ISO 789-6:2019

Agricultural tractors - Test procedures - Part 6: Centre of gravity

This document specifies a method of determining the position of the centre of gravity of agricultural tractors. The method is applicable to agricultural tractors having at least two axles fitted with wheels or tracks.

Tracteurs agricoles — Méthodes d'essai — Partie 6: Centre de gravité

General Information

• Status: Published
• Publication Date: 04-Feb-2019
• ICS: 65.060.10 - Agricultural tractors and trailed vehicles
• Technical Committee: ISO/TC 23/SC 2 - Common tests
• Drafting Committee: ISO/TC 23/SC 2/WG 6 - Test procedures for agricultural tractors
• Current Stage: 9093 - International Standard confirmed
• Start Date: 29-Jul-2024
• Completion Date: 13-Dec-2025

Relations

• Revises: ISO 789-6:1982 - Agricultural tractors - Test procedures - Part 6: Centre of gravity
• Effective Date: 30-Apr-2016
• Revises: ISO 789-6:1982/Amd 1:1996 - Agricultural tractors - Test procedures - Part 6: Centre of gravity - Amendment 1
• Effective Date: 30-Apr-2016

Overview

ISO 789-6:2019 - "Agricultural tractors - Test procedures - Part 6: Centre of gravity" specifies a practical laboratory method to determine the position of the centre of gravity (CG) of agricultural tractors (wheeled or tracked) with at least two axles. The method is based on the suspension and ground-reaction technique and requires common test apparatus such as a weighbridge, crane, decking with knife edges and a scribing board to mark verticals. The standard is intended to produce repeatable x (fore–aft), y (lateral) and h (vertical) CG coordinates relative to tractor reference planes.

Key technical topics and requirements

• Scope and applicability: Applies to agricultural tractors with two or more axles (wheels or tracks).
• Required apparatus: weighbridge or load cells, crane, decking with knife edges, level, plumb rule, squares, scribing board, tape measure and marking materials.
• Test condition requirements:
  • Tractor clean and in normal working condition.
  • Radiator, sump and hydraulic reservoirs filled; fuel tank full.
  • Driver replaced by a 75 kg mass on the driver’s seat.
  • Tools, spare wheel and accessories in normal storage positions.
  • Suspension locked in normal height position, or tires inflated to manufacturer’s max pressure when locking is not possible.
  • Measurement tolerances: distance ±0.5%, mass ±0.5%, tyre pressure ±5%.
• Procedure overview:
  • Determination of fore–aft coordinate x: for wheeled tractors use axle loads and wheelbase; for tracked tractors use decking with knife edges and weighbridge reactions to compute x from moment balance.
  • Determination of vertical coordinate h: suspend tractor at one end (20°–25°, minimum 15°) with other end on weighbridge, measure ground reaction and geometry to compute horizontal offset c, draw verticals on scribing board. Repeat from opposite end; intersection marks h. An alternative calculation method (clause 6.3.8) with supporting figures and tables is provided.
  • Determination of lateral coordinate y: measure left/right side loads and compute offset using track gauge or wheel track as the moment arm.
• Reporting: the standard defines data and formats to include in the test report.

Practical applications and users

ISO 789-6:2019 is used by:

• Tractor manufacturers (OEMs) for design verification and stability analysis.
• Test laboratories and certification bodies performing compliance and safety tests.
• Agricultural safety engineers and R&D teams assessing rollover, load-handling and implement coupling behavior.
• Regulators and technical inspectors who evaluate tractor stability claims or type-approval.

Keywords: ISO 789-6:2019, centre of gravity, agricultural tractors, weighbridge, test procedures, wheelbase, axle loads, tracked tractors, wheeled tractors.

