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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, the scent of fresh rain on hot tarmac fills your nostrils, you notice a pedestrian 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.

What's the deal? why do my disc brakes squeal when its wet, and is it possible to prevent this? Is it more common for disc brakes to squeal and why?

I am currently riding a Scott Aspect 30.

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Related question: bicycles.stackexchange.com/questions/26/… –  amcnabb Apr 27 '14 at 1:36
    
Mine do the same thing, but instead of both squealing only the front one squeals(when wet). the back one will sometimes make this weird noise which im not really sure how to explain. I kind of like it to be honest, since sometimes i ride on sidewalks it helps people know im behind them :) –  MoeTheMTBer Dec 17 '14 at 1:03

8 Answers 8

up vote 12 down vote accepted

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?

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So what would be the quietest compound to use? –  Wezly Feb 10 '12 at 12:52
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An organic compound might be quieter but wont provide the same stopping power. There's a good chance they'll squeal when wet too, though. My advice is that you write it off as part of riding in wet weather. –  joelmdev Feb 10 '12 at 12:55
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Fair play, the pedestrians will just have to buy ear plugs. –  Wezly Feb 10 '12 at 15:46
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Squealy brakes are one of the few noises that'll cut through loud music in earbuds. Very handy on trails. (When my brakes are squealy, I tap the brake levers about ten feet back if it looks like I'm about to enter into an ipod-person situation.) –  Neil Fein Feb 16 '12 at 23:31

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.

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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.

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

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Whilst you're right that car rotors get corroded quite quickly, it's very unlikely to be that on a bike. The OP probably would have mentioned it if the rotor was rusty. –  brendan Mar 27 at 3:30
    
@brendan - It only takes a very thin haze of corrosion to create the problem. –  Daniel R Hicks Mar 27 at 11:19
    
True, but it's a very rare reason for MTB rotors to make noise. Much more common is contamination from oil or grease on the rotors or pads. –  brendan Mar 31 at 6:34

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.

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I ride a Scott Scale 10 2010, Avid Elixir CR Mag + Avid 160mm rotor rear, Avid XX Elixir Cr/Mag (Awaiting new replacement F Elixir CR Mag) + Shimano 180mm rotor front. Brake pads A2Z Superior Sintered Brake Pads High Temp resistant F & R, these babies do not like water but will stop you dead under any condition!

Pads & Discs when wet will screech & loudly, add rolling down dusty trails will make it worse. The effect of cleaning pads and rotors with alcohol is that alcohol evaporates faster than water but the screeching disappears only after downhill riding and heating up the rotors enough to bed in and dry fully the pads and rotors after cleaning but only until wet and dirty again.

Noisy is better than poor braking, upside is, your riding buddies will know when you are coming up behind them downhill and make way.

Roll on the long dry summer.

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Welcome to Bicycles SE. This site does not function like a typical forum, it operates on a Q&A basis. This post doesn't really answer why disc brakes squeal when wet or whether it's preventable, which is what the OP was asking about. As such, it would have been better placed as a comment. Once you have asked and answered a few questions, you'll have sufficient reputation, you'll be able to comment everywhere. –  jimirings May 2 '14 at 14:55

I suffered from terrible front brake squeal when wet. I'd tried cleaning the rotors and pads, I'd tried different organic and sintered pads, none of it made any difference. But finally I have solved it - by changing the rotor. The original rotor was the Avid one that came with the brake calipers (BB7s), which is pretty light and spidery. The replacement is a Shimano rotor (SM-RT61) that has far smaller cutouts. Now with the shimano rotor and sintered pads the brake only rarely, and very briefly, squeals.

Interestingly the rear brake was identical and never made any noise. I guess that the difference is to do with the frame stiffness and resonant modes being quite different front and back. This is on a Surly Ogre frame, with an eZee hub motor on the front (I mention the hub because the motor hub has vastly more inertia than a normal front hub, and this might be a factor in the resonance).

As usual, YMMV. By all reports disc brakes are pretty fickle things and what fixed my problem might make yours worse.

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My brakeset is Tektro Auriga 160mm front and rear, both of them squeal like a beaten pooch during rainy rides. It is caused by the particles of the pads trying to grip the rotors but because of the wet condition, some of the particles are unable to grip the rotors. Pretty much the same when you rub your wet finger on a stainless steel. It makes a "crieeek" sound and you can feel your finger like rubbing a rugged surface

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protected by Gary.Ray Mar 25 at 12:33

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