EPA-AA-LDTP 78-12
The Effect of Wheel Alignment on Rolling
Resistance - A Literature Search and Analysis
by
John Yurko
July,1978
Standards Development and Support Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air and Waste Management
U.S. Environmental Protection Agency
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Introduction
The purpose of this report is to examine the effect of front-end wheel
alignment on rolling resistance. More specifically the effect of toe-in
and toe-out are considered.
With the vital need to conserve the nations resources, there is an
increasing demand for vehicle fuel economy. One factor that may con-
siderably affect fuel consumption is tire rolling resistance. The
rolling resistance is a function of many physical tire properties, such
as rubber compound, tread design, inflation pressure, etc. With the
exception of inflation pressure, most of these properties are fixed to
the design of the tire. However, another factor that may vary during
normal use of the tire, which has a significant effect on rolling resis-
tance, is slip angle.
Slip angle may be defined as the angle between the plane of the tire and
the forward direction of the vehicle. Slip angle is more commonly
referred to as "Toe", which is measured as the difference in inches
between the front centers and the back centers of the front wheels.
(See Figure IA.) For most vehicles, 1/2 inch toe corresponds approximately
to 1/2° slip angle. Toe-in refers to the tires directed inward toward
the front-center of the vehicle. Conversely, toe-out refers to the
tires directed outward from the vehicle (Figure IB). Since suspension
effects cause the front wheels to toe outward as the vehicle moves
forward, most vehicles have a manufacturer's recommended alignment
setting at some toe-in condition. This is to ensure that the vehicle
jnaintains...a.;straight line during normal driving.
A misaligned wheel introduces an angle of slip which results in two
forces which will affect fuel consumption: 1) a lateral force, (Fy),
which is normal to the plane of the tire. 2) a longitudinal force (Fx),
which lies in the plane of the tire (see figure 1C)). The components of
these two forces, in the direction opposite to the direction of forward
motion, are summed to give the effective rolling resistance (F ) at the
given slip angle (a). Mathematically, this rolling resistance can be
expressed as:
FD •= Fy SIN a + Fx COS a 1
R
It is the change in the effective rolling resistance at slip angle (a),
as compared to the base rolling resistance at zero slip angle, which
causes the overall effect on fuel consumption.
This report analyzes the effect of toe-in and toe-out on rolling resistance,
based on the road load results of an EPA test program. Secondly, it
correlates these results with the results of slip angle vs. rolling
resistance data, found in the existing literature. Then a survey of
vehicle safety inspections is used to approximate the number of vehicles
in a misaligned state, and finally an estimate of the effect on fuel
economy is discussed.
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3-
Discusslon
A. EPA Test Program - Design/Results
A test program was conducted in late October (1977), using a 1971 Vega
station wagon, at the Transportation Research Center of Ohio, to deter-
mine the effect of front-suspension alignment on vehicle road.load (1).
The driveshaft torque was measured, on a flat roadway, at approximately
50 mph for three different alignment states: (1) manufacturer's recommended
setting (1/4" toe-in), (2) 1/2" greater than the maximum recommended
setting (7/8" toe-in), (3) 1/2" less than the minimum recommended setting
(3/8" toe-out). A minimum of three tests was conducted in each direction
for four different sets of tires: steel belted radials, bias ply, bias
belted, and fiberglass belted radial tires. (The fiberglass belted set
was not tested at the toe-out condition.) Each set of tires was warmed
up for 30 minutes at steady state 50 mph prior to testing.
For the 3/8" toe-out condition, the three sets of tires tested all
showed a significant increase in driveshaft torque as compared to the
baseline 1/4" recommended toe-in. The increase was approximately 30%
for the radial tire, while bias and bias belted tires showed increases
of 75% and 22%, respectively (see Table 1).
Similarly, for the 7/8" maximum toe-in condition, with both the fiber-
glass and steel belted radials, driveshaft torque increased about 23% as
compared to the baseline. However, a slight decrease was detected at
-the same condition for the bias ply and bias belted tires.
B. Literature Search
The significance of these results, along with the discrepancy that
occurred with the bias tires, warranted further investigation into this
area. However, due to the development of equipment problems and the
unavailability of another vehicle equipped with a driveshaft torque-
meter, a limited data base was obtained. Therefore, a search of the
literature for supporting data was conducted. Although no evidence of
any road tests involving vehicles with various alignment conditions has
been found, there were some available data on the effect of slip angle
on rolling resistance. In order to compare these data with the results
of toe-out and toe-in conditions on the Vega, it was assumed that the
change in driveshaft torque was mainly related to the change in effective
rolling resistance due to the angle of slip. However, the front suspension
properties should also contribute to the overall effect. All the slip
angle vs. rolling resistance data that have been acquired were measured
with either a flat bed machine or a large roll dynamometer on tires not
connected to a vehicle front-end suspension system. Furthermore, the
test conditions were different for each of the four available sources .of
slip angle data.
