ASTM F538-22

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: F538 − 21 F538 − 22
Standard Terminology Relating to
Characteristics and Performance of Tires
This standard is issued under the fixed designation F538; 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 terminology primarily covers definitions for technical terms that occur in ASTM Committee F09 standards on the
characteristics and performance of tires.
1.2 Definitions for terms that may also be used in other technologies, such as vehicle behavior, are worded to cover both areas.
1.3 When any definition in this terminology (that does not have the limiting phrase) is quoted out of context, editorially insert the
limiting phrase in a tire after the dash following the term. This will properly limit the field of application of the term and definition.
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. Terminology
2.1 Definitions:
accelerometer, n—an instrument that senses inertial reaction to measure linear or angular acceleration. F811
accuracy, n—a measurement concept that describes the degree of correspondence between an average measured value and an
accepted reference or standard value for the object, material or phenomenon under test.
age, v—to apply conditions so as to promote change of material properties. F2838
aging, accelerated laboratory (also: aging, laboratory), n—increased rate of tire material property changes under specified
conditions, including temperature, inflation pressure, oxygen concentration in the filling gas, and time. F2838, F3015
aging, in-service, n—material property changes within tires due to consumer usage. F2838, F3015
aging, oven, n—accelerated laboratory aging in an elevated temperature environment. F2838, F3015
aging, thermal oxidative, n—the process whereby chemical and physical material properties of a tire change with exposure to
heat and oxygen. F2838
This terminology is under the jurisdiction of ASTM Committee F09 on Tires and is the direct responsibility of Subcommittee F09.94 on Terminology.
Current edition approved April 1, 2021Oct. 1, 2022. Published May 2021October 2022. Originally approved in 1977. Last previous edition approved in 20132021 as
F538 – 13.F538 – 21. DOI: 10.1520/F0538-21.10.1520/F0538-22.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F538 − 22
aligning stiffness [FL/γ], n—of a tire, the rate of change of tire aligning torque with respect to change in tire slip angle, usually
evaluated at zero slip angle.
aligning stiffness coefficient, n—of a tire, the ratio of tire aligning stiffness to the tire normal force.
aligning torque [FL],n—of a tire, the component of a tire moment vector tending to rotate a tire about the Z'-axis, positive
clockwise when looking in the positive direction of the Z'-axis. F424
alignment, n—the adjustment of various parts of the vehicle’s suspension system to ensure proper handling stability and to
minimize abnormal tire treadwear. F1922
all-season tread, n—tread design providing dry, wet, and snow traction performance for an optimized balance for year-round
performance and which may meet the U.S. Tire Manufacturers Association (USTMA) definition for an M&S, M+S, M/S, MS,
etc. marked tire (see USTMA “Snow Tire Definitions for Passenger and Light Truck (LT) Tires”). E1136, F2493
alternate element wear, n—type of element wear characterized by a pattern of an increased wear rate on alternating isolated
projections; the wear may be every second projection, every third projection, every fourth projection, etc. or a combination
thereof. F1426
analysis, n—an act of inspecting a shearographic or holographic (S/H) system image and associating this image with a known
calibration reference. F1364
anomaly, n—a change in the strain pattern of the rubber surface of a straining block as a result of applied stress brought about
through a change in atmospheric pressure on the rubber surface.
anti-lock braking system (ABS), n—a collection of sensing and control hardware installed on a vehicle to prevent wheel
lock-up during brake application. F1649
average tire tread depth [L],n—the average of all tire groove (void) depth measurements. F1016
axle efficiency [nd],n—in a vehicle, the ratio of the sum of the wheel torques at the driven wheels to the product of driveshaft
torque and axle ratio.
