An experimental study of disc brake squeal
Disc brake vibrations are a cause of concern for automobile users and manufacturers.
The user can find the noise of brake squeal annoying and sometimes worrying. The
manufacturer suffers the immediate costs involved in vehicle warranty claims and
may lose sales if a particular vehicle model develops a reputation for noise problems.
The measurement of disc brake squeal and its relationship to audible and structureborne
vibration measurement is difficult to determine without influencing the
vibration by the measurement technique. Ideally, measurements would be taken
under 'real' conditions. The complexity and cost of 'in-car' testing was prohibitive.
Instead, the approach taken was to use a real car brake system, but mounted in such a
way that the support structure carrying the knuckle was as rigid as practically
possible. This allowed an array of non-contact displacement transducers to measure
the vibration at the disc surface.
This experimental approach was used to investigate how the displacement signals
produced by a squealing disc could be processed to determine the mode shapes of the
disc and the behaviour of the waves that generate the audible squeal noise.
The two methods used in the analysis of the squeal data were the fast Fourier
transform of each data channel and the least-squares analysis of the time domain
data. The least-squares approach was the more difficult to achieve, but successfully
produced detailed analysis of the behaviour of the wave motion in the disc from the
available data.A finite element analysis of the brake structure compared favourably with the
frequencies and mode shapes obtained by both modal hammer tests and squeal tests.
The use of this type of dynamometer, measurement system and analysis method has
proved successful in providing a deeper understanding of the mechanisms involved
in disc brake squeal.