This section describes a methodology for quantifying aerodynamic drag for use in determining input values for tractors as described in §1037.520.

(a) General provisions. The GEM input for a tractor's aerodynamic performance is a Cd value for Phase 1 and a CdA value for Phase 2. The input value is measured or calculated for a tractor in a specific test configuration with a trailer, such as a high-roof tractor with a box van meeting the requirements for the standard trailer.

(1) Aerodynamic measurements may involve any of several different procedures. Measuring with different procedures introduces variability, so we identify the coastdown method in §1037.528 as the primary (or reference) procedure. You may use other procedures with our advance approval as described in paragraph (d) of this section, but we require that you adjust your test results from other test methods to correlate with coastdown test results. All adjustments must be consistent with good engineering judgment. Submit information describing how you quantify aerodynamic drag from coastdown testing, whether or not you use an alternate method.

(2) Test high-roof tractors with a standard trailer as described in §1037.501(g)(1). Note that the standard trailer for Phase 1 tractors is different from that of later model years. Note also that GEM may model a different configuration than the test configuration, but accounts for this internally. Test low-roof and mid-roof tractors without a trailer; however, you may test low-roof and mid-roof tractors with a trailer to evaluate off-cycle technologies.

(b) Adjustments to correlate with coastdown testing. Adjust aerodynamic drag values from alternate methods to be equivalent to the corresponding values from coastdown measurements as follows:

(1) Determine the functional relationship between your alternate method and coastdown testing. Unless good engineering judgment dictates otherwise, assume that coastdown drag is proportional to drag measured using alternate methods. This means you may apply a constant adjustment factor, Falt-aero, for a given alternate drag measurement method using the following equation, where the effective yaw angle, ψeff, is assumed to be zero degrees for Phase 1 and is determined from coastdown test results for Phase 2:

(2) Determine Falt-aero by performing coastdown testing and applying your alternate method on the same vehicles. Consider all applicable test data including data collected during selective enforcement audits. Where you have test results from multiple vehicles expected to have the same Falt-aero, you may either average the Falt-aero values or select any greater value. Unless we approve another vehicle, one vehicle must be a Class 8 high-roof sleeper cab with a full aerodynamics package pulling a standard trailer. Where you have more than one tractor model meeting these criteria, use the tractor model with the highest projected sales. If you do not have such a tractor model, you may use your most comparable tractor model with our prior approval. In the case of alternate methods other than those specified in this subpart, good engineering judgment may require you to determine your adjustment factor based on results from more than the specified minimum number of vehicles.

(3) Measure the drag area using your alternate method for a Phase 2 tractor used to determine Falt-aero with testing at yaw angles of 0°, ±1°, ±3°, ±4.5°, ±6°, and ±9° (you may include additional angles), using direction conventions described in Figure 2 of SAE J1252 (incorporated by reference in §1037.810). Also, determine the drag area at the coastdown effective yaw angle, CdAeffective-yaw-alt, by taking the average drag area at ψeff and −ψeff for your vehicle using the same alternate method.

(4) For Phase 2 testing, determine separate values of Falt-aero for a minimum of one high-roof day cab and one high-roof sleeper cab for 2021, 2024, and 2027 model years based on testing as described in paragraph (b)(2) of this section (six tests total). For any untested tractor models, apply the value of Falt-aero from the tested tractor model that best represents the aerodynamic characteristics of the untested tractor model, consistent with good engineering judgment. Testing under this paragraph (b)(4) continues to be valid for later model years until you change the tractor model in a way that causes the test results to no longer represent production vehicles. You must also determine unique values of Falt-aero for low-roof and mid-roof tractors if you determine CdA values based on low or mid-roof tractor testing as shown in Table 4 of §1037.520. For Phase 1 testing, if good engineering judgment allows it, you may calculate a single, constant value of Falt-aero for your whole product line by dividing the coastdown drag area, CdAcoastdown, by CdAalt.