balancing, n—a process to correct for heavy or light areas of a tire and wheel assembly. F1922
bandwidth [1/ T],n—the range of frequencies within which certain performance characteristics occur; specific limits normally
apply. F811
bead, n—of a tire, the part of a tire that comes in contact with the rim and is shaped to secure the tire to the rim. F1922, F1923,
F2663, F2803
bead separation, n—a breakdown of bond between tire components in the bead area. F1922
bead unseating block, n—machined block of cast aluminum (also known as “shoe”) used on the bead unseating fixture to press
against the tire sidewall. F2663
belt, n—in a tire, a breaker that substantially restricts the carcass in a circumferential direction. F3015
F538 − 22
belt edge (BE) temperature, n—in the cross section of a radial tire, the temperature at the edge of the stabilizer plies or belts,
for example, in the rubber region of the two belt edges. F2779, F2869
belt separation, n—in a tire, a breakdown of bonding between the belts or plies or tread, or combination thereof. F1922, F2838,
F3015
bias, n—the difference between the average measured test result and the accepted reference value; it measures in an inverse
manner the accuracy of a test. F337
black sidewall, n—a sidewall on which only black compounds comprise the outer visible surface of the tire. F724
block, n—synonym for element. F1426
bottom out, v—to deform a tire by radial load on the tread until radial movement of the inside surface is stopped by the rim
or other tire inside surface. F414
brake skid/flat spot wear, n—a type of irregular wear characterized by an excessive amount of wear in a footprint area of a
tread due to the tread sliding on the road surface; the tread surface may show abrasion marks, may be smooth with the entire
tread pattern removed, or may be worn into the internal components of the tire. F1426
braking coefficient [nd],n—the ratio of the braking force to the normal force on a tire.
braking force [F],n—of a tire, the negative longitudinal force resulting from braking torque application. F408
braking force coefficient, n—of a tire, the ratio of braking force to normal force. F408
braking force coefficient, 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. F408
braking force coefficient, slide, n—of a tire, the value of braking force coefficient obtained on a locked wheel. F408
2 2
braking torque [ML /T ],n—of a vehicle, the negative wheel torque. F408
brand, n—of a tire, the name under which one or more tire lines are marketed.
brand, tire, private, n—a brand name used by a tire seller or group of sellers who are not manufacturers.
break, n—in a tire, a crack or tear extending into or through the reinforcing material. F1922
breaker, n—in a tire, one or more plies under the tread region of a tire that are additional to those which extend from bead to
bead. F3015
break-in [L],n—one or more periods of initial standardized tire operation during which tire is brought to the state which will
lead to more consistent test results. F762, F1016
calibration tire, n—a witness tire designed to provide a fixed or known test value for selected properties.
F538 − 22
candidate tire, tire (set), n—a test tire (or test tire set) that is part of an evaluation program; each candidate tire (set) usually
has certain unique design or other features that distinguish it from other candidate tires (sets) in the program. F1572, F1649,
F1650, F1805, F1806, F1922, F2803
caravan, n—for purposes of a tire test, two or more vehicles running in the same time frame, over the same test course(s), under
similar but independent conditions. F1922
carcass, n—of a tire, the part of a tire structure that does not include the tread and sidewall rubber.
casing, n—of a tire, a used or treadless tire to which additional tread rubber may be attached for the purpose of retreading. F1922
center of tire contact, n—the intersection of the wheel plane and the vertical projection of the spin axis of a wheel onto the road
plane.
center rib, n—in a tire, a rib at or near the circumferential centerline of the tread band. F1426
center row, n—in a tire, a row located at or near the circumferential centerline. F1426
center wear, n—a type of irregular wear characterized by a wear rate continuously increasing from shoulder to center of the tire
tread band. F1426
2 2
centripetal acceleration [ML /T ],n—of a vehicle, the component of the vector acceleration (of a point in a vehicle)
perpendicular to the tangent of the path of the point and parallel to the road plane.
chip and tear (also cut and chip), n—a special type of irregular wear characterized by a rough tire tread surface which may
contain cracks, abrasion pits or surface ruptures. F1426
circumferential line, n—on a tire, any real or imaginary circle on the surface of a tire, lying in a plane that is perpendicular
to the spin axis. F421, F870
clinch strip, n—in a tire, high-modulus or high-hardness compound applied between the carcass and the sidewall in the bead
area to reinforce the bead. F724
cold inflation pressure, n—the gauge pressure of a tire, measured after equilibration at ambient temperature. F2838, F3015
connection point, n—any point on the wheel or metal loading plate where the resistance measuring instrument’s leads are
connected. F1971
contained air temperature, n—the temperature of the air contained within the tire cavity when the tire is mounted and inflated
on the proper rim. F2779, F2869
control tire (set), n—a reference tire (or reference set) repeatedly tested in a specified sequence, typically in conjunction with
a candidate tire (set), throughout an evaluation program.program, that is used to adjust data sets generated from an evaluation
program or the statistical procedures used on data sets, or both, in order to offset or reduce variation in test results. F1572, F1649,
F1650, F1805, F1806, F1922, F2803
DISCUSSION—
Control tires (sets) are used for adjustment of data sets generated from an evaluation program or the statistical procedures used on data sets, or both,
in order to offset or reduce variation in test results. They can also be used to improve the accuracy of candidate tire (set) data and to detect variation
in test equipment.
