SDSB 79-14
Technical Report
March, 1979
Ranking Tires Using a Transient
Speed-Time Cycle
by
Richard N. Burgeson
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 developments 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
-------
Abstract
The ability to rank passenger car tires according to their respec-
tive rolling resistance facilitates emission and fuel economy testing
and can serve as a consumer buying aid. However, at the present time a
single, universally accepted tire rolling resistance measurement method
is not available. Current practices measure rolling resistance while
the tire is operated at steady state conditions which are atypical of
actual tire use.
ECTD is concerned that tires ranked according to typical steady
state practices may perform differently when operated according to current
emissions and fuel economy (transient) tests or in real life. This
study was conducted to determine the difference in tire rolling resis-
tance rankings from steady state and transient testing.
Tires and equipment used in previous ECTD tire rolling resistance
experiments were utilized for this study. These consisted of two vehi-
cles equipped with driveshaft torques and speed sensors, a single large-
roll dynamometer and 13", 14" and 15" tires of various construction
types (radial, bias belted and bias). The power transmitted by each
vehicle was summed during its operation of accelerations and cruises of
the Federal Test Procedure. Immediately following this transient oper-
ation, steady state rolling resistance measurements were conducted.
The results indicate that a significant correlation between the
transient and steady state procedures exist but that test variability
would not permit any concrete conclusions. In general, the data in-
dicate that both procedures tend to rank tires in the same manner.
It is recommended that, due to the high test variability, the
equipment used in this study not be used for future programs of this
type. It is also recommended that further investigation be conducted as
a part of a recently awarded tire testing contract (#68-03-2763).
-------
I. Introduction
Since the advent of the coast down procedure to determine the road
force on a vehicle, the role of the tire has become more and more
important. In order to facilitate emission and fuel economy testing,
for which the road force is determined, ECTD has sought a tire ranking
system based on tire rolling resistance. This ranking system would be
used in vehicle selection prior to emission and fuel economy testing and
could be a consumer buying aid. However, no good, universally accepted
method of measuring rolling resistance is available. Current methods
tend to measure rolling resistance while the tire is at a steady state
condition. ECTD has reservations about these techniques, since the tire
is rarely used in this manner during emission and fuel economy (trans-
ient tests) testing or in typical consumer applications.
ECTD is concerned that tires during transient emission and fuel
economy testing may perform differently from tires under a steady state
condition. This report discusses the results of an experiment designed
to determine the differences in tire rolling resistance rankings from
transient and steady state tests.
II. Program Design
Two vehicles instrumented with driveshaft torque-transducers and
speed-sensors were utilized for this study. Each vehicle was installed
on a single large-roll dynamometer and operated at 50 mph for 30 minutes
to stabilize both engine and drivetrain lubricant temperatures. Tires
at ambient temperature (75°F) were then installed and the tire pressure
was set to 26 psi. The vehicle was then operated according to the
Federal Test Procedure speed-time cycle. The dynamometer was set for a
nominal road load force with only system inertia applied (approximately
1,900 pounds).
The power expended by the vehicle during the acceleration and
cruise modes of the speed-time cycle was monitored and recorded on a
once-per-second basis. Upon completion of the transient speed-time
cycle, the vehicle was accelerated to 50 mph and maintained at that
velocity for a period of approximately 15 minutes while steady state
tire rolling resistance measurements were conducted. The above pro-
cedure was repeated a minimum of two times for each pair of test tires
•for a total of 80 tests.
III. Analysis
In order to determine the power expended by the vehicle, the drive-
shaft torque and speed were monitored throughout the vehicle's operation.
The instantaneous power was then computed and summed for all accelera-
tions and cruises during the speed-time cycle according to the following
equations:
P± = T± W± (1)
-------
-2-
where
P. = ith output power point
T. = ith^driveshaft torque observation
W. = The angular velocity of the driveshaft during the ith
driveshaft torque measurement.
The total power was then computed as follows:
n
PTOT = Zi=l P
where
PTOT = The total output power during accelerations and cruises
n = Total number of observations.