(5) Determine Falt-aero to at least three decimal places. For example, if your coastdown testing results in a drag area of 6.430, but your wind tunnel method results in a drag area of 6.200, Falt-aero would be 1.037 (or a higher value you declare).

(6) If a tractor and trailer cannot be configured to meet the gap requirements, test with the trailer positioned as close as possible to the specified gap dimension and use good engineering judgment to correct the results to be equivalent to a test configuration meeting the specified gap dimension.

(c) Yaw sweep corrections. Aerodynamic features can have a different effectiveness for reducing wind-averaged drag than is predicted by zero-yaw drag. The following procedures describe how to determine a tractor's CdA values to account for wind-averaged drag and differences from coastdown testing:

(1) For Phase 2 testing with an alternate method, apply the following method using your alternate method for aerodynamic testing:

(i) For all testing, calculate the wind-averaged drag area from the alternate method, CdAwa-alt, using an average of measurements at −4.5 and +4.5 degrees.

(ii) Determine your wind-averaged drag area, CdAwa, rounded to one decimal place, using the following equation:

(2) For Phase 2 coastdown test results, apply the following method:

(i) For all coastdown testing, determine your effective yaw angle from coastdown, CdAeffective-yaw-coastdown.

(ii) Use an alternate method to calculate the ratio of the wind-averaged drag area (using an average of measurements at −4.5 and +4.5 degrees, CdAwa-alt) to the drag area at the effective yaw angle, CdAeffective-yaw.

(iii) Determine your wind-averaged drag area, CdAwa, rounded to one decimal place, using the following equation:

(3) Different approximations apply for Phase 1. For Phase 1 testing, you may correct your zero-yaw drag area as follows if the ratio of the zero-yaw drag area divided by yaw-sweep drag area for your vehicle is greater than 0.8065 (which represents the ratio expected for a typical Class 8 high-roof sleeper cab):

(i) Determine the zero-yaw drag area, CdAzero-yaw, and the yaw-sweep drag area for your vehicle using the same alternate method as specified in this subpart. Measure the drag area for 0°, −6°, and +6°. Use the arithmetic mean of the −6° and +6° drag areas as the ±6° drag area, CdA±6.

(ii) Calculate your yaw-sweep correction factor, CFys, using the following equation:

(iii) Calculate your corrected drag area for determining the aerodynamic bin by multiplying the measured zero-yaw drag area by CFys, as determined using Eq. 1037.525-4, as applicable. You may apply the correction factor to drag areas measured using other procedures. For example, apply CFys to drag areas measured using the coastdown method. If you use an alternate method, apply an alternate correction, Falt-aero, and calculate the final drag area using the following equation:

(iv) You may ask us to apply CFys to similar vehicles incorporating the same design features.

(v) As an alternative, you may calculate the wind-averaged drag area according to SAE J1252 (incorporated by reference in §1037.810) and substitute this value into Eq. 1037.525-4 for the ±6° drag area.

(d) Approval of alternate methods. You must obtain preliminary approval before using any method other than coastdown testing to quantify aerodynamic drag. We will approve your request if you show that your procedures produce data that are the same as or better than coastdown testing with respect to repeatability and unbiased correlation. Note that the correlation is not considered to be biased if there is a bias before correction, but you remove the bias using Falt-aero. Send your request for approval to the Designated Compliance Officer. Keep records of the information specified in this paragraph (d). Unless we specify otherwise, include this information with your request. You must provide any information we require to evaluate whether you may apply the provisions of this section. Include additional information related to your alternate method as described in §§1037.530 through 1037.534. If you use a method other than those specified in this subpart, include all the following information, as applicable:

(1) Official name/title of the procedure.

(2) Description of the procedure.

(3) Cited sources for any standardized procedures that the method is based on.

(4) Description and rationale for any modifications/deviations from the standardized procedures.

(5) Data comparing the procedure to the coastdown reference procedure.

(6) Additional information specified for the alternate methods described in §§1037.530 through 1037.534 as applicable to this method (e.g., source location/address, background/history).