Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Minimum Requirements for Motor Vehicle Brake Linings – SAE J998-1968
SAE J998-1968, titled ‘Minimum Requirements for Motor Vehicle Brake Linings’, is a recommended practice established by the SAE Brake Committee. It sets baseline coefficient of friction criteria for brake linings used in service brakes on public-road vehicles. While advisory and voluntary, this standard provides a common benchmark for lining performance under defined temperature conditions.
Scope and Purpose of SAE J998-1968
According to the standard’s first section, this specification covers brake linings for motor vehicles operated on public roads, with the exception of linings intended solely for parking brakes. The performance requirements are based on engineering data available at the time, and the recommended practice is intended to be reviewed periodically as knowledge advances.
The second section clarifies that the purpose is to establish minimum coefficient of friction requirements for linings used in the service brake system. Importantly, linings meeting these minimums may not be suitable for every vehicle due to differences in brake design or application.
📋 All SAE technical reports, including standards and recommended practices, are advisory only. Their use by anyone engaged in industry or trade is entirely voluntary. There is no agreement to adhere to any SAE Standard or SAE Recommended Practice.
Minimum Friction Requirements and Testing Methodology
To meet the minimum requirements, a brake lining must achieve specific coefficient of friction values under both normal and hot conditions, tested and evaluated per SAE J661 and SAE J866 respectively. Five complete tests are conducted, and the results are averaged.
| Parameter | Requirement |
|---|---|
| Normal friction coefficient (average of 5 tests) | Over 0.25 |
| Hot friction coefficient (average of 5 tests) | Over 0.15 |
| Individual test at each specified temperature point | 0.15 or above |
| Maximum variation below the average at each temperature point | 20% or 0.050 (whichever is greater) |
The temperature points are defined as follows: for the second fade run, between 200°F and 550°F inclusive; for the second recovery run, between 300°F and 600°F inclusive. Each of the five test specimens must maintain a coefficient of friction of 0.15 or over at every temperature within these intervals.
The variation limit ensures consistency: at each temperature point, no individual test value may fall more than 20% or 0.050 below the average of all five tests, whichever is greater.
⚠️ Important: Meeting the minimum friction requirements does not guarantee that a lining will perform adequately in all vehicle applications. Brake system design, operating conditions, and vehicle-specific demands must be carefully evaluated.
🛠️ Engineering Design Insight: When selecting brake linings for a specific application, consider not only the coefficient of friction but also the temperature sensitivity and fade-recovery characteristics. The variation limit of 20% or 0.050 ensures a degree of stability, but the average values at elevated temperatures must still satisfy the hot friction coefficient of 0.15. For consistent brake performance, linings should be tested across the full temperature range specified, and the results should be compared with system-level requirements.
Frequently Asked Questions
1. What is the difference between normal and hot friction coefficient?
Normal friction coefficient is the average from five tests measured under standard conditions before heat buildup. Hot friction coefficient is the average measured after heating, representing performance under high-temperature conditions such as brake fade and recovery.
2. What happens if a brake lining does not meet the variation limit?
The lining fails the minimum requirements. The variation must not exceed 20% or 0.050 below the average at each temperature point. Exceeding this limit indicates inconsistent friction behavior that could compromise brake performance under real-world conditions.
3. Is compliance with SAE J998 mandatory?
No. SAE J998-1968 is a recommended practice, entirely advisory and voluntary. However, it is frequently referenced as a benchmark for brake lining quality and is often incorporated into procurement specifications by manufacturers and suppliers.
4. What roles do SAE J661 and SAE J866 play?
SAE J661 defines the test procedure for brake linings, including the sequence of burnish, fade, and recovery runs. SAE J866 provides the evaluation method for determining normal and hot friction coefficients from the test data. Both are integral to the qualification process under SAE J998.
