Southwest Research Institute® San Antonio, Texas

Coastdown and 

Constant-Speed Testing 

Brent Shoffner - SwRI 

12/11/2014 
SwRI®

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Background

• Current procedure

– US Environmental Protection Agency (EPA) 

Code of Federal Regulations (CFR) 

§1066.310 “Coastdown procedures for 

vehicles above 14,000 pounds GVWR”.

– This procedure is valid for calculating roadload coefficients

 Chassis dynamometer testing

 Aerodynamic input (CD) for the Greenhouse Gas 

Emissions Model (GEM)

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Objective

• Investigate modified and new on-road testing 

methods for evaluating aerodynamic 

characteristics of Class 7 and 8 tractors. 

– Modified EPA Code of Federal Regulations (CFR) 

§1066.310 procedure for Class 8 trucks

– Constant speed procedure measuring driveshaft 

torque and drive wheel torque

• Evaluate the effect of trailer aerodynamic 

devices

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Matrix Variables

• Three trucks evaluated

– 2012 Freightliner Cascadia®

– 2012 Kenworth T700

– 2014 International ProStar®

• Two truck tire sets

– Primarily a set from a previous SmartWay test program

 “Low rolling resistance”

– Two tests with as-received truck tire set

• Same 53’ box trailer in three configurations

– Same “SmartWay” tire set for all tests

 “Low rolling resistance”

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2012 Freightliner Cascadia and

2012 Kenworth T700

• High Roof Sleeper Cabs

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T700

Cascadia
2014 International ProStar

• Donated for use in this program by Environment 

Canada

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Two Test Methods

• Modified CFR 40 1066.310 Coastdown

• Constant Speed 

– Driveshaft torque

– Drive wheel torque

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Coastdown

• Modified CFR 40 1066.310 Coastdown

– Coastdown 70 to zero mph

 Specification – minimum speed at or below 15 

mph

– Seven pairs of runs

 The specification is 8 minimum

 Time for each run increased - 15 to zero mph

– On-board anemometer

– A few tests were conducted with drive 

wheel torque meters

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Constant Speed

• Speeds

– 10, 20, 30, 50, 70 mph

– Torque Measurements

 Driveshaft

 Four drive wheels

– Calculate horsepower 

 Based on torque and shaft speeds

– Calculate road load force

 Truck speed

 Horsepower

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Additional Instrumentation

• On-board anemometer

• Drive shaft torque meter

• Drive wheel torque meters

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On-Board Anemometer

• RM Young 

– Model 81000

– Trailer mounted

 A = .85 m

 B = .97 m

– Constant speed 

calibration

 Truck speed

 Wind speed

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Top of Trailer
Driveshaft Torque Meter

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• Drive shaft torque meter

 Modify drive shaft and 

mount antenna

 HBM Model T40B

o -5K to +5K Nm 

o Shaft speed calculated

o ERPM and trans gear &/or

o Axle ratio & wheel speed

Antenna
Constant Speed Tests

• Drive wheel torque meters

– Michigan Scientific 

Corporation

 Mounted between the hub 

& the wheels as shown

 Torque range 

o -2000 to +2000 lb-ft

o Over-range protection

 Accuracy

o 0.1% full scale

– Wheel speed (RPM)

 Truck speed from the 

optical 5th wheel 

o R2=1.000 correlation

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Strain gauges 
Trailer Information

• “Standard trailer” for high roof tractors

– CFR 40 CFR §1037.501

– No Ballast

– Cab to trailer gap – 45-47”

 No gap reducer

• Same tire set for all tests

• “Low rolling resistance” 

• Used in a previous EPA SmartWay test program

• Size 295 / 75 / 22.5

• 100 PSI

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Standard trailer plus two aero 

configurations tested

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Designation Description

Configuration 1 Standard Trailer – no enhanced 

aero

Configuration 2 Trailer skirt

Configuration 3 Trailer skirt and tail aero
53’ Box Trailer Configurations

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Configuration 1 Configuration 2

Configuration 3
Test Road

• Texas Farm to Market Road 70

– East-West section between Chapman Ranch and 

Bishop

– Meets 40 CFR 1066.310 specifications

– Survey data used to calculate the effect of slight grade 

changes on road load force

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Google Maps

Seven pair of 

runs
Road Load Force

• Force to maintain a given constant speed

– Level road

– Standard air temperature and pressure

– No Wind

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Tractive Road

Load Force

Road Load Force
Typical Road Load Force Curve

Aerodynamic drag

Drag Area (CDA) (m2) = 2xDadj/ρ

ρ=air density at reference 

conditions = 1.17 kg/m3

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Mechanical

Road Load Force = Dadjustedx(Truck Speed)2+Amech adjusted
Coastdown

• Additional Considerations

– Rotational inertia of 18 

wheel and tire assemblies

 6% of the total truck mass

– Rotational resistance of 

the axles and transmission

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Constant 

Speed

• Additional 

Considerations – Driveshaft torque 

includes energy 

losses in the 

axles under load

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Results – Test Truck 1

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Configuration #1 No Aero #2 Trailer Skirt

#3

Trailer Skirt &

Tail Aero

Method Coastdown Driveshaft 

Torque

Coastdown Driveshaft 

Torque

Coastdown

 CDA (m^2) 6.91 6.62 6.37 5.93 5.60

 Delta vs Conf 1 - -0.54 -0.69 -1.31
Truck 1 Coastdown Results

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Results – Test Truck 2

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Configuration #1 No Aero #2 Trailer Skirt

#3 Trailer Skirt & 

Tail Aero

Method Coastdown Driveshaft 

Torque

Wheel

Torque

Coastdown Coastdown

Rerun

Driveshaft 

Torque

Coastdown

 CDA (m^2) 6.66 6.23 5.90 6.26 6.20 6.02 5.73

 Delta versus Conf 1 - - - -0.40 -0.46 -0.21 -0.93
Truck 2 Coastdown Results

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Truck 3

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Configuration #1 No Aero #2 Trailer Skirt #3 Trailer Skirt & Tail

Method Coastdown

*

CoastdownDrive

shaft 

Torque

Wheel

Torque

Coastdown

*

CoastdownDrive

shaft 

Torque

Wheel

Torque

CoastdownDrive

shaft 

Torque

Wheel

Torque

 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 vs. 

Conf 1 - - - - -0.72 -0.45 -0.38 -0.42 -.85 -0.99 -1.01

* As-received 

truck tires
Truck 3 Coastdown

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Truck 3 – Constant Speed – Driveshaft Torque

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Truck 3 – Constant Speed – Wheel Torque

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Truck 3 – Configuration 1 (no trailer aero)

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Truck 3 – Configuration 2 (trailer skirt)

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Truck 3 – Configuration 3

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Summary / Learning

• The constant speed test method is potentially a viable 

approach for determining road load coefficients

• In this program both the coastdown and the constant 

speed test methods did discriminate between the three 

trailer aero configurations in terms of the drag area / 

Dadjusted term

• The delta in the drag area was similar between the three 

trailer configurations with all three trucks

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Southwest Research Institute®

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