Brake noise has been recognized as a problem since the mid‐1930s. Research was initially conducted on drum brakes, but recent work has concentrated on disk brakes since they are now widely used on cars and trucks. The disk is bolted to the wheel and axle and thus rotates at the same speed as the wheel. The brake caliper does not rotate and is fixed to the vehicle. Hydraulic oil pressure forces the brake pads onto the disk, thus applying braking forces that reduce the speed of the vehicle.
From a dynamics point of view a braking system can be represented as two dynamic systems connected by a friction interface [31]. The normal force between the two systems results from the hydraulic pressure and is related to the friction force. The combination of the friction interface and the dynamic systems makes it difficult to understand and reduce brake noise. Brake noise usually involves a dynamic instability of the braking system.
There are three overlapping “stability” parameters of (i) friction, (ii) pressure, and (iii) temperature. If the brake operates in the unstable area, changes in the parameters have little or no effect, and the brake is likely to generate noise. Such conditions may be caused by excessively low or high temperatures, which cause changes in the characteristics of the friction material. The “stable” area may be regarded to represent a well‐designed brake in which the system only moves into the “unstable” region when extreme changes in the system parameters occur. Brake noise and vibration phenomena can be placed into three main categories: (i) judder, (ii) groan, and (iii) squeal.
Judder occurs at a frequency less than about 10 Hz and is related to the wheel rotation rpm or a multiple of it. It is a forced vibration caused by nonuniformity of the disk, and the vibration is of such a low frequency that it is normally sensed rather than heard. There are two types of judder: cold and hot judder. Cold judder is commonly caused by the brake pad rubbing on the disk during periods when the brakes are not applied. Hot judder is associated with braking at high speeds or excessive braking when large amounts of heat can be generated causing transient thermal deformations of the disk.
Groan occurs at a frequency of about 100 Hz. It usually happens at low speed and is the most common unstable brake vibration phenomenon. It is particularly noticeable in cars and/or heavy trucks coming to a stop or moving along slowly and then gently braking. It is thought to be caused by the stick–slip behavior of the brake pads on the disk surface and because the friction coefficient varies with brake pad velocity.
Squeal normally occurs above 1 kHz. Brake squeal is an unstable vibration caused by a geometric instability. It can be divided into two main categories: (i) low‐frequency squeal (1 kHZ to 4 kHz), diametrical nodal spacing in the disk, and (ii) high‐frequency squeal (> 4 kHz). See Ref. [31] for a more detailed discussion of brake noise. References [32–37] describe recent research on brake noise.
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