Since n and therefore PTnT is dependent upon throttle perturbations,
a weighted mean value was computed for all tests on the same pair of
tires. The following equation illustrates this computation:
m
AVPTOT = k£l "k PTOTk
N
where
AVP = Average Output Power,
n, = The number of observations in the kth test,
m = The number of tests on a particular pair of tires,
and
N = The total number of observations.
The method utilized to determine the tire rolling resistance during
the steady state portion of the test, was to measure the power transmit-
ted by the vehicle and the power absorbed by the dynamometer. The dif-
ference was considered to be the power dissipated by the tire. From the
tire power dissipation, a value for the tire rolling resistance, FT,R>
was then derived. A more complete derivation for the computation or
tire rolling resistance is contained in Appendix A. A mean tire rolling
resistance value was then calculated for all tests on a particular pair
of test tires.
IV. Results
The total vehicle output power, PTQT and the steady state tire
rolling resistance, V data were analyzed to determine the relation-
RR,
-------
-3-
ship between the transient and steady state procedures. This analysis
indicates that a significant correlation exists between the two proce-
dures (i.e., the two procedures are not independent). Figure 1 presents
PTn as a function of F for each test. The general trend of increasing
vehicle output power with increasing steady state rolling resistance can
be easily discerned. A complete listing of these data by tire identifi-
cation number is presented in Appendix B.
It should be noted that Figure 1 may be misleading. By computing
the ratio PTOT to Fpp and plotting these data versus tire identification
number, the magnitude of the test variability for each tire tested may
be realized. The causes of this variability could be numerous and are
not easily identifiable. The general test methods and equipment used
are considered to be the major contributors. Figure 2 presents these
data.
As a method of reducing the variability, a weighted mean total
vehicle output power, AVPTfyr and a mean steady state rolling resistance
value F were computed. An'analysis of variance was then performed on
these data with respect to tire type within each of the nominal tire
sizes tested (13", 14", and 15" tires). Significant differences between
the various tire types could not be discerned due to data variability.
The relative rankings of the tire types tested by the two procedures are
presented in Table 1 below. Note that the general trends of the tire
type mean values for each of the two procedures are the same.
TABLE 1
Rankings by Cycle and Tire Type Within Each Tire Size
Ranking
Size
13"
14"
15"
N
5
1
3
4
3
—
12
2
2
Tire
Type
Radial
Bias Belted
Bias
;
Radial
Bias Belted
Bias
Radial
Bias Belted
Bias
Transient
1
2
3
1
2
-
1
2
3
Steady
State
1
2
3
1
2
—
1
2
3
Mean Value
Transient
(watts)
4478300
4748900
4805100
4649000
5192900
-
4978700
5500600
5595800
Steady State
(Ib/k-lb)
13.057
15.174
15.558
12.286
16.495
-
13.711
14.959
17.152
N = Number of tires in the sample.
V. Conclusions/Recommendations
The results of this experiment indicate that, in general, either
-------
SCATTER PLOT
N= 80 OUT OF 80 3.PTOT VS. 4.FRR
PTOT (WATTS)
.61000
* *
.58778
.56556 +7+
o
c
•a
c
.54333 +7+
.52111 + 7+
.49889 +7+
.47667 +7+
**
* *
* * *
*
* *
* *
*
*
*
* *
* *
* *
* *
§
O
Ml
H
H-
$
09
s?
