ISO 21994:2022

INTERNATIONAL ISO
STANDARD 21994
Second edition
2022-11
Passenger cars — Stopping distance
at straight-line braking with ABS —
Open-loop test method
Voitures particulières — Distance d'arrêt de freinage en ligne droite
avec ABS — Méthode d'essai en boucle ouverte
Reference number
© ISO 2022
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Published in Switzerland
ii
Contents Page
Foreword .v
Introduction . vi
1 S c op e . 1
2 Nor m at i ve r ef er enc e s . 1
3 Terms and definitions . 1
4 P r i nc iple . 1
5 Va r i able s . 2
5.1 R eference system . 2
5.2 V ariables to be measured . 2
6 Mea su r i n g e qu ipment . 2
6 .1 D e s c r ip t ion . 2
6.2 T ransducer installation . 3
6.3 Calibration . 3
6.4 D ata processing. 3
7 Te s t c ond it ion s .3
7.1 G eneral test conditions . 3
7.2 G eneral data . 3
7.3 T est track . 4
7.4 Environmental conditions . 4
7.5 T est vehicle . 4
7.5.1 G eneral vehicle condition . 4
7.5.2 Tyres . 4
7.5.3 Braking system . 5
7.5.4 L oading conditions of the vehicle . 5
8 Te s t pr o c e du r e .5
8.1 T est preparation . 5
8.1.1 Defining the measurement distance . 5
8.1.2 Conditioning tyres and brake system . 5
8.2 Measurements . . 6
8.2.1 Brake disc temperature . 6
8.2.2 I nitial driving condition . 6
8.2.3 Brake pedal actuation . 6
8.2.4 C onditions during braking . 7
8.2.5 N umber of measurements . 7
9 D ata evaluation and presentation of results . 7
9.1 G eneral . 7
9.2 N omenclature of distances and decelerations . 8
9.3 Determination of normalized stopping distance s . 8
A100,norm
9.3.1 Determination of mean deceleration of a single test run . 8
9.3.2 D etermination of the normalized stopping distance of a single test run . 9
9.3.3 Determination of mean deceleration and mean normalized stopping
distance . 9
9.4 D etermining of ABS-braking distance s (100) (optional) . 9
L90,norm
9.5 D etermination of normalized build-up distance s (optional) . 10
F10,norm
Annex A (informative) Test report − General data .11
Annex B (informative) Test report − Test conditions and results .13
Annex C (informative) Test sequence, specific terms and background information .15
Annex D (normative) Method for determination of F .20
ABS
iii
Annex E (normative) Requirements for measurements and measuring equipment .24
Annex F (normative) Structure of the stopping distance calculation .26
Bibliography .27
iv
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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electrotechnical standardization.
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described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
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This document was prepared by Technical Committee ISO/TC 22, Road vehicles, Subcommittee SC 33,
Vehicle dynamics and chassis components.
This second edition cancels and replaces the first edition (ISO 21994:2007), which has been technically
revised.
The main changes are as follows:
— variables in formulae have been corrected.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
v
Introduction
The stopping distance of a road vehicle is an important part of vehicle performance and active vehicle
safety. Any given vehicle, together with its driver and the prevailing environment, constitutes a unique
closed-loop system. The task of determining the stopping distance is therefore, very difficult, since
there is a significant interaction between these driver-vehicle-environment elements, each of which is
complex in itself.
Test conditions and tyres have a strong influence on test results. Therefore, only vehicle stopping
distances obtained under comparable test and tyre conditions are comparable to one another.
vi
INTERNATIONAL STANDARD ISO 21994:2022(E)
Passenger cars — Stopping distance at straight-line
braking with ABS — Open-loop test method
1 S cope
This document specifies an open-loop test method to determine the stopping distance of a vehicle
during a straight-line braking manoeuvre, with the anti-lock braking system (ABS) fully engaged. This
document applies to passenger cars as defined in ISO 3833 and light trucks.
This document specifies a reference method and is especially designed to ensure high repeatability.
2 Normat ive references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 8855, Road vehicles — Vehicle dynamics and road-holding ability — Vocabulary
ISO 15037-1:2019, Road vehicles — Vehicle dynamics test methods — Part 1: General conditions for
passenger cars
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8855, ISO 15037-1 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
F
ABS
pedal force required for ABS activation
4 Principle
This document specifies a method to determine the braking distances characterizing the deceleration
build-up phase at the beginning of a braking manoeuvre and at full braking until the vehicle comes to a
standstill.
The driving situation represents an emergency or panic braking phase (pushing the brake pedal with a
very high activation speed) during straight-ahead driving on an even and dry road surface with a high
coefficient of friction.
