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SAE J937b: Service Brake System Performance Requirements for Passenger Cars
This article provides an in-depth look at SAE J937b, a recommended practice that establishes minimum service brake system performance requirements for new passenger vehicles. It covers stopping ability, pedal force limits, brake stability, fade and recovery, water recovery, and system integrity. Engineers and test technicians can use this guide to understand the key criteria and avoid common testing pitfalls.
Purpose and Scope
SAE J937b applies to brake systems of new motor vehicles intended for roadway use and falling in Uniform Vehicle Code Classification A, excluding special-purpose passenger vehicles. The standard sets minimum performance baselines for stopping ability under various conditions, including cold and hot brakes, emergency and inoperative power assist scenarios, and wet conditions. It also defines pedal force limits and stability requirements to ensure safe and predictable braking.
Key Performance Requirements 🛠️
The standard specifies strict pedal force ranges for achieving a deceleration of 20 ft/s² from different speeds. These are summarized in the table below:
| Vehicle Speed | Required Deceleration | Pedal Force Range (lb) | Pedal Force Range (N) |
|---|---|---|---|
| 30 mph (48 km/h) | 20 ft/s² (6.1 m/s²) | 15 – 100 | 67 – 445 |
| 60 mph (97 km/h) | 20 ft/s² (6.1 m/s²) | 15 – 120 | 67 – 534 |
| 80 mph (129 km/h) (where applicable) | 20 ft/s² (6.1 m/s²) | 20 – 150 | 89 – 667 |
Additionally, the first fade test requires that pedal force for the first four 15 ft/s² stops not exceed 120, 147, 173, and 200 lb respectively. The second fade has a more gradual increase over eight stops.
⚠️ Note: The vehicle must remain within a 12-ft wide lane during all braking maneuvers to meet stability requirements.
Design Insights and Testing Compliance
Engineers should design brake systems to meet these pedal force vs. deceleration requirements across a range of speeds. Fade resistance is critical; the brake system must manage heat dissipation to keep pedal forces within limits during repeated stops. Water recovery testing ensures that after wetting, the system returns to baseline performance within 20 lb of pedal force by the 15th stop.
🔍 Engineering Insight
Brake lining integrity and hydraulic system integrity are part of the final inspection. Minor cracks that do not impair attachment are acceptable for linings, but all components must be intact and functional. Leak-free hydraulic systems are mandatory.
Common mistakes during testing include confusing the first and second fade stop sequences, neglecting pre-burnish check requirements, and misinterpreting recovery test criteria. It is essential to reference SAE J843d for detailed test procedures.
⚠️ Important
Never overlook the stability requirement: uncontrolled braking that causes the vehicle to leave a 12-ft lane is not permissible below 20 ft/s² deceleration. This is a key safety criterion.
Frequently Asked Questions
What are the pedal force limits for different speeds?
From 30 mph, pedal force must be between 15 and 100 lb for 20 ft/s² deceleration. At 60 mph, the range is 15-120 lb, and at 80 mph it is 20-150 lb.
How is the fade test conducted?
The first fade test involves making four stops at 15 ft/s² deceleration from a specified speed, with allowable pedal force increasing from 120 lb to 200 lb. The second fade test has eight stops with a more gradual force increase.
What does the water recovery test require?
After wetting, pedal force for 8 ft/s² stops must not exceed 200 lb during stops 4-6, 100 lb during stops 7-14, and must be within 20 lb of the baseline average at stop 15.
What are common mistakes in testing to SAE J937b?
Common errors include mixing up the fade stop sequences, applying incorrect pedal force limits for different speeds, neglecting the pre-burnish check, and failing to ensure the vehicle stays within the 12-ft lane during braking.
