AUTO RESEARCH CENTER SCALE MODEL WIND TUNNEL 

STUDY USING A SPECIFIC LONG HAUL SLEEPER TRUCK 

WHICH WE WILL CALL MODEL “A” AND A 2008 WABASH 

TRAILER

Test conducted on:

May 26, 2011

Conducted by:

Auto Research Center

On Behalf of

The Environmental Protection Agency

CONFIGURATION:

Truck Model “A” with a 2008 53’ Wabash Trailer

Report Release Signature _____ Mike Camosy CEO

Date of Report Release _____June 7, 2011

Revision - 5
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TABLE OF CONTENTS

Section Title Page

Table of Contents 2

 1.0 Objective, Key Individuals, Distribution list 3

 2.0 Approach 3

 3.0 Model Configuration 4

 4.0 Test Matrix 5

 5.0 Summary of Results 5

 6.0 Description of Test Vehicle 6

 7.0 Description of Test Facility 8

 8.0 Test Procedure 11

 9.0 Data Reduction 11

 10.0 What Are the Differences Between On-Road 12

And A Wind Tunnel?

 11.0 Conclusions 15

APPENDIX A 22
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1.0 OBJECTIVE

The EPA conducted a wind tunnel test at the Auto Research Center (ARC) in Indianapolis, 

Indiana. The first objective was to determine the Cd value of the vehicle in different 

configurations at the 9 SAE standard yaw points and calculate the wind averaged drag as per 

SAE standards for Class-8 trucks and trailers. The second objective was to report the fuel 

economy change of each change over a range of varying road speeds. This test was a validation 

test and thereby followed a standard EPA verification process for each change.

Key individuals at ARC:

Mike Camosy – CEO/Chief Aerodynamicist was present during the test, created the excel 

spreadsheet for the raw data and wrote the report

Fritz Marinko - Director of Business Development coordinated the project and prepared the test 

matrix

Paul Cavanagh – Lead Technician operated the wind tunnel during the test

Winston Bush – Chief Model Maker prepared the model for the test and made all changes to the 

model during the test

Report Distribution List:

ARC - Mike Camosy, Fritz Marinko

EPA - Arvon Mitcham, Sam Waltzer

URS – Tariq Hasan, Bob Specht

2.0 APPROACH

The EPA provided a detailed test matrix. All test items already existed and were manufactured 

by ARC. All items tested were pre-manufactured and pre-fit on the model prior to the test date.

The truck model used was a highly detailed scale model of a specific class 8 long haul sleeper 

tractor which we will call model “A” with a 2008 Wabash 53 foot Trailer. The models were

detailed 1/8 scale model of these configurations. Every detail of the full scale tractor/trailer that 

touches the air was accurately modeled. This model had articulating suspension and rotating 

wheels. Every part of this model was adjustable (ie trailer/tractor gap or bogey position). ARC 

also provided the rolling road testing facility, model makers, technicians, aerodynamicists and 

computer hardware and software for data reduction.
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The test consisted of two phases: 1) Setting the initial baseline; 2) Component testing of 12

changes to the tractor/trailer. To set the baseline, a reference tractor trailer gap was used with a 

California trailer bogey position. All testing was completed at 50 m/s. All tests were conducted 

with multiple yawing angles sweeping the model through the following yaw angles (0 deg; 1

deg; 3 deg; 6 deg; 9 deg; 0 deg; -1 deg; -3 deg; -6 deg; -9 deg; 0 deg). Preceding every 

individual test run were two model tares. The first was a static weight tare, which removed the 

static weight of the model. The second was a rolling tare without wind completed at 50 m/s 

which removed any mechanical drag imparted on the balance through the rotating wheels. For 

the second tare, multiple tares were recorded for each yaw angle listed in the sequence above for 

EACH run.

All aerodynamic forces were measure using an AEROTECH 6 component load cell balance. 

This balance provided three forces: Lift, Drag and Side force as well as three moments: Pitch, 

Roll and Yaw moments. The repeatability recorded during our baseline setting was under 0.25%

for fuel economy. 

3.0 MODEL CONFIGURATION

Tractor: Model “A” Truck 

 Wheel Base of 238”

Full Sleeper

Hood Mounted Mirrors

Cab Extenders

Chassis Fairings without Extenders

275/60/22.5 Tires

Tractor – Trailer Gap = 45”

Trailer: 2008 Wabash 53’ Dry Van

Bogey in California Position

King Pin at 36”

Width = 102”

Front Corner Radius = 5”

Landing Gear in Standard Position

Note: California bogey position is defined as 40’ from the center of the king pin to the center line 

of the rear most bogey axle.
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4.0 PROGRESSIVE TEST MATRIX

Run# Description

1 Baseline Configuration

2 Repeat

3 Repeat

4 Baseline – Static Floor

5 Closed Grill – Rolling Road

6 Closed Grill – Static Floor

7

Trailer Gap = 50"; 3" Trailer Corner Radii; Open Grill – Rolling 

Road

8

Trailer Gap = 50"; 5" Trailer Corner Radii; Open Grill – Rolling 

Road

9

Trailer Gap = 50"; 5" Trailer Corner Radii; Open Grill – Static 

Floor

10 Closed Grill – Rolling Road

11 Closed Grill – Static Floor

12 Remove Chassis Fairings; Open Grill - Static Floor

13 Remove Chassis Fairings; Open Grill - Rolling Road

14 Trailer Gap = 45" - Rolling Road

15 Trailer Gap = 45" - Static Floor

Note: Pictures and results for each run can be found in Appendix A.

