ISO 20918:2007

Road vehicles - Braking threshold pressures for heavy commercial vehicle combinations with fully pneumatic braking systems - Test with roller brake tester

ISO 20918:2007 describes a method to evaluate the braking threshold of heavy commercial vehicle combinations with pneumatic braking systems, by means of a roller brake tester. ISO 20918:2007 describes procedures for workshops and garages and provides a recommended pressure range of the system threshold pressure for motor vehicles and trailers, and a recommended practice for determining the system threshold pressure. ISO 20918:2007 covers neither high-pressure compatibility, including the control of the coefficient of friction between lining/pad and drum/disc, nor dynamic influences. ISO 20918:2007 is not for use at Type Approval.

Véhicules routiers — Seuil de pressions de freinage pour les ensembles routiers lourds à systèmes de freinage uniquement pneumatiques — Essais sur banc d'essai à rouleaux

General Information

• Status: Published
• Publication Date: 08-Oct-2007
• ICS: 43.040.40 - Braking systems
• Technical Committee: ISO/TC 22/SC 33 - Vehicle dynamics and chassis components
• Drafting Committee: ISO/TC 22/SC 33 - Vehicle dynamics and chassis components
• Current Stage: 9093 - International Standard confirmed
• Start Date: 17-Jun-2021
• Completion Date: 13-Dec-2025

Overview

ISO 20918:2007 - Road vehicles: Braking threshold pressures for heavy commercial vehicle combinations with fully pneumatic braking systems - provides a standardized method for measuring the system threshold pressure using a roller brake tester. Intended for use in workshops and garages, the standard defines how to identify the control-line (trailer coupling-head) pressure at which brakes begin to generate measurable braking force above rolling resistance. ISO 20918:2007 is not for Type Approval and does not address high‑pressure compatibility, friction control, or dynamic influences.

Key topics and requirements

• Purpose: Determine the pressure at which braking force starts to develop (system threshold pressure) for heavy vehicles and trailers with fully pneumatic braking systems.
• Recommended threshold: A nominal value of around 70 kPa measured at the trailer coupling head is given as the target system threshold pressure, with specified allowances for small actuators and certain vehicle classes (see standard for exact conditions).
• Measurement method: Use a roller brake tester to detect when braking force exceeds rolling resistance while increasing control-line pressure.
• Apparatus requirements:
  • Minimum sampling rate recommended: 5 Hz
  • Pressure gauges: ±10 kPa accuracy below 500 kPa, ±2% above 500 kPa
  • Roller brake tester force resolution: 100 N (≤5 000 N) and 500 N above, with accuracy ±100 N (≤5 000 N) or ±2% thereafter
  • Compressed air source able to reach 1 000 kPa and control within 10 kPa up to 200 kPa
  • Adapter to measure control line pressure (or digital diagnostic tools meeting accuracy requirements)
• Test procedure (practical steps):
  • Charge air system, apply/release brakes several times
  • Fit pressure adapter or connect diagnostic tool
  • Place axle on roller brake tester, gradually increase service/control pressure
  • Note minimal coupling-head pressure when braking force rises above rolling resistance
  • Repeat three times per wheel/axle and record average
• Limitations: Excludes high‑pressure compatibility (coefficient of friction effects) and dynamic influences; not for regulatory Type Approval.

Applications and who uses it

• Vehicle maintenance workshops and heavy-vehicle garages for brake diagnostics and set-up
• Fleet maintenance and inspection teams for ensuring braking balance and wear optimization
• Brake equipment manufacturers and roller brake tester providers for device specification and calibration
• EBS system developers and vehicle OEMs as a minimum reference for default threshold values (EBS defaults should, at minimum, comply with this standard)

Related standards

• ISO 21069-1 - Roller brake tester testing for pneumatic systems (normative reference)
• ISO 611 - Braking vocabulary and terms used in ISO 20918

Keywords: ISO 20918:2007, braking threshold pressure, roller brake tester, heavy commercial vehicle, pneumatic braking systems, system threshold pressure, trailer control line, roller brake test, workshops and garages, EBS compliance.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 20918
First edition
2007-10-15
Road vehicles — Braking threshold
pressures for heavy commercial vehicle
combinations with fully pneumatic
braking systems — Test with roller brake
tester
Véhicules routiers — Seuil de pressions de freinage pour les ensembles
routiers lourds à systèmes de freinage uniquement pneumatiques —
Essais sur banc d'essai à rouleaux

