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  1. #1
    pbj
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    Squealing Disc Brake Facts

    With all problems people have with squealing brakes a brief FACT based on science might be helpful plus some physics behind the root cause.

    1) Alcohol, ethyl (drinking), methyl or isopropyl (rubbing) does not dissolve mineral based lubs and oils. So using it thinking it will degrease a pad is a waste of time. To degrease the pad you need to use a real degreaser like carbon tetrachloride, xylene or some other chlorinated solvent like trichlorethylene. Freon TF used to be the best but that another era. Mineral spirits are a no-no because they often contain paraffin, also a high molecular weight hydrocarbon lubricant.

    2.) Simply wiping the surface of a pad will not remove the oil, the pad is very porous. You need to soak the pad in the solvent for some time (~1 hr) to allow the solvent to penetrate, dissolve and eventually dilute the oils, then before the solvent dries out you need to rinse them off in fresh non-contaminated solvent. You might be concerned the solvent would dissolve the pad material. If its a pad manufactured by a reputable company soaking it for weeks should not be problem.

    3) "Organic" pads are actually more like Bakelite, remember the old black pot handles (thermoset phenolic) with non-organic high temperature fillers. This is why the material resists intermittent high temperature >400F. The metallic and semi metallic pads are even more resistant to solvents and high temperature >700F. However all are relatively soft. Which brings up another point, sanding rotors. You really don't need to or should sand disc rotors to remove the glaze, they are metal and don't develop glaze. If anything they develop an very thin (angstroms* thick) oxide layer which is actually beneficial because its as hard as ceramic. If they have uniform blue/black appearance that means the pads are evenly heating the rotor and hard oxide has properly developed, leave it on and let them naturally polish. If you do need to sand them us a #600-800 wet sand paper (silicon carbide) and do it on the bike as you spin the wheel. Don't get freaky about removing all the small grooves, the pad will quickly adopt these small grooves into its surfaced. Sanding pads to reinitialize them is OK if you want to deglaze from serious overheating. However if you are reinitializing them often to stop squealing all you are doing is sanding down your money. For them to work as designed they need to be seasoned or broke-in as noted in previous posts. This has two purposes, a.) create uniform contact between the rotor and disc (remember the little grooves) and b.) to slightly bake the pad surface to both burn away and residual oil and to carburize (burn) the pad surface harden to it up a bit.

    4.) The best way I've found to degrease the pad is to bake or roast it. ~350 in the oven for 30 min or over a low flame for couple minutes <400F. Use caution on the flame method not to overheat the pad. You'll know if it contaminated with oil if it smokes profusely after taking away from the flame (be careful). Keep doing this until the smoke produced subsides to slightly smoking when pulling it out of the flame. This removes the oil by carburizing it which is the same thing as doing it the hard way.

    5.) If after doing all this they are still squealing out of the gate then look at mechanical alignment issues. Forget about trying to damp the squeal by looking at frame related resonance, its physically impossible for the frame to resonate at that high frequency. Neither will air in the line cause it. Take note of the disk rotor when you press the brake lever to insure its equally loading and not flexing to one side or the other.

    6.) If they seem to work OK after break-in but after a while for no reason start squealing, suspect a slight brake piston leaking,

    7.) Be aware of what you are using to wash or clean your bike, even small amounts of grease, lubricants, waxes can easily make their way into and through the porous pad material. Remember the mechanical interface between the pad and rotor is quite literately only 1000 angstroms* thick, the oil causing the problem is only a molecular layer thick so a little goes a long way. Thats why ALL the disc manufactures warn against even touching the disc rotor, not because your fingers are dirty and greasy but because of body oil on your finger.

    8.) Silicone is an evil compound when it comes to disc brakes, it can cause the same problem as petroleums but is even worse because almost nothing will dissolve it, except xylene. (remember above) and it's decomposition temperature is higher >400. Roasting in a flame has the best chance of decomposing because it can chemically react with the gas in the flame and decompose easier to silicon dioxide SiO2 (glass dust).

    So at this point you probably wonder what is the root cause of the noise. It's a combination of things, resonance (natural frequency) of the rotor, slip-stick characteristics of the rotor/pad interface and compliance or stiffness of the mechanical system. For those with technical curiosity follow the link below. Point being, about the only variable under our (end user) control is the slip-stick part of the equation, the other two variable have been "set in stone" so to speak by the disc brake manufacture.

    http://webphysics.davidson.edu/facul...tick-slip.html

    *In everyday terms, a sheet of paper is approximately 1,000,000 angstroms thick.

    I hope some of you find this helpful..

  2. #2
    Nightriding rules SuperModerator
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    wow.. very nice overview!

  3. #3
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    Iso propyl alcohol will dissolve most oils to a degree that is useful.

