This section describes a methodology for quantifying aerodynamic drag for use in determining input values for tractors as described in § 1037.520. This coastdown testing is the reference method for aerodynamic measurements.

(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 and later. 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. Specify this functional relationship as Falt-aero for a given alternate drag measurement method. The effective yaw angle, Ceff, is assumed to be zero degrees for Phase 1. For Phase 2 and later, determine Ceff from coastdown test results using the following equation:

(2) Unless good engineering judgment dictates otherwise, assume that coastdown drag is proportional to drag measured using alternate methods and apply a constant adjustment factor, Falt-aero, for a given alternate drag measurement method of similar vehicles.

(3) 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. 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.

(4) Measure the drag area using your alternate method for a Phase 2 and later 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, see § 1037.810). Also, determine the drag area at the coastdown effective yaw angle, CdAalt(Ceff), by taking the average drag area at Ceff and−Ceff for your vehicle using the same alternate method.

(5) For Phase 2 and later testing, determine separate values of Falt-aero for at least one high-roof day cab and one high-roof sleeper cab for model year 2021, at least two high-roof day cabs and two high-roof sleeper cabs for model year 2024, and at least three high-roof day cabs and three high-roof sleeper cabs for model year 2027. These test requirements are cumulative; for example, you may meet these requirements by testing two vehicles to support model year 2021 certification and four additional vehicles to support model year 2023 certification. 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)(5) 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 § 1037.520(b)(3)(ii). 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 drag area from your alternate method, CdAalt.

(6) 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).

(7) If a tractor and trailer cannot be configured to meet the gap requirements specified in § 1037.501(g)(1)(ii), 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. For example, we may allow you to correct your test output using an approved alternate method or substitute a test vehicle that is capable of meeting the required specifications and is otherwise aerodynamically equivalent. The allowance in this paragraph (b)(7) applies for certification, confirmatory testing, SEA, and all other testing to demonstrate compliance with standards.

(8) You may ask us for preliminary approval of your coastdown testing under § 1037.210. We may witness the testing.

(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 as specified in § 1037.520:

(1) Apply the following method for all Phase 2 and later testing with an alternate method:

(i) 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) Apply the following method for Phase 2 and later coastdown testing other than coastdown testing used to establish Falt-aero:

(i) Determine your drag area at the effective yaw angle from coastdown, CdAcoastdown(ψeff).

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

(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, see § 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).