Using this document, three results become available:
— stopping distance from initial brake pedal contact until the vehicle comes to a standstill (s );
A100
— ABS-braking distance describing the distance travelled under full ABS-controlled braking from
90 km/h until the vehicle comes to a standstill (s ); and
L90
— estimation of the build-up distance from initial brake pedal contact until a velocity reduction of
10 km/h is achieved (s ).
F10
Apart from the technical equipment and especially the braking characteristics of the vehicle, the
distance travelled after the first pedal contact very strongly depends on the individual pedal actuation
of the driver. To minimize this influence, this document specifies rules for brake pedal actuation.
To achieve reproducible, reliable and comparable measurement results, a multitude of further test
conditions shall be observed.
Measurement results can only be compared if measurements take place under identical conditions. In
particular, this means:
— same track (see also Annex C); and
— very similar weather and ambient conditions (e.g. wind, temperature).
5 Variable s
5.1 Reference system
The reference system specified in ISO 15037-1 shall apply.
5.2 V ariables to be measured
The following variables shall be measured:
— longitudinal velocity: (v );
X
— time of brake pedal actuation: (t );
— longitudinal distance: (s);
— brake pedal actuation force: (F ).
P
The variable longitudinal velocity is defined in ISO 8855.
6 Measur ing equipm ent
6.1 Description
All variables shall be measured by means of appropriate transducers, and their time histories shall
be recorded by a multi-channel recording system. Alternatively, data measured may be recorded and
processed directly in a calculation unit of the measuring system without the possibility to access time
histories. Typical operating ranges, recommended maximum errors of the transducer, and recording
system are given in Table 1. If initial longitudinal velocities different from 100 km/h are chosen, the
following operating ranges shall be changed accordingly, but maximum errors shall be unchanged.
Table 1 — Variables, their typical operating ranges and recommended maximum errors —
Additions and exceptions to ISO 15037-1
Recommended maximum error
Variable Typical operating range of the combined transducer
and recorder system
a
Initial longitudinal velocity 102 km/h – 98 km/h ±0,5 km/h
b
Longitudinal velocity 93 km/h – 5 km/h ±0,5 km/h
Longitudinal distance 100 m ±1 % (u 50 m) ± 0,50 m (>50 m)
Brake force trigger u 10 N (triggering point) ±5 N
c
Brake pedal actuation force 0 N – 1 000 N (maximum 1 500 N) ±2 %
a
Determined in averaging interval 0,2 s-0 s before brake pedal contact.
b
Deviations of the measured velocity are normally found in the transition area from steady-state driving to full braking.
c
It is recommended to use a lateral force compensated brake force transducer.
The trigger signal for brake pedal contact shall be activated at a pedal force of 10 N or less. The
time delay of the trigger signal shall be 5 ms or less. If the pedal force transducer does not fulfil this
specification, it is recommended to use a contact switch on the brake pedal’s step pad.
To monitor test preparation (run-in) and test conditions, the following measuring devices are required:
— brake (disc/drum or pad/lining) temperature sensor; and
— device for measuring and displaying vehicle deceleration (run-in).
6.2 Transducer install ation
The requirements of ISO 15037-1:2019, 4.2 shall apply. In addition, it shall be ensured that transient
vehicle pitch angle changes during braking do not affect the measurement of the velocity and distance
variables for the chosen transducer system.
6.3 Calibration
All transducers shall be calibrated according to the manufacturer’s instructions. The transducer
manufacturer’s recommended application software and firmware version shall be used. If parts of the
measuring system used can be adjusted, such calibration shall be performed immediately before the
beginning of the tests.
A detailed procedure of calibration shall be followed as specified in Annex E.
6.4 Data processing
The recording system and data processing requirements contained in ISO 15037-1:2019, 4.3 shall apply.
7 Test conditions
7.1 Gener al test conditions
The test conditions shall be in accordance with ISO 15037-1:2019, Clause 5, unless otherwise specified
in this document.
7.2 General data
General data on the test vehicle and test conditions shall be recorded as specified in ISO 15037-1:2019,
5.4.1 and may be using the templates as proposed in Annexes A and B, with the additions of the braking
system and tyre data. Annex A provides further information.
7.3 Test track
All tests shall be carried out on a smooth, clean, dry and uniform paved road surface.
The gradient of the test surface to be used shall not exceed 1 % longitudinal inclination and 2 %
transversal inclination when measured over any distance interval between that corresponding to the
vehicle track and 25 m.
It is recommended to use a lane width of 3,5 m or more.
