Testing of brake Materials
Testing of brake material began starting in 1920 by the National Bureau of Standards (NBS). The testing set up used was then sent to manufacturers who wanted them so that they could begin to test their own products. Over time the NBS continued to develop new instruments and procedures for testing pads and lining, and these standards eventually became the standards for the American Engineering Standards Committee's Safety Code for Brakes and Brake Testing.
SAE J661 testing is used to determine the friction of different brake pad materials by testing a 1 inch square liner with a brake drum. This testing yields values both a hot and cold coefficient of friction, which is then paired with a letter designation. The table below outlines what letter goes with each range for the coefficient of friction. An example of the designation would be "GD", where "G" is the normal coefficient, while the "D" represents heated.
| Letter Designation for Friction Coefficients | |
| C | <0.15 |
| D | 0.15 to 0.25 |
| E | 0.25 to 0.45 |
| F | 0.35 to 0.45 |
| G | 0.45 to 0.55 |
| H | >0.50 |
| Z |
unclassified |