ISO 4251-2:2019

Overview

ISO 4251-2:2019 specifies the tyre load ratings for code-designated diagonal (ply‑rating marked, PR) tyres used on agricultural tractors, trailers and machines. It defines how load capacities are marked and assessed, establishes basic tyre loads (BTL) and reference inflation pressures, and sets rules for load variation with speed and special services (e.g., drive wheel, steering wheel, implement tyres). Tyre designation/dimensions and rim contours are covered in ISO 4251‑1. Note: construction (R‑4/IND) and forestry (LS) code‑designated tyres are handled in ISO 13442 and ISO 18807 respectively.

Key topics and technical requirements

• Scope and definitions: Terms such as ply rating, supplementary service description, cyclic loading, low/high torque and drive/free‑rolling tyres are defined in alignment with ISO 4223‑1.
• Tyre markings: Mandatory PR (ply rating) marking and optional service description (load index + speed symbol). Examples from the standard include:
  • 14.9-28 8PR 130A6 (126A8)
  • 6.50-16 6PR 91A6 (88A8)
• Basic tyre loads (BTL): BTL values for single tyres and corresponding reference inflation pressures are given in normative Annex A (Table A.1/A.3). Where no service description is marked, BTL at reference speed 30 km/h and the load/speed relationship in Table A.2/A.4 (column A6) apply.
• Load/speed relationship: Tyre loads vary with service speed; specific tables quantify permitted load reductions or equivalences for speed symbols (A6, A8, etc.). Supplementary service descriptions have their own rules and are exempt from some tables.
• Dual and triple mounting: Load factors to adjust BTL when tyres are run as duals (multiply BTL by 0.88) or triples (multiply by 0.82).
• Special applications:
  • Combine harvesters (cyclic loading): up to 170% of BTL for speeds ≤10 km/h with ~30% higher inflation pressure (manufacturer consultation required).
  • Marking distinctions for drive vs free‑rolling (trailer) and mixed‑application tyres.
• Normative annexes: Annex A (BTL and load/speed relationships) and Annex B (tyres with specific rim codes and small agricultural tyres).

Applications and who uses this standard

• Tyre manufacturers: to determine and mark load ratings, produce compliant diagonal PR tyres.
• Agricultural equipment OEMs and wheel/rim manufacturers: to specify compatible tyres, consult wheel strength and rim contours, and ensure safe load/pressure pairings.
• Fleet owners and operators: to select tyres matched to expected loads, speeds and applications (e.g., field work, transport, hillside combines).
• Test laboratories and certification bodies: to verify conformity with load/marking requirements.
• Regulators and procurement teams: to reference objective load‑rating rules in specifications and regulations.

Related standards

• ISO 4251‑1 (tyre designation and dimensions)
• ISO 4223‑1 (tyre industry terminology)
• ISO 18805 / ISO 18807 / ISO 13442 (construction, forestry, industrial tyre classifications and related rules)

Keywords: ISO 4251-2:2019, tyre load ratings, code designated diagonal tyres, ply rating (PR), agricultural tyres, basic tyre loads, load/speed relationship, tyre markings, combine harvester tyres.