There are two common explanations of the self-excitation mechanism. The first one is based on a coefficient of friction
decreasing with increasing relative velocity. In this case, the self-excitation can be explained with single-degree-of-
freedom models (cf. [3]). However, such a dependence of the friction coefficient does not necessarily occur in a brake
system tending to squeal. The second explanation is based on non-conservative forces in the frictional contact. In this case,
self-excitation may occur even if the coefficient of friction increases with increasing relative velocity [5]. The
corresponding mathematical mechanical models need to have at least two degrees of freedom (see e.g. [6,7]).
An overview of minimal models explaining the onset of disk brake squeal is given in [8].