
to within 1 psi (6j3 kPa). The gauges used to measure this tire
inflation pressures shall be accurate to within 0.5 psi (3.4
kPa).
B. Tire Break-in
Tires may undergo significant permanent growth upon first
operation and therefore require an initial break-in and cooling
period prior to the start of the test. A break-in run consisting
of installing the tire on the tire test machine and operating the
system under the test conditions for a period of 1 hour is re-
quired.
C. Thermal Conditioning
After initial break-in the tire shall be placed in the thermal
environment of the test conditions for a minimum period of 3 hours
before the test. During this period the tire inflation pressure
should be checked and adjusted if necessary, to the design cold
inflation pressure of the tire.
D. The Rolling Resistance Measurement
The test consists of loading the tire, a final pressure check;
the tire warm-up, during which the tire temperature and inflation
are allowed to increase as they should in typical service; followed
by the rolling resistance measurement.
1. Installation on the Test Machine
.The tire shall be installed on the test machine if not
presently installed, and the load on the tire perpendicular to the
test surface shall be adjusted to 80 percent of the design load of
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-24-
th e tire. At this time, the inflation pressure of the tire shall
be checked and adjusted if necessary. The inflation pressure
immediately prior to the test shall be correct to within 0.1 psi
(0.68 kPa) . The gauges used to determine this pressure shall be
accurate to within 0.05 psi (0.34 kPa).
2. Tire Warm-up
The test tire shall be conditioned by operation at a speed of
50 mph for a minimum of 45 minutes.
3. Rolling Resistance Measurements
Following the tire warm-up and with the test dynamometer
operating at 50 mph, the following parameters shall be recorded:
a. Tire spindle force
b. Normal load on the tire
c. Loaded radius of the tire
d. Angular velocity of the tire
4. Measurement of Parasitic Losses
As a final measurement, the parasitic machine losses shall be
determined. The test machine speed shall be maintained at 50 mph
while the load on the tire is reduced to the minimum value which
will maintain the angular velocity of tire measured during the
test. Under this condition the following parameters shall be
determined: .
a,. Tire Spindle Force
b. Normal load on the tire
IV. Data Analysis
The data reduction consists of the correction for the machine
parasitic loses, conversion to a tire energy dissipation force,
and finally the computation of the tire rolling resistance co-
efficient.
A. Subtraction of Parasitic Losses
The spindle force measurement of the machine parasitics losses
obtained in III. -E^shall be subtracted of the spindle forces
measured during the test, III. £-?. to obtain the net spindle
reaction force. P^
B. Tire Energy Dissipation Force
1 ~ ~"~- " ~" /
The tire energy dissipation force shall be calculated from the
net spindle reaction force by the following equation:
Fd = FV .(1 + r/R) (1)
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-25-
Where: F
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-26-
ATTACHMENT II
DRAFT
EPA Recommended Practice
for Grading and Labeling of
Tire for Fuel Efficiency
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-27-
DRAFT
EPA Recommended Practice For
Grading and Labeling of
Tires For Fuel Efficiency
by
Glenn Thompson
March 1980
Standards Development and Support Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air, Noise and Radiation
U.S. Environmental Protection Agency
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-28-
I. Introduction
Tires have a significant effect on the fuel consumption of a
vehicle.JY2_/ This effect occurs because the energy dissipation in
the tire affects the force required to propel the vehicle and hence
the fuel consumed by the vehicle engine. Consequently the fuel
efficiency of tires may be graded by their energy dissipation
characteristics.
This document provides a grading classification system for
tires, and provides the testing requirements necessary for a tire
manufacturer to determine the grade of a tire design. Tires of a
graded design may be labeled with their determined fuel economy
grade. The final section of this document provides the criteria
for the configuration of the label.
II. Tire Grade Classification
The grade of a spec ific tire is to be determined by the
tire rolling resistance coefficient according to Table 1. The
tire rolling resistance coefficients (RRC) are to be measured
in accordance with the EPA Recommended Practice for Determin-
ation of Tire Rolling Resistance Coefficients.^/
III. Determination of the Grade of a Tire Design
A manufacturer may determine a grade for a tire design
and use this grade for labeling all tires of this design.
In order to determine the grade of a tire design, a manufac-
turer shall randomly select and test sufficient tires from diverse
production sites such that there is 90 percent statistical confi-
dence that 90 percent of the tires produced will have rolling
resistance coefficients less than the upper bound rolling resis-
tance of that grade.UJ
For example, if a manufacturer wishes to determine a RRC
grade for his "Super Rounder" tires, 24 sample tires might ran-
domly be selected from the 4 production lines manufacturing
these tires. These 24 tires might than be tested by the EPA
Recommended Practice obtaining the results of Table 2.
The requirement for labeling "Super Rounders" as a grade
A tire is that the value computed from the sample data, such
that there is 90 prcent confidence that 90 percent of the tires
produced will have RRC below this value is less than 0.012.
NOTE: All references referred to in this paper are shown as /.
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-29-
Table 1
Tire Rolling Tire Fuel
Resistance Efficiency
Coefficient Grade
RRC < 0.012 A
0.012<^ RRC < 0.014 B
0.014< RRC < 0.016 C
0.0l£< RRC D
-------
Measured Tire Rolling Resistance Coefficients
Production Line
Test
Tire No. A
1 .0123
2 .0126
3 .0125
4 .0110
5 .0127
6 . .0126
B
.0113
.0101
.0104
.0132
.0101
.0115
C
.0134
.0121
.0119
.0115
.0133
.0104
D
.0115
.0111
.0113
.0119
.0115
.0134
Mean RRC = 0.01182
Std. dev. S = 0.00102
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-31-
That is: .
Xu = RRC + K-S < 0.012 (1)
Where,
Xu = the upper bound such that there is a given confi-
dence that a given percentage of the tires produced
will have rolling resistance coefficients below
this value.
RRC = the mean of the measured rolling resistance coef-
ficients.
K = the "K factor" (given in Table 3).
S = the sample standard deviation.
In this example K for a sample size of 24 is given by Table 3
as 1.712. Therefore:
Xu = 0.01182 + 1.712 (0.00102) (2)
= 0.0136
Since Xu > .0120, Super-Rounders cannot be labeled grade "A*. But
.0120 < Xu < .0140; thus, we are 90 percent sure that 90 percent of
Super-Rounders have RRC < .0136 < .0140; consequently Super-Round-
ers can be labeled as grade 'B1.
IV. Tire Labeling
For any tire design meeting the criteria of section III,
determination of the Grade of a tire Design, all tires of this
design may be labeled with their Fuel Economy Grade.
The Fuel Economy GRADE label shall be permanently affixed to
the tire (molded into or onto the tire would meet this condition)
in a location which is conspicious where it is installed on a
vehicle. The label may take either of the following forms, in
which a Grade B tire is presented as an example.
"Fuel Economy GRADE B as Determined By EPA Procedures"
"EPA Fuel Economy GRADE B"
All letters shall be of "Futura Bold, Modified, Condensed or
Gothic" type. The word "grade" and the letter designation of
. the grade shall be at least one-half inches (12mm) tall. The
remaining words shall be in letters at least three-sixteenth
inches (5mm) »tall. Example labels are shown in Figures 1 and 2.
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-32-
Table 3
Factors For One-Sided Tolerance Limits For
Normal Distributions
Factors K such that there is 90 percent probability that
at least 90 percent of the distribution will be less than X +
K S (or greater than X - K S), where X and S are estimates
of the mean and the standard deviation computed from a sample
size of n.
n
3 4.258
4 3.187
5 2.742
6 2.494
7 2.333
8 2.219
9 2.133
10 2.065
11 2.012
12 1.966
13 1.928
14 » ' 1.895
15 1.866
16 1.842
17 1.820
18 1.800
19 1.781
20 1.765
21 1.750
22 1.736
23 1.724
24 1.712
25 1.702
30 1.657
35 1.623
40 1.598
45 1.577
50 1.560
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f \J E
r
FIGURE 1
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.
