ISO 23671:2021

Overview

ISO 23671:2021 specifies a standardized method to measure the relative wet grip braking performance of loaded, new passenger‑car tyres on a wet‑paved surface. The standard defines how to compare a candidate tyre’s braking performance to a reference tyre (SRTT) to reduce test variability. It applies to most passenger‑car tyres, excluding special‑use tyres marked “ET”, T‑type temporary spares and tyres fitted with traction devices (e.g., studded tyres).

Key topics and technical requirements

• Scope and methods: Two accepted approaches - the vehicle method (tyre set mounted on a commercial vehicle) and the trailer/tyre test vehicle method (instrumented trailer or dedicated tyre test vehicle).
• Reference tyre requirement: Use of a controlled reference tyre (SRTT) is mandatory to limit variability and enable calculation of a relative wet grip index (G).
• Measured quantities: For the vehicle method the average braking force coefficient (BFC) is used; for the trailer/test vehicle method the peak braking force coefficient (µ_peak) and dynamic braking force coefficient (µ(t)) are used to derive the wet grip index.
• Track and surface conditions:
  • Dense asphalt surface with uniform gradient ≤ 2 % and max 3 m straight‑edge deviation ≤ 6 mm.
  • Macro texture depth (MTD) measured per ASTM E965 is specified at around 0.7 ± 0.3 mm.
  • Surface must be free of loose material and have uniform composition and wear.
• Environmental and procedural controls:
  • Wetting, atmospheric and temperature correction procedures are defined (BFC temperature corrections).
  • Homogeneity requirement: BFC values from two reference tyre tests across the braking area shall not differ by more than 10% of their average.
  • Test cycles, conditioning, tyre load, inflation and break‑in requirements are specified to reduce variability.
• Edition updates: The 2021 (3rd) edition improves accuracy of calculations and revises track validation procedures compared to ISO 23671:2015.

Applications and users

• Who uses it:
  • Tyre manufacturers for product development, benchmarking and internal quality control.
  • Independent test laboratories and certification bodies conducting wet braking performance evaluations.
  • Vehicle OEMs assessing tyre‑vehicle interactions for safety and homologation.
  • Regulators and standards bodies establishing performance criteria or labelling requirements.
• Practical uses:
  • Establishing comparative wet‑braking ratings for tyres.
  • Supporting tyre design optimization and R&D.
  • Providing evidence for regulatory compliance, type approval or consumer information.
  • Ensuring repeatable, reproducible wet‑surface braking tests across test facilities.

Related standards

• ISO 4000‑1 (Tyres), ISO 4223‑1 (Tyre terminology) and ASTM E965 (macro texture measurement) are normatively referenced. The reference tyre (SRTT) definition is linked to ASTM F2493.

Keywords: ISO 23671:2021, wet grip performance, passenger car tyres, wet braking test, reference tyre, BFC, tyre testing standard, macro texture depth.