5.0 SUMMARY OF RESULTS

The individual results for each yaw angle were measured and put through the SAE recommended 

process for wind averaging. The wind averaged Cd values for the individual speeds were then 

cross referenced to a study completed by NASA that related the fuel economy savings for each 

percentage change in drag at various speeds. A summary of the results are shown in table format 

in the appendix which displays the final fuel savings for each component at various speeds. A 

negative value reflects an improvement over the baseline tractor/trailer combination. All values 

are given in percent (%) improvement. 
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6.0 DESCRIPTION OF TEST VEHICLE

The test vehicle used was a 1/8 scale tractor and trailer. The tractor was a model “A” sleeper. 

The trailer was a 53 foot Wabash trailer. The model had articulating suspension. It also 

included pressure taps that were not used for this test. Any part of the tractor/trailer that touched 

the air was accurately represented in this model. This model also included internal flow 

modeling from the tractor grill into the radiator and then into the engine bay. The bogey was 

adjustable for multiple positions. Pictures of the model used are below.

Rivets on Side of Wabash Trailer
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Rear Door & Bumper of Wabash Trailer

Starting Bogey Position was California.
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7.0 DESCRIPTION OF TEST FACILITY

Wind Tunnel

Type Closed single return tunnel with 3/4 open-jet working section and 

moving ground plane

Construction Tubular steel frame with birch plywood skin, 20mm thick, 9 ply, 

controlled grain panel

Corner turning vanes Rolled steel vanes welded into assemblies

Air settling Two stainless steel settling screens 1800mm apart at the entrance to 

the contraction

Mesh screen specification 57.4% open area, 1mm aperture size

Air straightening Honeycomb wall in settling chamber, 75mm thick x 10mm core size

Tunnel volume 1,111 m3(39,235 ft3)

Surface area 2,147 m2 (23,110 ft2)

Average duct area 13.94 m2 (150 ft2)

Circuit length 75.84 m (249 ft)

Temperature control Water cooled to ± 1° C (1.8° F) by a cooling coil

Working Section

Type 3/4 open jet contained in a plenum

Length Adjustable up to 5.75 m (18.9 ft)

Contraction ratio 4.46:1

Maximum air velocity 50.0 m/s (164.0 ft/s) CDP

Maximum dynamic pressure 1,513 Pa (31.60 lb/in. ) at standard conditions

Nozzle width 2.500 m (7.414 ft)

Nozzle height 2.060 m (6.824 ft)

Plenum dimensions 10.62 m (34.8 ft) W, 16.46 m (54.0 ft) L, 8.28 m (27.2 ft) H

Plenum NET volume 1,141 m3(40,294 ft3)

Maximum model scale 50% open wheel, 40% yawed +/-8° NASCAR

Maximum model height above 

road

330 mm

Strut movement rate 8 mm/sec

Strut movement time from 

10 mm to 330 mm

39.5 sec

Model support A vibration isolated balance frame surrounds the working section and 

supports the model strut. The strut position is fully adjustable to 

accommodate model geometry.

Primary boundary-layer slot 0.050 m (0.197 ft) high

Boundary-layer elimination Three stage fully adjustable system
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Fan Section

Fan description Fixed geometry single stage axial pusher fan with nine equally 

spaced carbon fibre blades

Fan diameter 3.790 m (12.434 ft)

Fan power 373 kW (500 hp) variable speed DC motor

Maximum rotational speed 610 rpm

Maximum tip speed 121.1 m/s (397.2 ft/s)

Fan support Fourteen upstream stators with pre-rotation shrouds, eleven 

downstream stators with flow straightening shrouds

Fan mechanical drive Two-piece drive shaft on axis of tunnel through turning vane

Fan section total weight 21,729 kg (48,000 lb.)

Moving Ground Plane (Rolling Road)

Construction Tubular Steel Frame, concrete filled with Cast Iron Platen

Yaw table 5.18 m diameter

Yaw range +/- 8.0o

Moving ground length 4.180 m (13.58 ft)

Moving ground width 1.66 m (5.44 ft)

Belt type Ammeraal Flexam with Sandblast Profile

Maximum belt speed 50.0 m/s (164.0 ft/s) CDP

Belt suction 33 individually controlled suction chambers

Platen instrumentation 33 pressure sensors and 71 K-type thermocouples

Temperature control Water cooled to +/-3.0° C (+/-5.4° F) with 110 cooling tubes

Steering Hydraulically actuated steering roller controlled using two optical beltedge sensors and a roller tachometer

