Why do my disk brakes squeal when wet?

Question

Picture the scene... It's just been raining and you are getting ready to commute home after work, you cycle out on to the tarmac and as you do so, you notice a bunch of pedestrians push the button on the traffic lights up ahead, the lights turn red and you gently squeeze your brake to stop before the line but at the same time your brakes let out a horrific squeal frightening the pedestrians and ruining your street cred.

Whats the deal? why do my disk brakes squeal when its wet, and is it possible to prevent this? Is it more common for disk brakes to squeal and why?

I am currently riding a Scott Aspect 30 - spec

Answer 1

They squeal because water acts as a mild lubricant. That's why things are "slippery when wet" ;)

The water makes it harder for the pads to grab onto the rotors. They'll grab for a small fraction of a second and then let go again, and they do this really fast. Imagine the squeaky sound that a your finger makes when your run it over a smooth wet surface, or the sound your shoes make on a tile floor when you just came in out of the rain. Same sort of thing, only constant. With bike brakes and disc brakes especially, the braking surface (the rotors for disc brakes, the rim for rim brakes) will find a harmonic resonance and thus the really loud tonal squeal.

All brakes have the potential to squeal, especially when wet. I wouldn't worry much about it unless you find that your braking power is being significantly reduced. In such a case, try some different compound pads. Metallic pads are generally considered to be superior in adverse conditions but they're also loud as hell in the rain/mud.

Everything is a trade-off, don't ya know?

Answer 2

Street Cred? I've almost lost trail ridding buddies due to squeaking brakes. Keep the pads clean and you'll brake much more quietly (unless your skidding and trying to be loud).

Its most likely oil or other grimy substance built up on the pads. It can fly off your chain if you spin the cranks to help work in fresh lube. I try to be extra careful on my mountain bike with disc brakes when I lube my chain. Rotate very slowly putting one drip per bushing, then pedal very softly just a few rotations, so none flys off and potentially onto the pads. Let the lube settle in and wipe off any excess lube.

Answer 3

My all-weather commuter bike has discs. In my experience, cleaning the discs with window cleaner (e.g. Windex) reduces the squealing.

Allegedly, better results can be had by cleaning with automotive brake disc cleaner, but I've not yet tried this.

Answer 4

With auto brakes it's because of the thin layer of corrosion that's built up on the rotors. No experience with bike disks.

Answer 5

Some manufacturers specifically state not to use automotive brake cleaners,they suggest rubbing alcohol.Idon't know why, perhaps it might looseen the adhesive bonding the friction material and the backing plate.