    It will also dissolve degreasers, and detergents, that may contaiminate the pads.

    Science aside, when my rotors went blue black they squealed and did not stop well at all.

    Baking the pads works well.

  4. #4
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    So, riddle me this:

    I have Hayes 9s. I also have 2 wheelsets, one for dirt and one for road. The rotors that are on my dirt wheels are the rotors that came with the brakes. The rotors on the road wheels are also Hayes but I bought them separately. When I ride dirt, my brakes are nice and quiet. However, when I ride with my road wheels, the front brake squeals like a pig, and the back brake vibrates horribly. If I remove the pads and scuff them very lightly with sandpaper to remove any glazing, the problem goes away...temporarily. Within a few commutes, its back.

    Any ideas on why one set of rotors would be fine, but another set would cause such issues?
    "If you suck, that means I'm better. The more you suck, the better I am. So. Let me count the ways you suck." - Scribb

  5. #5
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    Have you tried swapping rotors so the road rotors get the same break-in treatment as the off-road rotors (which I assumed are used more)?
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    Yikes. If I had to go through all that, I would have new pads or rotors or brakes or all of them.

    Ohhh.. .and yes. I know is costs more money... whine.. whine.. cry. I don't live under a bridge.

  7. #7
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    Quote Originally Posted by Speedub.Nate
    Have you tried swapping rotors so the road rotors get the same break-in treatment as the off-road rotors (which I assumed are used more)?
    I haven't, but I've had enough time on both wheelsets that the rotors *should* be broken-in.

    Could the wheels themselves have something to do with my problem?
    "If you suck, that means I'm better. The more you suck, the better I am. So. Let me count the ways you suck." - Scribb

  8. #8
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    Could you use (auto) brake cleaner?

  9. #9
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    Ah, back to science.

    I wore out three Shimano rotors so far, the braking surface was clean, dry and smooth enough. but the damn brakes wouldn't perform, and squealed.

    I replaced them and with new pads the brakes were back to new performance.

    I am convinced that Shimano applies a procedure to harden the surface of the rotors in some way, perhaps flame, induction, peening, carburizing, etc.

    Once the brake rotor is worn through the surface hardening the rotor begins to shed larger bits of metal into the pad, and this begins to reduce performance.

    If the only difference is the rotors, (set-up etc all good), then it must be some difference in the rotor. (Duh)

    Examine the difference in the surfaces (the wear marks), with some magnification, also compare rotor thickness. (hardness testing is difficult and destructive.)

    Just some things that might be instructive.

  10. #10
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    Quote Originally Posted by pixelninja
    I haven't, but I've had enough time on both wheelsets that the rotors *should* be broken-in.

    Could the wheels themselves have something to do with my problem?
    Don't know... doubtful (in my mind) the wheels could play a role. I'd swap 'em out for a few rides just to check that possibility off of the list of suspects.
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  11. #11
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    The wheel will affect squeal and brake performance.

    If the bearings have play so that the rotor can shimmy a bit (tune -up fix hub).

    Worn dropouts may also cause problems.

  12. #12
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    Quote Originally Posted by jeffscott
    The wheel will affect squeal and brake performance.

    If the bearings have play so that the rotor can shimmy a bit (tune -up fix hub).

    Worn dropouts may also cause problems.
    Thanks. I'll double check to make sure the hub is snug.

    The dropouts on my bike are fairly shallow (Yeti 575), but the frame is fairly new (bought new in Feb). I have had issues with skewers coming loose because the gnarled surface of the skewer had become smooth.
    "If you suck, that means I'm better. The more you suck, the better I am. So. Let me count the ways you suck." - Scribb

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    Oh Oh.

    I just replaced my chainstay and rear wheel cause the axle stubs and dropouts were worn. (11000 km).

    If the axle can move around a bit, you should notice some strange shifting issues that come and go as well.

  14. #14
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    Quote Originally Posted by jeffscott
    Oh Oh.

    I just replaced my chainstay and rear wheel cause the axle stubs and dropouts were worn. (11000 km).

    If the axle can move around a bit, you should notice some strange shifting issues that come and go as well.
    My skewer issues had more to do with worn out skewers than worn dropouts. New skewer and no more problems with that issue.

    I'm not experiencing any strange shifting issues, but I'll double-check my hub anyway.
    "If you suck, that means I'm better. The more you suck, the better I am. So. Let me count the ways you suck." - Scribb

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    Does or has anyone tried running a set of rotors through a cryogenic processor? I'm new to bike disk brakes, but what are the pad materials available? Organic, semi-metalic, Kevlar, ceramic(negative heat coeffiecent)? What's out there?
    -Scott

  16. #16
    pbj
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    Road Squeal

    Quote Originally Posted by pixelninja
    So, riddle me this:

    I have Hayes 9s. I also have 2 wheelsets, one for dirt and one for road. The rotors that are on my dirt wheels are the rotors that came with the brakes. The rotors on the road wheels are also Hayes but I bought them separately. When I ride dirt, my brakes are nice and quiet. However, when I ride with my road wheels, the front brake squeals like a pig, and the back brake vibrates horribly. If I remove the pads and scuff them very lightly with sandpaper to remove any glazing, the problem goes away...temporarily. Within a few commutes, its back.

