Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
SAE J1247 Simulated Mountain-Brake Performance Test: Key Insights and Best Practices
Understanding the Scope and Purpose of SAE J1247
SAE J1247 (Stabilized May 2014) provides a standardized flat-road procedure to simulate the power and energy duty cycles encountered during mountain descents. It applies to light-duty trucks, multipurpose passenger vehicles up to 4500 kg GVW, and all passenger cars. The test evaluates brake temperature relative to fluid boil, fade resistance and reserve pedal travel, overall structural durability, and subjective stability. By replicating the longer descents typical of western U.S. mountain roads, this procedure complements the fade schedules in SAE J843, which require higher average power input for shorter durations.
Essential Instrumentation and Test Conditions 🛠️
Accurate instrumentation is critical for repeatable results. The standard recommends using line pressure gauges rather than pedal force for a more precise input measure. The table below lists key instruments and their roles.
| Instrument | Measurement | Key Requirement |
|---|---|---|
| Line pressure or pedal force gauge | Brake apply input | Line pressure preferred for precision |
| Decelerometer | Deceleration rate | Readings not taken below 8 km/h |
| Speedometer (fifth-wheel type recommended) | Vehicle speed | Calibrated |
| Thermocouple assembly | Brake lining, fluid, drum/rotor temperatures | Install per SAE J843, J291, J79 |
| Ambient thermometer | Ambient temperature | 4–32 °C range; record wind (<16 km/h) |
The test must be performed on a dry, level surface (±1% grade) with the vehicle at GVW unless otherwise specified. The powertrain should be selected to provide the least engine braking for production vehicle evaluations, and vehicle drag characteristics must be recorded.
Engineering Design Insights and Common Mistakes ⚠️
🔍 Design Insight: Line pressure usage provides a more precise measure of brake input than pedal force, eliminating variability from mechanical linkages and driver technique. New drums or rotors are recommended for consistency, and component uniformity (surface finish, mass, runout) should be controlled to avoid affecting high-temperature brake balance.
⚠️ Common Mistakes: Not controlling ambient temperature (4–32 °C) and wind velocity (<16 km/h), using incorrect test weight (not at GVW), interrupting the continuous procedure (paragraphs 5.3 to 5.10 must run uninterrupted), and using pedal force instead of line pressure. Also ensure proper thermocouple installation per SAE standards.
The test includes a burnish cycle (200 stops from 64 to 0 km/h at 3.66 m/s² deceleration), followed by warming brakes to 65–95 °C, then a continuous sequence of snubs. The snub final speed of 27 km/h is based on full-size domestic vehicles with automatic transmissions. The test must be terminated if pedal force reaches 900 N or if pedal travel becomes excessive.
Frequently Asked Questions
Why is line pressure recommended over pedal force?
Line pressure removes the influence of friction losses and pedal ratio variations, giving a direct and repeatable measure of hydraulic input to the brakes. This is especially important during high-temperature fade conditions where pedal feel can change.
What ambient conditions are required for the test?
Ambient temperature must be between 4 °C and 32 °C (40–90 °F), and wind velocity should be less than 16 km/h (10 mph). These limits ensure that external factors do not significantly affect cooling rates or brake performance.
How does brake component uniformity affect results?
During the high-temperature portion of the test, asymmetry in drum/rotor mass, surface finish, or runout can cause unbalanced braking (pull or swerve). For evaluations of lining stability, components should be matched left-to-right to isolate the effect of the friction material.
When should the test be terminated?
The procedure should be aborted if pedal force reaches 900 N (200 lb) or its line pressure equivalent, if pedal travel becomes excessive, or if the vehicle becomes unsafe. This prevents damage and ensures driver safety.