Related standards

• ISO 789-13:2018 - vocabulary and specimen test report (normative reference).
• Other parts of the ISO 789 series addressing common tests and definitions.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 789-6
Second edition
2019-02
Agricultural tractors — Test
procedures —
Part 6:
Centre of gravity
Tracteurs agricoles — Méthodes d'essai —
Partie 6: Centre de gravité
Reference number
©
ISO 2019
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Fax: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Apparatus . 2
5 General requirements . 2
6 Procedure. 3
6.1 General principle . 3
6.2 Determination of horizontal fore-and-aft coordinate ( x ) . 3
6.2.1 Tracked tractors [see Figure 1 a)] . 3
6.2.2 Wheeled tractors [see Figure 1 b)] . 3
6.3 Determination of vertical coordinate ( h ) (see Figure 2) . 3
6.4 Determination of lateral coordinate in the horizontal plane ( y ) (see Figure 3) . 4
7 Test report .22
Annex A (informative) Example calculation: Height of centre of gravity, h (Table 2) .23
Bibliography .26
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.
The procedures used to develop this document and those intended for its further maintenance are
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
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso
.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 23, Tractors and machinery for agriculture
and forestry, Subcommittee SC 2, Common tests.
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.
This second edition cancels and replaces the first edition (ISO 789-6:1982), which has been technically
revised. It also incorporates the Amendment ISO 789-6:1982/Amd.1:1996. The main changes compared
to the previous edition are as follows.
— The terms and definitions have been updated to reference ISO 789-13.
— A requirement for a mass in the driver’s seat has been added in 5.2.
— A requirement for locking the suspension has been added in 5.4.
— A new method has been added for alternative determination of vertical coordinate ( h) in 6.3.8.
— Figures 4 to 12 and Tables 1 to 4 have been added to support the method described in 6.3.8.
A list of all parts in the ISO 789 series can be found on the ISO website.
iv © ISO 2019 – All rights reserved

Introduction
This document specifies test procedures for agricultural tractors. It deals with the centre of gravity.
Although there are many possible methods of determining the centre of gravity, the purpose of this
document is to specify a simple, practical method, which requires the use of a weighbridge and crane.
Alternative methods can be used if they locate the centre of gravity with respect to the specified
reference planes and within the specified tolerances.
INTERNATIONAL STANDARD ISO 789-6:2019(E)
Agricultural tractors — Test procedures —
Part 6:
Centre of gravity
1 Scope
This document specifies a method of determining the position of the centre of gravity of agricultural
tractors.
The method is applicable to agricultural tractors having at least two axles fitted with wheels or tracks.
2 Normative 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 789-13:2018, Agricultural tractors — Test procedures — Part 13: Vocabulary and specimen test report
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 789-13 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
agricultural tractor
self-propelled agricultural vehicle having at least two axles and wheels, or endless tracks, particularly
designed to pull agricultural trailers and to pull, push, carry and operate implements used for
agricultural work (including forestry work), which may be provided with detachable loading platform
Note 1 to entry: The agricultural vehicle has a maximum design speed of not less than 6 km/h and may be
equipped with one or more seats.
[SOURCE: ISO 12934:2013, 3.1]
3.2
wheelbase
L
horizontal distance between the two vertical planes passing through the rotational centrelines of the
wheels, where one plane is for the front wheels and the other for the rear wheels
Note 1 to entry: In the case of a tractor equipped with a rear tandem, it is the distance between two vertical
planes passing through the centres of the front wheel and the vertical plane midway between the wheel centres
of the two axles of the tandem.
[SOURCE: ISO 789-13, 3.2, modified]
4 Apparatus
The following apparatus are required.
4.1 Weighbridge or load cells.
4.2 Crane.
4.3 Decking with knife edges.
4.4 Level.
4.5 Plumb rule.
4.6 Squares.
4.7 Scribing board, shall be at least 600 mm high by 450 mm wide, rigidly constructed, and attached
to the tractor in a suitable position with a smooth face vertical and parallel to the side or other
appropriate plane.
4.8 Marking materials.
4.9 Tape measure.
5 General requirements
5.1 The tractor shall be clean and shall be tested in normal working conditions or in a specified
condition agreed between the manufacturer and the testing authority.
5.2 The radiator, sump, hydraulic and other reservoirs shall be filled to specified working levels. The
fuel tank shall be full and the driver replaced by a weight of 75 kg on the driver’s seat, the tractor being
otherwise unballasted.
5.3 Tools, spare tyre, and loose accessories and equipment shall be complete as supplied and shall be
in the normal storage positions.
5.4 Any suspension system shall be locked in the normal height position. If locking the suspension
system is not possible, then inflate all tires up to the maximum permissible pressure as specified by the
tyre manufacturer. The difference in radius of the wheels on the fixed axle between horizontal and raised
position shall not be higher than 1,5 % of the wheel radius.
5.5 Articulated tractors shall normally be tested locked in a straight line, but it may be necessary to
conduct the test with the joint set at the maximum or any intermediate angle.
5.6 If testing a sprung tractor, no special measures shall be taken to lock the suspension of the machine.
5.7 In conducting the test, the following measurement tolerances shall be observed:
a) distance: ±0,5 %;
b) mass: ±0,5 %;
2 © ISO 2019 – All rights reserved