Data Acquisition and Control

Data acquisition Pi Research Mistral system

Motor control 6 Control Techniques ‘Mentor’ DC drives

Tunnel Control PLC direct to Control Techniques drives

Model balance Aerotech 6-component internal balance

Model pressure measurement Two 32 port PSI pressure scanners

Wheel drag balances 4 x single component strain-gauge load cells 

Wing/body panel balances 2 x three component strain-gauge load cells

Model exhaust simulation Maximum flow 2.1 m3/s (75 SCFM)
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Equipment Ratings

Main fan motor 373 kW (500 hp) variable speed air cooled DC motor

Rolling-road motor 186 kW (250 hp) variable speed air cooled DC motor

Primary boundary-layer motor 93 kW (125 hp) variable speed air cooled DC motor

Top-plate boundary-layer motor 11 kW (15 hp) variable speed air cooled DC motor

Secondary boundary-layer motor 22 kW (30 hp) variable speed air cooled DC motor

Belt suction motor 11 kW (15 hp) variable speed air cooled DC motor

Hydraulic power pack 2.5 kW (3.4 hp) AC motor

Road chiller capacity 281 kW (80 Tons)

Air chiller capacity 440 kW (125 Tons)

The tunnel is a scale model rolling road tunnel that tests models of vehicles. The model is 

mounted to a balance cradle that houses a 6-component load cell balance. The balance cradle is 

attached to a sting that is attached to a steel beamed structure that is mounted on vibration pads 8 

feet underground. This mounting allows for the model to be completely isolated from any 

building noise. The model’s wheels run on top of our rolling belt that has a surface roughness to 

match the average surface roughness of highways and roads. The belt speed matches the wind 

speed to +/- 0.01 m/s. 

Within the balance cradle is mounted a vehicle model motion system. The VMMS allows the 

vehicle to automatically yaw, pitch, roll, heave and front wheel steer during a test session. To 

maintain the road flatness, a multi-suction port system is used throughout the platen. To 

eliminate any belt edge curl, a pneumatic tensioning system is used to apply constant tension to 

the belt during a test session. To eliminate the static electricity build up before it gets to the 

model and effects test repeatability, a static electricity discharge system is utilized.

To minimize the road boundary layer that builds up in wind tunnels, a four stage boundary layer 

suction system is utilized.
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8.0 TEST PROCEDURE

8.1 Pretest Inspection and Warm up

At the beginning of each testing session, the tunnel and its data acquisition systems are run for a 

minimum of 30 minutes to warm up the facility. The balance and laser systems are warmed up 

for a minimum of four hours prior to use. The model was inspected for any wheel bearing issues 

or suspect/damaged parts that could affect testing. The tunnel and road system were inspected. 

The yaw position, heave position and roll position were all measured and maintained.

8.2 General

Each test series began with a static weight tear of the model. Following the static weight tear, a 

rolling wheel tare was taken for each model position that data was to be recorded. The model 

position remained the same for all of the runs. The positions were standard road height at yaw 

equaling 0, 1, 3, 6, 9, 0, -1, -3, -6, -9, 0 degrees. There were three 0 degree points to allow for a 

“first to last” run match that is used to determine potential belt stretch, or in test model issue. 

Positive 9 degrees is yawing the truck 9 degrees to the driver’s left. All rolling wheel tears were 

completed with the belt running 50 m/s without any air speed.

Following the rolling wheel tares, the data sampling process took place for all 11 yaw angles 

listed above. After all model point’s data samples were measured, the tunnel was shut down and 

a change was made to the model. Following a model change, the general process would repeat 

itself.

9.0 DATA REDUCTION

All force and moment data measured using an AEROTECH 6 component load cell balance, 124 

static pressures and all associated tunnel parameters (ie DC motor information, belt temperature, 

wind speeds etc.) were recorded using a PI Mistral system. The data was organized through a 

program called PI AERO which utilizes an ACCESS database. PI AERO was used to display 

the reduced and raw data into Excel. Using Excel, the data was then summarized for reporting 

purposes.
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10.0 WHAT ARE THE DIFFERENCES BETWEEN ON-ROAD AND A WIND TUNNEL?

This is a question that is consistently asked by our new trucking industry partners before they utilize one 

of our wind tunnel facilities. It is a fair question that deserves a detailed answer. Currently, the trucking 

industry relies on real world testing where the standards for these tests were determined by SAE in the 

early 1980s. The standards were developed in an attempt to improve the accuracy of a real world test 

as much as possible. The SAE standards helped to focus the real world results, but still left large 

variances in accuracy due to the impossibility of limiting at least 10 other test critical factors. In a wind 

tunnel, we are able to eliminate these 10 variables and offer a more realistic and true aerodynamic 

representation with regards to trucking development. The following top 10 variables of concern with 

“real world” testing are as follows:

1) Constant Temperature: It is impossible to conduct “real world” testing at a constant 

temperature. Because of the temperature varying throughout this type of test, the 

aerodynamic forces of the vehicle being tested will change. The only way to compensate for 

variable temperature during real world testing is to actually measure and record the ambient 

temperature throughout the entire test using an on board data logging system or a chase 

vehicle with similar data logging. After the temperature is recorded, the varying temperature 

would have to be taken into account throughout all the sections of the test through reduction 

formulas which would integrate the results (varying with temperature) throughout the whole 

test. In the aerodynamic testing community it is common practice to reference all data to a 

standard temperature such as “sea level” temperature.

