ASTM F408-21

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Designation: F408 − 99 (Reapproved 2015) F408 − 21
Standard Test Method for
Tires for Wet Traction in Straight-Ahead Braking, Using a
Towed Trailer
This standard is issued under the fixed designation F408; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers the measurement of braking traction of tires designed for passenger cars or light trucks. Such braking
traction measurements are applicable to conditions wherein the vehicle is traveling straight ahead on a wet, paved surface.
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
E274 Test Method for Skid Resistance of Paved Surfaces Using a Full-Scale Tire
E1337 Test Method for Determining Longitudinal Peak Braking Coefficient (PBC) of Paved Surfaces Using Standard Reference
Test Tire
F377 Practice for Calibration of Braking/Tractive Measuring Devices for Testing Tires
F457 Test Method for Speed and Distance Calibration of Fifth Wheel Equipped With Either Analog or Digital Instrumentation
F538 Terminology Relating to Characteristics and Performance of Tires
F1650 Practice for Evaluating Tire Traction Performance Data Under Varying Test Conditions
3. Terminology
3.1 Definitions:
3.1.1 braking force, [F], n—of a tire, the negative longitudinal force resulting from braking torque application. F538
3.1.2 braking force coeffıcient, n—of a tire, the ratio of braking force to normal force. F538
3.1.3 braking force coeffıcient, peak, n—of a tire, the maximum value of tire braking force coefficient that occurs prior to wheel
lockup as the braking torque is progressively increased. F538
This test method is under the jurisdiction of ASTM Committee F09 on Tires and is the direct responsibility of Subcommittee F09.20 on Vehicular Testing.
Current edition approved May 1, 2015Sept. 1, 2021. Published June 2015September 2021. Originally approved in 1975. Last previous edition approved in 20082015 as
F408 – 99 (2008).(2015). DOI: 10.1520/F0408-99R15.10.1520/F0408-21.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
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F408 − 21
3.1.4 braking force coeffıcient, slide, n—of a tire, the value of braking force coefficient obtained on a locked wheel. F538
2 2
3.1.5 braking torque, [ML /T ], n—of a vehicle, the negative wheel torque. F538
3.1.6 lockup, n—of a wheel, the condition of a wheel in which its rotational velocity about the wheel spin axis is zero and it is
prevented from rotating in the presence of applied wheel torque. F538
3.1.7 longitudinal force, [F], n—of a tire, the component of the tire force vector in the X' direction. F538
3.1.8 normal force, [F], n—of a tire, the component of a tire force vector in the Z' direction. F538
3.1.9 skid number (SN), n—slide braking force coefficient multiplied by 100. F538
3.1.10 test run, n—in tire testing, a single pass of a loaded tire over a given test surface.surface, or the acquisition of a sequence
of data, or both, in the act of testing a tire or tire set under selected test conditions. F538
3.1.11 tire axis system, n—the origin of the tire-axis system is the center of the tire contact where the X'-axis is the intersection
of the wheel plane and the road plane with a positive direction forward, the Z'-axis is perpendicular to the road plane with a positive
direction downward, and the Y'-axis is in the road plane, its direction being chosen to make the axis system orthogonal and
right-hand. (See Fig. 1.)
3.1.11.1 Discussion—
See Fig. 1. F538
3.1.12 tire forces, [F], n—the external forces acting on a tire by the road. F538
3.1.13 torque, [FL], n—of a wheel, the external torque applied to a tire from a vehicle about the wheel spin axis.
3.1.13.1 Discussion—
Driving torque is positive wheel torque; braking torque is negative wheel torque. F538
3.1.14 vertical load, n—the normal reaction of the tire on the road which is equal to the negative of the normal force. F538
4. Summary of Test Method
4.1 The measurements are conducted on tires mounted on a trailer towed by a vehicle. Brakes are applied firmly until one or both
test tire(s) are locked and then held locked for a period of at least 1.5 s at speeds ranging from 32 km/h (20 mph) to 96 km/h (60
mph).
FIG. 1 Tire-Axis System
F408 − 21
4.2 Recommended vehicle test speeds are: 32, 64, and 96 km/h (20, 40, and 60 mph).
5. Significance and Use
5.1 The measured values are traction properties of tires obtained with the towed trailer described here on a given road surface,
under given environmental conditions (ambient and road surface temperature, humidity, wind speed and direction, purity and film
depth of water used to wet the road surface). They are carried out in accordance with the stated test procedures and reflect the
performance of the total tire-vehicle environmental system. A change in any of these factors may change the measurements on a
subsequent run of the test.
5.2 These test methods are suitable for research and development purposes, where tires are compared during a single series of
tests. They may not be suitable for regulatory statutes or specification acceptance, because the values obtained may not necessarily
agree or correlate either in rank order or absolute traction performance level with those obtained on other road surfaces (or the same
surface after additional wear), under other environmental conditions, or other towed trailers, or other test devices, or with results
obtained with other test procedures.
6. Apparatus
6.1 The apparatus consists of tow vehicle and trailer. Vehicle and trailer must comply with all applicable state and federal laws
when operated on public roads.
6.2 Tow Vehicle:
6.2.1 The tow vehicle shall have the capability of towing the trailer at specified speeds in the range from 32 to 96 km/h (20 to
60 mph), which are to be kept constant to within 0.8 km/h (0.5 mph), even at maximum level of application of braking forces.
