i

TABLE OF CONTENTS

I. BACKGROUND ....................................................................................................... 1

II. OBJECTIVE .............................................................................................................. 1

III. TEST TRUCKS AND TRAILER.............................................................................. 1

IV. TEST MATRIX ......................................................................................................... 2

V. TEST METHODS...................................................................................................... 2

A. Test Site ............................................................................................................ 2

B. Additional Instrumentation ............................................................................... 3

1. On-board Anemometer ..................................................................................... 3

2. Driveshaft Torque Meter................................................................................... 3

3. Drive-Wheel Torque Meters............................................................................. 3

C. Coastdown......................................................................................................... 3

D. Constant Speed.................................................................................................. 7

VI. CALCULATIONS................................................................................................... 12

VII. RESULTS ................................................................................................................ 14

VIII. SUMMARY............................................................................................................. 15

APPENDICES

Photographs of the Test Trucks ........................................................................................ A

Truck Information ............................................................................................................. B

Photographs of the Test Trailers........................................................................................ C

Trailer Information............................................................................................................. D

Additional Instrumentation ................................................................................................ E

Test Truck and Trailer Specifications................................................................................ F

Coastdown and Constant Speed Test Results.................................................................... G
ii

FOREWORD

This work was funded by the United States Environmental Protection Agency (EPA). 

The Southwest Research Institute®Project Manager was Mr. Brent Shoffner, Manager, 

Fuel Performance Evaluations. Technical staff members who contributed to this work 

were Mr. James Chadwell, Staff Technician, and Mr. Matthew Hinojosa, Research 

Engineer. Mr. Prashanth Gururaja, Engineer, Office of Transportation and Air Quality, 

served as the EPA technical contact for this project. Mr. Arvon L. Mitcham, Program 

Manager/Engineer, HD GHG Aerodynamics Team Leader, and Dr. Houshun Zhang,

Engineer, Office of Transportation and Air Quality, were also involved in this program.
Page 1 of 15

I. BACKGROUND

The current procedure in the US EPA Code of Federal Regulations (CFR) §1066.310 is entitled 

“Coastdown procedures for vehicles above 14,000 pounds GVWR.” This procedure is valid for 

calculating road-load coefficients for chassis dynamometer testing and aerodynamic input (coefficient of 

drag) for the greenhouse gas emissions model (GEM). SwRI engineers modified this procedure and, at 

the direction of EPA, developed a constant speed procedure measuring driveshaft torque and drive 

wheel torque. Using these methods, the test matrix evaluated three trucks and one box trailer in three 

aerodynamic configurations.

II. OBJECTIVE

The objective of the program detailed in this report was to investigate modified and new on-road testing 

methods for evaluating the aerodynamic characteristics of Class 8 trucks. 

III. TEST TRUCKS AND TRAILER

The three trucks tested in this program are listed below. Photographs of the trucks are shown in 

Appendix A.

1. 2014 International ProStar® 

2. 2012 Freightliner Cascadia®

3. 2012 Kenworth T700

One set of tires, designated the “SmartWay tire set,” was used for all of the evaluations except for two 

tests. The SmartWay tire set had been used in a previous EPA SmartWay program and is categorized as 

having “low rolling resistance.” One of the test trucks was tested twice with its “as-received tire set.”

Information regarding the trucks is listed in Appendix B.

A single 53-foot box trailer was tested in the three aerodynamic configurations (designated 1, 2, and 3) 

noted in Table 1. The aero configurations are shown in Appendix C. The trailer in configuration 1 was a 

standard trailer for high-roof trucks as specified in CFR 40 §1037.501. Specific trailer information is 

included in Appendix D.

The trailer configurations are listed in Table 1.

Table 1. Test Trailer Configurations

Configuration 1 Standard trailer – no enhanced aero

Configuration 2 Trailer skirt

Configuration 3 Trailer skirt and tail aero
Page 2 of 15

IV. TEST MATRIX

The test matrix is given in Table 2. For each test, road load coefficients of Dadj in units of lbf/(mph2) and 

Aadj in units of lbf. were calculated.

Road Load Force = Dadj[Truck Speed]

2

+ Aadj

Road Load Force (lbf)

Truck Speed (mph)

Table 2. Test Matrix – Number of Evaluations

Trailer Configuration

Truck Test Method Measurement 1 2 3

Truck 1 Coastdown Vehicle speed 2* 2* 1

Constant speed Driveshaft torque 1 1 1

Wheel torque 1 1 1

Truck 2 Coastdown Vehicle speed 2 1 1

Constant speed Driveshaft torque 1 1 1

Wheel torque 1

Truck 3 Coastdown Vehicle speed 1 2 1

Constant speed Driveshaft torque 1 2 1

Wheel torque 1 1 1

* One of the two coastdown tests was conducted with the as-received tires.

