Creep groan is an annoying brake noise at very low speeds of the vehicle. In
general, stick-slip between brake disk and brake pads is believed to be the most
dominating vibration mechanism of creep groan phenomena. This paper will show by
sophisticated measurement techniques that stick-slip and speed-dependent
friction is an important trigger. However, the overall vibration is much more
complex than stick-slip reproduced by simple conveying belt minimal models. It
turns out that in typical brake systems of passenger cars, creep groan appears
from 15 to 25 Hz as well as 60 to 100 Hz. The mechanism from 15 to 25 Hz is
highly impulsive and “hard”. Transitions between stick and slip phases trigger
coupled nonlinear vibrations of the complete brake and suspension system. From
60 to 100 Hz, the vibrations show a more harmonic-like and “soft” signature,
caused mainly by a speed-dependent friction behavior. Basically, the growth of
self-excited vibrations is stimulated by flexibilities in the suspension
bushings as well as elastic deformations of suspension parts, wheel and tire.
For practical vehicle operating conditions, the frequency range from 60 to100 Hz
is the most relevant one. The 15 to 25 Hz nonlinear vibrations are frequently
not detected, because these creep groan phenomena require high brake pressures.
This fact often leads to inappropriate model reductions, focusing on the
pads-disk subsystem for a simulation of just one vibration state. To completely
address the physical nature of the vibrating system, large-scale models of the
entire suspension system - including wheel, brake, link arms and bushings - are
used within this work. Thus, existence and properties of bifurcating vibration
states are revealed and the study of “hard” and “soft” creep groan
characteristics is possible.