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Test results published by W.W. Curtiss for a steel belted radial tire
showed about a 25% increase in rolling resistance for a 1° change in
slip angle (2). (See Figure IIA.) These tests were conducted on a
large roll dynamometer at various speeds up to 60 mph.
Similar test results, presented by Walter and Conant of Firestone Tire
Co. (3) for a bias ply tire also showed about a 25% increase in rolling
resistance for a 1° increase in slip angle (see Figure IIB). These
tests were conducted at 30 mph on a large roll dynamometer.
A much smaller effect was detected by tests conducted at Calspan Corpor-
ation (4). Three tires were tested (bias ply, bias belted, radial ply)
on a flat bed machine at 50 mph. Rolling resistance increased for an
increase in positive slip angle as well as negative slip angle for both
bias tires (see Figure III). At 2° slip angle in either direction the
increases in rolling resistance ranged from 3% to 9%. However, a decrease
of approximately 25% was observed at +2.5° slip angle in the radial ply
tire, and a decrease of about 15% was seen at -1.5° slip angle. Rolling
resistance reached two distinct minima at these points, after which it
increased with increasing magnitudes of slip angle. Similar tests were
conducted at Calspan on three different truck tires for both positive
and negative slip angles (5). The results showed a very symmetric curve
about 0.0° slip angle with a parabolic shape of increasing rolling
resistance with increasing magnitude of slip angle (see Figure IV).
Approximately a 40% increase in rolling resistance was observed at 1°
slip angle, while at 2° slip angle the increase was almost 100%. The
results were nearly identical for each of the three different tires
tested.
C. In-Use Condition
In order to estimate the number of in-use vehicles that are misaligned
to an extent which will have some effect on fuel economy, a survey of
state and city vehicle safety inspections was conducted. However, only
four of the twenty-two states contacted include a front-end alignment
check. Of these four, only two have responded with data.
Both the State of New Jersey and Cincinnati, Ohio reported approximately
7% of the vehicles inspected annually, failed inspection due to wheel
alignment (6). In all cases, these vehicles were tested on scuff gages,
with failure criterions of 40 - 50 ft/mile or approximately 0.5° slip
angle. (A scuff guage measures the number of feet of side scuff per
mile of forward motion.) However, this equipment is not very accurate
and tends to underestimate the degree of misalignment. Also, those few
states that do conduct wheel alignment inspections are expected to have
a lower percentage of misaligned vehicles than those that do not.
Another, perhaps more accurate investigation, was conducted by the U.S.
Department of Transportation which included a front end alignment inspec-
tion of 125,000 vehicles in five states (7). (Alabama, Tennessee,
Arizona, Pureto Rico, and Washington, D.C.) The vehicles were inspected
for toe-in, toe-out, caster, and camber on Hunter alignment guages. The
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criterion for failure was the manufacturer recommended maximum settings
which may range from 1/16" to 1/2". The irate of failure was approximately
19%.
D. Estimated Effect on Fuel Economy
In order to estimate the effect of front-end misalignment on fuel eco-
nomy, the worst and best cases were considered.
The greatest effect (worst case) is observed from the actual road test
data in which driveshaft torque was measured. A criterion that a 10%
change in road load yields approximately a 3% change in combined fuel
consumption, was used (8). The change in driveshaft torque for all 7
conditions, presented in Table 1, was averaged to obtain a mean increase
in driveshaft torque of 23% over the aligned condition. This corres-
ponds approximately to a 7% decrease in fuel economy.
A less severe effect on fuel consumption is obtained when only the
effect of slip angle on rolling resistance is considered. From the
literature data, approximately a 25% increase in rolling resistance was
observed for a 2° change in slip angle.
It has been observed at EPA that a 10% change in rolling resistance will
yield approximately a 1% to 2% change in fuel consumption (9). This
slip angle effect might be considered as a best case for the effect of
wheel alignment on fuel economy. The average vehicle with a front-end
misalignment, is expected to lie somewhere between these best and worse
cases, that is, a 3% to 6% decrease in fuel economy is estimated for the
average misaligned vehicle.
Conclusions
Overall, a large number of in-use vehicles operate with a front sus-
pension system in a significantly misaligned state. Although the major
incentive to correct such a condition would be to prevent increased tire
tread wear, an additional incentive may be to improve the overall fuel
efficiency of the vehicle. The fuel economy benefit, in some worse
cases, may be in excess of 7% based on the road tested vehicles. How-
ever, data indicates that for some front-end systems with certain tires,
the optimum state with respect to both tire wear and fuel economy may be
at some misaligned condition. Both the EPA road tests and the slip
angle vs. rolling resistance data indicate that the latter is the excep-
tion to the rule. Therefore, in general, one may conclude that improper
wheel alignment will tend to decrease the fuel efficiency of most motor
vehicles. The magnitude of this effect is most appropriately obtained
from the actual road tests. It is estimated that approximately 10% of
the motor vehicles in-use, operate with about a 4% decrease in fuel
efficiency, due to improper front-end wheel alignment. With more than
105 million registered passenger motor vehicles in the U.S., that travel
almost 1 trillion miles annually(10), an overall annual savings of over
5 million barrels of gasoline (approximately 0.3 billion dollars) may
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be gained by nation-wide proper wheel alignment.