ID
"3
M
O
.45444
* * *
* *
* * *
.43222
.41000
12.222
14.444
16.667
18.889 FRR (LB/K ....
11.111 13.333 15.556 17.778 20.000 '
-------
FLUi
N= 80 OUT OF 80 l.TID VS. 5.RATIO
TID
430.00 +
* *
•f * *
**
He *
382.22 -f * *
* *
+ * *
* *
* *
334.44 +
* *
+
# #
286.67 -f * *
* *
•f * *
238.89 + * *
* *
+
* *
* *
191.11 + -
+
* * 2 * *
143.33 +
* *
* * * *2 * *
•f * ** *
* *
* * *
95.556 +
* * * 2 * *
* *
•f * *
* • • . *
47.778 + * *
+ **
* *
0. +
^
ft
S
re
o
n
H-
H-
S
Ji
o
9
a
|
<
re
n
S
if
S*
it
o
o
•rj
H M
n I
h-" M
< to
re
y
H«
O
1"*
re
S
•a
c
*a
§
S
rt
0
S
M
re
50
£L
f^
y
re5
a
CO
rt
B>
I
.24000 -f<5 .29111 +6 .34222 +6 .39333 +6 .44444 +6 RATIO TOT
.26556 +6 .31667 +6 .36778 +6 .41889 +6 .47000 +6
-------
-4-
procedure can be used to rank tires on the basis of tire rolling re-
sistance. The significant correlation between the two procedures
implies that the two parameters analyzed (PTf)T and FRR) are interdepen-
dent (i.e., controlling variations of either parameter will affect the
other parameter).
Several potential areas for concern to ECTD are revealed upon
examination of the data presented above. Figure 1 indicates that
several tires display the characteristic of having low steady state
rolling resistance, F and high vehicle output power, PTnT» and some
which have high F ana low PTOT- These data points could be the result
of test variability. However, the question "Do tires with these char-
acteristics actually exist?" must be answered. If these data are
representative of actual tire characteristics and not just a function of
test variability, it behooves ECTD to require tire rolling resistance
information obtained via a transient speed-time test procedure with each
vehicle certification request. This type of information would also be
of interest to the consumer when replacing existing tires.
It is therefore recommended that tests of a similar nature be
conducted using more sophisticated equipment to confirm the results of
this experiment. This area could be investigated as part of the re-
cently awarded "Tire Energy Dissipation" contract (contract #68-03-
2763).
It is also recommended that the equipment used for this experiment
not be used for future tire rolling resistance investigations since it
is a major source of variability.
-------
References
1. Schuring, D. J., "Rolling Resistance of Tires Measured Under
Transient and Equilibrium Conditions on Calspan's Tire Research
Facility", DOT-OST-76-9, March 1976.
2. Thompson, C.D. and Myriam Torres, "Variations in Tire Rolling
Resistance", EPA Technical Support Report for Regulatory Action,
LDTP-77-05, October 1977.
3. Elliot, D. R., ; Klamp, W. K., and Kraemer, W. E., "Passenger
Tire Power Consumption", Society of Automotive Engineers, SAE
710575.
4. Floyd, C. W., "Power Loss Testing of Passenger Tires", Society
of Automotive Engineers, SAE 710576.
5. Clark, S.K.; Dodge, R. N.; Banter, R. J., and Luchini, J. R.,
"Rolling Resistance of Pneumatic Tires", University of Michigan
Report DOT-TSC-74-2; Prepared for the Department of Transportation,
Transportation Systems Center, Cambridge, Mass., July 1974.
6. Curtis, W. W., "Low Power Loss Tires", Society of Automotive
Engineers, SAE 690108.
7. Clark, S. K., "Rolling Resistance Forces in Pneumatic Tires",
University of Michigan Report DOT-TSC-76-1; Prepared for the
Department of Transportation Systems Center, Cambridge, Mass.,
January 1976.
-------
APPENDIX A
Methodology for Determining Tire Rolling Resistance
-------
A-l
Methodology Utilized to Determine Tire Rolling Resistance
The power absorbed by the tire was computed each second of data
collected according to the following equations:
p = p _p _p
AT engine abs. diff. bearing losses dyno (1)
Teng WE ~Tdiff WE ~TLC WD "TBL WD . (2)
" (Teng -Tdiff) WE - (TLC + V WD (3)
where
P. = the power absorbed by the tire at the test speed
AJL
T = torque from the engine/transmission (measured by
the driveshaft torque sensor)
torque required to revolve the rear axle and
associated bearings and gearing which make up the
differential. NOTE; This quantity includes any
effects due to brake drag.
T = total torque measured by the dynamometer load cell
LiC
T = torque due to bearing and frictional losses in the
dynamometer
WF and Wn = the angular velocities of the vehicle driveshaft
and the dynamometer roll, respectively.