V. TEST METHODS

Two test methods were used for this program: a modified 40 CFR §1066.310 procedure and a constant

speed procedure. For both methods an anemometer was mounted above the front of the trailer to 

measure relative wind velocity, wind yaw angle, and relative wind elevation. For the constant speed 

method an in-line torque meter was used to measure driveline torque, and four wheel torque meters were 

installed to measure torque at each drive wheel.

A. Test Site

For the evaluations in this test matrix a 3.8-mile east-west section of Texas Farm to Market Road (FM) 

70 between Chapman Ranch and Bishop was used. This “test road” meets 40 CFR 1066.310 

specifications. In this report, directions are noted as “east” and “west.” However, other test roads would 

potentially be oriented in other directions. Survey data, which included elevation, were used to correct 

the calculation of road load force due to the effect of slight grade changes.

The tests were conducted at night and the early hours of the morning, typically 11:00 PM to 4:00 AM, to 

achieve the lowest potential wind speed and minimize the traffic on the test road. However, there were 

still a few instances of other vehicles either passing the test truck or approaching the test truck. The
Page 3 of 15

observer noted these instances in the paperwork for each run. Test validity of the runs in each direction 

was determined by 40CFR 1006 310(6) and no tests were invalidated for traffic issues.

B. Additional Instrumentation

1. On-board Anemometer

An R.M. Young Model 81000 anemometer was mounted on a pole above the trailer at the front 

of the trailer. A photograph is shown in Appendix E. The goal was to mount the anemometer in a 

location where the air stream is not affected by the truck and trailer rig. However, that is not

practical in an on-road application. The height was limited by the clearance needed to avoid 

power lines. A long boom to mount the anemometer in the front of the truck is unwieldy, adds 

weight, and presumably would have affected the aerodynamics of the rig.

2. Driveshaft Torque Meter

For the constant speed evaluations a calibrated HBM Model T40B torque meter was installed in 

the test trucks to measure driveshaft torque. A photograph of the driveshaft torque meter 

installation is shown in Appendix E. The measurement scale was –5,000 Nm to +5,000 Nm. The 

production driveshaft of each truck was modified to install a flange, which facilitated mounting 

of the torque meter. The driveshafts were then balanced. A mounting bracket was fabricated to 

mount the telemetry antenna for the torque meter. The data acquisition rate of the torque in 

Newton meters was 10 Hertz. The shaft speed was calculated from the engine speed and the 

transmission gear ratio and/or the wheel speed and the axle ratio.

3. Drive-Wheel Torque Meters

Wheel torque meters were installed between the axle hubs and the drive wheels. A photograph of 

the torque wheel installation is shown in Appendix E. The units and modified wheel rims were 

purchased from Michigan Scientific Corporation. The measurement range of each torque meter 

was –2000 to +2000 lb-ft. The torque wheels had “over-range” protection for heavy accelerations 

and braking. The accuracy was 0.1% full scale. Wheel speed in RPM was also recorded with the 

torque wheels. Brackets were fabricated for the wire harnesses.

C. Coastdown

At the beginning of the program, in the first quarter of 2013, the coastdown procedure for the program 

was defined based on CFR §1066.310, entitled “Coastdown procedures for vehicles above 14,000 

pounds GVWR,”. The procedure included the following modifications:

1. The coastdown range was defined as seventy to zero mph. 

2. Because of the added time to conduct the tests to zero mph, seven valid pairs of runs were 

conducted instead of eight. (Validity of run pairs is defined in 40 CFR§1066.310.)

The modified coastdown procedure is given below.
Page 4 of 15

Coastdown Setup

1. Record the truck and trailer specifications. The list of specifications given in Appendix F is a

recommended guideline.

2. Make sure the vehicle has the correct documentation and license to drive on the road.

3. Reset the maximum truck speed governor to 75 mph, if the maximum speed is set less than 75 

mph.

4. Check and set the tire pressures and inspect the tires for the following issues. Make repairs as 

required.

a. Low tire pressure (more than 5 psi below the cold set pressure), which might indicate a 

leaking tire

b. Missing wheel balance weights or dented wheels

c. Missing or dented wheel covers

d. Tire tread or scuffing issues

e. More than one wheel size and/or tire size

f. Lubricant leaks in the drive train or in the wheel bearings

5. Record the make, model, and size of each tire. Each group of tires (steer tires, truck drive tires, 

and trailer tires) should consist of the same tire make, model, and tire size.

6. Install a calibrated fifth wheel. Make sure the fifth wheel is calibrated to the readout. Record the 

serial number and calibration due date.

7. Program the fifth wheel to measure the vehicle speed at 10 Hertz.

8. Install a GPS unit that will read the truck position into the same data base as the fifth wheel 

information.

