Investigation of brake pad vibration under dynamic squeal conditions

Brake squeal is a highly complex noise mechanism resulting from friction induced vibration
which excites many separate but mechanically connected components within the brake
system. Often this can be broken down into the whole corner assembly, including suspension,
at low frequencies while at the higher frequencies only the friction couple of disc and pads are
of interest. The brake disc plays the role of the noise emitter and therefore must be excited
through the friction pair interface in order to generate noise. It is therefore of interest to
identify how the brake pad vibrates in relation to the disc mode of vibration. This paper
presents measurements of pad vibration under dynamic squeal conditions on a laboratory
dynamometer. Measurements are made using piezo-electric beams mounted about the inboard
and outboard brake pad. Various squeal frequencies are investigated including a phenomenon
where a pair of squeal frequencies, close in value, are seen to emanate at different brake
pressure applications. This phenomenon is of particular interest as the noisy regions are
separated by quiet zones within the brake application. This “switching” between the two
frequencies can be related to the pad mode of vibration (torsion or bending) which in turn
may be related to the disc mode “fixing” about the pads. Finite element analysis is used to
investigate the free-free pad modes of vibration with experimental “shaker” testing used to
correlate and confirm the results.