From each P. the rolling force was then derived as follows:
A J.
P = T W
AT LI T (4)
where T is the torque at the tire/roll interface and W is the
angular velocity of the tire. However, T_ can be defined as the
product of a force and a lever arm as follows:
TT = FR x r (5)
where F is the rolling force of the tire and r is the tire radius.
K •
Substituting equation 5 into 4 yields:
PAT= (FR x r) WT
-------
A-2
Since the angular velocity W can be represented as a ratio of the
linear velocity, V , and the radius of the tire, r, a substitution
for W in equation 6 produces:
_ (F x r) V
PAT " —^ ". FR VT (7)
the linear velocity VT is in actuality the ground or test surface velo-
city. However, with all vehicle tests on dynamometers, a certain amount
of slip between the tire and the dynamometer roll occurs. Therefore,
the vehicle linear velocity, the one parameter common to both dynamo-
meters, rather than the dynamometer-roll linear velocity was utilized for
this analysis. Therefore, FR can be expressed as:
F = ?AT
V
T (8)
where V is the vehicle speed.
Of all the parameters affecting tire power absorption, the vertical
load on the tire has yet to be discussed,. In general, tire power absorp-
tion is directly proportional to the load upon it [1]*. As the vertical
load increases, the tire power absorption also increases. Therefore, all
the above computations are a function of the vertical load under which a
particular set of tires were tested. The vertical load used for this
experiment was arrived at by weighing the rear portion of each test
vehicle with a full tank of fuel and a driver. Fuel was added to each
test vehicle at the completion of every second test in order to maintain
as constant a vertical load as possible. However, the vertical load of
the two test vehicles differed, therefore, making direct tire rolling
force, F , data comparisons difficult. By calculating the ratio of F
K K
to the test vertical load, F , all tire test results could then be
i-* JL
directly compared. This is expressed in the equation below:
FRR
ZT (9)
Numbers in [] refer to references listed in the References section
of this paper.
-------
APPENDIX B
Test Data by Tire Identification Number
-------
TABLE B-l
Total Vehicle Output Power and Tire Rolling Resistance by
Tire Identification Number
TIRE
ID
010
010
020
020
050
050
060
060
07.0
070
080
080
090
090
090
090
090
090
090
100
100
100
110
110
121
121
122
]>'3/?
131
131
131
131
131
13.1.
131
131
132
132
151
151
TIRE
SIZE
13
13
13
13
14
14
15
15
15
15
15
15
15
15
15
15
15
15
15
13
13
13
14
14
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
TIRE
TYPE
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
BIASBE
BIASBE
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
BIASBE
BIASBE
BIASBE
BIASBE
BIASBE
RADIAL
RADIAL
RADIAL
RADIAL
BIAS
BIAS
BIAS
BIAS
BIAS
BIAS
BIAS
BIAS
BIAS
BIAS
BIASBE
BIASBE
PTOTy
TOTAL
VEHICLE OUTPUT
POWER (UATTS)
4699607.70110
4456460.98574
4169058.63029
4459429,52088
4861544.48247
4976103.17255
5437440.26569
5082784.62920
4503038.11836
5003666.10788
5193074.84290
5422514.44701 .
4569432.59125
4829152.70572
4673362.90804
4835103.88043
5157718.88194
5011721.31631
5031882.96196
4936358.72204
4678714.39497
4646476.97175
5251387.67691
5547526.54437
5225096.70084
4722578.47916
4946343.12039
5204202.99512
5880754.75908
5599430.11113
5487245.65375
5905336.77896
5332775.93030
5678084.85648
5897708.69109
6079214.81062
5434690.59457
5480499.63302
5659061.59892
5494032,17758
FRR ,
ROLLING
RESISTANCE
(LB/K-LB)
15.481
11.671
13.604
14*681
13.890
14.649
13,148
15.082
15.771
12.084
17.368
15.204
10.732
11.289 .
