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June 21, 2011

Proposed GHG/FE Rule

Aerodynamic Certification Issues

I. Reference Method

A. An aerodynamic drag “reference method” should be used to correlate 

manufacturers’ individual methods. Manufacturers would test vehicles using the 

reference method and compare the Cd results to the results from their own 

methods.

B. Coastdown appears to be an acceptable “reference method” for the near term. 

However, the results from coastdown testing of heavy-duty vehicles are 

significantly affected by wind conditions, track conditions, and tire rolling 

resistance. The existing SAE Recommended Practices for coastdown testing (i.e.,

J1263 and J2263) do not sufficiently address those issues for heavy-duty vehicles. 

Accordingly, EMA/TMA is supporting SAE in developing a new procedure for 

heavy-duty vehicles. In the meantime, there must be allowances in the rule to 

minimize and/or account for those influences. The following enhancements are

warranted to the coastdown procedures proposed in Part 1066, Subpart C:

1. Average and cross wind speeds ≤2.0 mph. Even wind conditions within 

the specifications of section 7.3 of SAE J1263 (i.e., average speeds no 

more than 10 mph (or peaks no more than 12.3 mph) and cross winds no 

more than 5.0 mph) may significantly influence the measured Cd.

2. No anemometer may be mounted to the tractor; relative wind speed should 

be measured with one or more track-mounted anemometers. If an 

anemometer is mounted to the vehicle, it must be mounted to the trailer to 

avoid affecting air flow around the tractor.

3. Test track or road must be dry, clean, straight, smooth, hard-surfaced, not 

have excessive crown, and have a consistent grade of no more than 0.02%. 

For surfaces that are not level, the force contribution due to gravity must 

be determined per section 11.5 of SAE J2263. 

4. Tractor tires

a. Size: 295/75R22.5 or 275/80R22.5

b. CRR ≤6.0 kg/metric ton 

c. Broken in per section 8.1 of SAE J1263

d. Pressure per section 8.5 of SAE J1263

e. No uneven wear

f. No re-treads

5. To achieve consistent results, the assumptions used in the aerodynamic 

drag calculations should be standardized (e.g., using specified software).
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6. Test from 70 mph

7. Unloaded “standard trailer”

8. Drivetrain rotational inertia = 125 lbs. per tire

9. Data is acceptable if within two standard deviations of the average of runs 

in the same direction.

C. The “reference method” should be used to make adjustments to the Cd bin values 

proposed in §1037.510. Those adjustments include overall bin Cd values and day

cab values relative to sleeper cab values.

1. Based on six comparisons, day cabs are 2.5% to 13.1% worse than the 

same model in a sleeper configuration. The average is 8.5% worse.

2. Cd values for day cab bins should be 8.5% higher than for sleeper cabs.

D. Data is needed for analysis of the proposed coastdown procedure and 

readjustment of bins as appropriate.

II. Correlation Factor

A. Each manufacturer tests a high-roof “golden tractor” using the “reference 

method” and establish a “correlation factor” (or, “CF”) for each of their own 

methods. For example, the “correlation factor” for a manufacturer’s CFD 

analysis is:

CFCFD = Cdref method golden tractor / CdCFD golden tractor

B. The manufacturer calculates the Cd for determining the applicable bin category

for a particular tractor (per §1037.520(b)(2)) using the results of the 

manufacturer’s CFD analysis times the “CFD correlation factor” as follows:

Cd bin = CdCFD * CFCFD

III. Standard Trailer

A. To ensure that all certification is done with trailers that are identical in all aspects 

that affect aerodynamics, EPA should revise the definition of “standard high-roof 

trailer” proposed in §1037.501(g) as follows:

1. Gap (back of cab to front of trailer): 44" ±2.0". If only a smaller gap is 

achievable, use the largest gap achievable.