9. Install an anemometer on the trailer to collect relative wind speed data. Refer to Section V.B.1.

10. Wipe the anemometer sensor heads with a damp cloth to remove dust from the transducer face.

11. Install a manual trigger, which can be actuated by the test observer in the truck.

12. Install a datalogger capable of recording the following parameters at a data acquisition speed of 

10 Hertz into a common data base.

a. Date

b. Time in hours, minutes, and one-tenth second intervals

c. GPS latitude

d. GPS longitude

e. GPS elevation

f. Vehicle speed (mph)

g. Anemometer relative wind speed (mph)

h. Anemometer relative wind yaw angle (degrees)

i. Anemometer relative wind elevation angle (degrees)

j. A manual trigger which is activated by the test observer when the steering axle of the truck 

passes a landmark identified on the survey of the test road

13. Set the datalogger clock time and the weather station clock time based on a device that connects 

to an Internet clock.

14. Make a test run to try out the system and record data including two trigger applications. 

15. Analyze the data for completeness and accuracy.
Page 5 of 15

16. Set up the calibrated weather station to read the following parameters and take sample data.

a. Air temperature

b. Relative humidity 

c. Wind speed (mph)

d. Wind direction (degrees from a zero reference)

e. Atmospheric pressure

17. Analyze the sample weather data for completeness and accuracy.

18. Pack the weather station in the auxiliary vehicle with weather station batteries. The auxiliary 

vehicle is used for transportation and is equipped with safety lights. The driver of the auxiliary 

vehicle drives behind the test truck at a distance which does not affect testing but warns other 

traffic that the slow-moving test truck is ahead. 

19. Pack a calibrated tire pressure gauge in the rig.

Coastdown Pre-Test

20. Fill the fuel tanks with diesel fuel.

21. Fill the diesel emissions fluid (DEF) tank if it is less than 3⁄4 full.

22. Weigh the truck with driver, observer, and instrumentation. This is the weight that will be used 

for calculating the road load force. Record the weight values:

Weight (lbs)

Steer axle

Truck drive wheels

Trailer tandem

Total rig

23. Allow the truck and trailer to “soak” at ambient temperature until just prior to test.

24. Measure the tire pressure with the calibrated gauge and reset the tire pressure to the tire pressure 

specification.

25. Set up the weather station as specified in 40 CFR §1066.310.

26. Record the odometer reading at the start of test.

27. Record the date and time at the start of test.

28. Record the fuel gauge reading at the start of test.

29. Check the weather station parameters at the test site. Make sure that it is recording all the 

parameters. The battery voltage should be above 12 volts.
Page 6 of 15

Anemometer Calibration Runs

30. Drive east for a minute on the coastdown road at 30 mph while recording all the data.

31. Accelerate to 50 mph and continue driving east for a minute on the coastdown road at 50 mph 

while recording all the data.

32. Accelerate to 70 mph and continue driving east for a minute on the coastdown road at 70 mph 

while recording all the data.

33. Proceed to the turnaround and drive west for a minute on the coastdown road at 30 mph while 

recording all the data.

34. Accelerate to 50 mph and continue driving west for a minute on the coastdown road at 50 mph 

while recording all the data.

35. Accelerate to 70 mph and continue driving west for a minute on the coastdown road at 70 mph 

while recording all the data.

36. If the truck and trailer rig have been driven for at least 30 minutes, the warm-up requirement has 

been met. If not, continue to drive the truck until a minimum of 30 minutes of warm-up time has 

been achieved.

Coastdown Runs

37. Run 7 pairs of valid coastdown laps. 

a. Place all windows in the “rolled up” position.

b. Accelerate to 75 mph, place the transmission in neutral, and start the coastdown.

c. Activate the datalogger.

d. Manually activate the trigger when the steering axle passes a known reference position on the 

road survey. 

e. Coast down from 70 mph to zero mph.

f. Remain at zero mph for 20 seconds, then deactivate the instrumentation.

g. As much as possible, use exactly the same stretch of road for the actual coastdown in both 

directions.

h. While coasting, avoid lane changes if at all possible. If necessary, they should be done as 

slowly as possible and over a distance of at least a half kilometer (a quarter mile). If such a 

gradual change cannot be made, abort the run.

i. The ambient temperature must be 41–95oF.

38. Record the odometer reading at the end of test.

39. Record the date and time at the end of test.

40. Record the fuel gauge reading at the end of test.

41. Pack up the weather station.

42. After the coastdown weigh the truck with driver, observer, and instrumentation. Record the 

weight values:

Weight (lbs)

Steer axle

Truck drive wheels

Trailer tandem

Total rig
Page 7 of 15

D. Constant Speed

The constant speed procedure developed for this program is given below:

Constant Speed Setup

1. Record the truck and trailer specifications. The list of specifications given in Appendix F is a 

recommended guideline.

2. Make sure the vehicle has the correct documentation and license to drive on the road.

3. Reset the maximum truck speed governor to 75 mph if the maximum speed is set less than 75 

mph.