FIGURE 2
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-35-
The labeling of tires is intended to allow manufacturers of
truly fuel efficient tires to advertise this beneficial aspect of
their tires. Advertising which uses the designated label without
proper adherance to the test procedures, including procedural
documentation, is prohibited. Evidence of such prohibited label
use will be referred to the Federal Trade Commission (FTC) for
their consideration for appropriate action.
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-36-
References
!_/ Klamp, W. K., "Power Consumption of Tires Related to How They
Are Used," Procedings of the 1977 SAE-DOT Conference, Tire
Rolling Losses and Fuel Economy - An R&D Planning Workshop.
2/ Yurko, J. "Tire Related Effects on Vehicle Fuel Economy," U.S.
Environmental Protection Agency Technical Report SDSB.
3f Thompson, G. "EPA Recommended Practice for Determination of
Tire Rolling Resistance Coefficients," U.S. Environmental
Protection Agency Technical Report (Draft March 1980).
4/ Experimental Statistics National Bureau of Standards: Hand-
~~ book 91 U.S. Dept. of Commerce (1966).
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-37-
ATTACHMENT III
DRAFT
Conunents Received in Response to
the Draft EPA Recommended Practices
-------
-38-
ATTACHMENT III
Comments Received in Response to
the Draft EPA Recommended Practices
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-39-
IVITS SYSTEMS CORPORATISM
BOX 24O12. MINNEAPOLIS. MINNESOTA 55424
TELEPHONE 612-844-4000 TELEX 29-0521 CABLE MTSSYSTEMS
15 April 1980
Mr. Charles L. Gray Jr.
Director
Emission Control Technolgy Division
United States Environmental Protection Agency
Ann Arobr, MI 48105
Subject: EPA Recommended Practice for Grading and Labeling
of Tires for Fuel efficiency, EPA Recommended
Practice for Determination of Tire Rolling Resistance
Coefficient; March 1980
Dear Mr. Gray:
ThiS"±s to communicate my thinking concerning the subject test
and grade labeling procedures, as follows:
1. The machine alignment accuracy range specified in
paragraph II.Al of the proposed EPA Grade Labeling
procedure will result in machine to machine differences
at least 5 times larger than the EPA Proposed Tire Fuel
Efficiency Grade ranges. More particularly, an Fx align-
... , ^ent error of Fx error = P sin +0.3
~ where P = tire load
will result in a rolling resistance coefficient error
range of RRC = FX error B s±n (+Q>3) _ sin (_0>3)
error p
range
= 2 sin 0.3
= 0.0105
In fact the entire range from A thru D of Table 1 of the
Proposed Grading and Labeling Procedure is only 0.016 -
0.012 = 0.004. I suggest that one of the following four
courses of action be pursued.
A) Specify machine alignment to +0.03°, producing a cross-
coupling error range of 0.00105. This is still a size-
able error of 1 Ib. per 1000 Ibs. of F tire load.
z
tB) Specify that test be run both clockwise and counter-
clockwise and the results averaged to remove the cross-
coupling bias error.
C) Measure and eliminate the cross-coupling error using
suitable calibration procedures.
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-40-
D) Test using sensing techniques that are insensitive to
the load application alignment (e.g. , roadwheel drive
torque/ carriage reaction measurement) .
2. The specification of only a 67.23 inch diameter roadwheel .
machine unduly restricts the adoption of today's new
generation of flat surface testing machines. An example
of such a machine is shown on pages 4 and 5 of the en-
closed product brochure, "MTS Tire and Wheel Testing Cap-
abilities." There are currently six such machines on
order, two of which have rolling resistance measurement
capability with one of these to be shipped within the
month. Previously experienced tire support bearing pro-
blems cited in EPA test reports have been solved on this
machine and, although this system has not yet been widely
distributed in the tire industry, it is now a practical
reality.
I suggest that the following SAE statement be added to para-
graph II. 2, "The Development of Preferable Flat Surface
Laboratory Apparatus is progressing a pace which may soon
bring it into common use. The recommended practice is writ-
ten in such a way that it can be used without modification
when such equipment becomes commonly available."
3. Paragraph III.A2 and III.Dl do not agree as to tire pres-
,sure accuracy specified.
4. .Reference to paragraph III.E3 and III.E2 in paragraph IVA
should read III.D3 and III.D2.
Thank you for the opportunity to respond to the subject proce-
dures. If I may be of further service in clarifying these matters,
please contact me at 612/944-5409.
Sincerely yours ,
S SYSTEMS CORPORATION
Gerald R. Potts
Manager
Tire/Wheel Test Systems
GRP/jmr
cc : Glenn Thompson - EPA
Encl: MTS Brochure 114.08-02
MTS Tire and Wheel Testing Capabilities
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LAND
SEA
AIR
SPACE
-41-
Society of Automotive Engineers, Inc.
Committee:
Rolling Resistance Sub-Committee
Reply To:
T. P. Baker, Chairman
SAE Rolling Resistance Sub-Committee
Uniroyal, Inc.
6600 E. Jefferson Ave.
Detroit, Michigan U8232
Mr. Charles L. Gray, Jr., Director
Emission Control Technology Division
United States Environmental Protection Agency
Ann Arbor, Michigan U8105
April 1U, 1980
Dear Mr. Gray:
Thank you for your recent letter and the two attached Draft Recommended Practices
for the Measurement and the Grading of tires for Rolling Resistance. I am
very pleased to have this opportunity to respond, on; behalf of the SAE Tire
Rolling Resistance Sub-Committee. My comments are as follows:
We are very pleased that the EPA has recognized the merits of our SAE Rolling
Resistance Measurement Procedure for Passenger Car Tires - SAE J12o9 by
adopting many of its features as the framework for the EPA Draft Procedure.
We believe that with present technology a steady state test of this nature is
quite satisfactory for the purpose you intend. SAE J1269 has the added
advantage that it is a familiar test and already widely accepted, as you
mention in your letter.
With regard to in-use and test inflation pressures, I would like to point out
to you that the pressures specified in J1269 were arrived at in anticipation
of higher pressures being adopted generally throughout the automotive industry,
and by the motoring public. Our intention was to draft a document which would
be in tune with the 1980*s, as opposed to one whose pressures would soon be
obsolete. To the extent that the industry has been slower than we expected
in making this change, our recommended pressures are ahead of their time.
Time has not stood still, however. Manufacturers are specifying higher pressures
for some new vehicles and more will be added to the list in the 198! model
year. In addition, the public is receiving more and more messages extolling
the virtues of maintaining higher inflation pressure for safety, fuel economy,
and treadwear. Therefore, it is likely that T&RA design pressures are no
better a reflection of in-use pressures than are the J1269 pressures at the
present time; a*nd in-use pressures may approach J1269 pressures as time goes on.
-------
J. VJJLCIJ, Wi», */J.J. v,._v/w.k -
Emission Control Technology Div.
U.S. Environmental Protection Agency -42-
Ann Arbor, Mich.
In view of the foregoing, I would encourage the EPA to publicly endorse higher
, inflation pressures and to help lead the way by adopting higher pressures in
the Draft Recommended Practice. To do otherwise implies satisfaction with
lower pressures, which I am sure you do not intend.
The EPA Draft Measurement Procedure is more restrictive than J1269 in several
important features which we believe should be broadened to include current
practices. One is the specification of the tire spindle reaction force
measurement, which implies the exclusion of the torque method and the energy
.f method. The torque and energy methods are in widespread use, and are
. demonstrably as reliable as the force method. Therefore, we can see no
reason for their exclusion.