F538 − 22
convoy, n—in tire testing, two or more vehicles running at the same time, over the same test course, under the same
interdependent conditions. F1922
cord, n—in a tire, filament(s) or plied yarns used in making a tire ply. F1922
2 2
cornering force [ML /T ],n—of a tire, the horizontal force acting perpendicularly to the instantaneous motion vector of the
center of contact for a tire operating at a slip angle.
cornering force coefficient [nd],n—the ratio of cornering force to the normal force on a tire.
cornering stiffness [F/γ],n—of a tire, the negative of the rate of change of tire lateral force with respect to change in tire slip
angle, usually evaluated at zero slip angle.
cornering stiffness coefficient, n—of a tire, the ratio of tire cornering stiffness to tire normal force.
cornering traction coefficient, n—the ratio of the cornering tractive force vector and normal force.
cornering traction vector angle, n—the angle between the resultant cornering traction force vector and the X' axis.
2 2
cornering tractive force [ML /T ],n—the vectorial sum of lateral and longitudinal tractive force components.
coverstrip, n—a thin layer of black compound which covers the unexposed white sidewall portion of a finished tire. F724
crazing, n—on a tire, minute, closely grouped, generally superficial cracks that usually results from light activated oxidation.
F724
critical slip angle, (rad or degree),n—the value of the slip angle at the peak lateral force coefficient.
cross-country track, n—surface not subject to repeated traffic in addition, no roads, routes, well-worn trails or man-made
improvements; can consist of tank trails with crushed rock or having large exposed obstacles (rocks, boulders, etc.). F1922
cupping (also scalloping), n—a type of irregular wear characterized by a variation in wear rate that may be periodic (essentially
cycloidally shaped) around the tire tread band circumference in one or more rows; the variation of loss is essentially independent
of individual projections if the pattern contains these projections. F1426
curved equivalent test severity—in tire testing, the test conditions (load, rotational speed, tire inflation pressure) on the flat or
highway surface that will provide equivalent internal tire temperatures, for example, at the belt edge, to a known set of curved
1.707-m roadwheel surface test conditions. F2779, F2869
determination, n—the application of the complete measurement procedure to one piece, specimen or object to produce one
numerical measured value to be used to form an average or median.
developed footprint length [L],n—the maximum footprint dimension in the circumferential direction of the tire, under stated
conditions of measurement. F870
developed footprint width [L],n—the maximum lateral dimension of a tire footprint under stated conditions of measurement.
F870
F538 − 22
diagonal wear, n—a type of irregular wear characterized by an increased wear rate region or band oriented transversely (from
shoulder to shoulder) at some non-90° angle with respect to the circumferential centerline of the tire tread band. F1426
driving coefficient [nd],n—the ratio of the driving force to the normal force. F424, F1572, F1805
driving coefficient, peak[nd],n—the maximum value of the driving coefficient. F424
driving force, [F], n—of a tire, the positive longitudinal force resulting from the application of driving torque. F424, F1572,
F1805
2 2
driving torque [ML /T ] , n—of a wheel, the positive wheel torque. F424
effective rolling radius, n—the ratio of the linear velocity of the wheel center of the free rolling tire in the X' direction to the
spin velocity. F424
element, n—an isolated (totally bounded by void) projection. F421, F762, F870, F1426
element wear, n—a type of irregular wear characterized by an increased wear rate on various, random isolated projections either
laterally, longitudinally, or both around the tire with no distinct or repetitive irregular wear pattern. F1426
endurance, n—of a tire, the ability of a tire to perform as designed in its intended usage conditions such as load, inflation
pressure, speed, time, and environmental conditions. F2779, F2869
erosion/river wear, n—a type of irregular wear characterized by a greater wear rate along the circumferential rib edges next to
the major longitudinal grooves; the wear may be wavy in appearance and vary in width around the tire. F1426
fastest wearing groove, n—the circumferential groove with the minimum life expectancy. F1016
fastest wearing location, n—that location which exhibits the highest percent tire tread (depth) loss (as calculated in 6.1.1 of
Practice F1016). F1016
feather wear, n—in a tire, a type of element irregular wear characterized by thin strips of rubber extending from the edge of
the element. F1426
flange, n—that part of the rim which gives lateral support to the tire.
