In that photo you see the shiny brake disc, the frame of the caliper, and the outer pad. The pad consists of a metal plate with friction material laminated to the backing. The rule is: the thickness of the friction material must be greater than the thickness of the backing plate.
The reason for that is heat insulation. If the pads wear down to nothing there is no longer any insulation and the heat of braking will transfer from the rotor, to the backing plate and from there into the pistons and fluid. If the fluid gets too hot it boils and you lose your brakes.
This photo shows the same pad and caliper seen from behind. The thing in the center is the wear sensor, which as you see is in contact with the rotor. That put the brake wear light on in this car. IN some European cars the light has to be reset with a test tool even after the sensor has been changed.
Other cars - like BMW - also calculate how many miles the pads have run and they will light the warning based on mileage even if the pads are good (for example, if someone replaced them without a reset)
John Elder Robison
(c) 2016 John Elder Robison
John Elder Robison is the general manager of J E Robison Service Company, celebrating 30 years of independent BMW/MINI, Mercedes, Land Rover, Rolls-Royce and Bentley restoration and repair in Springfield, Massachusetts. John is a longtime technical consultant to the car clubs, and he’s owned and restored many fine British and German motorcars. Find him online at www.robisonservice.com or in the real world at 413-785-1665
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