Another unnecessary restriction is the requirement of a break-in run for all
tires, whether they need it or not. We suggest that only tires which under-
go significant permanent growth need be broken in, and that the break-in be
omitted where supporting evidence can be produced to justify the omission.
The EPA Draft Test Procedure calls for the measurement of parasitic losses by
what we know as a "skim reading". Many laboratories have found that a "skim
reading" is not as reproducible as a "machine reading" in which the tire is
removed completely from the test surface. The difference, of course, is in
the tire spindle bearing loss and the aerodynamic loss of the tire, as
>- explained in SAE J1270, section 7-1- For the sake of more reliable end
reproducible test data we recommend that the EPA Draft be modified to allow
the use of a "machine reading" which is current practice in most rolling
resistance test laboratories today.
Another point of difference, which may at first seem trivial, is that the EPA
Draft calls for loading the tire prior to the cold inflation pressure check;
whereas the SAE procedure is unclear as to the sequence of these events.
In actual practice, most test laboratories make the pressure check prior to
loading the tire. In some cases it makes little difference to the test
operator which comes first; but in laboratories where the test is operated
from a remote location, sometimes even from another room the requirement of
. , loading first and then checking pressure imposes a penalty on test efficiency.
' > We agree that the sequence should be specified, but we recommend that it be
specified in the opposite order in order to bring it into line with current
practice.
I have only one comment on the EPA Draft for Grading and Labeling. It concerns
an apparent inconsistency between the definition of the grades as given in
Table 1 and the procedure for establishing a grade as given in Section III,
paragraph 2. Although the grades in Table 1 have upper and lower bounds,
-V only the upper bound is used to establish a tire's grade. Furthermore, Grade
D has no upper bound to compare the test results to. I think the intention
of the Draft can be inferred by the reader, but it needs more work to make it
unambiguous in this area.
-------
U.S. Environmental Protection Agency
Ann Arbor, Mich.
-43-
I trust these comments will prove useful to you as you give further attention
to this subject.
Sincerely,
T. P. Baker, Chairman
SAE Rolling Resistance Sub-Committee
cc: Mr. R. T. Northrup - SAE
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-44-
1465 N. Hametown Road
Akron, Ohio 44313
April 17, 1980
Mr. Charles L. Gray, Jr., Director
Emission Control Technology Division
United States Environmental Protection Agency
Ann Arbor, Michigan 48105
Dear Mr. Gray:
We received a copy of your letter and Glenn Thompson's drafts from Gerald
R. Potts of MTS through ASTM. With the exception of one procedural comment
all other comments will be technical. The exception is that: either desired
deadlines should be extended or a more prompt means of communication should be
established as many of us never see the Federal Register.
Our technical comments will refer to Glenn Thompson's draft, "EPA Recom-
mended Practice for Grading and Labeling of Tires for Fuel Efficiency," March,
1980. .
Page la, III. Determination of the Grade of a Tire Design
We basically agree with the statistical idea of deciding the grade for a
tire design, but do see significant problems: the scheme of tire grading, the
method of sampling, certain statistical points, and labeling difficulties.
The scheme of tire grading into four distinct categories does not reward
those who improve their product within a grade, it unduly punishes those who
are only slightly worse, and it could lead to endless haggling and law suits.
We would like to amplify these points by bending Glenn Thompson's example.
Let us suppose that company "A" makes "super rounders" and company "B"
makes "maxi-tractions." Suppose now for simplicity that "super rounders" and
"maxi-tractions" have precisely identical standard deviations, S = 0.00102,
but different means. Also assume that we are using the proposed grades in
Table I with the necessary misprint correction that for grade D, 0.016 ^ RRC.
Suppose that for "super rounders," RRC_= 0.01030, then their X = 0.01205
(grade B) and that for "maxi-tractions," RRC = 0.01215, then their k = 0.01390
(grade B). Is the difference significant? Yes. Is Company "A" rewarded
competitively for a better job? No. The customer is misinformed.
Suppose now that for "super roundersA"_RRC = 0.0102, then their X = 0.01195
(grade A) and that for "Maxi-tractions," RRC = 0.0103, then their X =U0.01205
(grade B). Is the difference significant? No. Isn't company "B" at an
exaggerated competitive disadvantage with respect to company "A"? Yes. The
customer is misinformed.
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-45-
Mr. Charles Gray, Jr. Page 2 April 17, 1980
For the preceding two cases the use of actual numerical grades similar in
form to the EPA estimated fuel economy for automobiles would serve to better
inform the customer and to foster competition. To best use numerical grades
point of sale information like the estimated fuel economy brochure should be
used instead of molded-on grade labeling. We believe this would be advisable
for other reasons and will return to this subject a bit later.
Suppose that company "A" tests its sample of "super rounders" and gets
RRC = 0.0102, then_their X = 0.01195 (grade A) and that later EPA tests a
sample and gets RRC = O.Oltb, then their X = 0.01205 (grade B). Are the
answers significantly different? No. Will an insuing fight over this help
anyone? No. Therefore, we should seek a way to prevent this type of thing
from occurring. Again numerical grades might help by not creating sharp
arbitrary distinctions which blow insignificant differences up to monumental,
time wasting, arguments. Also, it really won't help customers if companies
must be hyper-conservative in self defense. NHTSA has already been concerned
with how grades are set for UTQG wear ratings. They are looking at self
defense in the face of uncertainty.
Of course, the validity of the whole procedure depends on adopting a
standardized sampling method for use by both industry and the EPA.
Practical considerations and past experience indicate that the same tire
design made in different plants is going to vary in rolling resistance, depending
on where it is made. Larger size tires usually have somewaht lower rolling
resistance coefficients (RRC) than smaller sizes, everything else being equal
and since all sizes are not in production at once, tire age must be considered.
Therefore, the 90% confidence level value will depend on how the sample is
taken. Ideally, all plants and all sizes should be weighted to reflect the
contribution of each plant and each size to the total population of that given
tire design. But this would be a very unwieldy, perhaps impossible, situation.
A simple, reasonable cost, sampling technique is, thus, an absolute prerequisite
to implementation. . .
Page 4a, IV. "Tire Labeling"
We have just suggested that point of sale information be used not molded
on labeling. If, however, molded on labeling does indeed occur, it would be
very desirable if the various agencies would get together and look at what
already exists. Standardization would be very nice, and if we keep going
willy nilly, the companies will run out of space on the sidewall*
Page Ib, II. "Test Equipment^
It would be well to recognize other types of test equipment as well as
the spindle force type in the standard. There are not many spindle force
machines now in existence. Spindle force machines are physically fragile in
routine use as will be pointed out below. SAE J1269 NOV79 recognizes other
methods.
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-46-
Mr. Charles Gray, Jr. Page 3 April 17, 1980
Page Ib, I.IA1. "Test Machine Alignment"
For a spindle force machine we do not believe that the stated tolerances
which are from SAE J1269 are adequate.. A load alignment error of 0.3° which
is allowed in the proposal will produce an interaction of 5.74 lbs/1000 Ib
load. This is quite high, about 1/2 of the actual data amplitude. Inter-
actions should be limited to a much lower order, about 0.2 Ib per 1000 Ib, and
still accounted for. We would, therefore, recommend the load be normal to the
surface within 0.01°. It is worth noting that the preceding comments illustrate
that the spindle force machine is very sensitive to small misalignments.
The procedure specifies that slip angle, wheel plane alignment parallel-
ness to the test surface direction of motion, must be less than 0.1°.. A 0.1°
slip angle could cause a drag error of 0.21b, which is greater than the speci-
fied accuracy. Furthermore, tires are usually a little assymetrical with
respect to slip angle, so a slip angle to the right could have a different
effect than a slip angle to the left. Usually the minimum value of drag
occurs at some small value of slip angle in a given direction. We would
suggest that slip angle be within 0.01°.
