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Measurement of Disc Brake Friction Material Underlayer Distribution According to SAE J2724-2016
Why Underlayer Distribution Matters
The underlayer, also known as the backing layer, is the material between the friction compound and the shoe plate. Its distribution across the pad directly influences brake performance, lining life, and wear uniformity. Measuring this distribution per the SAE J2724-2016 recommended practice provides a quantitative metric that engineers can use for quality management and product development. By determining the fraction of underlayer at various heights above the shoe plate, you can compute the usable lining thickness and identify potential issues such as non-uniform application or material inconsistencies.
The Measurement Procedure
The procedure requires a friction material thickness grinder, a micrometer, and either a transparency with a 10 mm grid or a digital camera with image analysis software. Before grinding, visually inspect the lining for any defects (cracks, delaminations, edge lift, un-bonded insulators, or contamination) and record them. Grind the lining to the specified target thickness within ±0.1 mm tolerance.
At each thickness, overlay the grid and count squares that are 50% or more backing layer versus squares that are 50% or more friction material. Sum and calculate the percentage. Alternatively, use photographic image analysis for faster, automated quantification.
Typical target thicknesses for development and production are listed in the table below.
| Application | Target Thicknesses Above Shoe Plate |
|---|---|
| Development | 3 mm, 2.5 mm, 2 mm, 1.5 mm, 1 mm |
| Production | 1.8 mm (e.g., 90% minimum friction material) |
⚠️ Safety Reminder: Always use dust collectors when grinding friction materials and follow all applicable safety procedures.
🛠️ Accuracy Tip: Calibrate all measurement equipment per local standards and ensure consistent lighting for image analysis to obtain reliable results.
Design Insights and Quality Applications
Quantifying underlayer distribution at multiple thicknesses during development gives engineers deep insight into pad construction and performance consistency. The SAE J2724 method standardizes this measurement, enabling confident comparisons across batches and designs. In production, the test verifies that specific criteria (e.g., a minimum friction material percentage at a given thickness) are satisfied, supporting robust quality control.
Engineering Insight: A consistent underlayer distribution ensures that the brake pad wears evenly and that the usable lining thickness is accurately predicted. This directly contributes to brake performance, noise reduction, and lining life optimization.
Frequently Asked Questions
Can this test be performed on used brake pads? Yes, as long as the backing layer is visually distinguishable, the procedure applies to both new and used pads.
What is the allowed tolerance for grinding? The measured pad thickness must be within ±0.1 mm of the target thickness.
How do I count grid squares if a square contains both materials? Count squares that are at least 50% backing layer as backing layer; otherwise count as friction material. The sum of both types gives the total, and the ratio is calculated accordingly.
What should I do if defects are found? Record all defects per Section 5.1 of the standard, and consult your quality protocols to determine if the sample is acceptable for testing.