flex cracking, n—cracking primarily caused by application of mechanical stress-strain cycling. F724
footprint area [L ],n—the gross contact area of a tire that is loaded (under stated conditions) against a smooth flat surface. F870
free rolling tire, n—a loaded tire rolling without applied driving or braking torque. F424
front end breakaway, n—in cornering vehicle, the point in the curved trajectory of a vehicle when it can no longer be
maintained on its intended path because of front wheel departure toward the outside of the curve.
full shoulder wear, n—a type of row/rib wear characterized by a greater wear rate across the entire shoulder rib to a longitudinal
groove, usually on one shoulder of the tire only. F1426
F538 − 22
2 2 2
g [L/T ],n—a unit of acceleration where 1 g is equal to the acceleration of gravity, 9.8 m/s (32.2 ft/s ). F811
global testing, n—testing conducted at two or more laboratories or test sites for the purpose of comparing candidate tire
performance at each location for selected characteristic properties. E1806
gravel road, n—two lane, all-weather, occasionally maintained, hard or loose surface (for example, large rock, paved, crushed
rock, gravel) intended for medium-weight, low-density traffic, in accordance with the U.S. Federal Highway Administration.
F1922
grooming, v—in tire testing, mechanically reworking a snow test surface in order to obtain a surface with more consistent
properties. F1572, F1805
groove, n—a void that is relatively narrow compared to its length. F414, F421, F762, F870, F1046, F1426, F1922, F1923
groove, average depth [L],n—the average of all tire groove depth measurements in a single groove.
F421, F762, F1016, F1046
groove (void) area [L ],n—that portion of tire footprint area which is not contacted by ribs or elements. F870
groove (void) area fraction [nd],n—the ratio of the groove (void) area to the footprint area of a tire. F870
groove (void) depth [L],n—a measurement of the perpendicular distance from a real or calculated reference plane defined by
edges of two adjacent ribs (lugs) to the lowest point of contact in the groove (void). F421, F762, F1046, F1922, F1923
gyro-stabilized accelerometer, n—a precision vertical gyroscope fitted with one to three accelerometers to provide orthogonal
measurements referenced to the earth-fixed axis system. F811
heel-toe wear, n—a type of irregular wear characterized by different wear rates at the leading and trailing edges of a projection
(element). F1426
high speed performance, n—of a tire, the rotational speed capability of a tire to perform as designed in its intended usage
conditions such as load, inflation pressure, speed, time, and environmental conditions. F2779, F2869
highway equivalent test severity—in tire testing, the test conditions (load, rotational speed, tire inflation pressure) on the
1.707-m roadwheel that will provide equivalent internal tire temperatures, for example, at the belt edge, to a known set of
highway or flat surface conditions. F2779, F2869
hot inflation pressure, n—the gauge pressure of a tire after equilibration in an oven, measured between 60 and 80 min after
removal from oven. F2838
ice, dry, n—smooth ice without loose surface materials. F1572, F1805
inclination angle, n—of a tire, the angle between the Z'-axis and the wheel plane. F424
inflation gas, n—the specific filling medium used to pressurize the tire cavity and maintain a specified gauge pressure (for
example, oxygen/nitrogen gas mixture, air). F2838, F3015
F538 − 22
inflation pressure loss rate, n—rate of change of normalized inflation pressure, determined from the slope of the linear portion
of the log pressure versus time curve. F1112
inner liner, n— of a tire, the innermost layer(s) of a tubeless tire that limit(s) diffusion of the inflation medium into the carcass.
intended trajectory, n—the intended or ideal path (rectilinear or curvilinear) to bring a vehicle to a stop, that is, under controlled
angular orientation. F1649
intermediate rib, n—one or more rib(s) located between the centerline and the shoulder ribs of the tire tread band. F1426
intermediate row, n—a row located between the circumferential centerline and the shoulder ribs/rows of the tire tread band.
F1426
inter-projection wear, n—a type of irregular treadwear characterized by different wear rates on one or more adjacent projections
(either transverse or circumferential orientation); this results in a step-off in tread depth between the adjacent projections. F1426
intra-projection wear, n—a type of irregular wear characterized by a different wear rate at two or more locations within a given
projection. F1426
inverse wear rate [nd],n—the distance traveled by a tire, after break-in, per unit loss in tread depth. F1016
irregular wear, n—a type of treadwear characterized by substantial variations of tire tread loss both from projection to
projection and frequently from point to point on a given projection. F421, F762, F1426
juncture, n—the interface between two different tire components or different compounds within the same component. F724
juncture cracking, n—a crack with opening originating at a juncture between two components. F724
juncture opening, n—a separation developing in a juncture. F724
kerf, n—synonym for sipe.