2. Overall height (from top of trailer to ground on landing gear, in level 

condition): 13' 6" +0.0" -1.0"

3. Front height (from top of trailer to fifth wheel plate): 115" ±1.0"

4. Length: 53' ±1.0"

5. Width: 102" ±0.5:"

6. Front corners: 5.5" radius ±0.5"

7. King pin (from front of trailer to king pin center line): 36"±.5"
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8. Landing gear (front of trailer to centerline of landing gear): 105" ±4.0"

9. Rear axles (axle centerline to rear of trailer): 146" ±3.0"

10. Underride guard per 49 C.F.R. §393.86

11. Tires

a. Size: 295/75R22.5 or 275/80R22.5

b. CRR<5.1 kg/metric ton (In addition, the CRR for trailer tires in GEM 

should be updated to 5.0 kg/metric ton.)

c. Broken in per section 8.1 of SAE J1263

d. Pressure per section 8.5 of SAE J1263

e. No uneven wear

f. No re-treads

B. The rule should not include “standard trailers” for low- and mid-roof tractors.

IV. Wind Averaged Drag

A. The rule should recognize wind-averaged aerodynamic drag performance.

B. Allow manufacturers to adjust emissions levels based on a vehicle’s windaveraged drag performance compared to an industry baseline”

1. Use a ±6° yaw to establish an industry baseline 6°/0° Cd ratio

a. Preliminary industry results indicate a baseline ratio of 1.20

b. Additional data made be needed

2. Using GEM, establish the emissions of the subject tractor using its Cd 

times the industry baseline ratio.

3. Establish the emissions of the subject vehicle using its ±6° yaw Cd.

4. Use the ratio of the two emissions levels above to adjust its regular 

emissions (i.e., its GEM output using 0° yaw Cd)

V. Low- and Mid-Roof Tractors

A. Due to unpredictable air flow, lower VMT, lower sales volumes, and more weight 

sensitive and more on/off road applications, manufacturers place a lower priority 

on low- and mid-roof tractor aerodynamics than they do on high-roof tractors.

1. To simplify certification of low- and mid-roof tractors, a manufacturer 

should be allowed to “carry-over” the performance of the high-roof 

version of the same model. Tractors would be considered the same model 

as long as the tractors have the same bumper, hood, cab and sleeper (if 

applicable).

2. There should be only two bins for low- and mid-roof tractors: “Classic” 

and “Smartway.” If the high-roof version of a model is Conventional, 

Smartway, Advanced Smartway, or Advanced Smartway II, the low- and 

mid-roof versions would be in the “Smartway” bin. If the high-roof 

version of a model is Classic, the low- and mid-roof versions would be in 

the “Classic” bin.
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B. Alternatively, a manufacturer may measure and establish a Cd in the same manner 

as high-roof tractors to determine if the low- or mid-roof tractor would fall in the 

“Classic” bin or the “Smartway” bin. To do so, the manufacturer would test or 

analyze the tractor in its bobtail configuration.

VI. Enforcement

A. If one of the Agencies audits a manufacturer’s Cd determination, they may assign 

a new bin and require that the manufacturer make up any credit deficit and change 

the bin category of the tractor for the next model year.

1. The Agencies may test using either the “reference method” or the 

manufacturer’s method.

2. If the average or cross winds exceed 2.0 mph, the results must be 

corrected to account for wind influences.

3. Manufacturers should be afforded a +/- one bin “compliance margin” (i.e.,

a tractor would have to be off by more than one bin).

B. If one of the Agencies determines that the tractor must be moved to different bin, 

only “identical” tractors would be affected. “Identical” tractors must be the same 

in all aspects that affect aerodynamics.

VII. Aerodynamic Descriptions

A. The rule should not include the requirement in §1037.520(b)(4) for a 

manufacturer to obtain approval from EPA to move a tractor to a higher bin than 

its bin determined by the vehicle design characteristics.

B. The rule should not include §1037.141.

VIII. Frontal Area

A. The rule should use the tractor’s actual frontal area for determining Cd. 

B. The ranges in Table 2 in §1037.520 should be specified in CdA.

IX. Drag Coefficient Equation

A. Revise the proposed equation in §1037.520(b)(i)(A) to read as follows:

CDA = D/(1/2)ρ

B. ρ = actual air density (so that D does not need to be corrected)

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