6.2.2 The tow vehicle shall be equipped with the following accessories:
6.2.2.1 Equipment to actuate brakes on towed trailer,
6.2.2.2 A water tank to store sufficient water to supply the watering system unless external watering is used,
6.2.2.3 Recording equipment to record signals from transducers installed on the towed trailer,
6.2.2.4 Hitch with adjustable height for towing the trailer, and
6.2.2.5 Optional equipment to monitor and control brakeline pressure, brake application rate, and water application rate.
6.3 Towed Trailer—The trailer shall be equipped with a tow hitch and one or two test wheels which shall be instrumented as
described. A self-watering system is optional (see 6.3.6). Trailers shall be designed to accommodate the range of passenger car and
light truck rim sizes to be tested.
6.3.1 The hitch height shall be no higher than the loaded tire radius, and the distance from hitch ball to axle centerline shall be
no less than ten times the hitch height. For external watering, the trailer hitch on the tow vehicle shall be so arranged that the trailer
tires will run in tracks separate from the tire tracks of the tow vehicle. The minimum track offset of the test tire(s) with respect
to the track of the tow vehicles should be 0.3 m (12 in.).
6.3.2 The trailer shall have provisions for adjustment of vertical load from 65 to 110 % of the appropriate maximum load as
specified in the current yearbook of the appropriate tire and rim standards organization. To meet this requirement for all rim sizes,
two or more trailers designed for different load ranges and equipped with appropriate wheel adaptors may be necessary.
6.3.3 Each of the test wheels shall be equipped with a typical or special automotive hydraulic brake system which can apply
sufficient braking torque to initiate and maintain lockup of the test wheel(s) for the duration of the test.
6.3.4 Each of the test wheels shall have a suspension capable of limiting change of toe and camber to within 60.05° with
maximum vertical suspension displacements. Suspension arms and bushings shall have sufficient rigidity necessary to minimize
F408 − 21
free play and compliance under application of maximum braking forces. The suspension system shall provide adequate
load-carrying capacity and be of such a design as to isolate suspension resonance.
6.3.5 The brake application system shall be able to control the time interval between initial brake application and wheel lockup
within a range of 0.2 to 0.5 s with a repeatability of 60.1 s.
6.3.6 The trailer may be optionally equipped with a pavement-wetting system, less the storage tank, which is mounted on the tow
vehicle. The water being applied to the pavement ahead of the test tires shall be supplied by a nozzle suitably designed to ensure
the water layer encountered by the test tire has a uniform cross section at all test speeds with a minimum splash and over-spray
(see Note 1). The nozzle configuration and position shall ensure that the water jets shall be directed toward the test tire and pointed
toward the pavement at an angle of 20 to 30°. The water shall strike the pavement 0.25 to 0.46 m (10 to 18 in.) ahead of the center
of tire contact. The nozzle shall be located 25 mm (1 in.) above the pavement or the minimum height required to clear obstacles
which the tester is expected to encounter, but in no case more than 100 mm (4 in.) above the pavement. The water layer shall be
at least 25 mm (1 in.) wider than the test tire tread and applied so the tire is centrally located between the edges. The volume of
water per unit of wetted width shall be directly proportional to the test speed. The quantity of water applied at 32 km/h (20 mph)
shall be 300 mL/min per mm of width (2.0 gal/min per inch of width) of wetted surface. The rate of water application at speeds
of 64 and 96 km/h (40 and 60 mph) shall be 600 and 900 mL/min per mm of width (4.0 and 6.0 gal/min per inch of width) of
wetted surface, respectively. The nominal values of rate of water application shall be maintained within 610 %. The water shall
be reasonably clean and contain no chemicals such as wetting agents or detergents.
NOTE 1—A suitable nozzle design is described in detail in Test Method E274.
6.4 Instrumentation—Each test wheel position on the trailer shall be equipped with a wheel rotational velocity measuring system
and with transducers to measure the braking force and vertical load at the test wheel.
6.4.1 General Requirements for Measurement System—The instrumentation system shall conform to the following overall
requirements at ambient temperatures between 0 and 43°C (32 and 110°F):
6.4.1.1 Overall system accuracy, force: 61.5 % of vertical load or traction force from 450 N (100 lbf) to full scale,
6.4.1.2 Overall system accuracy, speed: 61.5 % of speed or 60.8 km/h (60.5 mph), whichever is greater,
6.4.1.3 Shunt Calibration—All strain-gage transducers shall be equipped with shunt calibration resistors that can be connected
before or after test sequences. The calibration signal shall be at least 50 % of the full scale for the transducer, and
6.4.1.4 Ruggedness—The exposed portions of the system shall tolerate 100 % relative humidity (rain or spray) and all other
adverse conditions such as dust, shock, and vibrations which may be encountered in regular operation.
6.4.2 Vehicle Speed—To measure vehicle speed, a fifth wheel or non-contact precision speed-measuring system should be used.
Output shall be directly visable to the driver and shall be simultaneously recorded. The fifth wheel or precision speedometer used
to measure vehicle speed shall have specifications in accordance with Test Method F457.
6.4.3 Braking Forces—The braking force-measuring transducers shall measure longitudinal force generated at the tire-road
interface as a result of brake application within a range from 0 to at least 125 % of the applied vertical load. The transducer design
and location shall minimize inertial effects and vibration induced mechanical resonance. The transducer shall have an output
directly proportional to the force with less than 1 % hysteresis and less than 1 % nonlinearity at full scale. It shall have less than
2 % cross-axis sensitivity at full scale. The transducer shall be installed in such a manner as to experience less than 1° angular
rotation with respect to its measuring axes at a maximum expected braking torque.
6.
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