Squeaky disc brakes

JackPozzi

I'm a new MTBer in need of help please! Recently bought a 2nd hand MTB with hydraulic discs and the front is very squeaky when applied, on a road bike I'd just adjust the blocks to a slight angle to get rid of this but with discs I haven't got a clue. Is there an easy remedy (or even a non-easy one) please?

Ouija

The rotors need cleaning. They've got a small amount of grease on them (which causes the squealing noise). Use a bike specific degreaser on them and then wipe them clean. Probably have a bit of grease on the pads as well, but this will come off after a couple of uses/cleans.

JackPozzi

Sounds nice and easy, thanks very much

nicklouse

it does really depend on the type of noise you are getting have a read.

viewtopic.php?f=40073&t=12674934

cooldad

Maybe, or they just squeal sometimes, lots do depending on weather, pad compound etc.
How do they stop you? If they work fine, leave them.

The Rookie

Grease tends to stop brakes squealing, stops them working all together as well of course, despite it often being said that it causes squeaking, if you think about it it's very unlikely.

70\ʹspenguin

Invest in a small bottle of IPA (not the beer) from fleabay. Cleans rotors really nicely. If i were you i would get some new pads, seeing as you got this second-hand. No matter how much cleaning you do, if the pads are contaminated, you will still get squeal. Sintered squeal a lot. Ive found Kevlar ones have made my life so much quieter (better stopping power too). Superstar components sell em. I get mine from discobrakes.

The Rookie

Can someone explain why they think contamination causes squealing? Logically it does the opposite, no friction, no noise....

The squeal is a resonance and is to do with the physics of the system, stiffness and weight, sintered pads are worse than organic as the pad is stiffer and has significantly less internal hysterisis (which would help abosrb the vibration). I've found avid to be worse and have thught it likley to be the CPS washers reducing the stiffness of the mounting.

cooldad

I was playing tunes on mine in the snow yesterday, different notes back and front, but it doesn't stop them working.

70\ʹspenguin

The Beginner wrote:

Can someone explain why they think contamination causes squealing? Logically it does the opposite, no friction, no noise....

The squeal is a resonance and is to do with the physics of the system, stiffness and weight, sintered pads are worse than organic as the pad is stiffer and has significantly less internal hysterisis (which would help abosrb the vibration). I've found avid to be worse and have thught it likley to be the CPS washers reducing the stiffness of the mounting.

Perhaps you could send an Email to Formula and ask them?

Ouija

The Beginner wrote:

Can someone explain why they think contamination causes squealing? Logically it does the opposite, no friction, no noise....

Because you get variable grip. The constant fluctuation between grip/nogrip sets up resonance in the rotor.

The Rookie

Maybe, but it makes no sense, the resonance is far more liley to come from the holes./grooves/slots in the disc, not that they are needed, I once workd ona car where you could creep along (auto) just touching the brakes and paly tunes with the squeal...

As for Formula, sounds like a handy way to divert blame prehaps!

As an engineering explanation contamination doesn't stack up as obviously as just a resonancae through hitting a resonant frequancy....my lightweight aligator squeals very readily, where the replacement Avid G2 didn't, same pads, both on cleaned disc, so contamination not possibly an issue.

pesky_jones

To be fair it does sound wierd that grease would cause squeaking - if there was the tiniest bit of grease of lubrication to the pads or rotor, surely you'd feel it in performance. More likely to be bad weather or dirt or maybe a small stone or maybe its just your pads.

thelonegroover

We have four bikes, all with different disk brakes: Avid Elixir 5, Shimano 575, Shimano XT and Formula RX. The Avid Elixir 5s are the only ones that squeal.
I’ve cleaned all eight disks exactly the same with methylated spirit. I’ve tried organic and sintered pads in the Avids, but can’t stop them squealing. They work perfectly though.

Uli

As mentioned above disk squealing is caused by resonance due to interval periods of grip and lack of it (Ouija's post). It is very often caused by contamination, water, compound uneven wear and so on. but as long as they work fine I don't bother (usually stops after few good stops anyway). With current weather, bike cleaning etc they do get contaminated to certain degree pretty quick so keep them quiet would require detailed rotor cleaning after every ride and new set of pads every couple of rides. No time for this

The Rookie

Uli wrote:

It is very often caused by contamination, water, compound uneven wear and so on.

