Everybody loves a number, especially when it comes to selling something. We often see it in the grocery store when something has 50 percent less sodium or is 25 percent larger. When it comes to brake pads, it can get you in trouble.
Recently, a co-worker brought an estimate to me that said "25 percent of the material left." What really troubles me is that there is no actual "work" shown on the invoice telling how many millimeters of material is actually left on the backing plate. The "percentage" is just an "eyeball estimate." This estimate is by no means accurate, and if you asked other technicians, you would get different numbers every time.
If you were to properly estimate the percentage of material worn on a brake pad, you would first have to know how much was there in the first place. This number is near impossible to find in books or on a database.
Every vehicle has a "minimum wear specification" for the brake pads. This number is typically between two and three millimeters. If the rotor was worn to discard specifications, caliper piston travel maybe too excessive, or the friction material attachment method may lose its effectiveness and cause the friction material to shear away from the backing plate.
The minimum thickness specifications can be found on any repair computer-based information database or in a repair manual. For the most part, these specs are between two and three millimeters. But, some vehicles will have a higher specification.
This measurement is very credible in the eyes of the customer because you can tie it to an OEM specification. I love a percentage just as much as the next guy, but "eyeballed" percentages should not be used when inspecting brakes.