The friction coefficient of the test surface shall be a minimum of 0,9 and its variation shall not
exceed ±5 % over the length of the test surface. These requirements are generally fulfilled on concrete
and rough asphalt surfaces. See also C.2.2 and C.2.3.
7.4 Environmental conditions
The weather conditions shall remain unchanged during a sequence of measurements. The ambient wind
velocity (regardless of the wind direction) shall either not exceed 3 m/s or, if the wind velocity ranges
between 3 m/s and 5 m/s maximum, an equal number of measurements specified shall be carried out in
both driving directions. The total number of measurements shall remain the same (see 8.2.5).
The ambient temperature shall be between +5 °C and +35 °C and its variation during a sequence of
measurements shall not exceed 10 °C.
The surface temperature of the test track shall be between +10 °C and +40 °C and its variation during a
sequence of measurements shall not exceed 10 °C.
Additionally, the variation in surface temperature along the length of the test track (e.g. due to changes
from sunlit to shaded areas) shall not exceed 10 °C.
Measurements performed within acceptable temperature ranges as specified above can only be
compared if, additionally, the temperature difference between one another is below 10 °C. Special tests
with specific structural components such as tyres can require much smaller tolerance ranges in order
to become comparable.
7.5 Test vehicle
7.5.1 General vehicle condition
The condition of the test vehicle shall be in accordance with the vehicle manufacturer’s specifications,
particularly with respect to the complete brake system, the suspension geometries, power train (e.g.
differentials and locks) configuration and tyres used.
7.5.2 Tyres
Generally, all measurements shall be conducted with summer tyres.
For a general tyre condition, new tyres shall be fitted on the test vehicle according to the manufacturer’s
specifications. If not specified otherwise by the tyre manufacturer, they shall be run in on the test
vehicle for at least 150 km on a road surface with high friction or on an equivalent vehicle without
excessively harsh use, for example, braking, acceleration, cornering, hitting the kerb. Therefore,
longitudinal and lateral accelerations shall not exceed 3 m/s during run-in. After run-in the tyres shall
be used at the same vehicle locations for the tests.
The existing tread depth and the type of wear have an impact on the length of the braking distance (see
C.2.5). Therefore, when comparing vehicles or tyres, new tyres shall be used for the measurements as a
general rule. If no new tyres are used, the tyre parameters and tread widths should show a steady wear
condition with a tread depth of at least 90 % of the original value across the whole breadth of the tread
and around the whole circumference of that of the new tyre.
Tyres shall be manufactured not more than one year before the test. The date of manufacturing (DOT-
stamp) shall be noted in the presentation of test conditions (see Annex A).
Tyres shall be inflated to the pressure as specified by the vehicle manufacturer for the test vehicle
configuration. The tolerance for setting the cold inflation pressure is ±5 kPa for pressures up to 250 kPa
and ±2 % for pressure above 250 kPa.
Tyre data, the inflation pressure and tread depth of the tyres determined before tyre warm-up and
after the test runs shall be recorded in the test report (see Annex B).
7.5.3 Braking system
The braking system shall be in a technically perfect condition (see also C.2.9). Any newly installed
wheel brakes (brake discs, brake drums, brake pads) shall be burnished in accordance with the vehicle
manufacturer specifications. Alternatively, the burnishing procedure for brakes as specified in C.2.5.2
may be applied. Hydraulic systems shall be fully bled (free of air residuals) in accordance with the
manufacturer’s instructions.
7.5.4 Loading conditions of the vehicle
The fuel tank shall be full and, in the course of the measurement sequence, the indicated fuel level
should not drop below “half-full”.
The total load of the driver plus instrumentation should not exceed 150 kg.
If the vehicle is to be tested in any other load condition (e.g. GVM), then the additional payload shall be
evenly distributed such that cross-axle variations do not exceed 50 kg (see C.2.6).
8 Test procedure
8.1 Test preparation
8.1.1 Defining the measurement distance
To ensure constant friction characteristics, all test runs shall be performed on the same track section.
It shall be ensured that neither tread wear nor frequent braking can cause a relevant change of the
track surface and hence a different road friction coefficient.
Comparative measurements should always be started at the same spot to avoid different friction
coefficients.
However, to avoid punctual road contamination or damage in the long run, the initial braking point
should vary along the track when carrying out entirely different measuring sequences.
Since friction coefficients often vary considerably across the driving track, it shall be ensured that the
tests are all performed on the same driving track to achieve reproducible test results.
8.1.2 Conditioning tyres and brake system
The tyres, and at the same time the brakes, are submitted to a two-step conditioning procedure on the
test track directly before the braking distance measurements:
1) Five ABS controlled braking from about 100 km/h to a stop without excessively heating the brake,
i.e. the brake disc temperatures shall not exceed 120 °C at the beginning of each braking; and
2) cooling down the tyres (normal ride for about 10 km recommended).