11.649
11.498
11.036
12.217
11.068
15.373
15,439
14.709
15.849
15.853
14.575
12.241
13.989
14.406
16.810
15.705
15.363
14.121
18.912
17.534
16.622
15.378
16.369
19,294
16,081
12,850
PTOT/FRR
i
303572.618
381840.544
306458.294
303755.161
350003.202
339688.933
413556.455
337009.987
285526.480
414073.660
299002.467
356650.516
425776.425
427775.065
401181.467
420516.949
467354.012
410225.204
454633.444
321105,752
303045.171
315893,465
331338.739
349935.441
358497,201
385800.055
353588.042
361252.464
349836.690
356538.052
357172.795
418195.367
281978.423
323832.831 \
354813,421 [
395318.950
332011.155
284052.018
351909.807
427551,142
-------
TABLE B-l (continued)
Total Vehicle Output Power and Tire Rolling Resistance by
Tire Identification Number
TIRE
ID
151
151
151
151
200
200
210
210
230
230
240
240
260
260
270
270
290
290
300
300
320
320
340
340
350
350
360
360
370
370
380
380
390
390
400
400
410
410
420
420
TIRE
SIZE
15
15
15
15
15
15
15
15
15
15
15
15
14
14
14
14
15
15
14
14
14 .
14
14
14
13
13
13
13
13
13
13
13
13
13
15
15
13
13
15
15
TIRE
TYPE
BIASBE
BIASBE
BIASBE
BIASBE
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL .
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
BIASBE
BIASBE
BIASBE
BIASBE
BIAS
BIAS
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
BIAS
BIAS
RADIAL
RADIAL
BIAS
BIAS
RADIAL
RADIAL
PTOTy
TOTAL
VEHICLE OUTPUT
POWER (WATTS)
6085531*48439
5912991*88684
52 1404 1,40.1.96
6078585*89076
5015769.50980
4725925*94608
4678619,58792
5081775,55830
5474233,67325
5048233.57910
5210731*75113
5174624*18976
4548570.03879
4517766*91246
.4477099.54956
4453785*67033
5035755*49335
4669527*44244
4863804,01492
4502121,83540
4651382*81145
5063672,56870
5178608,69943
5452987,08649
4734697,99628
4676142,38075
4535218,38450
4532181,99243
4429162,46963
4601722,82945-
4378467*70093
4523452*27654
4862117*06833
4769476*88401
4646382,52911
4910079,64209
4982682,78344
4796418*02577
4760031*74965
5116505,39162
FRRf
ROLLING
RESISTANCE
(LB/K-LB)
16.611
16.166
15.793
16.154
12,766
13,548
12.521
13.280
13.325
11.738
12.775
15,480
11,944
12,378
12,218
12.768
13.870
17.099
10.980
10.620
15,257
16.080
18.514
17.830
16,508
13,010
12.497
11,624
11.492
14.320
14*660
11*872
19.736
14.737
13.463
14.384
13,680
14.993
13,577
12,929
PTOT/FRR
366355,516
365767.159
330143.889
376289.829
392900*635
348828.310
373661.815
382663.822
410824.291
430076.127
407885.069
334278.048
380824.635
364983.593
366434.732
348824,066
363068.163
273087.750
442969.400
423923.610
304868.769
314905.010
279713.120
305832,142
286812.333
359426.778
362904*568
389898.657
385412,676
321349.360
298667,647
381018.554
246357.776
323639.607
345122.375
341357.039
j
364231.198
319910.493
350595,253
395738,680
-------
TABLE B-2
Average Total Vehicle Output Power and Tire Rolling Resistance
by Tire Identification Number
TIRE
ID
010
020
050
060
070
080
090
100
110
121
122
131
132
151
200
210
230
240
260
270
290
300
320
340
350
360
370
380
390
400
410
420
TIRE
SIZE
13
13
14
15
15
15
15
13
14
15
15
15
15
15
15
15
15
15
14