4. Check and set the tire pressures and inspect the tires for the following issues. Make repairs as 

required.

a. Low tire pressure (more than 5 psi below the cold set pressure), which might indicate a 

leaking tire

b. Missing wheel balance weights or dented wheels

c. Missing or dented wheel covers

d. Tire tread or scuffing issues

e. More than one wheel size and/or tire size

f. Lubricant leaks in the drive train or in the wheel bearings

5. Record the make, model, and size of each tire. Each group of tires (steer tires, truck drive tires, 

and trailer tires) should consist of the same tire make and model.

6. Install a calibrated fifth wheel. Make sure the fifth wheel is calibrated to the readout. Record the

serial number and calibration due date.

7. Program the fifth wheel to measure the vehicle speed at 10 Hertz.

8. Install a GPS unit that will read the truck position into the same data base as the fifth wheel 

information.

9. Modify the production driveshaft to facilitate the installation of a calibrated in-line torque meter. 

10. Dynamically balance the driveshaft and torque meter assembly.

11. Install the assembly in the truck.

12. Fabricate a bracket to mount a torque meter antenna or slip-ring wires.

13. Modify the drive-wheel rims to accommodate installation of calibrated wheel torque plates.

14. Install the wheel rim and torque plate assemblies on the drive axle hubs.

15. Fabricate brackets to mount the wheel torque meter antenna slip-ring wires.

16. Install an anemometer on the trailer to collect relative wind speed data. Refer to Section V.B.1.

17. Wipe the anemometer sensor heads with a damp cloth to remove dust from the transducer face.

18. Install a manual trigger which can be actuated by the test observer in the truck.

19. Install a datalogger capable of recording the following parameters at a data acquisition speed of 

10 Hertz into a common data base.

a. Date

b. Time in hours, minutes, and one-tenth second intervals

c. GPS latitude

d. GPS longitude

e. GPS elevation

f. Vehicle speed (mph)
Page 8 of 15

g. Anemometer relative wind speed (mph)

h. Anemometer relative wind yaw angle (degrees)

i. Anemometer relative wind elevation angle (degrees)

j. A manual trigger which is activated by the test observer when the steering axle of the truck 

passes a landmark identified on the survey of the test road

k. Torque reading at each drive wheel

l. Wheel speed (RPM) at each drive wheel

m. Driveshaft torque

n. Driveshaft RPM or engine speed if the transmission gear ratios are known

20. Set the datalogger clock time and weather station clock based on a device that connects to an 

Internet clock.

21. Set up the calibrated weather station to read the following parameters and take sample data.

a. Air temperature

b. Relative humidity 

c. Wind speed (mph)

d. Wind direction (degrees from a zero reference)

e. Atmospheric pressure

22. Analyze the sample weather data for completeness and accuracy.

23. Raise the drive wheels so that the tires are not contacting the ground and make sure the wheels 

and the driveshaft torque meter have no actual torque on them. Record the torque values:

As-received 

indicated 

torque

(lb-ft)

Value after 

the offset 

has been 

“zeroed”

(lb-ft)

Shunt calibration 

value

(lb-ft)

Reading on the 

datalogger

(lb-ft)

Positive Negative Positive Negative

Left front

Right front

Left rear

Right rear

24. Drive the truck and try out the instrumentation including two trigger applications.

25. Decide on the best transmission gear for constant speed testing. Record the gear selections:

Nominal

Speed (mph)

Gear

Selection

10

20

30

50

70
Page 9 of 15

26. Send the data file to the project manager for approval

27. Charge 3 weather station batteries and check them with the battery checker.

28. Pack the weather station in the auxiliary vehicle with the weather station batteries. The auxiliary 

vehicle is used for transportation and is equipped with safety lights. The driver of the auxiliary 

vehicle drives behind the test truck at a distance which does not affect testing but warns other 

traffic that the slow-moving test truck is ahead.

29. Pack the calibrated tire pressure gauge in the rig.

Pre-Constant Speed Test

30. Fill the fuel tanks before leaving for the road so that all the constant speed tests can be completed 

without having to stop and fill the vehicle.

31. Fill the diesel emissions fluid (DEF) tank if it is less than 3⁄4 full.

32. Weigh the truck with driver, observer, and instrumentation. Record the weight values:

Weight (lbs)

Steer axle

Truck drive wheels

Trailer tandem

Total rig

33. On the scale, remove the pre-load (torque) on the drive shaft and the torque wheels. Record the 

torque values:

Indicated zero reading lb-ft

Left front torque wheel

Right front torque wheel

Left rear torque wheel

Right rear torque wheel

Drive shaft

34. Measure the tire pressure with the calibrated gauge and reset the tire pressure to the tire pressure 

specification.

35. Drive to the test site and set up the weather station per the regulations in 40 CFR §1066.310.

36. Set out cone(s) at a distance of 3.8 miles. The constant speed tests will be conducted in 

approximate 3.8 mile segments.

37. If the truck and trailer rig have been driven for at least 30 minutes, the warm-up requirement has 

been met. If not, continue to drive the truck until a minimum of 30 minutes of warm-up time has 

been achieved.

