There are lots of mechanical myths about what causes a car's brakes to produce that biting howl. The all too familiar piercing squeal is heard at road junctions and traffic lights the world over as drivers hit the brakes. But, understanding what causes brake squeal might help researchers find a way to stop it.
It is not so simple a problem as worn out brake pads or grit on the pads, nor rusty discs.
Rather, the "squeal in a disk brake is initiated by an instability due to the friction forces leading to self-excited vibrations," an engineer would say.
Utz von Wagner and Stefan Schlagner of the Technische Universität Berlin, Germany, have developed a mathematical model that helps explain what physical conditions are needed for brake squeal to happen. Their model predicts how different braking setups affect the squeal frequency and perhaps the level of pedestrian annoyance caused by the brake noise. They have developed a rig to simulate braking and so allow engineers to test different approaches to preventing brake squeal.
Well, that's the scientific definition, but what can be done about this frictional instability?
Oliviero Giannini of the University of Rome 'La Sapienza', points out that despite a century of driving with brake squeal little progress has been made in combating this annoyance until now. He is developing the idea of a fuzzy damper that could one day suppress brake squeal altogether.
"Brake squeal is caused by vibrations induced by friction forces," explains Giannini. A damper that responds to the level of frictional forces and absorbs some of the energy, without interfering with braking, could prevent the kind of unstable vibrations in the brake disks that cause the high-pitched screeching.
Giannini has thus successfully modelled and tested the concept of a fuzzy damper for his braking test-rig and demonstrated that for this lab-based system, at least, brake squeal can be totally suppressed. Patent applications are underway and the next step is to test real braking systems with the fuzzy damper.
The idea is presented this month in the International Journal of Vehicle Safety.