A method has been developed to simulate worn-in-service brake friction materials and rotors. The simulated thin linings and worn rotors were prepared by grinding the materials to a predetermined level and then subjecting the prethinned friction couple to a moderate inertial-dynamometer conditioning sequence. The friction materials were characterized by optical microscopy and thermogravimetric analysis. The rotors were characterized by surface roughness measurements and scanning electron microscopy.The simulated components were shown to have the same microstructure and chemical compositions as worn-in-service components. The extent of thermal and mechanical damage was found to be proportional to the duty cycles experienced. By the method described, the friction-affected layers of the simulated friction couple were found to be intermediate between those of a similar couple with 40,000 miles of light use and those of a couple with 3300 miles of medium-duty durability use. These simulated components were then used in a study of the effects of worn brake components on braking system performance.