2) Constant Density: Along with varying temperature, varying density will occur throughout the 

entire “real world” test. The variable density also has a direct effect on the aerodynamic forces 

acting on a semi-truck through the relationship of D = Pd*Cd*A where the aerodynamic drag, D, 

on a vehicle is equal to the product of the dynamic pressure, Pd; the drag coefficient, Cd and the 

vehicle’s frontal area, A. The dynamic pressure is equal to one half the product of the density, d, 

and the square of the velocity, V as seen in the following equation: Pd = 1⁄2*d*V2. It is obvious 

because of this that with the density varying on the day or through a vehicle traveling across 

state lines, the forces on the truck during a specific test will definitely change and be 

inconsistent. The only way to eliminate this is to measure the density change locally (on the 

truck during the test at every moment it is actually testing) and then to compensate through a 

speed variation with the truck. This can only be done accurately through using a computer 

system to vary the speed continuously throughout the entire test.

3) Driver Input: The driver input will vary dramatically depending on many different factors. The 

only way to eliminate this variable is through elimination of the drive by using a computer 

system to maintain control of the truck.

4) Tire Grip/Degradation: Over a test, the tire will degrade due to wear and weight of the vehicle, 

along with speeds run during the test. The tire pressures will also change throughout the test 

which will have an effect on the sidewall stiffness of the tire and ultimately the aerodynamic 
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shape of the tire and an associated ride height and roll attitude of the truck itself. In a “real 

world” test, this is a variable that cannot be completely eliminated.

5) Engine: Selecting the size and type of engine will play a role of the fuel economy numbers 

based upon the engine’s efficiency and specific fuel consumption. Does the system use a fuel 

burn to clean the system? If so, what is the frequency and amount of fuel used? How accurate 

is the system? Does it dump the same amount of fuel exactly each time?

6) Cross Winds: Differential cross winds during “real world” testing are a fact of life. Even for 

straight line testing this presents inaccuracies. One way to eliminate this inaccuracy is to mount 

a 5 hole probe to the test vehicle. This probe will measure the wind direction at all times while 

the vehicle is testing. However, the data must then be used to correct the results over the 

whole test through integrating the change in cross wind in parallel with the measured fuel burn.

7) Proper Fuel Measurements: The accuracy of measuring the fuel put into the fuel tank versus 

what is left in the tank allows for inaccuracy. Was the tank truly “dry” of fuel to begin with? 

Was all the fuel removed from the tank in the end (ie fuel residue left in or on the tank due to 

viscosity or gaseous build up)? Were there any evaporation losses of the fuel?

8) Traffic: Traffic conditions will have an effect on the fuel burned during a test. Not only would 

traffic affect the driver’s input due to changes associated with the gas pedal; but, traffic can help 

or hurt the drag another vehicle sees based upon its positioning around this vehicle. Traffic is a 

random variable that can have a significant impact on fuel economy runs over the entire test.

9) Type of Fuel: The type of fuel used will also affect the fuel economy runs for “real world” 

testing. Not all diesel mixtures are the same. Also if pumps are used for specific fuel/water 

mixtures this will also offer variability. This can play a role throughout “real world” testing.

10) Static Weight Changes: Throughout a “real world” test, the static weight of a truck will change 

as it burns fuel and looses rubber from its tires. This overall reduction in static weight will 

reduce the amount of fuel required to move the truck over the road due to a reduction in 

mechanical weight. This variability happens throughout the entire test, but also requires an 

exact starting fuel amount for all the tests. However, all the other variables will play a role into 

how much fuel is burned and during what part of a test this will happen. Therefore, it is feasible 

that the static weight changes on the truck (even when one begins with the “exact” amount of 

fuel in the beginning) variably throughout the truck’s “real world” testing cycle.

Testing in a scale model, rolling road wind tunnel eliminates all 10 of these “real world” issues. All of our 

testing is temperature controlled to within 2 degrees Fahrenheit. The testing is all completed at a 

constant dynamic pressure. We use models and electronically control them so there is no driver input. 

The tires we use do not degrade and do not affect the aerodynamic data we record. Although we model 

an engine, the engine does not burn fuel so it does not influence the aerodynamic data we measure. 
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The wind generated in our tunnel is a very controlled wind and therefore has no cross wind. For specific 

crosswind testing, we yaw our model and scientifically control the angle of cross wind. Since we are 

measuring the raw aerodynamic forces acting on the test vehicle, there is no fuel involved in this type of 

testing and therefore, no ability for fuel measurement errors or different types of fuel errors. Our 

vehicles are tested individually which eliminates the variable of traffic influencing the aerodynamic 

outcome. The static weight of our test vehicles is taken out of the equation through our ability to “tear” 

out this weight before the test run.

