[lectio]

my car's breaks make some noise time to time when I'm driving
in a slow traffic. the noise comes and goes. the noise is very sharp like the rotor is rubbing against fully used break pads. but I check the break pads and they are still good. any suggestion?

 

[j brosseau]

in my experience most disc brakes sqeek. usually when cold. once they heat up a bit and teh pad warms up it will go away. now if is making a grinding noise than you know you have metal to metal. im not sure if you car has audible wear sensors on the brake pads or not. on alot of cars the brake pads have a piece of metal riveted to the pad. when the pad gets to say 10% life, the small piece of metal will begin to rub on the disc making a loud sqeeking sound. when that becomes constant it is time to get a brake job done.

 

[Harry]

Noise from the breaks comes from the so-called stick-slip-effect. The stick friction is higher than the slip friction. If there is some play between the pad and its seat in the caliper, the pad sticks at the disk until it hits the seat, then it "breaks loose" from the disk, starts to slip and will be pushed back because it is a little bit elastic. Then it sticks again and the whole thing starts from the beginning. The normal play for the pad is in the some 1/10 mm range so you can imagine the frequency is quite high if you break at 30 mph. This is what you hear.
How to avoid it? Small play for the pad or some damping, i.e. copper paste between the caliper and the pads. Also a different pad or disk material can help to get the resonance frequency out of reach.
Regards
Harry

 

[lectio]

thanks Harry for the greatly detailed explanations and suggestion, I spoke to my mechanic about your suggestion and he recommends that maybe I should try another type of pads. however he doesn't think the noise will go away though. well, we'll see. I'll post a follow up when the pads come in.