Constant Speed Test

38. Record the odometer reading at the start of test.

39. Record the date and time at the start of test.

40. Record the fuel gauge reading at the start of test.

41. Place all windows in the “rolled up” position.

42. Conduct the test only when the ambient temperature is 41-95oF.
Page 10 of 15

Eastbound

a. For eastbound, accelerate to the specified constant speed and activate the instrumentation 

before reaching the “west-end cone.”

b. Trigger the marker at a landmark that is identified on the survey.

c. Hold the constant speed(s) for the time specified or until reaching the “east-end cone”,

whichever is longer.

Westbound

a. For westbound, accelerate to the specified constant speed, activate the instrumentation, and 

achieve steady state before reaching the east-end cone.

b. Hold the constant speeds(s) for the time specified or until reaching the west-end cone,

whichever is longer.

c. Trigger the marker at a landmark that is identified on the survey.

43. Record the data with the installed instrumentation:

Lap Direction Gear* Constant Speed

1 East 70 mph for 3 min 20 sec

1 West 70 mph for 3 min 20 sec

2 East 70 mph for 3 min 20 sec

2 West 70 mph for 3 min 20 sec

3 East 10 mph for 7 min 30 sec

Accelerate to 20 mph

20 mph for 7 min 30 sec

3 West 20 mph for 7 min 30 sec

Decelerate to 10 mph

10 mph for 7 min 30 sec

4 East 30 mph for 7 min 30 sec

4 West 30 mph for 7 min 30 sec

5 East 50 mph for 4 min

5 West 50 mph for 4 min

6 East 50 mph for 4 min

6 West 50 mph for 4 min

7 East 70 mph for 3 min 20 sec

7 West 70 mph for 3 min 20 sec

* Determined during setup
Page 11 of 15

44. Record the odometer reading at the end of test.

45. Record the date and time at the end of test.

46. Record the fuel gauge reading at the end of test.

47. Pack up the weather station.

48. Weigh the truck with driver, observer, and instrumentation.

Weight (lbs)

Steer axle

Truck tandem

Trailer tandem

Total rig

49. On the scale, remove the pre-load (torque) on the drive shaft and the torque wheels. Record the 

torque values:

Indicated zero reading lb-ft

Left front torque wheel

Right front torque wheel

Left rear torque wheel

Right rear torque wheel

Drive shaft

50. The technician saves the data files on the server.

51. The project manager analyzes the data and calculates the results.
Page 12 of 15

VI. CALCULATIONS

A. Coastdown

The test results were calculated from data collected during the tests. Calculations were performed per 40 

CFR 1066.310, with the following enhancements:

1. The coastdown calculations were performed on the whole speed range (70-0 mph).

2. The distance from the steering axle to the mass center of the truck and trailer rig was calculated. 

The location of the longitudinal mass center of the truck and trailer rig was calculated where the 

sum of the moments about the mass center was zero, using the following information:

a. The steering axle, truck tandem axle, and trailer tandem weights

b. The distance between the center lines of the axles

3. The manual marker was triggered by the observer when the steering axle was at a landmark that 

was identified on the survey of the road. The position of the mass center on the survey was 

calculated by integrating the recorded truck speed over the 10-Hertz intervals to determine the 

distance from the landmark. 

4. The slight grade in the coastdown road was then determined from the change in altitude 

indicated by the survey.

5. The road load force of each 10-Hertz deceleration rate was corrected for grade.

6. An enhancement added during the program was that the only data taken during the actual runs 

(not setup and turnaround time) were used to calculate temperature and barometer for 

determining the corrected Dadj term and the Aadj term.

7. The Amech, was corrected using a formula given in SAE J1263 with input from EPA regarding 

the estimated value for kt.

Aadj = Amech[1+kt(T-T0)]

kt = .006

T = ambient temperature in Kelvin

T0 = 20°C (standard temperature) in Kelvin

B. Constant Speed

As noted in the procedure, the constant speed evaluations were conducted at 10, 20, 30, 50, and 70 mph 

in opposing directions in approximately the same location on the test road. 

1. Calculation by Driveshaft Torque

a. The driveshaft torque of the 10-Hertz readings during steady-state operation was

averaged. This included the data from all the laps at a specific speed in both directions.

b. The 10-Hertz truck speed readings during the same time periods were averaged and 

converted to units of feet/second.
Page 13 of 15

c. The average driveshaft speed (RPM) during the same intervals was determined from the 

engine speed (RPM) and the transmission gear. 

d. The horsepower at the driveshaft torque meter was calculated using the following 

formula:

Average Horsepower = [average torque (lbft)][(average driveshaft speed (RPM)]/5252

e. The road load force (RLF) for each steady-state speed (10, 20, 30, 50, and 70 mph) was 

calculated using the following formula:

RLF(lbf) = 550[average horsepower]/[average truck speed (ft/sec)]