    Any ideas on why one set of rotors would be fine, but another set would cause such issues?
    ----------------
    Try swapping the tires on the wheel sets and see if the squeal follows the wheel sets. If it does then the rotors, axle, or hub is clearly different and further investigation is necessary.

    If they follow the dirt/road, best "guess" is the dust and dirt thats kicked up during trail riding is making it way into the disc/pad interface acting as a dry anti-slip-stick lubricant of sorts. Also check for oil weeping around the pistons, dirt/dust is an excellent oil absorbent (kitty litter).

  17. #17
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    I always thought dirt = noise?

    pbj, the info you posted is all good. I am curious as to how the old classic disc brake info from cars and motorcycles that dirt causes noise plays into all this. It's pretty easy to see that when driving a car or motorcycle that hasn't been washed in a while that the brakes squeal. Then you wash them with simple soap and water and they are quite again for about a week. I find the exact same thing with my bike; after washing it and for the first 5 - 10 miles they are quite then after that I get some squealing always from the rear. And what makes sense about the rear squealing is that where most of the dust ends up as the front tire kicks it up and it settles on the rear because of the movement of the bike. Then after I wash the bike with just hose water the rear is quiet again until 5- 10 miles on a dusty trail.
    If you're not falling, then you're not riding fast enough!
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  18. #18
    pbj
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    Quote Originally Posted by baraant
    pbj, the info you posted is all good. I am curious as to how the old classic disc brake info from cars and motorcycles that dirt causes noise plays into all this. It's pretty easy to see that when driving a car or motorcycle that hasn't been washed in a while that the brakes squeal. Then you wash them with simple soap and water and they are quite again for about a week. I find the exact same thing with my bike; after washing it and for the first 5 - 10 miles they are quite then after that I get some squealing always from the rear. And what makes sense about the rear squealing is that where most of the dust ends up as the front tire kicks it up and it settles on the rear because of the movement of the bike. Then after I wash the bike with just hose water the rear is quiet again until 5- 10 miles on a dusty trail.
    Unfortunately...

    Disc brake squeal in a industry wide problem. Really smart people and vast technical resources are spending years and millions of dollars trying to solve this seeming simple problem, it still eludes everyone. (do a quick web search +disc +brake +squeal and see what pops up). That said there are some factors known to help minimize it.

    Slip-stick or non-linear friction is where it all starts. The people who formulate the pad material cannot design for all the possible contaminates (dust, grit, grime, grease) that might get between the rotor and pad plus they're expected to provide durable long lasting pad with both durability and a high coefficient of friction (braking power vs lever force). These two factors are at odds with each other. Its a engineering performance balance. Possible solutions might be to use a good quality disc break cleaner regularly, be careful about getting it on the pistons seals, some rubbers can be deteriorated and swelled by some solvents. Also might want to try a different pad manufacturer/material. I've had pretty good luck with the EBC gold sintered bronze on my Magura brakes, the stock ones wore out in less than 6 months in light trail riding. These have been on for 2 years with little visible wear. They don't squeal in light or heavy breaking at normal speeds, just creak a little a real slow speeds and very heavy pressure.

    Reducing the tendency for resonance to begin in the first place is a possible solution. Some people have has success beveling 30-45 the leading and trailing edges of the pad surfaces. I have not tried this but it make technical sense from a energy distribution standpoint. Problem is you may need to re-bevel them occasionally as they wear down.

    Damping, since the squeal is a resonance, primarily of the brake pad themselves and the in-plane resonance of the rotor. Placing a relatively soft damping material between the mechanical structure and vibrating member (pad) may absorb enough of resonant energy to allow it to die out. ie, reduce the phased shifted mechanical feedback. This might be a problem on bike disc brakes due to limited clearance in the pad/rotor area in the first place. A layer of thin high temp plastic like mylar (laser printer transparency material) ~0.5mm thick glued to the brake pad might work. Polyamide film would be better due to higher temp resistance and would be necessary for DH riders. Some people have had success with copper colored anti-size compound, it has good temp resistance, it may provide sufficient damping plus better thermal management. Just don't get it on the pad surface.

    In the end, what counts is what works.

    Science is only a logical construct attempting to describe how the world around us is [U]supposed/U] to work. It really only exists in our meager mammalian brains.

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