Although ARC can eliminate all 10 of the main variables that directly influence the outcome of a “real 

world” test, ARC is still utilizing a scale model rolling road tunnel. The two areas of question in this 

arena are Reynolds and scaling effects. Reynolds number is a number that demonstrates the viscous 

effects on an object. For instance, if you take a truck running on the highway at 65 mph and scale it 

down to 50%, you would need to test this scale model at a speed greater than 65 mph to maintain the 

same kind of effects the air has on this vehicle. The number used to coordinate just how fast you need 

to be testing at is called the Reynolds number.

At ARC we test 1/8 scale models. Tests conducted by NASA on the Reynolds number necessary for an 

1/8 scale semi-truck being simulated as a full scale truck running at 65 mph shows that ARC’s tunnel 

testing speed of 50 m/s (111.8 mph) is within the acceptable Reynolds number range require for 

accurate comparisons to full scale trucks at 65 mph. 

The scaling concern covers two main areas. The first area is accuracy and the second is choking or 

separation. With regards to the accuracy of ARC’s modeling, our highly experienced staff grew from the 

racing industry. This meant that if our model makers made mistakes on a scale model during a test, 

their errors were magnified in the “real world” due to scaling. It also meant that the potential for these 

mistakes could literally kill a driver. Therefore, ARC established a culture based around accuracy. This 

culture is still driven with the view of accuracy means safety. Therefore, through our maintaining the 

same principles we have operated with from our racing heritage, our accuracy is second to none.

With regards to choking or separation due to scaling effects, imagine a small tube that has its opening 

directly facing the airflow. When scaling this tube down, the actual air molecules that would normally 

flow through this tube remain the same size. Therefore, it is possible to scale something down too small 

and not allow the air molecules (that are remaining the same size) to flow through this tube (or “choke.” 

Because of this, care must be taken in the areas where these types of flows are critical to scaling. Over 

the years “rules of thumb” are established to eliminate these issues with regards to scaling. ARC has the 

capabilities to eliminate these types of choking or separating effects on its models due to ARC’s 

collective experience of scale model testing.
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11.0 CONCLUSIONS

For a final analysis, we will examine trends associated with the wind averaged drag coefficient at 

65mph. The various wind averaged drag coefficients for other speeds can be found in 

APPENDIX A along with all individual run data.

Results Summary

Run# Description Cdw @ 

65mph %F (to baselines)

1 Baseline Configuration 0.629

2 Repeat 0.629

3 Repeat 0.631

4 Baseline – Static Floor 0.616 -1.26%

5 Closed Grill – Rolling Road 0.617 -1.22%

6 Closed Grill – Static Floor 0.605 -1.03%

7 Trailer Gap = 50"; 3" Trailer Corner Radii; Open Grill – Rolling Road 0.642 0.93%

8 Trailer Gap = 50"; 5" Trailer Corner Radii; Open Grill – Rolling Road 0.640 0.72%

9 Trailer Gap = 50"; 5" Trailer Corner Radii; Open Grill – Static Floor 0.628 1.01%

10 Closed Grill – Rolling Road 0.628 -0.25%

11 Closed Grill – Static Floor 0.616 -0.05%

12 Remove Chassis Fairings; Open Grill - Static Floor 0.633 1.44%

13 Remove Chassis Fairings; Open Grill - Rolling Road 0.648 1.43%

14 Trailer Gap = 45" - Rolling Road 0.642 0.92%

15 Trailer Gap = 45" - Static Floor 0.630 1.15%
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From the results listed above for 65 mph, all of the static floor versus rolling road changes are 

directionally consistent with this vehicle configuration. This vehicle configuration is a high drag 

level.

The above chart clearly shows not only similar trends between rolling road and static floor, but 

also close results. This is the opposite to the previous vehicle tested for the EPA. These results 

show that sometimes both rolling road and static floor can be consistent and other times they 

can’t be. It is results like this that cause people to take this consistency for granted and later run 

into problems.

The cooling chart above shows that cooling changes are more sensitive in rolling road with the 

45” trailer gap; however, with static floor, the cooling is more sensitive with the larger 50” trailer 

gap. These differences are due to the flow development over the distance from the tractor to the 

front of the trailer as well as the tire pumping effect caused by the rolling road. It seems that 

with the 45” trailer gap, the front of the trailer is still with in the wake effect of the tire pumping 

flow, but at 50” could be outside of this effect due to the similar differences at this trailer gap 

measure with static floor.
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Comparing the chassis fairings, one sees some interesting trends. Although both the rolling road 

and static floor results are in the same direction, we see a much larger effect going to the trailer 

gap of 45” with the static floor than the rolling road. In both cases the 45” trailer gap produces 

the bigger effect with chassis fairings. With the 45”trailer gap, the static floor over predicts this 

change, while with the 50” trailer gap, the static floor under predicts this change compared to the 

rolling road.