For consistency we would make the wheel plane to test surface angle, 90°
to within 0.01°.
Page 2b, IIA3. "Test Machine Instrument Accuracy"
The spindle force accuracy of 0.1 Ibf will require the use of very light
duty load cells preferably 100 Ibs or less. This requires very careful design
and operating procedures to avoid incessant breakage. This is a cautionary
note based on our own experience with very light duty load cells in tire force
measuring heads.
Page 2b, IIB1. "Thermal Control"
Not many machines are now in air conditioned environments. Time for
putting the machines in such environments must be allowed or else it might be
better to use a 100°F environment as used for endurance testing. This envi-
ronment is already widely available.
Page 4b, IIID1. "Installation on the Test Machine"
The pressure gauge which meets SAE J1269 doesn't really exist. There is
a definite precision problem even for electronic gauges in measuring to ±0.05
PSI while covering a range up to over 60 PSI. We are also concerned that
measuring the pressure will produce changes in excess of the intended infla-
tion precision for the test, 0.1 PSI. This is an area that needs careful
attention so that a realistic standard can be drawn. In our own work, we try
for .an inflation accurate to within 0.25 PSI which is one part in one hundred
for the lowest test pressure we use.
Page 4b, IIID4. "Measurement of Parisitic Losses"
*
The tire angular velocity depends on load. It will go down as load
decreases. Thus, an unchanged angular velocity cannot be called out in the
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-47-
Mr. Charles Gray, Jr. Page 4 April 17, 1980
process of attempting to measure windage or bearing losses.. It would be
better to call out a measure of angular velocity variation or to use a com-
pletely unloaded condition as the tare condition.
Page 4b, IVA. "Subtraction of Parasitic Losses"
E3 should be D4 and E2 should be D3 .
The comments in this letter are our own and shou.ld not be construed as a
statement of position by our employer, The BFGoodrich Company.
Sincerely,
_.
.Marion G.. Pottinger.
'
Nicholas M. Trivisonno
MGP/NMT/cs
2Z
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-48-
RUBBER MANUFACTURERS ASSOCIATION
1901 PENNSYLVANIA AVE., N. W. WASHINGTON, 0. C. 20006 (202)735-2602
May 23, 1980
Mr. Charles Gray, Director
Emission Control Technology Division
Environmental Protection Agency
2565 Plymouth Road
Ann Arbor, Michigan 48105
SUBJECT: 1) EPA Draft Recommended Practice for Determining Tire
Rolling Resistance Coefficients
2) EPA Draft Recommended Practice for Grading and
Labeling of Tires for Fuel Efficiency
Dear Mr. Gray:
On March 24, 1980 we received your undated letter which contained copies of
the two subject documents. We are pleased to note that the draft test
procedure is similar to SAE J1269 Recommended Practice, Rolling Resistance
Measurement Procedure for Passenger Car Tires. However, since the SAE
J1269 does not include procedures for truck tires, we have restricted our
comments to include passenger car tire rolling resistance considerations
only.
There are some technical reservations we have concerning the EPA draft
grading system which we have not addressed here, since we believe that we
should first try to resolve the technical details of the test procedure.
Once these have been settled, your proposed grading system can then be
adequately evaluated for technical comment.
Specific comments concerning the EPA draft test procedure follows:
II. The EPA Draft measurement procedure is more restrictive
than SAE J1269 and should be broadened to include
current practices. The specification of the tire
spindle reaction force measurement implies the exclu-
sion of the torque method and the energy method. The
torque and energy methods are in widespread use, and
are demonstrably as reliable as the force method.
Therefore, the EPA specification should be revised to
include as alternatives both the torque and energy
methods.
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-49-
Mr. Charles Gray
Environmental Protection Agency
May 23, 1980
Page Two
SAE Paper 780636 ("Interlaboratory Tests for Tire
Rolling Resistance" by D.J. Schuring and S.K. Clark)
discusses machine to machine variability. Absent a
calibration method, grade ranges would have to take
into account anticipated machine to machine variability.
II.B.I. Federal Motor Vehicle Safety Standard 109 requires that
the air surrounding the test area be 100 +_ 5°F. Since
it would be practical and necessary in some cases to
conduct rolling resistance tests at .the same location
as the FMVSS 109 tests, II.B.I. should be revised to
permit other room testing conditions, provided a
suitable conversion is used to compensate for the
different conditions as permitted by paragraph 4.6.1
> of SAE J1269.
Conversion from english to metric units should be made
in accordance with recognized practice. It appears that
the implied temperature measurement precision in Celsius
is greater than that required in the Fahrenheit units.
We suggest that temperatures in Celsius be rounded to
the nearest 0.5° which is approximately the same implied
precision as 1°F. In this case as well as in other
places in the specification, conversion from metric to
english should be made .in accordance with recognized
practice.
III.A.I. It is not certain what the effect of wide versus narrow
rim width is on rolling resistance. Consequently, it
is recommended that test rims be those specified by
the Tire & Rim Association, Inc. as "design rim width,"
+_ one-half inch.
For tire sizes not standardized by the Tire & Rim
Association, Inc., reference should be made to the
standardizing organizations listed in Federal Motor
Vehicle Safety Standard 109.
III.A.2. The inflation pressures should be rounded to the nearest
whole number, whether in metric or. english units, to be
consistent with recognized practice. In the case of
P-type tires therefore, 26.1 psi should be changed to
26 and 31.9 psi should be changed to 32.
»
We have noted that certain typographical errors contained
in the preprint of SAE J1269 have been carried over
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-50-
Mr. Charles Gray
Environmental Protection Agency
May 23, 1980
Page Three
into your document, for example, 41.4 kPa should be
414 kPa, 68 kPa should be 6.9 kPa.
The test pressures specified in SAE J1269 (32 psi for
alpha-numeric and 35 psi for P tires) is considered to
be more representative of current and near future
recommended inflation pressures for new vehicles.
Also, tire manufacturers and others are recommending
higher tire inflation pressures for improved fuel
economy and treadwear. Consequently, we believe test '
pressures more consistent with the capped pressure of
paragraph 5.2 of the SAE recommended practice and tire
industry recommendations are preferable.
Since T-type tires and collapsible spare tires are
intended for use one tire at a time over short dis-
tances until the vehicle owner can repair the standard
tire on his car, their use on a continuing basis
should not be encouraged by including them in your
rolling resistance considerations. Although SAE J1269
encompasses all kinds of passenger car tires, inclu-
sion of temporary spare tires will not impact on
energy conservation.
III.B. Tires vary widely in their rate of growth during
break-in. In SAE J1269, the need for break-in is left
to the judgment of the tester depending on the nature
of the tire being tested. Similarly, the draft proce-
dure should be revised to permit the tester to deter-
mine the need for break-in. Data available at this
time indicate that radial tires do not require any
break-in to stabilize tire dimensions.
III.D.I. In most testing facilities it is impractical to check
inflation pressures while the tire is loaded on the
test machine. Since there is no significant difference
between inflation pressures measured while the tire is
loaded versus unloaded, checking inflation pressure
when the tire is unloaded should be specified in accor-
dance with typical test practice.
The inflation pressure accuracy is unnecessarily
restrictive for the requirements of the .draft test
procedure. To be consistent with our comments under
* III.A.2., we believe the inflation pressure accuracy
should be 1 psi (7 kPa). Therefore, the gauge
accuracy should be 0.5 psi (3 kPa).