2. Calculation by Wheel Torque

f. The 10-Hertz readings of the torque at each drive wheel during steady-state operation 

were averaged. This included the data from all the laps at a specific speed in both 

directions.

a. The 10-Hertz truck speed readings during the same time periods were averaged and 

converted to units of feet/second.

b. The average wheel speed (RPM) of each drive wheel during the same intervals was 

determined from the instrumentation readings. 

c. The total horsepower at the drive wheels was calculated using the following formula:

Total Average Horsepower = Sum of the power at each drive wheel

(n is the number of drive wheels)

where the power at each wheel is equal to:

[average wheeln torque (lbft)][(average wheeln speed (RPM)]/5252

d. The road load force (RLF) for each steady-state speed (10, 20, 30, 50, and 70 mph) was 

calculated using the following formula:

RLF(lbs) = 550[average horsepower]/[average truck speed (ft/sec)]

For both the driveshaft torque and wheel torque cases the road load coefficients Dunadjusted and Amech 

were determined by the linear trend-line on a scatter plot of the road load force at each of the tested 

speeds (10, 20, 30, 50, and 70 mph). This is shown in Figure 1. Dadj was calculated by correcting for 

temperature and pressure and Aadj was corrected for temperature.
Page 14 of 15

Figure 1. Determination of Road Load Force Coefficients with Constant Speed Data

VII. RESULTS

The results of the tests that were considered valid by both the SwRI and EPA project managers are given 

in Appendix G. The tests described in this document were conducted from January 2014 through April

2015. In several cases valid tests were re-performed at the direction of the EPA technical contact. These 

tests are indicated by test numbers (Example: Test #1, Test #2). 

Tests which had average crosswinds of more than 5 mph were not included. Note that this is a validity 

limit defined in SAE J1263. Tests which had instrumentation issues that might have potentially 

compromised the results or did not provide adequate data for a complete analysis were also not included.
Page 15 of 15

VIII. SUMMARY

1. The constant speed method using either the driveshaft torque meter or the torque wheels seems 

to be viable to determine drag area. The difference in the drive shaft drag area and the torque 

wheel drag area is probably due to the additional power required at the driveshaft to overcome 

any losses in the axles. One of the potential reasons that the coastdown drag area is greater than 

either constant speed method is that there are additional energy losses in the transmission.

2. There is a difference in the magnitude of the drag area among the three measuring techniques: 

coastdown, constant speed driveshaft torque, and constant speed wheel torque. However, using 

the same technique with the same truck produces similar drag aero reduction with the trailer skirt 

and the trailer tail aero.

3. Tests were conducted in a range of ambient temperatures and slight differences in weight. 

However, the Aadj term is similar for all the truck tests with the SmartWay tire set.

4. Two tests were conducted with crosswinds greater than 5 mph, which would be invalid by the 

SAE J1263 procedure, and the CFR 1066.310 procedure since it references the SAE procedure. 

These tests had higher than expected drag area and were rerun with valid wind conditions, which 

produced the results reported in this document. It is recommended that each pair of runs be 

evaluated against the wind criteria in SAE J1263. If either run in a pair exceeds the limits, that 

pair should not be used for calculating the final results.
APPENDIX A

Photographs of the Test Trucks
A-1 of 2

Freightliner Cascadia (Left) and Kenworth T700 (Right)
A-2 of 2

2014 International ProStar

This truck was donated for use in this program by Environment Canada.
APPENDIX B

Truck Information
B-1 of 1

Truck Specifications

Truck make International Freightliner Kenworth

Model ProStar Cascadia T700

VIN 3HSDJSNR0EN785904 1FUJGLDR6CSBE3545 1XKFD49XXDJ342758

Truck skirt to ground (approx.) 3.5 to 4.4 inches Approx. 10.5 inches 5.4-6.0 inches

Gap measurement trailer to cab 47 inches* 45 inches 47 inches**

Two Tire Sets Used in the Program

Tire Set SmartWay tire set An as-received tire set

Truck steering tires

Tire size 295 / 75 / 22.5 11R / 22.5

Tire manufacturer Hankook Michelin

Set designation AL-11 XZA-3

Set pressure 110 PSI 110 PSI

Truck tandem tires

Tire size 295 / 75 / 22.5 11R / 22.5

Tire manufacturer Bridgestone Bridgestone

Set designation M-710 726-EL

Set pressure 100 PSI 100 PSI

* The gap was measured to be 47 inches and was not reset. This is the same dimension as the Kenworth truck.

