ISO 26866:2009 Brake Lining Friction Materials — Standard Wear Test Procedure for Commercial Vehicles with Air Brakes

1. Introduction and Scope

ISO 26866:2009, developed by ISO/TC 22/SC 2 (Braking systems and equipment), defines a standardized wear test procedure for brake lining friction materials used in commercial vehicles equipped with air brake systems. The standard covers vehicle categories M2, M3, N2, N3, O3, and O4 as specified in UNECE R.E.3 — essentially trucks, buses, and trailers with pneumatic braking systems.

The standard applies during product development, prototype evaluation, specification validation, and ongoing series production as defined in ISO 15484. It is part of the global harmonization program for friction material testing, developed in collaboration with major vehicle manufacturers, brake system suppliers, testing services, and standards organizations including SAE and JIS/JASO.

2. Test Equipment and Measurement Methodology

2.1 Inertia-Dynamometer Requirements

The test uses a single-ended or double-ended brake inertia-dynamometer with computer-controlled test sequencing. Key capabilities include continuous recording of rotational speed, stopping time, brake torque, control line pressure, and disc/drum temperature via embedded thermocouples located 0.5 ± 0.1 mm below the friction surface. The inertia must be matched within ±5 % of the specified value from Table 1.

2.2 Wear Measurement Precision

Pad wear measurements require at least six equally spaced locations (eight if a centre groove is present), with a distance of approximately 12 mm from edges and corners. Thickness measurements demand ±0.01 mm accuracy, while mass measurements require ±0.1 g precision. For drum brakes, the standard explicitly notes that drum wear measurement is inherently inaccurate and therefore not recommended; only mass change is tracked with ±1 g accuracy.

3. Test Sequence and Wear Cycles

3.1 Disc Brake Procedure

The disc brake test comprises 30 steps organized into 5 wear cycles, totaling over 3,000 brake applications. Each cycle includes performance-versus-pressure characterization, a defined number of wear applications at controlled deceleration (1.5 m/s²), and intermediate weigh/measure intervals. The test systematically increases temperature from ambient through 100 °C, 200 °C, 300 °C, 400 °C, and finally 500 °C, with both 60 km/h→10 km/h and 100 km/h→50 km/h speed regimes per temperature level.

3.2 Drum Brake Procedure

The drum brake procedure follows a parallel structure with adjusted temperature targets (120 °C, 180 °C, 230 °C, 290 °C, 340 °C) reflecting the different thermal dynamics of drum brake systems. The test also covers 5 wear cycles with 3,092 total brake applications, using green performance and bedding procedures identical to the disc brake protocol. Characteristic and recovery checks are interspersed between wear cycles to monitor friction stability throughout the test.

The expression of results follows ISO 611:2003 Annex B and ISO 11157:2005 Annexes A and B for mean fully developed deceleration. Test reports must include graphical presentations of wear measurements showing pad/lining wear in millimetres and grams, disc/drum wear, and wear per unit of energy (mm/GJ). The standardized wear rate normalized to 250 stops per step enables direct comparison across different test campaigns and material formulations.

4. Engineering Design Insights

For friction material engineers, ISO 26866 represents a critical tool for generating comparable wear data across suppliers and formulations. The standardized wear rate index — normalized to 250 stops per step — enables direct comparison regardless of minor procedural variations. This normalization is particularly valuable when evaluating material batches over time or comparing competing formulations from different suppliers, as it removes the confounding effects of different test durations or stop counts. Key practical considerations include:

Cooling air management: Ambient temperature cooling air directed perpendicular to the rotation axis is permitted, but the speed must be recorded. The standard allows maximum system cooling speed to reduce test time, provided initial brake temperature targets are achieved — a pragmatic balance between test efficiency and thermal accuracy.

Bedding procedure: The 200-stop bedding sequence establishes a stable transfer layer on the friction interface before baseline measurements, critical for repeatable wear data. If torque variation exceeds 5 % between snubs 3 and 5, additional burnish cycles are required, ensuring consistent surface conditions.

Disc versus drum considerations: The standard’s explicit acknowledgment that drum wear measurement is inaccurate (6.4.4) reflects fundamental metrological limitations — engineers should prioritize mass loss measurements for drum systems over thickness-based approaches.

5. FAQs