Looking at the next chart showing the changes to their respective baselines, one can see a fair 

amount of consistency between the rolling road and the static floor results. In two cases, the 

static floor is under predicting the rolling road. In another two changes, the static floor is over 

predicting the rolling road. In one case, the static floor is giving the same delta as the rolling 

road. This shows the inconsistencies between rolling road and static floor. One cannot predict a 

simple delta offset when testing in a static floor environment in an attempt to predict what 

happens in a rolling road environment.
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Another question raised by the EPA is if multiple yaw angles on both sides of zero yaw are really 

necessary. To examine this question, it is important to look at comparison runs instead of just 

individual runs. To keep things simple, we will limit our comparison runs to rolling road versus 

static floor for a few isolated changes. In a previous report to the EPA we did the same analysis 

on a different tractor/trailer combination with the same results.

The chart below shows the initial baseline configuration chosen by the EPA and reflected in our 

“model configuration” section. The first column is the yaw angle. A positive yaw angle is when 

the driver’s side is moved to the driver’s left (rear of trailer moved to the right). The “Rolling” 

column shows the measured Cd (not wind averaged CdW) with the rolling road. The “Static” 

column shows the measured Cd (not wind averaged CdW) with the static floor. The “Delta” 

column represents the rolling road Cd subtracted by the static floor Cd. The “Delta Refl % Diff” 

column represents the percentage difference of the “Delta” column in the reflective yaw angles. 

For example, the “Delta” column’s 9.0 yaw result has the “Delta” column’s (-9.0) yaw result 

subtracted and then this result is divided by the “Delta” column’s 9.0 yaw result. By finally 
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multiplying the answer by 100, we get the percentage difference between the two “Delta” 

column’s results for positive and negative 9.0 degree yaw cases. In this case, the result is 83.5%. 

The “Rolling Refl Diff” column shows the rolling reflective difference for the Cd value in the 

“Rolling” column. For positive and negative 9.0 yaw, we get a value of 0.038. The “Static Relf 

Diff” column shows the static floor reflective difference for the Cd value in the “Static” column. 

For positive and negative 9.0 degree yaw cases we see a value of 0.023.

Baseline Configuration: Trailer Gap = 45"; Trailer Corner Radii = 5"; Open Grill

Yaw 

AOA Rolling Static Delta Delta Refl % Diff Rolling Refl Diff

Static Refl 

Diff

0.000 0.562 0.549 0.013 1.0% 0.002 -0.001

1.000 0.572 0.558 0.014 6.7% 0.009 0.008

3.000 0.606 0.596 0.010 -26.1% 0.011 0.014

6.000 0.688 0.673 0.015 46.1% 0.016 0.009

9.000 0.798 0.780 0.018 83.5% 0.038 0.023

0.000 0.564 0.551 0.013

-1.000 0.563 0.550 0.013

-3.000 0.595 0.582 0.013

-6.000 0.672 0.664 0.008

-9.000 0.760 0.757 0.003

0.000 0.561 0.550 0.010

Taking a second example, in this case changing the trailer gap to 45” with a trailer radius of 5” 

and closing the grill, we find a large variance for 6.0 and 9.0 degree yaw cases when comparing 

the positive and negative yaw results from the “Delta” column. It should also be noted that the 

reflective differences in the respective rolling road and static floor runs shows a 13 count or 

greater drag difference. Since the tunnel’s repeatability is well below this at around 2 – 3 counts, 

these results are significant.

Baseline Configuration: Trailer Gap = 45"; Trailer Corner Radii = 5"; Closed Grill

Yaw 

AOA Rolling Static Delta Delta Refl % Diff Rolling Refl Diff

Static Refl 

Diff

0.000 0.550 0.538 0.012 15.3% 0.000 -0.002

1.000 0.556 0.545 0.010 -5.0% 0.002 0.002

3.000 0.591 0.582 0.010 6.5% 0.010 0.010

6.000 0.678 0.663 0.014 72.3% 0.023 0.013

9.000 0.783 0.768 0.015 60.8% 0.027 0.018

0.000 0.550 0.540 0.010

-1.000 0.554 0.543 0.011

-3.000 0.581 0.572 0.009

-6.000 0.654 0.650 0.004

-9.000 0.756 0.750 0.006

0.000 0.549 0.540 0.010
4012 Championship Drive, Indianapolis, Indiana 46268; Tel: 317-291-8600; Fax: 317 – 291-8700 Pg. 20

Examining a third case, where we have a 50”trailer gap with a 5” trailer corner radii and an open 

grill, we still see a similar trend for 6.0 and 9.0 degree yaw cases. The drag count changes 

between the respective yaw angles are, as large as 58 counts, which is very large. If one was 

only using half of the acceptable yaw range, they would really misunderstand this change.

