Overview
ISO 789-3:2015 - "Agricultural tractors - Test procedures - Part 3: Turning and clearance diameters" specifies a standardized method for measuring the turning diameter and clearance diameter of wheeled agricultural tractors fitted with pneumatic tyres. The standard defines terms (track, wheelbase, turning and clearance diameters), required apparatus, test-area and tractor setup requirements, measurement procedures (direct and GPS-based) and reporting requirements.
Key topics and requirements
- Scope: Applies to wheeled agricultural tractors with at least two axles and pneumatic tyres.
- Definitions: Precise definitions of turning diameter (path of tyre contact centre) and clearance diameter (smallest circle enclosing outermost tractor projections) are provided.
- Apparatus:
- Tape measure (length > expected diameter) with tolerance ±0.5%
- Plumb-line (for clearance point marking)
- Corrected GPS equipment (alternate method) with linear accuracy sufficient for ±0.5% tolerance
- Test area: Compacted or paved, dry surface with good tyre adhesion and legible markings; visually flat with no more than 3% grade. For GPS tests, avoid obstructions that affect satellite reception.
- Tractor setup:
- Use tire, wheel and fender equipment as commonly used for intended market; record tyre pressures, ballast and dual-wheel use.
- Standard track (tread) setting: 1 500 mm ± 25 mm (or nearest manufacturer-specified setting).
- Test each operating condition for vehicles with steerable axles, disconnectable axles or selectable drive modes; record results for each condition.
- Procedure:
- Perform sharpest practicable right- and left-hand turns at steering full lock, speed not exceeding 2 km/h.
- Direct method: mark outer tyre contact points around at least one complete turn, use plumb-line to mark outermost projection, measure diameters at three equally spaced positions and average.
- GPS method: record antenna track and coordinates, calculate turning and clearance diameters per the ISO geometry.
- Brake-assisted test (where applicable): apply inside non-steered wheel brake to lock (pedal effort up to 60 daN) and repeat tests.
- Reporting: Test report and specimen form (Annex A) specify required recorded data and measured values.
Applications and users
ISO 789-3:2015 is used by:
- Tractor manufacturers and design engineers for validating steering geometry and packaging
- Test laboratories and certification bodies for performance verification and compliance
- R&D teams and dealerships evaluating manoeuvrability, clearance requirements and field suitability
- Fleet managers and agricultural equipment specifiers comparing tractors for confined-field and transport operations
Keywords: ISO 789-3:2015, agricultural tractors, turning diameter, clearance diameter, tractor test procedures, GPS measurement, track setting, wheelbase, tyre pressure.
Related standards
Part of the ISO 789 series (tractor test procedures). Related parts include ISO 789-1 (PTO power tests), ISO 789-2 (rear three-point linkage), ISO 789-11 (steering capability) and others in the ISO 789 family.