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-51-
Mr. Charles Gray
Environmental Protection Agency
May 23, 1980
Page Four
III.D.4. The EPA draft calls for measurement of parasitic loss
by what is commonly called "skim reading." Many
laboratories measure parasitic loss by "machine
reading." The different test equipment currently in
use in the industry requires that the alternate
parasitic loss methods contained in paragraph 6.6.1
and 6.6.2 of SAE J1269 be permitted. Choosing only
one method unnecessarily forces the selection of
specific test equipment. '
IV.A. There are apparent typographical errors in this para-
graph: E.3 should be D.4; E.2 should be D.3; "of" in
the second line should be "from."
Sincerely,
lomas E. Cole
Vice President
Tire Division
TECAk
-------
1909 K STREET. ,M W. SUITE 300 ' . 366 MADISON AVENUE
WASHINGTON. O.C 20006 ~~52_ NEW YORK. NEW YORK 10017
MOTOR VEHICLE MANUFACTURERS ASSOCIATION
of the United States, Inc.
300 NEW CENTER BUILDING DETROIT, MICHIGAN 48202 AREA 313-872-4311
BROOKS McCOKMICK. Chairman
V. J. ADDUCI. President and Chief Executive Q/"/7«r
THOMAS H. HANNA. Senior Vice President
April 28, 1980
Mr. Charles Gray, Jr., Director
Emission Control Technology Division
U. S. Environmental Protection Agency
2565 Plymouth Road
Ann Arbor, Michigan 48105
Dear Charles:
MVMA appreciates the extensions of time you granted to allow
preparation of our comments on the Draft of the EPA Recommended
Practice for Determination of Tire Rolling Resistance Coefficients.
These comments are attached.
The vehicle manufacturers will comment individually on the
Draft of the EPA Recommended Practice for Grading and Labeling of
Tires for Fuel Efficiency.
If you or your representative would like to discuss these
comments with the appropriate industry personnel, we will be pleased
to arrange for such a meeting.
Sincerely,
Harry By Weaver
Assistant Director, Engineering
AES/kd .
Attachment
TWX NO. 710-322-9245 AUTOMAKERS VVSH.
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-53-
Motor Vehicle Manufacturers Association
COMMENTS ON DRAFT EPA RECOMMENDED PRACTICE FOR DETERMINATION OF
TIRE ROLLING RESISTANCE COEFFICIENTS
II.A.2. Test Machine Control Accuracy
. SI Unit should be 1.6 km/h
II. Test Equipment
IV. Data Analysis
In addition to force techniques, others such as torque and
energy should be allowed as discussed in SAE Recommended
Practice J-1269 "Rolling Resistance Measurement Procedure
for Passenger Car Tires."
III.A.2. Inflation Pressure
Eliminate "T" Type Tires since these are temporary usage
tires.
Alpha Numeric Size Tires should be tested at 32 psi and
equal loads, in order to avoid the rank ordering within
the various tire load ratings. .
"P" Type Tires should be tested at 35 psi and equal loads,
in order to avoid rank ordering within the various tire
load ratings.
. Typographical errors: 41.4 kPa should be 414kPa, and
68 kPa should be 6.8 kPa.
III.B. Tire Break-in
. Change title to "Tire Break-in for Non-Radial Tires" and
reflect this in the first sentence.
III.D.I. Installation on the Test Machine
Set tire pressure prior to loading of the tire to
protect the operator
III.D.S.d. Rolling Resistance Measurements
The angular velocity measurement on the tire requires
instrumentation additional to the SAE practice, and is
not necessary for the determination of parasitic losses.
4/28/80
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-54-
III.D.4. Measurement of Parasitic Losses
The parasitic machine loss skimming method contained in
the draft may cause variations and therefore is not
adequate.
IV.A. Subtraction of Parasitic Losses
. III.E3 should be III.D4, and III.E2 should be III.D3.
4/28/80
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-55-
COMMITTEE F-9
ON
TIRES
I
ASTM, 1916 Race St., Philadelphia. PA 19103 (215) 299-5400
Choirmon: H. G. SCHWARTZ, E. I. do Ponl de Nemours and Co., Inc., 4330 Allen Rd.. Stow, Ohio 44224 (216-929-2941)
fittl Vice-Chairman: D. M. CODDINGTON, Exxon Chemical Co., Elastomers Tech. Div., 1600 linden Ave., linden, N.J. 07036 -(201-474-3365)
Second Vice-Choirmon.- J. W. DAVIS, Cooper Tire & Rubber Co., Western S, lima Aves., Findlay, Ohio 45840 (419-423-1321)
Recording Secretary: WAITER BERGMAN, 33664 Cindy, livonia, Mich. 48150 (313-323-1194)
Membership Secretory.- J. R. WEST, Continental Carbon Co., Box 22085, Houston, Tex. 77027 (713-965-5224)
Staff Monoger.- R. M. SHERWOOD (215-299-5510)
April 21, 1980
Mr.,Charles L. Gray, Jr., Director
Emission Control Technology Division
United States Environmental Protection Agency
Ann Arbor, Michigan U8105
Dear Mr. Gray:
Mr. Gerald R. Potts, Chairman, Subcommittee F9.20, ASTM Committee F-9, on
Tires transmitted your recent letter with attached drafts of the EPA Recom-
mended Practices (a) for Grading and Labeling of Tires for Fuel Efficiency,
and (b) For Determination of Tire Rolling Resistance Coefficient to F-9.20
Committee members and asked them to respond directly to you.
In response to Mr. Potts' request, I am forwarding to you my comments to the
proposed EPA Recommended Practices.
(a) EPA Recommended Practice for Determination of Tire Rolling Resistance
Coefficients.
1. The values of inflation pressure specified in the proposed procedure
do not reflect a current trend showing an increase in tire pressure
by the industry and driving public and, therefore, may become obsolete
in the near future. It is recommended to increase these values.
2. Loading the tire prior to checking inflation pressure is undesirable
because it may reduce test efficiency. An inverse sequence, commonly
used by most test laboratories, is recommended.
3. Requirements for determination of tire energy dissipation force from
measured values of spindle reaction force introduce unreasonable re-
striction in the test procedure. It is recommended to specify the
torque and energy methods as alternatives to spindle reaction force
measurements.
If. Measurement of parasitic losses on the tire contacting the drum surface
may adversely affect precision of measurements. To improve precision
it is recommended to perform the measurement on a tire completely dis-
engaged from the drum surface.
Standards for Materials. Products. Systems & Services
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-56-
2
5. Rolling resistance coefficient vary with ambient temperature. 1°F
increase of temperature produces approximately O.k% decrease of rol-
ling resistance coefficient. To illustrate the significance of the
effect of temperature variations within the +5°F limits specified
in the proposed procedure, it is assumed that the value of coefficient
at 75°F is equal to .012; then the values of rolling coefficient
at temperatures of 70°F and 8o°F become:
C = .012 (1 + 0.00^ (75-70) ) = .0122 (Grade B)
Cgo = .012 (1 + 0.00k (75-80) ) = .0117 (Grade A)
Since such temperature variation significantly effect the value of
rolling resistance coefficient which even results in change of tire
grading, it is recommended that rolling resistance coefficient should
be corrected for change of ambient temperature and normalized to a
nominal temperature.
6. There are few typographical errors in the proposed draft. Surface
speed specified in Section II A2 is 1 Km/h instead of 1.6 Km/h.
Inflation pressures specified in Section III A2 are Ul.U kPa instead of
kPa and 68 kPa instead of 6.8 kPa.
(b) EPA Recommended Practice for Grading and Labeling of Tires for Fuel
Efficiency.
1. Limitations of applicability of rolling resistance coefficient of a
free rolling tire as a criterion for grading of tires for fuel effici-
ency of driven automobiles should be clearly stated in the introduction
to the Proposed Recommended Practice. The reasons for this statement
are explained in the following discussion.