Trailer Gap = 50"; 5" Trailer Corner Radii; Open Grill

Yaw 

AOA Rolling Static Delta Delta Refl % Diff

Rolling Refl 

Diff

Static Refl 

Diff

0.000 0.561 0.549 0.012 29.9% 0.001 -0.002

1.000 0.569 0.557 0.012 -10.0% 0.004 0.005

3.000 0.610 0.601 0.009 2.8% 0.013 0.013

6.000 0.697 0.681 0.016 55.4% 0.027 0.018

9.000 0.841 0.831 0.010 23.3% 0.058 0.056

0.000 0.560 0.551 0.008

-1.000 0.566 0.552 0.013

-3.000 0.597 0.589 0.009

-6.000 0.670 0.663 0.007

-9.000 0.783 0.775 0.008

0.000 0.560 0.552 0.008

A fourth case; 

Trailer Gap = 50"; 5" Trailer Corner Radii; Closed Grill

Yaw 

AOA Rolling Static Delta Delta Refl % Diff

Rolling Refl 

Diff

Static Refl 

Diff

0.000 0.552 0.538 0.014 15.9% 0.001 -0.003

1.000 0.556 0.547 0.009 -49.4% 0.000 0.004

3.000 0.596 0.586 0.011 55.6% 0.015 0.010

6.000 0.691 0.674 0.017 62.9% 0.038 0.027

9.000 0.831 0.815 0.016 31.2% 0.052 0.047

0.000 0.553 0.541 0.012

-1.000 0.556 0.543 0.013

-3.000 0.581 0.576 0.005

-6.000 0.653 0.647 0.006

-9.000 0.778 0.768 0.011

0.000 0.551 0.541 0.010
4012 Championship Drive, Indianapolis, Indiana 46268; Tel: 317-291-8600; Fax: 317 – 291-8700 Pg. 21

A fifth case;

Remove Chassi Fairings; Open Grill 

Yaw 

AOA Rolling Static Delta Delta Refl % Diff

Rolling Refl 

Diff

Static Refl 

Diff

0.000 0.570 0.559 0.011 44.0% 0.003 0.001

1.000 0.577 0.566 0.011 -5.6% 0.004 0.005

3.000 0.617 0.609 0.007 -22.8% 0.004 0.005

6.000 0.715 0.699 0.016 25.8% 0.022 0.018

9.000 0.863 0.824 0.039 50.0% 0.024 0.005

0.000 0.565 0.559 0.006

-1.000 0.572 0.561 0.011

-3.000 0.613 0.604 0.009

-6.000 0.692 0.680 0.012

-9.000 0.839 0.819 0.019

0.000 0.567 0.558 0.009

Looking at these five examples, we find similar trends. These combined examples illustrate the 

change in forces caused by the asymmetry of a tractor/trailer. For each tractor/trailer these 

asymmetries are different and therefore cause different variances on the measure force values at 

the corresponding positive and negative yaw angles. This is the reasoning behind the SAE 

standard requiring the mapping of both yaw directions. There really is no short cut for tunnel 

testing, track testing or CFD.
4012 Championship Drive, Indianapolis, Indiana 46268; Tel: 317-291-8600; Fax: 317 – 291-8700 Pg. 22

APPENDIX A

Individual Run Pictures if Approved

&

Run Data
4012 Championship Drive, Indianapolis, Indiana 46268; Tel: 317-291-8600; Fax: 317 – 291-8700 Pg. 23

FUEL ECONOMY DATA FOLLOWS

MPG = Miles Per Hour

CDW = Wind Average Drag

Del CDw = Difference to Comparison CDW

%CDw = Percentage Difference to Comparision CDW

%F = Percentage Fuel Economy
4012 Championship Drive, Indianapolis, Indiana 46268; Tel: 317-291-8600; Fax: 317 – 291-8700 Pg. 24

Run # Description MPH CDW

Run 1 Baseline Configuration 75 0.614

70 0.621

65 0.629

60 0.640

55 0.654

50 0.671

Compared To Previous Run

Run # Description MPH CDW

del 

CDw % CDw %F

Run 2 Repeat 75 0.613 -0.001 -0.15% -0.10%

70 0.620 -0.001 -0.13% -0.07%

65 0.629 -0.001 -0.11% -0.06%

60 0.639 -0.001 -0.08% -0.04%

55 0.653 0.000 -0.05% -0.02%

50 0.671 0.000 0.00% 0.00%

Compared To Previous Run

Run # Description MPH CDW

del 

CDw % CDw %F

Run 3 Repeat 75 0.615 0.002 0.37% 0.25%

70 0.622 0.002 0.39% 0.22%

65 0.631 0.002 0.40% 0.21%

60 0.642 0.003 0.41% 0.20%

55 0.656 0.003 0.43% 0.20%

50 0.674 0.003 0.44% 0.19%
4012 Championship Drive, Indianapolis, Indiana 46268; Tel: 317-291-8600; Fax: 317 – 291-8700 Pg. 25

Compared To Previous Run

Run # Description MPH CDW del CDw % CDw %F

Run 4 Static Road 75 0.601 -0.014 -2.34% -1.56%

70 0.608 -0.015 -2.34% -1.34%

65 0.616 -0.015 -2.33% -1.26%

60 0.627 -0.015 -2.31% -1.16%

55 0.641 -0.015 -2.29% -1.07%

50 0.659 -0.015 -2.26% -0.98%

Compared To Run #3 - Baseline

Run # Description MPH CDW del CDw % CDw %F

Run 5

Closed Grill - Road 

Road 75 0.601 -0.014 -2.31% -1.54%

70 0.608 -0.014 -2.28% -1.31%

65 0.617 -0.014 -2.25% -1.22%

60 0.628 -0.014 -2.21% -1.10%

55 0.642 -0.014 -2.16% -1.00%

50 0.660 -0.014 -2.09% -0.91%

Compared To Run #4 - Baseline

Run # Description MPH CDW del CDw % CDw %F

Run 6

Closed Grill - Static 

Road 75 0.589 -0.012 -1.94% -1.29%

70 0.596 -0.012 -1.92% -1.10%

65 0.605 -0.012 -1.90% -1.03%

60 0.615 -0.012 -1.88% -0.94%

55 0.629 -0.012 -1.85% -0.86%

50 0.647 -0.012 -1.80% -0.78%
4012 Championship Drive, Indianapolis, Indiana 46268; Tel: 317-291-8600; Fax: 317 – 291-8700 Pg. 26