c) tyre pressure: ±5 %.
Tyre pressure should be in accordance with the manufacturer’s recommendations.
6 Procedure
6.1 General principle
The centre of gravity is determined by the suspension and ground reaction method. This involves
measuring the ground reactions with the tractor:
a) in a horizontal position;
b) tilted with one end lifted;
c) tilted with the other end lifted.
The calculated horizontal distance of the centre of gravity from a ground contact point is measured in
each case and verticals are drawn on the scribing board (4.7) fixed to the tractor. The intersection of
the verticals indicates the centre of gravity.
6.2 Determination of horizontal fore-and-aft coordinate ( x)
6.2.1 Tracked tractors [see Figure 1 a)]
Determine the mass (m) of the whole tractor on the weighbridge.
Measure the reaction (F ) under the knife edge due to its mass and part of the decking.
Move the tractor on to the decking, part supported by the weighbridge, and measure the reaction at
the front knife edge due to the mass of the tractor, the decking and knife edge (F + F ). Calculate the
1 2
reaction of the front knife edge due to the tractor mass only (F ) by subtraction.
Measure the distance (d) between the knife edges.
The horizontal fore-and-aft coordinate is given by Formula (1):
dF⋅
x = (1)
m
6.2.2 Wheeled tractors [see Figure 1 b)]
In the case of wheeled tractors, it is not necessary to use decking or knife edges. With the brakes off,
measure the axle loads and calculate x from the mass and the wheelbase of the tractor using Formula (1)
(using the wheelbase as the value for d).
6.3 Determination of vertical coordinate ( h) (see Figure 2)
6.3.1 Suspend the tractor from one end at an angle of 20° to 25° to the horizontal, the other end resting
on the weighbridge. For some tractors, it may not be practical or safe to use an angle this large. In this
case, a lesser angle may be used but shall not be less than 15° to the horizontal. (The method is applicable
either to wheeled or tracked tractors, the main difference being in establishing the exact location of the
point of application of the ground contact. In the case of wheeled tractors, which shall be unbraked, this
is vertically below the axle. In the case of tracked tractors, it is necessary to manoeuvre until the contact
grousers are in the line of ground contact BB’ on either side, or to make contact through a knife-edge on
the ground contact line BB’. In all cases, the suspension cable shall be vertical as tested by plumb rule.)
6.3.2 Measure the reaction (F ) at the ground contact on the weighbridge.
6.3.3 Measure the horizontal distance (d) from the ground contact to the line of suspension.
6.3.4 Calculate the horizontal distance (c) from the centre of gravity to the line of suspension using
Formula (2):
Fd⋅
c = (2)
m
where m is the mass of the tractor.
6.3.5 Draw a vertical line on the scribing board at a distance c from the line of suspension.
6.3.6 Repeat the procedures specified in 6.3.1 to 6.3.5 with the tractor suspended from the other end.
The suspension angle need not be the same for both ends.
6.3.7 The intersection of the two lines on the scribing board, determined as specified in 6.3.5 and 6.3.6,
gives the vertical coordinate of the centre of gravity ( h).
NOTE 1 The tractor can be conveniently run on to the weighbridge, square, using chalked lines. This will assist
in drawing the plan. If, in the case of tracked tractors, the grousers are not in the ground contact line BB’ (see
Figure 2), the tractor can be driven in varying circles until the required result is attained at the last approach.
NOTE 2 An alternative method is to use a tilting platform and load cells respectively.
6.3.8 Alternative determination of vertical coordinate ( h) (See Figures 4 to12).
Repeat the procedures specified in 6.3.1 to 6.3.4 with the tractor suspended from the other end. The
suspension angle need not be the same for both ends.
Vertical coordinate ( h) of the centre of gravity is determined by means of calculation (see Tables 1, 2,
3 and 4)
NOTE The tractor can be conveniently run on to the weighbridge, square, using chalked lines. This will assist
in drawing the plan. If, in the case of tracked tractors, the grousers are not in the ground contact line BB’ (see
Figure 2), the tractor can be driven in varying circles until the required result is attained at the last approach.
6.4 Determination of lateral coordinate in the horizontal plane ( y) (see Figure 3)
Measure the left-hand (F ) and right-hand (F ) wheel or track loadings. Calculate the offset (b) of the
4 5
centre of gravity using track gauge or wheel track (d ) as the moment arm, i.e. Formula (3):
t
Fd
5t
b= (3)
m
The lateral coordinate in the horizontal plane is given by the Formula (4);
d
t
y=−b (4)
NOTE The right-hand and left-hand side loads usually do not exactly total the mass of the tractor due to
small differences in level between the weighbridge deck and the surround. Any error is minimized by equalizing
the overlap of the side being weighed in both case
...