Tires have a significant effect on the fuel consumption of a vehicle
because of the energy dissipation in the tire. Energy dissipation in
the tire results from deformation of the tire produced by forces trans-.
mitted through the tire. Load carried by the tire produce vertical
deformation. Energy dissipation due to this deformation constitute
rolling resistance.
Driving forces propelling the vehicle produce circumferential deforma-
tion. Circumferential deformation produced by driving forces not only
cause a direct energy dissipation due to this deformation, but also
tend to modify pressure distribution along the length of tire contact
which results in shifting the point of application of the resultant
vertical force forward. This shift tends to further increase tire rol-
ling resistance. Therefore, energy dissipation in the driven tire is
considerably greater than that in the free rolling tire.
Furthermore, tires generate lateral forces which are necessary for
directional control of a vehicle and for maintaining direction sta-
bility. These forces result from lateral deformation of the rotating
tire operating at a slip angle. Additional dissipation of energy in a
tire occurs during this process. Since vehicle continuously tends to
change its direction of motion even in so-called "straight ahead"
driving, energy dissipation due to lateral deformation continuously takes
place. Vehicle fuel consumption is influenced by the total energy dissi-
pation resulting from vertical, circumferential, and lateral deformation
of the tire.
-------
The magnitude of these deformations and consequently the magnitude
of energy dissipation are effected by vehicle operating conditions,
environmental conditions, and tire construction. Tire vertical, cir-
cumferential, and lateral stiffnesses may be consideredas major tire
design factorsaffecting tire deformations and energy dissipation.
Tire grading for fuel consumption based exclusively on rolling resis-
tance (vertical deformation) can be justified only if one would assume
that tires showing low or high energy dissipation in a free-rolling
mode also display correspondingly low or high dissipation in other tire
modes and particularly in driving modes. Since there is no evidence
supporting such an assumption, grading of tires for fuel consumption
based on rolling resistance should be limited only to tires used on
towed vehicles and cannot be used for tires used on driven automobiles.
To evaluate the proposed EPA grading system, five different tires
tested in three different laboratories (identified as Facilities F,
G, and H in the SAE Paper 780636) were graded in accordance with this
system.. Test data used for grading was obtained from tests conducted
on a 6?" diameter drum in accordance with the EPA Recommended Practice
of 50 mph speed, 2k psi inflation pressure, and 80% rated load. The
rolling resistance force was determined from the measured value of drum
torque in Laboratories F and G, and from the spindle force in Laboratory
H. Measurements in different laboratories were conducted at different
ambient temperatures but the reported values of rolling resistance force
were corrected to a common temperature of 70°F. The values of rolling
resistance coefficient were calculated from measured values of rolling
resistance force and corrected to 70°F temperature as follows:
RRC75 = RRC70 (1 + O.OOU )70-75) )
The upper limit (X ) of rolling resistance coefficient was determined
in 'accordance to the EPA Recommended Practice. Because of lack of actual
data, I assume that the value of standard deviation for each of the test
tires is equal to O.OOOMl and the sample size is equal to 2k. By using
these assumptions I determine the value of K-factor from Table 3 in the
proposed EPA Recommended Practice, K = 1.712. These values were used
for calculation of the upper limit of rolling resistance coefficient for
all five tires. Calculated values are summarized in Table 1, which also
contains a complete data necessary for these calculations. The values of
the upper limit of rolling resistance coefficient were used for grading
of tires for fuel efficiency. The grading was performed in accordance
with the proposed EPA Recommended Practice. The results of grading are
summarized in Table 1.
Table 1 shows that tires tested in three different laboratories re-
ceived the same grading. Radial tires received grading B and C: how-
ever, all bias and bias-belted tires received the same gradeD. Un-
fortunately, the values of rolling resistance coefficient of bias and
bias-belted tires were beyond the lower limit of the grading scale.
Therefore, in spite of considerable difference between the rolling
resistance values of these tires, they received the same grading. To
enable one to grade the bias and bias-belted tire, it is recommended
to expand the proposed grading scale as it is shown in Table 2. By
applying the expanded scale.for grading of tires, whose data is sum-
marized in Table 1, one. may find that bias-belted tires received higher
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-58-
grades than the bias tire. Furthermore, it was also possible to dif-
ferentiate between the fuel efficiency of two different bias-belted tires,
A78-13 and L?8-15, which received grade E and D, respectively. Tire
grades established by using expanded scale are also incorporated into
Table 1. The expanded scale also has an additional grade AA at the upper
end of the scale. The addition of this grade will give tire manufacturers
an incentive to further improve tire fuel efficiency.
3. If sufficient evidence exists that the rolling resistance coefficient of
a free rolling tire can be used as a reliable criterion for grading of
free rolling-as well as driven tires for fuel efficiency, grading of
tires then may be used as a guide for selection of tires for fuel effici-
ency. However, it is still doubtful that the proposed grading and labeling
of tires will satisfy the needs of a consumer. In order to decide which
tire to buy, the consumer probably would like to know how much gasoline
he is going to save by purchasing higher grade tires. The difference in
average fuel consumption for different grade tires determined perhaps at
conditions used for specifying vehicle fuel consumption (EPA fuel mileage)
should be stated in order to provide customers with a meaningful guide.
Furthermore, it is also important to evaluate possible benefits of grading
of tires against expenditures (testing, labeling, etc.). It would be
highly objectionable if grading tires for fuel efficiency became another
contributor to the problem of national inflation.
h. There is a typographical error in Table 1, which shows that for Grade D
tire, the rolling resistance coefficient should be RHC > 0.016 instead of
RRC> 0.015.
I hope that my comments may be useful to you.
Sincerely,
V. Bergman/ .
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TABLE 1 - GRADING OF TIRES FOR FUEL EFFICIENCY
Rolling Resistance Force (ib)
Vertical Load - Lb.
Rolling Resistance Coefficient
at 70°F
Rolling Resistance Coefficient
at 75°F
Upper Limit of RRC at 75°F
Fuel Efficiency Grade
Determined by Using
Grading Scale
Fuel Efficiency Grade
Determined by Using
Modified Grading
IN ACCORDANCE
WITH THE
PROPOSED
EPA
RECOMMENDED PRACTICE
Test
Facility
>) F
G
H
F-G-H
rat F
G
H
mt F
G
H
F
G
H,
F
G
H
F
G
H
A78-13
Bias
llf.O
1U.O
1^.5
720
.019^
.019^
.0201
.0190
.0190
.0197
.0197
.0197
,020k
D .
D
D
E
E
F
Tire
A78-13
Bi-Belt
Ih.l
12.9
12.9
720
.0196
.0179
.0179
.0192
.0175
.0175
.0199
.0182
.0182
D
D
D
E
E
E
Size and
BR78-13
Radial
12.0
10.9
10.9
780
.015^
.0139
.0139
.0151
.0136
.0136
.0158
.01^3
.01^3
C
C-
C
C
C
C
Type
L78-15
Bi-Belt
23.3
23. ^
2U.8
13^0
.017*1
.017^
.0185
.0170
.0170
.0181
.0177
.0177
.0188
D
D
D
D
D
E
LR78-15
Radial
18.1
17.2
17.8
13^0
.0135
.0128
.0133
.0132
.0125
.0130
.0139
.0132
-0137
B
B
B
B
B
B
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60-
TABLE 2 - EXPANDED SCALE FOR GRADING OF TIRES
FOR FUEL EFFICIENCY
Tire Rolling Resistance Tire Fuel Efficiency
Coefficient Grade .
