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ISO 26867:2009 Brake Lining Friction Materials — Friction Behaviour Assessment for Automotive Brake Systems
Inertia-Dynamometer Friction Evaluation Replacing Traditional Drag-Based Test Protocols
1. Introduction and Scope
ISO 26867:2009, prepared by ISO/TC 22/SC 2, describes a comprehensive inertia-dynamometer test procedure for assessing the influence of pressure, temperature, and linear speed on the coefficient of friction of brake lining friction materials. This standard represents a fundamental shift from earlier test protocols that depended solely on drag brake applications, which failed to capture real-world driving conditions or vehicle-specific dynamic parameters.
The standard addresses a critical industry problem: drag-based friction evaluation does not correlate with real-world braking because it ignores the dynamic formation of transfer layers, speed-dependent friction variation, and the influence of vehicle mass distribution and dynamic load transfer during braking.
ISO 26867 is designed for comparing friction materials under identical conditions and for controlling friction behaviour against specifications or performance limits. It supports friction assessment throughout the entire product lifecycle — from early screening and benchmarking through development, production validation, quality control, and field issue evaluation. The standard includes provisions for additional sections and brake applications useful during product development testing.
2. Test Conditions and Preparation
2.1 Inertia and Wheel Load Calculation
Front axle inertia is calculated using 75 % of half the gross vehicle mass, while rear axle inertia uses 25 %, reflecting typical braking force distribution in passenger vehicles. Dynamic wheel load calculations incorporate static loading and mass transfer at 0.3 g deceleration, using Equations (4) and (5) that consider wheelbase, centre of gravity height, and gross vehicle mass.
| Parameter | Requirement | Tolerance |
|---|---|---|
| Pressure ramp rate | 25,000 kPa/s | ±5,000 kPa/s |
| Sampling rate | ≥100 Hz | For pressure and torque |
| Thermocouple depth | 0.5 mm below surface | ±0.1 mm |
| Lateral run-out (disc) | ≤50 μm | Measured 10 mm from OD |
| Cooling air temperature | 20 °C | ±5 °C |
| Absolute humidity | 7.29 g/kg | At sea level |
2.2 Fade Section Temperature Control
The fade sections use logarithmically increasing initial temperatures calculated using Equation (6), ensuring progressively severe thermal conditions. For disc brakes, the temperature rises from 150 °C to 550 °C over 15 stops; for drum brakes, from 100 °C to 300 °C. The logarithmic progression (rather than linear) better represents the thermal accumulation characteristics of real-world repeated braking scenarios.
3. Test Procedures and Friction Value Analysis
3.1 Product Monitoring vs. Development Testing
The standard defines two tiers of testing: a baseline product monitoring procedure (Table 2, 20 steps, ~253 brake applications) and an extended product development procedure (Table 3, 20 steps with additional applications at optional pressures and speeds). The product monitoring version includes characterisation, burnish, pressure line, speed line, failed booster, motorway, fade, and hot performance sections. The development version adds low-speed/low-pressure characterisation with finer resolution and optional high-pressure points up to 12,000 kPa.
| Section | Purpose | Control Method | Key Output |
|---|---|---|---|
| Green characteristic | Initial friction baseline | Fixed pressure (3,000 kPa) | Green friction level |
| Burnish | Transfer layer development | Variable deceleration (0.17–0.57 g) | Stabilized friction |
| Pressure line (1)/(2) | Pressure sensitivity | 1,000–6,000 kPa increments | Friction vs. pressure |
| Speed line (1) | Speed sensitivity | 80–200 km/h | Friction vs. speed |
| Failed booster | Emergency backup performance | 2,800 kPa or vehicle-specific | Stopping distance |
| Fade (1)/(2) | High-temperature fade resistance | 0.40 g deceleration, log temp ramp | Minimum fade friction |
| Hot performance | Post-fade recovery | Pressure line at 500 °C/300 °C | High-temp effectiveness |
3.2 Instantaneous Friction Statistics
A distinctive feature of ISO 26867 is its requirement for statistical analysis of instantaneous friction values. The standard mandates histograms and cumulative frequency curves with 0.02 class size, distinguishing between two friction distributions: all sections (excluding green and burnish) and low-speed/low-pressure sections only. Statistical parameters such as 10th, 50th, and 90th percentiles provide engineers with detailed insight into friction stability — critical for modern brake control systems including ABS, ESP, and VSC that depend on consistent friction behaviour.
The use of average-by-distance torque and pressure calculations (rather than simple arithmetic means) ensures that friction values accurately represent energy dissipation across the entire brake application, giving engineers reliable data for system-level performance modelling.
4. Engineering Design Insights
For brake system engineers, ISO 26867 represents a paradigm shift in how friction materials are characterized. Key practical considerations include:
Cooling air conditioning: The standard specifies nominal cooling air conditions of 20 ± 5 °C and 50 ± 10 % relative humidity. Absolute humidity of 7.29 g/kg at sea level is the reference — humidity significantly affects transfer layer formation and friction level, making environmental control essential for reproducible results.
Brake cooling rate measurement: Recording the cooling time from 500 °C to 200 °C (disc) or 300 °C to 100 °C (drum) immediately after the hot performance section provides valuable thermal characterization data that correlates with vehicle-level brake cooling performance.
Optional development applications: The provision for additional brake applications during development testing — such as 8,000–12,000 kPa pressure points and 90 % vmax motorway stops — gives engineers flexibility to explore the friction envelope beyond standard conditions without compromising the core monitoring protocol.
Statistical process control integration: Production verification testing can leverage the results from this test in conjunction with a statistical process control (SPC) system as part of a quality assurance plan. The standard notes that specific projects will detail applicable limits and assessment criteria, making it adaptable to different quality management frameworks across automotive manufacturers worldwide.
The standard explicitly states that ISO 26867 does NOT include performance requirements related to stopping distance or braking force distribution. It is a characterization tool, not a pass/fail specification. Engineers must establish project-specific acceptance limits based on their particular application requirements.
5. FAQs
Q1: What vehicles does ISO 26867 apply to?
The standard applies to passenger cars and light-duty vehicles, complementing ISO 26866 which covers commercial vehicles with air brakes.
The standard applies to passenger cars and light-duty vehicles, complementing ISO 26866 which covers commercial vehicles with air brakes.
Q2: How does this standard differ from older drag-based friction tests?
Drag tests apply constant low-speed sliding, which does not replicate real braking events. ISO 26867 uses inertia-dynamometer stops and snubs that reproduce the speed, pressure, and thermal dynamics of actual vehicle braking.
Drag tests apply constant low-speed sliding, which does not replicate real braking events. ISO 26867 uses inertia-dynamometer stops and snubs that reproduce the speed, pressure, and thermal dynamics of actual vehicle braking.
Q3: What is the significance of the histogram analysis?
Histograms of instantaneous friction reveal friction stability and spread — data that is critical for modern brake control algorithms (ABS, ESP) that require predictable friction behaviour across all operating conditions.
Histograms of instantaneous friction reveal friction stability and spread — data that is critical for modern brake control algorithms (ABS, ESP) that require predictable friction behaviour across all operating conditions.
Q4: Can the test be shortened for production monitoring?
Yes — the product monitoring procedure (Table 2) omits optional brake applications and uses fewer pressure line points, providing a streamlined protocol while maintaining all critical friction assessment sections.
Yes — the product monitoring procedure (Table 2) omits optional brake applications and uses fewer pressure line points, providing a streamlined protocol while maintaining all critical friction assessment sections.
