# Title 40

## SECTION 1037.525

### 1037.525 Aerodynamic measurements for tractors.

§ 1037.525 Aerodynamic measurements for tractors.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 *C*d value for Phase 1 and a
*C*d*A* 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, *F*alt-aero, for a given alternate
drag measurement method using the following equation, where the
effective yaw angle, ceff, is assumed to be zero degrees for Phase
1 and is determined from coastdown test results for Phase 2:

(2) Determine *F*alt-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 *F*alt-aero, you may either
average the *F*alt-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 *F*alt-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,
*C*d*A*effective-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
*F*alt-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
*F*alt-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 *F*alt-aero for low-roof and mid-roof
tractors if you determine *C*d*A* 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 *F*alt-aero for your
whole product line by dividing the coastdown drag area,
*C*d*A*coastdown, by *C*d*A*alt.

(5) Determine *F*alt-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,
*F*alt-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 *C*d*A* 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, *C*d*A*wa-alt, using an average of
measurements at −4.5 and +4.5 degrees.

(ii) Determine your wind-averaged drag area,
*C*d*A*wa, 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, *C*d*A*effective-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, *C*d*A*wa-alt) to the drag area at the
effective yaw angle, *C*d*A*effective-yaw.

(iii) Determine your wind-averaged drag area,
*C*d*A*wa, 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, *C*d*A*zero-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, *C*d*A*±6.

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

(iii) Calculate your corrected drag area for determining the
aerodynamic bin by multiplying the measured zero-yaw drag area by
*CF*ys, 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 *CF*ys to drag areas measured
using the coastdown method. If you use an alternate method, apply
an alternate correction, *F*alt-aero, and calculate the final
drag area using the following equation:

(iv) You may ask us to apply *CF*ys 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 *F*alt-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).