Overview
ISO 11795:2018 - "Agricultural tractor drive wheel tyres - Explanation of rolling circumference index (RCI) and speed radius index (SRI) and method of measuring tyre rolling circumference" - specifies a repeatable method for measuring tyre rolling circumference for new agricultural tractor drive‑wheel tyres (diagonal and radial). The standard also explains the Rolling Circumference Index (RCI) and the Speed Radius Index (SRI) used for tyre selection, coupling and theoretical speed calculations.
Key topics and technical requirements
- Test scope: Measurement of rolling circumference for new tyres mounted on representative drive axles under loaded conditions.
- Mounting and pairing: Tyres must be mounted singly on approved rims and paired (same size, type, brand) with exterior diameter matching within 0.5% when unladen and inflated.
- Load and inflation: Each tyre is loaded to the maximum load printed on the sidewall; inflation pressure is the reference pressure corresponding to the 30 km/h load (inflation with air only).
- Preconditioning: Tyres are run 30 minutes at 25 km/h ±5 km/h (or until pressure stabilizes) before measurement.
- Measurement procedure:
- Test speed: 10 km/h ±2 km/h.
- Track: straight, level, smooth and dry (asphalt or concrete).
- Distance: minimum 100 m; at least 1 000 impulses recorded per test.
- Instrumentation: each test tyre must provide at least eight pulses per wheel revolution; distance may be measured directly or with a calibrated fifth wheel.
- Two runs in each direction; average of eight observations (4 runs × 2 tyres) used to compute rolling circumference.
- Calculation: Rolling circumference RC (mm) = 1000 × S (m) × ipr / n, where S = track length, ipr = pulses per revolution, n = mean impulses recorded.
- Environmental limits: Ambient temperature 5 °C to 30 °C; wind ≤ 15 km/h.
- RCI and SRI:
- RCI provides a logical progression of rolling circumferences (each step ≈ 5.4% larger). RCI values have tolerances (±0.2). R‑1 and R‑1W tyres are recommended to use integer RCI values; R‑2 RCI values are typically +0.5.
- SRI is a convention for calculating theoretical tractor speed during homologation/interchangeability and is not a directly measurable circumference.
Practical applications and users
- Useful for tyre manufacturers, tractor OEMs, test laboratories and regulatory bodies to:
- Ensure correct tyre coupling for 4WD tractors (minimise driveline penalties from mismatched rolling circumferences).
- Define tyre designations and RCI/SRI markings for interchangeability and homologation.
- Produce authoritative test reports supporting tyre selection and vehicle calibration (speedometers, transmission matching).
Related standards
Keywords: ISO 11795:2018, rolling circumference index (RCI), speed radius index (SRI), tyre rolling circumference, agricultural tractor drive wheel tyres, method of measuring tyre rolling circumference.
Open preview (PDF)Agricultural tractor drive wheel tyres — Explanation of rolling circumference index (RCI) and speed radius index (SRI) and method of measuring tyre rolling circumference