Recommendations
Based on this study, the effect of front-end wheel alignment on fuel
economy can be considered quite significant. Not only does this phenom-
enon affect the overall energy efficiency of the nations in-use vehicle
fleet, but it may also be an important factor in motor vehicle test
programs. In fact, the possibility to under load a vehicle at a misaligned
condition, as compared to the recommended alignment setting, exists.
Therefore, further testing in this area is recommended. The most
appropriate tests would be actual road tests, such as the previous EPA
test program, with several vehicles and tires, representative of the
current market. Alternatively, slip angle vs. rolling resistance
testing, on an appropriate tire test machine, may be sufficient to
predict the effects of wheel alignment, given the physical character-
istics of the tire. Furthermore, a public awareness compaign is suggested
in order to inform the motor vehicle owners of this information.
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Forward Direction
(Top View)
Figure IA. Toe-in
Figure IB. Toe-Out
Forward Direction
Fy
o = Slip Angle
Fx = Longtudinal Force
Fy = Lateral Force
R
Rolling Resistance
Figure 1C.
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Table 1
Mean Torque for Each Tire/Alignment State Combination
Coefficient of
Tire Mean
Type Torque(f t-lbs)
Radial (1)* 41.535
Bias Belted
Bias
Radial (2)**
Radial (1)*
Bias Belted
Bias
Radial (2)**
Bias Belted
Bias
Radial (2)**
41.218
42.406
38.520
47.788
38.728
37.908
47.476
50.193
73.996
53.750
Variability
(o/xZ)
3.087
1.877
2.995
1.212
2.002
4.701
4.657
2.037
3.648
2.056
3.158
Alignment Setting
(Toe-in (inches))
0.250
0.250
0.250
0,250
0.875
0.875
0.875
0.875;
-0.375
-0.375
-0.375
Baseline
1/2^' greater.
than mfr .
recommended
max. setting
1/2" less
than mfr.
recommended
min- setting
(^oesfout)
* Fiberglass Belted
** Steel Belted
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Figure II
SLIP ANGLE-DEGREES
A. Percent Increase in Rolling
Resistance (Rr) vs. Slip Angle (2).
(100% Rr corresponds to Rolling Resistance at Zero Slip.)
£.08
Ltl
o
CO
13 .04
o:
o
§
2345
SLIP ANGLE-DEGREES
Rolling Resistance Coefficient
vs. Slip Angle (3).
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10
Figure III
RUN 22
G78-14 BIAS PLY
(NO. 601)
RUN 23
GR78-14 RADIAL PLY
(NO. 618)
SLIP ANGLE, deg
ROLLING RESISTANCE VS SLIP ANGLE {AT CONSTANT TEMPERATURE)
ON FLAT ROADWAY (4) .
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Figure IV
•TIRE17-MICHELINXZA
:T,RE 18 - FIRESTONE TRANSPORT 1
TIRE 12-GOODYEAR CUSTOM
CROSS RIB
X MO'3)
-2-10 1 2
SLIP ANGLE (DEC)
Rolling Resistance Coefficient
vs. Slip Angle for Truck Tires (5).
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12
References
1. Richard Burgeson, "The Effects of Wheel Alignment on Road Load",
EPA Memo, January 6, 1978.
2. W.W. Curtiss (Goodyear Tire & Rubber Co.), "Low Power Loss Tires",
SAE 690108, January 17, 1969, p.12.
3. J.D. Walter, F.S. Conant, "Energy Losses in Tires", TSTCA Vol. 2,
Firestone Tire and Rubber Co., November 1974, p.252-253,
A. D.J. Shuring, "Rolling Resistance of Tires Measured Under Transient
and Equilibrium Conditions on Calspan's Tire Research Facility",
March 1976, pp. 101-102, Report //DOT-TSC-OST-76-9.
5. D.J. Shuring, "Rolling Resistance of Truck Tires as Measured Under
Equilibrium and Transient Conditions on Calspan's Tire Research
Facility", Report //DOT-TST-78-1, October 1971.
6. State of New Jersey Division of Motor Vehicles; and Motor Vehicles
Inspection - City of Cincinnati.
7. Joseph J. Innes, Leslie E. Eder, "Motor Vehicle Diagnostic Inspec-
---- tion Demonstration Program", NHTSA Technical Report DQT HS-802-760.
8. Glenn D. Thompson, "Investigation of the Alternate Dynamometer
Power Absorption for the Ford Fiesta," Technical Report LDTP 78-07,
March 1978.
9. Glenn D. Thompson and Myriam Torres, "Variations in Tire Rq^ling
Resistance," Technical Support Report for Regulatpry Action,
75-5, October 1977.
10. MVMA of United States, Inc., "Motor Vehicle Facts and Figures,
1978," p. 60. '. ' ' : '
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