What actually causes the squealing? Is it air trapped between the pad and the rotor, vibrations or something completely different?

I think its the pads gripping and releasing setting up vibrations – sort of very high speed judderings. sometimes it seems to be accompanied by teh brake not working well but not always

I imagine it’s the abrasion of the pad on disc exciting resonant modes in the rotor – very similar to if you wet your finger and run it round the rim of a glass. There may be some correlation between a wet pad tending to squeal more than a dry pad, just as with a wet vs dry finger.

Now would a thicker rotor have a lower pitch due to the greater mass or would the greater stiffness raise the resonance back up? Bigger rotors will probably squeal at a lower frequency – but as they’re passing the pad at a greater linear speed the excited harmonics might be higher in the series…

Brake squeal is caused by high frequency vibrations. When the brakes are applied and the pads contact the rotors, tiny surface irregularities in the rotors act like speed bumps causing the pads to jump and skip as they scrape against the rotors. This, in turn, causes the pads to shake and vibrate in the calipers and against the caliper pistons. It also causes the calipers to shake and vibrate on their mounts and bushings. The greater the play between all of these parts, the greater the amplitude of the vibrations and the louder the squeal. The rhythmic vibrations of the pads rubbing against the rotors also creates harmonic vibrations in the rotors that causes them to ring like a cymbal. The areas generating the most noise can be seen with special laboratory equipment that uses a laser to scan the surface. Changes in color reveal the intensity and frequency of the vibrations. Researchers have found that rotor vibrations are not uniform all the way around a noisy rotor. The rotor has certain spots or “nodes” that oscillate more than other areas.

From here. It’s about automobile brakes but a great deal still applies to what you have on your bicycle.

I found a Specialized bulletin saying that one of there bikes fitted with Avid Something-higher-in-the-range-than-7s needed to have the rotors changed to solid ones due to noise issues. They were changing them for free.

I have just bought one second hand so will report back when it turns up and I’ve had it on a while.

My 7s squeel like hell and I can usually fix them temperarily by cleaning the pads(sintered) off on wet’n’dry and applying copper grease to their backs.

I reckon it is something to do with the damp getting into the pad matl at this time of the year and then not getting warm enough to dry out and keep the contact surface of the pad in the correct condition.

Just a guess though.

Wow – Something I actually know about! I work in aerospace braking…

The noise does come from the rotor, so different thickness’s or rotor designs will have different resonant frequencies. The pad sets off the vibrations, could be due to stick-slip or just the action of clamping in one place. My front and rear rotors definitely have a different pitch!

dirty rotors i find, if i clean my rotors with mythilated spirits it tends to go away, something i really need to do actually, sound awful right now.

If it is the rotor – why does applying copperslip to the rear of car pads prevent squealing?
Maybe I’ll put copperslip on the back of my bike pads and see what happens…

DrP

seosamh77 ::: you clean your rotars with metholated spirits… is this ok ???? my rotors are slightly rusty around the edges..will this spirit clean this off ?

elaine – meths is fine, it takes all the grease and crapp off the rotors, don’t know about rust though.

Probably adds a bit of damping to the pads, and when they’re clamped the pads are part of the system that is vibrating.

Probably adds a bit of damping to the pads

Didn’t it use to be the thing to smear copper grease on the backs of the pad to reduce squealing?

Is it a squeak or a squeal? If it’s a squeal it’s coming from your cake hole and if it’s a squeak it’s yer arse cheeks. 😉