Reference number
©
ISO 2007
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ii © ISO 2007 – All rights reserved

Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 System threshold pressure recommendation . 2
5 Apparatus . 2
6 Procedure . 3
6.1 General. 3
6.2 Vehicle test for truck and semi-trailer tractor. 3
6.3 Vehicle test for trailer . 4
Annex A (informative) Report form for the roller brake test if printouts not available . 5
Annex B (informative) Examples of roller brake test printouts for the measurement of system
threshold pressure . 6
Bibliography . 8

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 20918 was prepared by Technical Committee ISO/TC 22, Road vehicles, Subcommittee SC 2, Braking
systems and equipment.
iv © ISO 2007 – All rights reserved

Introduction
The braking performance compatibility between motor vehicles and trailers is governed by the compatibility
bands contained in ECE Council Directive 71/320/EEC, Annex II (including ECE Commission Directive
75/524/EEC) and ECE Regulation No.13, Annex 10, and by brake force development requirements contained
in ECE Regulation No.13, Annex 10. The purpose of these compatibility bands is to maintain the stability of a
vehicle combination in all braking conditions, while the brake force development requirements are to provide a
minimum standard regarding wear balance at low braking pressure.
The compatibility between motor vehicles and trailers relates not only to stability, but also to wear balance, or
rather, to wear optimization.
Wear optimization and low adhesion utilization requires good braking balance between axles in the pressure
1)
range up to 200 kPa . This improvement in balance is achieved by minimizing the variation in pressure when
all brakes start to develop a braking force.
This International Standard provides a best practice for determining the threshold pressure at which a brake
starts to develop a braking force. It is applicable to all vehicles equipped with a conventional braking system
and to trailers equipped with an electronic braking system (EBS) when used in conjunction with conventional
equipped vehicles, the connector being connected in accordance with ISO 7638.
For EBS equipped motor vehicles, it is intended that the default values for the threshold pressure values of the
EBS system comply with this International Standard as a minimum requirement. For EBS equipped vehicles, it
is advisable that the vehicle/system manufacturer's manual be followed in order to obtain the appropriate
threshold values.
1) 100 kPa = 1 bar.
INTERNATIONAL STANDARD ISO 20918:2007(E)

Road vehicles — Braking threshold pressures for heavy
commercial vehicle combinations with fully pneumatic braking
systems — Test with roller brake tester
1 Scope
This International Standard describes a method to evaluate the braking threshold of heavy commercial vehicle
combinations with pneumatic braking systems, by means of a roller brake tester.
This International Standard describes procedures for workshops and garages and provides
⎯ a recommended pressure range of the system threshold pressure for motor vehicles and trailers, and
⎯ a recommended practice for determining the system threshold pressure.
NOTE Motor vehicles and trailer combinations with conventional braking systems are covered by the procedures
given in this International Standard. Motor vehicles with a conventional braking system coupled to an EBS equipped trailer
can also be tested using the procedure given in this International Standard, as the pneumatic control line is used.
This International Standard covers neither high-pressure compatibility, including the control of the coefficient
of friction between lining/pad and drum/disc, nor dynamic influences. This International Standard is not for use
at Type Approval.
2 Normative references
The following referenced documents are indispensable for the application 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 611, Road vehicles — Braking of automotive vehicles and their trailers — Vocabulary
ISO 21069-1, Road vehicles — Test of braking systems on vehicles with a maximum authorized total mass of
over 3,5 t using a roller brake tester — Part 1: Pneumatic braking systems
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 611 and the following apply.
3.1
rolling resistance
A
resistance force generated by rolling causing a braking force or torque
3.2
trailer pressure
p
m
pressure at the coupling head of the control line to the trailer
3.3
brake actuator pressure
p
c
pneumatic pressure in a brake chamber of a braking system
3.4
system threshold pressure
p
t
pressure at which the braking force or torque starts to increase above the rolling resistance measured as
trailer pressure
NOTE Pressure is only to be measured with increasing pressure.
4 System threshold pressure recommendation
The recommended system threshold pressure, p , in kPa, measured as trailer pressure, p , is as follows:
t m
+10
p = 70 kPa (1)
()
t2−0
In the case of unladen vehicles (trailers) equipped with small size actuators (16''), it may not be possible to
achieve the above recommended value.
The following value may be accepted for N2 vehicles and O3 trailers:
+20
p = 70 kPa (2)
t2()−0
For EBS equipped motor vehicles, the default values for the threshold pressure val
...