Overview
ISO 19940:2017 defines a laboratory testing procedure to determine the tyre stiffness index (TSI) for passenger car extended mobility tyres (EMT) and run flat tyres (RFT/SST). The method characterizes tyre stiffness as the combined contribution of the tyre’s air stiffness and structural stiffness by measuring vertical force and absolute vertical deflection under controlled conditions. This standard applies to tyres designed to sustain basic vehicle functions in flat-running mode (at least 80 km/h for 80 km per ISO 16992) and is not intended for conventional tyres.
Key topics and technical requirements
- Measured parameters: vertical loading force and absolute vertical position (deflection) in the Z direction at multiple inflation pressures.
- Inflation pressure steps: reference pressure per ISO 4000-1 (typically 250 kPa standard or 290 kPa reinforced), plus 100 kPa, 70 kPa and 30 kPa. A mathematical approximation is used to derive the zero-inflation (0 kPa) structural curve.
- Stiffness components:
- Vertical stiffness (Kz) - tyre stiffness under inflated conditions (N/mm).
- Structural stiffness (Kstr) - stiffness measured/approximated at zero inflation pressure (N/mm).
- Air stiffness (Kair) - computed as Kz − Kstr (N/mm).
- Test machine and data:
- Plate dimensions larger than tyre footprint; tyre rotational axis parallel to plate.
- Maximum loading speed: 1 mm/s; acquisition rate: at least 5 data points per mm of displacement.
- Record channels: loading force (accuracy ±0.5% of cell capacity, max ±100 N) and absolute position (±0.5 mm).
- Measure at three circumferential positions (~120° apart) and repeat per pressure step.
- Loading limits: maximum test load = 150% of the tyre’s maximum load associated with its load index (per ISO 4000-1).
- Environmental/conditioning: assembly conditioning (e.g., 3 h at 25 ± 3 °C) and ambient test temperature (25 ± 5 °C).
Applications and who uses it
- Tyre manufacturers - R&D, design verification and product characterization of EMT/RFT technologies.
- Independent test laboratories - standardized TSI measurement for certification and comparison.
- Automotive OEMs - selection and validation of spare or run-flat tyre systems for vehicle integration and safety validation (linked to ISO 16992 requirements).
- Regulatory bodies & conformity assessors - harmonized test data to support compliance, labeling or performance claims.
- Quality assurance teams - production validation and incoming inspection of run-flat/extended mobility tyres.
Related standards
- ISO 16992 - requirements for spare unit substitutive equipment (SUSE) and flat-running performance.
- ISO 4000-1 / ISO 4000-2 - passenger car tyre dimensions, load/speed indexing and rim specifications referenced for test conditions.
ISO 19940:2017 is essential for anyone needing reproducible, standardized measurements of run-flat and extended mobility tyre stiffness (TSI), enabling fair comparison, product optimization and regulatory compliance.