RRC <[.010 . AA
.010 <£ RRC <^ .012 A
.012 <^ RRC < .Oil* B
.Oil*
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-61-
FE-2053
Environmental Activities Staff
General Motors Corporation
General Motors Technical Center
Warren. Michigan 48090
May 15, 1980
Mr. Charles L. Gray Jr., Director
Emission Control Technology Division
Environmental Protection Agency
2565 Plymouth Road
Ann Arbor, Michigan 48105
x
Dear Mr. Gray:
This letter with attachment is in response to your proposed recommended
practices for measuring tire rolling resistance coefficients and for
the grading and labeling of tires for fuel efficiency. However, General
Motors objects to EPA's assumption that it has the authority to grade
and label tires for fuel efficiency. Nowhere in the Energy Policy and
Conservation Act has Congress delegated to EPA, authority to grade and
label tires. GM wishes to point out that Congress is now considering
legislation which would authorize the Secretary of Energy to
.categorize tires according to their contribution- to automotive fuel
economy. Under the proposed legislation, it is the Secretary of Energy
who, in consultation with the Administrator of EPA and the Society of
Automotive Engineers, is to "...develop test procedures -for
determining the relative fuel economy attributable to the use of a
specific tire compared to the use of another tire in the same
category." See § 602(c) of S. 2015. What EPA purports to do by way of
issuing a so-called "recommended practice" Congress is considering
doing by way of legislation. General Motors submits that EPA cannot
exercise power it does not have. Moreover, the national highway
traffic safety administration has already promulgated requirements for
new pneumatic tires. See 49 CFR § 571.109.
General Motors will continue to utilize the more detailed aspects of
the SAE procedure which provide testing over a range of loads and
pressures and, therefore, provide us with the capability of evaluating
the rolling resistance at any number of specific conditions that would
relate to actual vehicles. The only difference in this regard is
related to the use of a regulated estimated "hot" pressure rather than
the capped air concept. General Motors plans to further evaluate both
methods.
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Currently, there are rolling resistance requirements for GM original
equipment tires on all new vehicle development programs and state-of-
the-art low rolling resistance technology tires are now being used on
over 50% of all GM passenger cars. They will be available on over 95%
of all GM passenger cars by 1982. We believe it is important, however,
that the fuel economy concerns of any governmental agency not be over
emphasized to the extent of possible diminution of other tire
performance characteristics such as handling, traction and wear, which
are not necessarily indpendent of rolling resistance.
GM's comments on the technical merit of the EPA draft procedures are
attached.
Sincerely,
pisher, Director
Automotive Emission Control
Environmental Activities Staff
8RGF/430/E
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-63-
GM COMMENTS ON EPA RECOMMENDED PRACTICES FOR
MEASURING TIRE ROLLING RESISTANCE COEFFICIENTS AND
GRADING/LABELING TIRES FOR FUEL EFFICIENCY
Procedural Comments on Measuring Tire Rolling Resistance Coefficients
o Inflation Pressure
The multiple test pressures used for tires having different load
ranges will likely cause the higher load range tires to obtain an
artifically better grade. This approach could lead some consumers to
purchase the higher load range tires for their application without
utilizing the higher inflation pressure. For example, if a
particular tire design is manufactured in LR-B, LR-C, and LR-D sizes,
the higher fuel economy grade would be assigned to the LR-D tire even
though we would expect it to exhibit poorer rolling resistance at a
constant inflation .pressure. Therefore, we would recommend that all
passenger car tires of mulitiple load ranges be evaluated at a common
inflation pressure.
Since the "T" type high pressure spares are intended only for
temporary usage, they should be deleted from this procedure.
o Tire Break-in
GM agrees that the one-hour break-in is most important for the non-
radial tires where tire growth could occur. The break-in growth
effect on belted radial tires, however, will be far less and
therefore the break-in is unnecessary. Omitting the break-in would
have the tendancy to directionally lower the fuel economy grade
(higher rolling resistance), but this is considered to be an
insignificant change. By omitting this step for radial tires, the
total elapsed time to conduct a test would be reduced by
approximately 75%.
o Inflation Pressure Accuracy
The stated accuracy of the pressure gauge (0.05 psi) is much greater
than what is known to be commercially available for manually applied
gages. Pressure losses greater than this could occur during the
process of taking the pressure reading, which must be a manual
operation. The estimated effect of a 0.5 psi (ten times the
recommended accuracy) error in initial inflation pressure would be
to affect rolling resistance by only 0.10 to 0.15 Ibs. , depending on
the tire construction type. A gauge with 0.25 psi accuracy should be
sufficient for this generalized characterization of tire rolling
resistance performance.
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Parasitic Loss Measurements
The angular velocity measurements on the tire present an additional
piece of instrumentation not needed in the normal SAE practice. Since
tire angular velocity is load sensitive, and the principal intention
of this measurement is to establish accurate parasitic loss
measurements, then an alternate approach is recommended. This
recommended approach would be to measure the rolling resistance and
load at both the desired load condition (80% TRA) as well as at a
light load condition (approximately 20-30 Ibs.). The change in
rolling resistance can be observed as well as the change in load,
which permits the calculation of the rolling resistance coefficient
as follows:
_
~
c ~ A Load
This approach eliminates the need for angular velocity measurements,
and also cancels the effects of any long-term instrumentation drift
that may occur.
TIRE GRADING AND LABELING COMMENTS
o Tire Handling and Traction
The EPA should be advised that tire handling and traction are not
necessarily independent of rolling resistance. While there
currently appears to be a significant technology gap in rolling
resistance between the OEM tires and some aftermarket tires, the EPA
should not over emphasize tire rolling resistance without considera-
tion of its impact upon other important tire performance
characteristics. The OEM tires produced under the TPC system attempt
to maintain a balance between these characteristics.
o Grade Labeling
The EPA proposed tire label appears to be a promotional advertisement
for the EPA and is far larger than other information on the tire,
while maintaintaining other desireable tire properties, and larger
than we believe necessary. A simplified version such as "fuel
economy grade B" in dimensions consistant with the tire quality
grading label should be adequate.
o Effect of Size and Load on Fuel Efficiency Grade
We believe the principal intent of the tire fuel economy grading
system is to encourage the development of lower rolling resistance
tires and to accurately inform the aftermarket buyer of the relative
fuel efficiency levels of different tire lines. The test data in
Figure 1. (attached) indicates that there is a size relationship to
rolling resistance coefficients such that smaller tire sizes have a
higher rolling resistance coefficient than larger sizes, even though
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' -65-
they represent the same level of tire technology. Our analysis of
this range of tires indicates that a full size line of equal
technology tires would have as many as three different fuel economy
ratings (i.e., A, B, and C). .We feel this would lead to considerable
confusion in the customer's mind, especially since some of the
smaller size tires may not be capable of reaching a grade "A" level
with todays best known technology. We would propose that an
alternate criteria be considered. This criteria would encompass the
inherent slope of the curve shown in Figure 1. The proposal would be
to calculate an index that would be dependent on both the rated load
and the upper limit on RR as described in the EPA proposed method.
Since the average trend (Figure 1) can be described by a best fit
linear relationship in the form
RR = 1 - KT L
c o L
where: 1 = intercept (index)
KT = Slope of curve = 736X10~9
Li
L = 80% TRA Load at design pressure - "(Newton)
]
then:
RR = Rolling Resistance Coefficient - (N/N)
1 = RR + (736X10~9)L
o c >.
We would expect.this trend to be valid over the range of tires shown
in Figure 1. If one assumes that the 90% confidence/90% population
limits have the same trend, then this equation would be:
1 = RR , ,,. + KT L
o c(ul) L
where: RR , .. -. = Upper limit for 90% confidence on
90% of the population
KT, L = Previously defined
L
f
Now, a fuel economy grading system can be developed around this index
(1 ) concept. A suggestion would be:
1 < 0.015. Grade A
0.015 < 1° < 0.017 Grade B
0.017 < 1° < 0.019 Grade C
1° > 0.019 Grade D
o
-------
T66-.