8.2 Measurements
8.2.1 Brake disc temperature
Before each measurement, the temperature of the front brake discs shall be between 80 °C and 120 °C
and that of the rear brake discs (brake drums) below 120 °C (100 °C). If required, cooling phases shall
be provided.
8.2.2 Initial driving condition
The initial driving condition is a steady-state straight ahead run (see ISO 15037-1:2019, 6.2.2). The
longitudinal acceleration shall not exceed ±0,3 m/s .
The specified vehicle velocity at the beginning of the braking is 100 km/h with a maximum tolerance
of ±2 km/h. To minimize dynamic effects, the vehicle should be driven at a steady velocity for at least
1,5 s (about 50 m) before braking is initiated (see also C.2.7).
Depending on the vehicle transmission type, one of the following driving conditions shall be selected:
— automatic transmission: standard drive mode D;
— manual transmission: starting; usually with the fourth or a higher gear engaged, disengaging in the
course of the braking, i.e. it should be disengaged at the latest at a velocity of about 80 km/h.
The gear chosen (for automatic transmissions, selected driving range) shall be documented in the test
record.
Alternatively, neutral gear may be selected before commencing the brake application. Comparisons of
braking distances are only possible if the condition of engagement is the same (gearbox: disengaged/
declutched, respectively in neutral mode “N”; or gearbox: engaged, respectively drive mode “D”).
On vehicles equipped with a vacuum brake booster, the brake force depends on the vacuum level of the
vacuum brake booster. Therefore, a sufficient vacuum shall be ensured at the beginning of braking. To
achieve a sufficient vacuum level, it is recommended to move the vehicle in a drag operation for a short
time during the cooling phases between the individual breaking. When doing so, the driving pedal can
be released for at least 10 s at high engine speed (e.g. by engaging a suitable gear). Afterwards, the
brake shall not be operated before the next measurement because this will reduce the vacuum level
that was established before.
8.2.3 Brake pedal actuation
8.2.3.1 Determination of the minimum brake pedal force
The brake pedal shall be applied very fast and with sufficient pedal force. The brake pedal force shall be
high enough to guarantee ABS-control throughout the whole braking phase of the test run. Therefore, a
minimum force of 500 N shall be applied. This force shall be at least 1,5 times F (pedal force required
ABS
for ABS activation) or higher. F shall be determined for the test vehicle as described in Annex D.
ABS
8.2.3.2 Brake pedal application
The measurement shall start at the instant of first foot contact with the brake pedal. This instant is
defined by either a signal of a contact switch or determined from the pedal force signal. The signal
representing the initial pedal contact shall be triggered at a pedal force of 10 N or lower.
The brake pedal shall be applied with a minimum force of 500 N or 1,5 times F (whichever is higher).
ABS
The gradient shall be higher than 3 333 N/s (i.e. 500 N in 150 ms). The minimum pedal force shall be
maintained until the vehicle comes to a standstill.
During the entire procedure, the pedal force shall not exceed a value of 1 500 N.
8.2.4 Conditions during braking
To be able to keep the vehicle properly on track, no major steering corrections shall be applied during
braking (see C.2.8). Any minor steering corrections during braking shall be documented in the test
report.
8.2.5 Number of measurements
One measurement sequence consists of 10 valid individual measurements (i.e. measurements
performed while observing all conditions specified).
9 Data e valuation and presentation of results
9.1 General
In the test report, general information should be presented as shown in Annex A. Each change in vehicle
equipment (e.g. different load conditions) shall be documented.
Applying this document delivers up to three results as shown below.
1) Normalized stopping distance s : distance travelled between the initial brake pedal contact
A100,norm
until the vehicle comes to a standstill. The stopping distance is normalized to the nominal initial
velocity (100 km/h).
2) Normalized ABS-braking distance s (100): distance travelled under full ABS-controlled
L90,norm
braking from 90 km/h until the vehicle comes to a standstill.
3) Normalized build-up distance s (100): distance travelled during deceleration build-up,
F10,norm
defined as interval of a velocity decrease of 10 km/h (“first 10 km/h”); distance between the point
of first brake pedal contact, normalized to the nominal velocity of 100 km/h, until the velocity
90 km/h is reached.
Positions 2 and 3 are optional.
All calculated longitudinal decelerations shall be determined according to Formula (1):
2 2
vv −
1 2
a = (1)
x
2⋅s
x
where
a is the calculated deceleration;
x
v is the initial velocity;
v is the velocity at end of measurement;
s
...