14
15
14
14
14
13
13
13
13
13
15
13
15
TIRE
TYPE
RADIAL
RADIAL
RADIAL
BIASBE
RADIAL
RADIAL
RADIAL
BIASBE
BIASBE
RADIAL
RADIAL
BIAS
BIAS
BIASBE
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
RADIAL
BIASBE
BIASBE
BIAS
RADIAL
RADIAL
RADIAL
BIAS
RADIAL
BIAS
RADIAL
fcVPTOTf
WEIGHTED
MEAN VEHICLE
POWER (WATTS)
4574553*58600
4317660,20300
4917026,70500
5259796.73000
4754096,65100
5305296,33200
4870421,62000
4748929.74100
5404428.26600
4968880,56700
5076651.16900
5733699.06400
5457839.64500
5741444.10400
4871960.53500
4878631*21600
5261616.49000
5192475,11900
4532920,80200
4465615.76600
4852751.44600
4680605.99100
4859595.21900
5314678,63300
4706746.25600
4533676,37600
4515774,67800
4449849,60300
4816153,46700
4778466,39000
4892407,22500
4933726.12700
FRR »
ROLLING
RESISTANCE
(LB/K-LB)
13*576
14.143
14.270
14.115
13.923
16.286
11,356
15,174
15.851
13.408
14,198
16,306
17,832
15.609
13.548
12,901
12,532
14,128
12,161
1.2*493-
15,484
10,800
15,668
18,172
14,759
12.061
12,906
13,266
17,236
13,924
14,337
13,253
AVPTOT/FRR
336958.868
305286.022
344570,897
372638.805
341333.763
325758.095
428885.313
312964,923
340951,881
370590.734
357561.006
351631.244
306069.967
367829,080
359607,362
378159,152
419854,492
367530.798
372742,439
357449.433
313404.253
433389.444
310160.532
292465.256
318906.854
375895.562
349897.310
335432.655
279424.081
343182.016
341243.442
372272.401
-------
APPENDIX C
Tire Description by Identification Number
-------
C-l
Tire Description
ID Number Manufacturer
010 Goodyear BR 70
020 Goodyear BR 70
050 Goodyear HR 78
060 Goodyear H 78
070 Goodyear HR 78
080 Goodyear HR 70
090 Goodyear HR 78
100 Goodyear B 78
110 Goodyear H 78
121 B. F. Goodrich HR 78
122 B. F. Goodrich HR 78
131 B. F. Goodrich H 78
132 B. F. Goodrich H 78
151 B. F. Goodrich HR 78
200 Goodyear HR 78
210 Uniroyal GR 78
230 General GR 78
240 Uniroyal LR 78
260 Uniroyal FR 78
270 Firestone FR 78
290 Firestone HR 78
300 Uniroyal ER 78
320 Goodyear E 78
340 Firestone E 78
350 Uniroyal B 78
360 Goodyear BR 78
370 Firestone BR 78
380 Uniroyal . BR 78
390 Firestone B 78
400 Uniroyal HR 78
410 B. F. Goodrich B 78
420 B. F. Goodrich GR 78
Size
X 13
X 13
X 14
X 15
X 15
X 15
X 15
X 13
X 14
X 15
X 15
X 15
X 15
X 15
X 15
X 15
X 15
X 15
X 14
X 14
X 15
X 14
X 14
X 14
X 13
X 13
X 13
X 13
X 13
x 15
X 13
X 15
Model
Polyglass Radial WT
Polyglass Radial
Polyglass Radial WT
Custom Power Cushion Polyglass
Polyglass Radial
Polyglass Radial WT
Custom Polysteel Radial
Cushion Belt Polyglass
Cushion Belt Polyglass
Silvertown Steel Radial
Silvertown Steel Radial
Custom Long Miler
Custom Long Miler
Silvertown Belted
Steel Belted Radial Custom Tread
Steel Belted Radial PR6
Dual Steel II Radial
Steel Belted Radial PR6
Steel Belted Radial
Steel Belted Radial
Steel Belted Radial
Steel Belted Radial
Custom Power Belted Cushioned Polyglass
Sup-R-Belted Champion
Fastrak Belted
Steel Belted Radial
Steel Belted Radial
Steel Belted Radial
Deluxe Champion
Steel Belted Radial
Silvertone Bias
Lifesaver 78 Steel Belted Radial
------- |