Overview
ISO 9112:2008 - "Truck and bus tyres - Methods of measuring tyre rolling circumference - Loaded new tyres" defines standardized test methods for determining the rolling circumference and revolutions per unit distance (per km) of new commercial vehicle tyres under loaded conditions. The standard establishes two measurement procedures (a drum method and a vehicle method) and provides environmental, loading and accuracy requirements so results are repeatable and comparable across laboratories. ISO 9112:2008 applies to all truck and bus tyres.
Key topics and technical requirements
- Two test methods
- Drum method: A test tyre on a free axle is loaded against a driven road wheel (drum). Rolling circumference is derived from the ratio of drum and tyre revolutions and the drum radius. Formula: C = 2π × (N_d / N_t) × R (where N_d = drum revolutions, N_t = tyre revolutions, R = drum radius).
- Vehicle method: A representative two‑axle vehicle with test tyres on the drive axle is driven over a measured straight road; tyre revolutions are counted over the measured distance and used to compute circumference (L / N) and revolutions per km ((N / L) × 10^3).
- Loading and inflation
- Standard test load = 85% of the tyre’s maximum single (or dual) load capacity.
- Inflation pressure = manufacturer-specified pressure corresponding to maximum load capacity (capped).
- Test conditions
- Drum diameter ≥ 1.7 m; drum surface preferably smooth steel.
- Temperature ranges and equilibration times specified (thermal equilibrium prior to test).
- Test speeds depend on tyre load index and speed symbol (e.g., 80 km/h or 60 km/h), with defined speed tolerances.
- Accuracy and instrumentation
- Drum method: minimum 10 impulses per revolution, total impulses > 1 000.
- Vehicle method: minimum 16 impulses per revolution, counting error ≤ 0.1%.
- Annex A tolerances: alignment (mrad), tyre load ±100 N, inflation ±5 kPa, surface velocity ±1 km/h, drum measurement ±1 mm.
Applications
- Standardizing tyre rolling circumference data used by vehicle electronic systems (speedometer, odometer, telematics).
- Ensuring tyre interchangeability and consistent distance/speed indication across tyres and vehicles.
- Laboratory testing and validation of tyre data supplied to OEMs, telematics/ADAS vendors, fleet managers and calibration houses.
Who uses ISO 9112:2008
- Tyre manufacturers (product data and conformity testing)
- Vehicle OEMs (speedometer/odometer calibration and integration)
- Independent test laboratories and certification bodies
- Fleet operators, telematics providers and regulators concerned with accurate distance and speed measurement
Related standards
- ISO 4209-1 (tyre and rim dimensions/specifications) - referenced for diameter and measurement procedures.
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