Compared To Run #3 - Baseline

Run # Description MPH CDW del CDw % CDw %F

Run 7

Open Grill; Trailer Gap = 

50" 75 0.624 0.008 1.36% 0.91%

3" Trailer Corner Radii 70 0.632 0.009 1.52% 0.87%

65 0.642 0.011 1.72% 0.93%

60 0.655 0.013 1.95% 0.97%

55 0.671 0.015 2.23% 1.04%

50 0.692 0.017 2.58% 1.12%

Compared To Run #3 - Baseline

Run # Description MPH CDW del CDw % CDw %F

Run 8 Trailer Corner Radii = 5" 75 0.621 0.006 0.98% 0.65%

70 0.629 0.007 1.14% 0.65%

65 0.640 0.008 1.34% 0.72%

60 0.652 0.010 1.57% 0.79%

55 0.668 0.012 1.85% 0.86%

50 0.689 0.015 2.21% 0.96%

Compared To Run #4 - Baseline

Run # Description MPH CDW del CDw % CDw %F

Run 9 Static Road 75 0.610 0.009 1.50% 1.00%

70 0.618 0.010 1.67% 0.95%

65 0.628 0.012 1.87% 1.01%

60 0.640 0.013 2.11% 1.05%

55 0.656 0.015 2.40% 1.12%

50 0.677 0.018 2.76% 1.20%
4012 Championship Drive, Indianapolis, Indiana 46268; Tel: 317-291-8600; Fax: 317 – 291-8700 Pg. 27

Compared To Run #3 - Baseline

Run # Description MPH CDW del CDw % CDw %F

Run 10 Close Grill - Rolling Road 75 0.610 -0.006 -0.91% -0.60%

70 0.618 -0.004 -0.71% -0.40%

65 0.628 -0.003 -0.47% -0.25%

60 0.641 -0.001 -0.18% -0.09%

55 0.657 0.001 0.18% 0.08%

50 0.679 0.004 0.61% 0.27%

Compared To Run #4 - Baseline

Run # Description MPH CDW del CDw % CDw %F

Run 11 Close Grill - Static Road 75 0.598 -0.003 -0.51% -0.34%

70 0.606 -0.002 -0.32% -0.18%

65 0.616 -0.001 -0.10% -0.05%

60 0.628 0.001 0.16% 0.08%

55 0.644 0.003 0.49% 0.23%

50 0.665 0.006 0.88% 0.38%

Compared To Run #4 - Baseline

Run # Description MPH CDW del CDw % CDw %F

Run 12 Remove Chassis Fairings; 75 0.615 0.014 2.36% 1.57%

Open Grill - Static Road 70 0.623 0.015 2.49% 1.42%

65 0.633 0.016 2.66% 1.44%

60 0.645 0.018 2.86% 1.43%

55 0.661 0.020 3.11% 1.45%

50 0.682 0.023 3.43% 1.49%
4012 Championship Drive, Indianapolis, Indiana 46268; Tel: 317-291-8600; Fax: 317 – 291-8700 Pg. 28

Compared To Run #3 - Baseline

Run # Description MPH CDW del CDw % CDw %F

Run 13 Remove Chassis Fairings; 75 0.628 0.013 2.14% 1.43%

Open Grill - Rolling Road 70 0.637 0.015 2.37% 1.35%

65 0.648 0.017 2.65% 1.43%

60 0.661 0.019 2.99% 1.50%

55 0.679 0.022 3.42% 1.59%

50 0.701 0.027 3.97% 1.73%

Compared To Run #3 - Baseline

Run # Description MPH CDW del CDw % CDw %F

Run 14 Trailer Gap = 45" 75 0.625 0.009 1.52% 1.01%

Rolling Road 70 0.632 0.010 1.60% 0.91%

65 0.642 0.011 1.70% 0.92%

60 0.654 0.012 1.82% 0.91%

55 0.669 0.013 1.99% 0.93%

50 0.689 0.015 2.21% 0.96%

Compared To Run #4 - Baseline

Run # Description MPH CDW del CDw % CDw %F

Run 15 Trailer Gap = 45" 75 0.613 0.012 1.96% 1.30%

Static Road 70 0.620 0.012 2.03% 1.16%

65 0.630 0.013 2.13% 1.15%

60 0.641 0.014 2.24% 1.12%

55 0.656 0.015 2.40% 1.11%

50 0.676 0.017 2.59% 1.13%