lateral force [F],n—of a tire, the component of the tire force vector in the Y' direction. F424
lateral force coefficient [nd],n—of a tire, the ratio of lateral force to normal force.
lateral groove, n—a groove that has its long dimension oriented at a direction non-parallel to the tire circumferential centerline;
it most frequently opens into a void at both ends. F870, F1426
light truck tire, n—a tire that has a “LT” prefix or suffix in the tire size description: this indicates that the tire was primarily
intended for service on light trucks with gross vehicle weights (GVWR’s) <4536 kg. F2869
line, n—of a tire, a group of similar tires of different sizes but common construction type (bias, belted bias or radial) all with
a common tire name.
line, neutral, n—of a tire, a line of tires to which a brand name may be added after their manufacture.
F538 − 22
load index, n—a numerical code associated with the maximum load a tire can carry at the speed indicated by its speed symbol
under specified conditions. F1923
load range, n—a letter designation (A, B, C, D) or, for P-metric tires, standard load (SL) or extra load (XL), used to identify
a given size tire with its load and inflation limits when used in a specific type of service. F414, F1922
load range, n—of a light truck tire, a letter designation (B, C, D, E) used to identify a given size tire with its load and inflation
limits when used in a specific type of service. F2869
load range, n—of a truck-bus tire, a letter designation (F, G, H, J, L, M) used to identify a given size tire with its load and
inflation limits when used in a specific type of service. F2779
load rating [ M], n—the maximum load a tire is rated to carry for a given usage at a specified cold inflation pressure. F414,
F1922
loaded radius [L],n—of a tire, the wheel plane distance from the center of tire contact in the footprint to the wheel center,
specified as a static or dynamic (rolling) measurement.
load symbol, n—a code associated with the maximum load a tire can carry at the speed indicated by its speed symbol under
specified conditions. F1923
local testing, n—testing conducted at one laboratory or test site for the purpose of comparing a number of candidate tires for
selected characteristic properties. F1806
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. F408
longitudinal force [F],n—of a tire, the component of the tire force vector in the X' direction. F377, F408, F424, F1572, F1805,
F2803
longitudinal (circumferential) groove, n—an endless groove that has its major (long) dimension substantially parallel to the
tire circumferential centerline; the walls of the groove may not be perfectly parallel planes, but may have short alternating
sections of the wall at angles to the tire circumferential centerline. F1426
longitudinal slip velocity [L/T],n—the effective rolling radius multiplied by the difference between the spin velocity (in rad/unit
time) of a driven or braked tire and that of a free rolling tire when each is traveling in a straight line. F424, F1572, F1805, F2803
manufacturer, n— of a tire, the name of a company or wholly owned subsidiary making the tire. F2663
master set, n—a selected group of witness tires, each with different test response characteristics to provide a range of values
for the measured property or properties. F1806
maximum load rating [M],n—of a passenger tire, the load rating at the maximum permissible cold inflation pressure for that
tire. F414, F2803
maximum plunger travel, [L],n—in tire testing, the relative displacement of tread surface by a plunger, measured from the point
of initial contact of the plunger with the tread surface to the point of maximum force at rupture or at the bottom-out point. F414
F538 − 22
maximum rated load, n—the load corresponding to the maximum tire load capacity at the rated inflation pressure in accordance
with the publications of tire and rim standards current at the time of manufacture. F1805, F2779, F2869
measured inflation pressure, n—gagegauge pressure of a tire measured at a given time under ambient temperature and
barometric pressure. F1112, F2779, F2869
measurement interval, n—the distance travelled, in kilometres (miles), between two successive groove (void) depth
measurements. F762
measuring rim, n—any ‘rim’ with a width as specified for the design or measuring rim for a particular tire size designation, and
with the ‘bead’ seat and flange dimensions in accordance with publications of tire and rim standards organizations current at the
time of manufacture. F2663
nominal plunger energy W = (F × P)/2, n—in tire testing, one half of the product of a peak force (required to rupture the tire
structure in tread area) and maximum plunger travel into a tire at the time of rupture. F414, F1923
normal force [F],n—of a tire, the component of a tire force vector in the Z' direction. F408, F424
normalized inflation pressure, n—measured pressure of a tire adjusted, according to the ideal gas law, to the nominal test
temperature and one atmosphere barometric pressure. F1112
notch, n—a groove smaller in both width and length than a lateral groove that contains one closed end. F870, F1426
one-sided wear, n—a type of irreg
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