Brake judder in mid-size vehicles is characterized by 10 to 20 Hz brake torque variations on the order of 100 N-m when braking from speeds of approximately 120 kph. These vibrations can be perceived by the vehicle’s occupants through the seat track, chassis, steering wheel and/or brake pedal. The source of these vibrations is often attributed to thickness variations on the rotor surface which generate brake torque variations and brake pressure variations. This thesis suggests considerations for experimental studies as well as examines the effects of brake pad geometry on brake torque and brake pressure variations measured under laboratory conditions. A dynamic friction laboratory experiment has been constructed and instrumented for the purpose of investigating these specific circumstances at the source. This dynamic friction experiment consists of a motor-driven flywheel which stores 1/5th the kinetic energy of a mid-sized vehicle traveling at 115 kph. The flywheel is connected to a shaft terminating into a rotor. A floating caliper braking system affixed to the rotor dissipates the kinetic energy by means of friction at the disc-caliper interface. During the controlled braking event, dedicated instrumentation allows for quantification of brake torque variations and brake pressure variations under cold judder conditions.
A hypothesis related to changes in the pad-caliper contact stiffness due to changes in pad geometry is formulated from experimental results. Supporting evidence for this hypothesis is provided by means of pad bending strain measurement, computational stiffness calculations, and torque calculations using a four degree-of-freedom linear model of the judder source (floating caliper braking system). Analytical and experimental results suggest a strong correlation between computational caliper-pad contact stiffness values and levels of torque and pressure variation, when normalized with respect to the baseline pad. A recommendation for future work involves verification of the calculated contact stiffness values.