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Federal Register / Vol. 86, No. 148 / Thursday, August 5, 2021 / Proposed Rules
khammond on DSKJM1Z7X2PROD with PROPOSALS

straight line on snowand ice-covered surfaces. Tires that meet the definition of snow tires are subject to less stringent performance test requirements compared to other tires subject to FMVSS No. 139.6
The SRTT is also used as part of the Uniform Tire Quality Grading Standards UTQGS, an information program to assist consumers in making informed decisions when purchasing tires. The UTQGS apply to passenger car tires and require motor vehicle and tire manufacturers and tire brand name owners to provide consumers with information about their tires relative performance regarding treadwear, traction, and temperature resistance.
The 14-inch SRTT is used as part of the determination of a tires UTQG
treadwear rating. As part of the UTQG
test procedures, treadwear is measured by running the tires being tested called candidate tires in convoys over a 400mile course of public roads near San Angelo, Texas. The performance of tires over this course can change daily due to variability in the road surface, temperature, humidity, and precipitation. To compensate for changes in condition of the test course, candidate tires are tested concurrently with course monitoring tires CMTs.
NHTSA has used the 14-inch SRTT as the exclusive CMT since 1991. CMTs must be not more than one year old at the time of commencement of the test and must be used within two months from being removed from storage in order to prevent variability resulting from aging of the CMT. The performance of the CMT is used to determine the base course wear rate BCWR by running four-vehicle convoys equipped with 16 CMTs for 6,400 miles over the test course four times per year. 7 The wear rate of the CMT over the prior four quarterly CMT
test runs are averaged to calculate the BCWR, which is published in Docket No. NHTSA20019395. The BCWR is used to determine a course severity adjustment factor, which is applied to the comparison between the candidate tires and CMTs to determine a tires rating.
II. Proposal To Replace 14-Inch SRTT
With 16-Inch SRTT
This proposal would amend NHTSAs safety standards and regulations to no longer reference the 14-inch SRTT.
Because of technological advancements in the development of tires and the general trend of increasing rim diameter sizes since the 1980s, the size and 6 See 7 See
71 FR 877, 880 Jan. 6, 2006.
65 FR 33481 May 24, 2000.

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materials of the 14-inch SRTT are no longer representative of modern tires sold in the U.S. Further, Michelin has ceased production of the 14-inch SRTT
because it has become difficult for Michelin to obtain the materials necessary to manufacture the SRTT.8
Thus, NHTSA seeks to reference a different standard reference test tire in the agencys safety standards and regulations and to transition seamlessly to the new tire in the agencys compliance and consumer information test programs.
ASTM International has developed an updated specification for an SRTT
designated F2493 16-inch SRTT. The 16-inch SRTT is size P225/60R16. The 16-inch SRTT is considered to be more representative of current tires because of its larger size and new material and design features that lead to traction that is more typical of modern passenger car tires.9 To the best of NHTSAs knowledge, the 16-inch SRTT is manufactured only by Michelin and sold under its Uniroyal brand.
To reference an SRTT that is more representative of tires on the road today, and in consideration of Michelins decision to cease production of the 14inch SRTT, NHTSA has determined that replacing the 14-inch SRTT in its regulations is warranted. The only suitable replacement for the 14-inch SRTT that has been suggested to NHTSA is the 16-inch SRTT. However, because the 16-inch SRTT is a larger size and uses more modern design and materials, it is likely that the 16-inch SRTT will not perform identically to the 14-inch SRTT. Therefore, NHTSA has been cooperating with Transport Canada, Natural Resources Canada, representatives of ASTM International committees F09 on tires and E17 on vehicle-pavement systems, the U.S. Tire Manufacturers Association including Michelin, currently the sole manufacturer of SRTTs, and the Rubber Association of Canada to conduct testing to determine the consequences of replacing the 14-inch SRTT with the 16inch SRTT. The results of the testing by these entities, in addition to NHTSAs own testing, have substantially contributed to this proposal to replace 8 See Discontinued Tire Will Lead to ASTM
Standard Changes July 30, 2015, available at https www.astm.org/cms/drupal-7.51/newsroom/
discontinued-tire-will-lead-astm-standard-changes last accessed April 13, 2021.
9 See New ASTM Specification Presents Requirements for Standard Reference Test Tire April 1, 2007, available at https www.astm.org/
cms/drupal-7.51/newsroom/new-astmspecification-presents-requirements-standardreference-test-tire last accessed April 13, 2021.

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the 14-inch SRTT with the 16-inch SRTT.10
A. Proposed FMVSS Amendments 1. Surface Friction Measurement As discussed above, other than for defining a snow tire, NHTSA uses the SRTT in the FMVSSs to define the surface coefficient of friction for the test surface for braking and electronic stability control ESC standards. The friction of the test surface is measured by the peak braking force prior to wheel lockup, which is referred to as a peak friction coefficient PFC or peak braking coefficient PBC. For the purpose of this preamble, NHTSA uses the term peak friction coefficient or PFC, but the terms are used interchangeably in the FMVSS.
In the FMVSS, the peak friction coefficient of a surface is determined using the 1990 version of ASTM E1337
test method. The ASTM E1337 test method involves mounting the SRTT to a test trailer, bringing the trailer to a test speed of 40 mph 64 km/h, and applying the brake to produce the maximum braking force prior to wheel lockup.
When NHTSA was informed that production of the 14-inch SRTT was to be discontinued, NHTSA evaluated the 16-inch SRTT to determine whether it would be a suitable replacement.
NHTSA carefully considered the effect of the 16-inch SRTT on the determination of PFC. NHTSA was concerned that the use of the 16-inch SRTT without further changes to the FMVSSs would increase the stringency of the braking and ESC FMVSSs. The reason for this was that the different materials used in the 16-inch SRTT and the increased size of the tire would result in the 16-inch SRTT having better traction performance than the 14-inch SRTT. If the 16-inch SRTT has improved traction performance relative to the 14-inch SRTT, then the same surface would have a higher PFC when tested with the 16-inch SRTT.
Alternatively stated, obtaining an identical PFC value using the 16-inch SRTT would require a road surface with lower friction. Testing braking systems using stopping distance on road surfaces with lower friction would require improved braking performance to stop in the same distance, which is not an outcome intended by this rulemaking.
Consequently, NHTSA sought a conversion factor to evaluate PFC of a test surface using the 16-inch SRTT
without altering the severity of any braking or ESC FMVSSs.
10 See
E:FRFM05AUP1.SGM

Docket No. NHTSA20200067.

05AUP1