** The king pin was moved to the forward-most slot on the truck. The tolerance for measurements in CFR §1037.501 is ± 2 inches.
APPENDIX C

Photographs of the Test Trailers
C-1 of 2

Standard Trailer – Configuration 1 – No Enhanced Aero
C-2 of 2

Trailer – Configuration 2 – Trailer Skirt

Trailer – Configuration 3 – Trailer Skirt and Tail Aero
APPENDIX D

Test Trailer Information
D-1 of 2

Test Trailer Information

Trailer Configuration 1 2 3

Manufacturer Wabash Wabash Wabash

Unit number U92947 U92947 U92947

Base trailer length 53 foot 53 foot 53 foot

Trailer width 102 inches 102 inches 102 inches

Ballast None None None

Top of trailer to the ground 161.25 inches 161.25 inches 161.25 inches

King pin to front of the trailer 36 inches 36 inches 36 inches

Trailer skirt Trailer skirt 

installed

Trailer skirt 

installed

Model number 24305 24305

Trailer skirt height 35.5” – 36.75” 35.5” – 36.75”

Tail aero

Tail aero 

installed

Distance from end of trailer to 

back side 48”

Trailer Tandem Location (“California Position”)

Kingpin to center of rear 

tandem axle 40 feet 40 feet 40 feet

Rear of trailer (not aero) to 

center of trailer tandem axles 141* inches 141* inches 141* inches

Trailer Tires

Tire size 295 / 75 / 22.5 295 / 75 / 22.5 295 / 75 / 22.5

Tire manufacturer Bridgestone Bridgestone Bridgestone

Set designation R-197 R-197 R-197

Set pressure 100 PSI 100 PSI 100 PSI

* Refer to the explanation on the next page
D-2 of 2

Rear of Trailer to Center of Trailer Tandem Axles

The trailer skirt used for this program was specified by the EPA. The locating holes for the 

trailer-tandem wheel fore and aft location were 4 inches apart. The tandem wheel position 

was set as far forward as possible or the trailer tires would have potentially contacted the 

trailer skirt as shown below.

Trailer Wheels with Skirt Installed

Front of Trailer
APPENDIX E

Additional Instrumentation
E-1 of 3

On-Board Anemometer

Anemometer mounting dimensions

A = .85 m

B = .97 m
E-2 of 3

Driveshaft Torque Meter

Drive shaft torque meter

 HBM Model T40B

 Modified drive shaft

 Antenna installed on mounting plate

 Range = –5K to +5K Nm 

 Shaft speed calculated

 Engine RPM and trans gear 

 Axle ratio & wheel speed

Torque Meter

Antenna Assembly Antenna Mounting Bracket
E-3 of 3

Drive Wheel Torque Meters

Drive wheel torque meters

 Michigan Scientific Corporation

 Mounted between hub and wheels as shown

 Torque range = –2000 to +2000 lb-ft

 Over-range protection

 Accuracy = 0.1% full scale

 Wheel speed (RPM)
APPENDIX F

Test Truck and Trailer Specifications
F-1 of 2

Recommended Test Truck and Trailer Specifications to Record

Truck Specification

Truck make

Truck model

Truck model year

Truck unit number (if applicable)

Take photographs of the truck and trailer rig

Odometer at start of test

Date

Truck skirt to ground clearance

Truck vehicle identification number

Truck tires

Truck steering tires

Tire size

Tire manufacturer

Set designation

Set pressure

Truck tandem tires

Tire size

Tire manufacturer

Set designation

Set pressure

Trailer tires

Tire size

Tire manufacturer

Set designation

Set pressure

Other
F-2 of 2

Trailer Specification

Manufacturer

Unit number

Base trailer length

Top of trailer to ground

King pin to front of trailer

Gap measurement from trailer to cab

Trailer tandem location

Kingpin to center of rear tandem axle

Rear of trailer (not aero) to center of rear tandem axle

Trailer tandem axles

Aero Devices

Trailer skirt

Trailer skirt manufacturer

Trailer skirt description

Model number 

Trailer skirt ground clearance

Tail aero (if applicable)

Manufacturer

Description

Distance from end of trailer to back side

Gap reducer (if applicable)

Manufacturer

Description

Distance from end of trailer to back side

Other

Fifth wheel location

Actual Weight for Coastdown

Steer axle weight (lbs.)

Tandem axle weight (lbs.)

Trailer axle weight (lbs.)

Total rig weight (lbs.)
APPENDIX G

Coastdown and Constant Speed Test Results
G-1 of 6

Truck 1

* As-received truck tires. The trailer tires were the same SmartWay set.

** Results were based on ten runs. The data files from the other runs were mistakenly over-written by the ring memory of the datalogger.

