By Gene Markel
The “Edge Code” can tell you information about the product you are about to sell. These letters and numbers help technicians select the right friction material for a vehicle and its driver.
Edge Code is a language written by engineers, federal entities and industry associations. Like any language, edge coding has its own “grammar” that has been defined by standardized vehicle and laboratory tests. If you are to take one thing away from this article, it should be how to read the letters that correspond with the friction levels of the brake compound. These two letters are midway through the code, but to understand them, you must first understand the tests behind the letters.
Friction is a resistive force that prevents two objects from sliding freely against each other. The coefficient of friction: The Greek letter μ (pronounced “mew”) is a number that is the ratio of the resistive force of friction (Fr) divided by the normal or perpendicular force (Fn) pushing the objects together. It is represented by the equation: m = Fr / Fn.
The coefficient of friction and resistance to fading is measured using Society of Automotive Engineers (SAE) practice J661 and marked with SAE Practice J688 (truck standard). Stay with me here, when you see a SAE JXXX code or practice don’t think that it is useless to a technician. Keep in mind that when SAE publishes these standards it has been reviewed and tweaked by all concerned parties like OEMs, suppliers and governmental bodies in some cases. In the cases of J661 and J688, it has direct bearing on the aftermarket technician. The purpose of J661 and J688 is to establish a uniform laboratory procedure for securing and reporting the friction and wear characteristics of brake linings.
The performance data obtained can be used for in-plant quality control and for the quality assessment of incoming shipments by the purchasers of brake linings. But, the data is also used in determining the edge code and the right friction material for a vehicle.
J661 has its roots in a Ford test that was developed in the 1960s. This is why the test takes place in a simulated drum brake. It is a simple test that just about every friction material formulation has to endure.
SAE Practice J661 and J688 testing procedure takes place on a machine that contains an 11-inch drum with three
temperature sensors.
The drum can also be heated during the test. But, the drum is turned at a constant speed. The SAE Practice J668 and J661 for the edge code on friction material notes the normal coefficient of friction and hot coefficient of friction or fade resistance. Letters are used to note the coefficient of friction as shown in the table above. In the edge code, the first letter notes the normal coefficient of friction and the second letter notes the hot coefficient of friction. The code appears in 0.25-inch letters on the edge of the friction material.
Wear measurement results are not indicated in the edge coding. It does not mean that the car will stop in a safe distance or will not make noise. Edge code should only be used to determine if the friction material will match what was installed on the vehicle for cold and hot friction.
