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.
