By Tycho de Feijter for 6th Gear Automotive Solutions.
Haarlem, November 2019.
An electric motor is almost completely silent. That means that, in an electric car, other sounds are more audible. Prime examples are wind, tire, and brake noise. In a traditional patrol-powered car these noises are masked by the much louder noise of the engine, so it may seem as if there are no other noises.
But in an electric car these other sounds come to the foreground, annoying driver and passengers. They will be even more of a problem in future autonomous cars, where occupants may be watching a movie or having a meeting while the car drives along all by itself. What an irritation it will be, when you are presenting your latest sales results to your boss in a fancy autonomous office van, when brake noise blows your numbers away..?
To prevent this misery from happening, Italian brake maker Brembo is looking for solutions to make brake systems on electric cars more quiet. Brake noise has three major causes:
1) the noise of the hydraulic system which connects the brake pedal to the brakes.
2) the noise of wind hitting the brakes and especially the brake calipers (this one doubles as a cause for wind noise).
3) the noise of the caliper hitting the brake disc when the brake pedal is applied.
For each cause there is a separate solution. The noise of the hydraulic system can be completely deleted by replacing it with a brake-by-wire system, where there is no physical connection between the pedal and the brakes. Many high-end cars and supercars are already fitted with brake-by-wire systems, and this will slowly spread out to affordable cars everyone can buy.
The wind noise can be reduced by designing a low drag caliper, essentially a ‘streamlined’ variant of the brake calipers we know. By making the edges of a caliper just a little bit more slippery, noise will go down significantly. Designing low-drag brake discs is less easy, as there is less room to alter their shape. But noise can be reduced by fitting thinner discs, reducing the frontal area, and thus limiting noise.
The noise of the caliper hitting the brake disc is by far the largest cause of brake noise. There are two main ways to reduce this. One way is to develop new low-sound materials for the area of the caliper that actually touches the brake. Another way is the use of stronger retraction springs. A retraction spring pushes the caliper back to normal position after the brake action is over. But the longer the caliper touches the disc, even only partially, noise remains. Stronger redaction springs basically shorten the ‘touch-time’, and so limit noise.
Brembo is testing some of these new technologies in the new Karma Revero GT, a high-end hybrid sedan. In time, and especially with the unstoppable transition to electric cars, these new brake technologies will soon become standard in every vehicle.