ISO 4251-1:2019

Overview

ISO 4251-1:2019 - "Code designated diagonal tyres (ply rating marked series) for agricultural tractors, trailers and machines - Part 1: Tyre designation and dimensions, and approved rim contours" defines the identification, dimensional data and approved rim contours for code designated diagonal tyres (ply‑rating marked series) used on agricultural tractors, trailers and machines. The standard is limited to diagonal construction tyres (marked with PR) and does not cover radial drive tyres or certain construction/forestry classifications (these are covered by other ISO standards).

Key topics

• Tyre designation and marking
  • Size designation formats for normal and low section height tyres (examples: 13.6–28, 9.5L–15, 9.5/85–15).
  • Required sidewall markings: tyre size, ply rating (PR), service description and category codes (e.g., R‑2 for tractor drive, F or FRONT for steering tyres, IMP/IMPLEMENT for implement tyres).
  • Optional/required markings such as TUBELESS, preferred rotation arrow, and the bead‑seating maximum pressure pictogram.
• Dimensional requirements
  • Design dimensions for new tyres, maximum dimensions in service (for vehicle clearance) and minimum new tyre overall diameter calculations. Example formula provided: D = d + 2 · 0.97 · H (rounded per the standard), where H is the design section height and d is rim diameter.
  • Measurement rims and dimensional tables are given in Annex A.
• Tyre and rim coordination
  • Approved rim contours listed in Annex B; rim dimensional details referenced to ISO 18804.
  • Guidance for correcting tyre section width when mounted on a rim width different from the measuring rim.
• Performance/vehicle reference parameters
  • Rolling circumference (RC), rolling circumference index (RCI) and speed radius index (SRI) used for forward ground speed calculations (Annex D). References ISO 3965 and ISO 11795 for homologation context.
• Supporting definitions & scope limits
  • Terms and definitions aligned with ISO 4223‑1; load ratings are specified in ISO 4251‑2.

Applications

• Used by tyre manufacturers to mark and dimension agricultural diagonal tyres and to specify approved rim contours.
• Used by rim and wheel manufacturers for compatibility and rim contour design.
• Used by agricultural equipment OEMs and vehicle designers for tyre fitment, clearance and speed/gear calculations.
• Relevant to certification bodies, test labs and regulators verifying tyre marking, dimensions and mounting safety.

Related standards

• ISO 4251‑2 (tyre load ratings)
• ISO 4223‑1 (terms and definitions)
• ISO 18804 (rims for agricultural/forestry/construction machines)
• ISO 18805 (tyre classification)
• ISO 8664, ISO 18807, ISO 3965, ISO 11795

Keywords: ISO 4251-1:2019, code designated diagonal tyres, ply rating, agricultural tyres, tyre designation, tyre dimensions, approved rim contours, tyre marking, tyre and rim coordination.