Hypothesis or fact, if the latter, evidence please?

I stated what I was saying was belief, you are saying it as a fact, so you need to back that up!

Uli

Well it is fact. You get uneven force applied by pads to the rotor while it spins. Rotor starts oscillating and squeals. The force difference can be a result of listed above as they cause sort of friction material slippage. It could be also due to friction material differences, dust and anything else getting between brakes and rotors. In more details it is the way how pads "bite" into the rotor and then let it go but that is more complex physic which I don't remember much.

The Rookie

So it's your belief it causes squealing, a belief I don't share, so you need some evidence.....the frequancy of that input is far to low to be anywhere near the 10,000 Hz (ish) of a squeal.

pesky_jones

Getting a bit deep now... theres infinite reasons that could cause it. The only thing that can be done is to mention the most probable reasons (contamination etc.) - which we have done. For all we know, it could be a mouse caught in the rotor - in which case contamination, frequencies and blah blah can do one

Uli

What evidence are you asking for? You could as well ask me to prove here that photons exists. You can build you own lab and test it if you don't believe. You can ask physicist for explanation with more details. I have only described the process (huge simplification) and not created a theory for it.
Human hearing range is between 20-18000 Hz (+/-) and rotor is drilled piece of metal fixed on one side only which makes it quite good oscillator. No more input from me on this.

The Rookie

How about a certain exmaple where decontaminating it (as far as possible being the only change) fixed it, I've never known it do diddly.

That's the point PJ, people always trot out contamination, but from an engineering viewpoint it makes no sense, so if it can't be shown reasonably to be true (empirically or by testing), then lets stop giving poor advice (If it is that).......

heez29

The Beginner wrote:

Grease tends to stop brakes squealing, stops them working all together as well of course, despite it often being said that it causes squeaking, if you think about it it's very unlikely.

I had a customer that lubricated his Elixirs because they were "too dangerous".

The Rookie

Indeed, bet they didn't squeal though right! ;-)

Lagrange

The Beginner wrote:

So it's your belief it causes squealing, a belief I don't share, so you need some evidence.....the frequancy of that input is far to low to be anywhere near the 10,000 Hz (ish) of a squeal.

..which would make the wavelength 34mm which is perfectly feasible in the circumference of a rotor.

The Rookie

Oh dear god, think about it, your answer is non sensical in the extreme

Say rotor has 2 contaminated patches, pedaling at 120rpm with a 2:1 gear ratio which is mid cassette on a 32t chainring (so rear wheel spinning at 240rpm). thats 480 input pulses per minute, or 8Hz....a long way from 10,000!
The 34mm would only be relevant if the if the surface of the disc was at the speed of sound, I'll pay to watch you do that.....as the tyre (and therefor bike) will be doing circa Mach 4.5.....

Too clever for your own good?

Lagrange

You can justify pollution by lubricant as the source of the squeal if you draw the force diagram. Two inward pointing forces from the brakes compressing the disk. On one pad there will be a circumferential force because of friction and less so on the other because of the grease. Resolving these will give you a force vector pointing, away from your left foot which will cause the rotor to vibrate with a wavelength where npi Lamda = circumference of the rotor at the point of the friction. n an integer or it would not resonate. The vibration is parallel to the axle...ish.

The input can be anything - beat a drum once and it will resonate with a frequency - beat it with a frequency of 480 per minute and it will still sound the same. The accoustics are associated with the rotor and not the disc which is exciting it.

Lagrange

Wavelengthxfrequency = speed of sound...work it out. That is not even o level physics.

The Rookie

I know it...the point is to get to what you said the bike had to be doing Mach4.5 which was also O-level physics....

As for your second one 'inward forces' Where on what? Differece front and rear, you think the frame resonates at that sort of frequancy, try it, they don't!

thelonegroover

Just to add another spoke, it's only my front brake that sqeals, the rear has always been quiet. I think I might try swapping the disks. However, I suspect the problems is the different frequency properties of the fork compared to the rear triangle.

The Rookie

Different stiffness wheel mounting (fork v frame), different caliper adaptor different stiffness caliper mounting (fork v frame), different hub, that's assuming you have the same pads and disc.

thelonegroover

Yeh, different just about everything apart from the disk and pads themselves. The point I was trying to make was: there are numerous factors that control squeal, not necessarily the pads and disk or what may or may not be on them.