Configuration

Measurement Method Coastdown Coastdown Driveshaft 

Torque

Wheel

Torque

Coastdown Coastdown Driveshaft 

Torque

Wheel

Torque

Coastdown Driveshaft 

Torque

Wheel

Torque

Weight (lb) 35300 36280 35960 35960 35,680 36,380 36,480 36,480 36,500 36,560 36,560

SwRI Calculated 

 Dadj (lbf/(mph^2)) 0.168 0.163 0.162 0.157 0.150 0.152 0.152 0.146 0.141 0.136 0.130

 Amadj (lbf) 240.2 195.2 213.1 177.9 234.2 211.1 215.0 186.2 208.7 202.4 175.7

 CDA (m^2) 6.41 6.21 6.16 5.97 5.69 5.76 5.78 5.55 5.36 5.17 4.96

 Delta versus Conf 1 - - - - -0.71 -0.64 -0.38 -0.42 -1.05 -0.99 -1.01

Average Temperature (F) 77.6 57.7 60.1 60.1 78.5 47.0 72.7 72.73 57.88 66.58 66.58

Average Barometer (in-Hg) 29.90 30.05 30.02 30.02 29.86 30.27 29.87 29.87 29.95 29.96 29.96

SmartWay Tire Set No* Yes Yes Yes No* Yes Yes Yes Yes Yes Yes

Trailer Skirt N o N o N o N o Yes Yes Yes Yes Yes Yes Yes

Tail aero N o N o N o N o N o N o N o N o Yes Yes Yes

Gap reducer N o N o N o N o N o N o N o N o N o N o N o

Gap cab to trailer (inches) 47 47 47 47 47 47 47 47 47 47 47

#1 #2 #3

Truck 1
G-2 of 6

* As-received truck tires. The trailer tires were the same SmartWay set.
G-3 of 6

Truck 2

Configuration

Measurement Method Coastdown 

Test #1

Coastdown 

Test #2 *

Driveshaft 

Torque

Coastdown Driveshaft 

Torque

Coastdown Driveshaft 

Torque *

Wheel 

Torque *

Weight (lb) 34,420 35,520 35,040 34,840 34,520 34,820 35,600 35,600

SwRI Calculated 

 Dadj (lbf/(mph^2)) 0.182 0.182 0.174 0.167 0.156 0.147 0.142 0.131

 Amadj (lbf) 187.3 192.3 203.5 196.7 175.7 196.2 213.8 178.2

 CDA (m^2) 6.91 6.91 6.62 6.37 5.93 5.60 5.42 4.97

 Delta versus Conf 1 - - -0.54 -0.69 -1.31 -1.20

Average Temperature (F) 53.65 55.48 78.58 40.76 75.19 67.90 43.84 43.84

Average Barometer (in-Hg) 29.97 30.07 29.77 30.03 29.97 29.90 29.83 29.83

SmartWay Tire Set Yes Yes Yes Yes Yes Yes Yes Yes

Trailer Skirt N o N o N o Yes Yes Yes Yes Yes

Tail aero N o N o N o N o N o Yes Yes Yes

Truck 2

#1 #2 #3
G-4 of 6

* Driver side truck aero trim piece inadvertently left off for these tests.

Black Trim Piece

Inner fender
G-5 of 6

Truck 3

Configuration

Measurement Method Coastdown Driveshaft 

Torque 

Test #1

Driveshaft 

Torque 

Test #2

Wheel

Torque 

Test #1

Wheel

Torque 

Test #2

Coastdown 

Test #1

Coastdown 

Test #2

Driveshaft 

Torque 

Test #1

Driveshaft 

Torque 

Test #2

Driveshaft 

Torque 

Test #3

Wheel

Torque 

Test #1

Wheel

Torque 

Test #2

Coastdown Driveshaft 

Torque

Wheel

Torque

Weight (lb) 35,100 35,080 34,860 35,080 34,860 35,100 35,260 35,400 35,720 35,200 35,720 35,200 35,740 35,700 35,700

SwRI Calculated 

 Dadj (lbf/(mph^2)) 0.175 0.164 0.164 0.155 0.154 0.164 0.163 0.158 0.159 0.155 0.150 0.145 0.151 0.145 0.136

 Amadj (lbf) 201.1 198.3 191.3 187.7 171.3 191.6 191.6 205.2 202.9 192.1 186.6 170.3 185.8 199.4 191.2

 CDA (m^2) 6.66 6.23 6.24 5.90 5.87 6.26 6.20 6.02 6.07 5.89 5.70 5.52 5.73 5.50 5.19

 Delta versus Conf 1 - - - -0.41 -0.47 -0.21 -0.167 -0.347 -0.185 -0.365 -0.93 -0.735 -0.695

Average Temperature (F) 44.73 74.59 67.46 74.59 67.46 42.19 49.24 72.47 40.78 57.44 40.78 57.44 57.72 68.26 68.26

Average Barometer (in-Hg) 30.17 29.77 29.91 29.77 29.91 30.13 30.12 29.77 30.11 29.92 30.11 29.92 30.15 29.94 29.94

SmartWay Tire Set Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

Trailer Skirt N o N o N o N o N o Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

Tail aero N o N o N o N o N o N o N o N o N o N o N o N o Yes Yes Yes

Gap reducer N o N o N o N o N o N o N o N o N o N o N o N o N o N o N o

Gap cab to trailer (inches) 47 47 47 47 47 47 47 47 47 47 47 47 47 47 47

#1 #2 #3

Truck 3
G-6 of 6