We would expect this fuel economy grading method to be valid over the
13, 14, and 15 inch passenger car tire sizes which represent the
majority of tires used today.
8RGF/430/E1
-------
nuuru. j.
ROLLING RESISTANCE COEFFICIENT VS 807, TRA LOAD
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RCM 4/24/80 .
-------
-68-
Helen O. Petrauskas
Assistant Director
Automotive Emissions and
Fuel Economy Office
Environmental and Safety
Engineering Staff
Ford Motor Company
The American Road
Dearborn. Michigan 48121
June 18, 1980
Mr. Charles L. Gray, Jr.
Director
Emission Control Technology Division
U.S. Environmental Protection Agency
2565 Plymouth Road
Ann Arbor, MI 48105
Dear Mr. Gray:
Enclosed are comments prepared -i by Ford Motor Company with
respect to. a document entitled "EPA Recommended Practice for
Grading and Labeling of Tires for Fuel Efficiency". We
believe grading and labeling of tires with respect to fuel
efficiency would be worthwhile only if such information could
be imparted to consumers in an accurate, meaningful, and
effective manner. Our principal concerns in this regard are
(i) the accuracy and technical validity of equating tire fuel
efficiency and rolling resistance, (ii) the difficulty of
stating tire efficiency in some objective manner which would be
understandable to the consumer, and (iii) the effectiveness
with which tire fuel efficiency ratings can be communicated
in light of the existing requirements to make known ratings
related to other tire characteristics.
These concerns have led us to conclude that, at present,
fuel efficiency grading and labeling of tires should not be
implemented. -
Sincerely,
H. 0. Petrauskas
Attachment
-------
-69-
COMMENTS OF FORD MOTOR COMPANY
WITH RESPECT TO
EPA RECOMMENDED PRACTICE FOR GRADING AND
LABELING OF TIRES FOR FUEL EFFICIENCY
I. Purposes of Grading and Labeling Requirement
If labeling of tires is intended to assure that low rolling
resistance tires are used as original equipment parts, labeling
is not necessary. There exists a high notivation for vehicle
manufacturers to provide optimum tire designs to maximize
fuel economyto meet consumer demand and federally mandated
fuel economy standards.
If labeling is intended to aid the consumer in malting an
informed choice in the purchase of replacement tires, several
questions regarding the relative merit of this additional
information must be considered.
II. Effectiveness
Tire manufacturers are already required by the Uniform Tire
Quality Grading Standard (Up C?R 575.10U) to provide ratings
of treadware, traction and temperature resistance in symbols.
In our opinion, without explanation these ratings offer limited
guidance to the consumer. If, as indicated above, EPA also
- .intends this labeling-of'the tire fuel economy to be an aid
to consumers, the adoption of the proposed EPA "Fuel Economy
Grade" label on the sidewall of tires would appear to compound
the existing complexity in the purchase of after-market tires
by adding another indicator (of unknown value) of tire per-
formance characteristics.
If the agency believes that fuel economy labeling would be
beneficial to the consumer, it should ensure that such labeling
is (a) understandable, meaningful and useful and makes clear
to the user, the differences in projected fuel econoiry at
each grade rating; and (b) that any tradeoffs introduced
into the existing Uniform Tire Quality Grading indicators
by this fourth variable are adequately investigated and
understood. Each of the present UTQG grades are footnoted
by explanations of the particular grade marking. It is be-
lieved that the fuel economy rating on the tire would also
require extensive explanation, which would dilute its value
to the consumer.
Further, it is believed that before any relevance to fuel
economy'can be claimed as a consumer aid, research into the
usefulness of the proposed information should be completed
to attempt to determine:
0 Does the buyer understand the information offered? -
Can he relate each grade level to some generally
understood measure (miles per gallon)? Would they
use it in making a purchasing decision?
-------
-70-
0 Are grade levels established such that clear break-
points in performance are differentiated? Is a high
"B" measurably different than a low "A" in MPG?
0 How would the information be distributed, and at
what cost (a) to the manufacturers and (b) ultimately
to all tire buyers?
0 Is there any substantial indication that additional
information will be any more widely used than past
automotive consumer information?
. Unless a reasonable level of usefulness can be established
by market research, it is believed that the need for molded
tire labeling is not justified.
III. Rolling Resistance as Sole Determinant of Tire "Fuel Efficiency"
The use of rolling resistance coefficient (PP.C) of a free
rolling tire as criterion for grading of tires for "fuel
efficiency" may be an over-simplification of a complex re-
lationship, and therefore may not be technically adequate.*
Tires have a significant .effect on the fuel consumption
of a vehicle because of the energy dissipation in the tire.
Energy dissipation in- the tire results .from deformation of
the tire produced by forces transmitted through the tire.
Load carried by the tire produces vertical deformation.
* Energy dissipation due to this deformation constitutes
rolling resistance.
Driving forces propelling the vehicle produce circumferential
deformation. Circumferential deformation produced by
driving forces not only causes a direct energy dissipation
due to this deformation, but also tends to modify pressure
distribution along the length of tire contact which results
in shifting the point of application of the resultant vertical
force forward. This shift tends to further increase tire
rolling resistance. Therefore, energy dissipation in the
driven tire is considerably greater than that in the free
rolling tire.
Furthermore, tires generate lateral forces which are necessary
for directional control of a vehicle and for maintaining
* Ford's views in regard to determination of tire rolling
resistance coefficients are reflected in the comments filed
by the Motor Vehicle Manufacturers Association with respect
to "EPA Recommended Practice for Determination of Tire
»
Rolling Resistance Coefficients".
-------
-71-
direction stability. These forces result from lateral defor-
mation of the rotating tire operating at a slip angle. Addi-
tional dissipation of energy in a tire occurs during this
process. Since a vehicle continuously tends to change its
direction of motion, even in so-called "straight ahead"
driving, energy dissipation due to lateral deformation con-
tinuously takes place. Vehicle fuel consumption is influenced
"by the total energy dissipation resulting from vertical»
circumferential, and lateral deformation of the tire..
The magnitude of these deforrnaticr.s and consequently the
magnitude of energy dissipation are affected by vehicle
operating conditions, environmental conditions, and tire
construction. Tire vertical, circumferential, and lateral
stiffnesses may be considered a major tire design factor
affecting tire deformations and energy dissipation. Tire
grading for fuel consumption based exclusively on rolling
resistance (vertical deformation) can be justified only if
one vould assume that tires shovir.g low or high energy
dissipation in a free-rolling mode also display correspond-
ingly lov or high dissipation in other tire modes and partic-
ularly in driving modes. Since there is no evidence supporting
such an assumption, grading of tires for fuel consumption
based on rolling resistance may not be applicable to tires
used on driven automobiles.
IV. Authority to Require 'Grading and Labeling of Tires
Ford believes that EPA lacks the authority to promulgate any
regulations concerning tire labeling. SPA has the authority,
pursuant to the Energy Policy and Conservation Act, P.L. 9^-lo3i
to measure vehicle fuel economy,.to calculate corporate average
fuel economy, and to require fuel economy labeling of vehicles;
but authority is not granted to rate or label the component
parts of the vehicle with regard to fuel economy effects.
This conclusion is supported by the fact that Amendment 1663
to S. 2015, proposes to require testing and labeling of tires
for fuel economy effects. Congress then seems to have con-
cluded as veil that EPA presently lacks the authority to perform
these functions.
V. Miscellaneous
If labeling of tires vere adopted, we recommend that:
0 It should not be applicable to "T" type tires (temporary
tires).
-------
-72-
For the sake .of completeness the RRC scale "be expanded
lower than R?.C = 0.012
...say RRC = 0.010
greater than ?5C = 0.015
...say RRC = 0.020
------- |