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Old 10-05-2008, 07:25 PM   #1
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Red face Brake squealing explained

Hope this helps my fellow BMW owners who just can't stop the squeal and squeaks from your brakes.

This unfortunately can happen more than you or I want it to. A certain amount of high-pitched brake noise is considered "normal" these days because of the harder semi-metallic brake pads that are used on cars now. In my experience, the smaller cars like Honda and Toyota seem to have the most trouble with this. Squeals heard the first few stops in the morning when the brakes are cold and somewhat damp from dew, and squeals that are heard the last few feet while coming to a stop are usually nothing to worry about.

Semi-metallic brakes are made of bits of metal shavings in place of the asbestos material that has been banned by the U.S. government. These semi-metallic brakes have great stopping power and have a long wear life, but can cause a high-pitched squeal that drives car owners crazy and frustrates mechanics who can't get it to go away to please their customers.

When are squeals not a problem?
Some brands of semi-metallic pads are inherently noisier than others because of the ingredients used in the manufacture of the friction material. Think of it this way, the longer life pads or pads that claim to have more stopping power usually contain more metallic material. Yes, they will last longer and could enhance braking but the chance of causing a squeal noise is very high. The squealing noise that might be caused from use of these pads does not affect braking performance and does not indicate a brake problem.

Brake squeal is caused by vibration between the brake pads, rotors, and calipers. Having the brake rotors refinished or trued (machining a small layer of the metal away from the brake rotor to make it smooth and "true" again) and a thin layer of a silicone compound placed on the back of the brake pads are a great way to reduce the squeal if the semi-metallic pads are the culprit of the noise and not due to a worn out brake pad.

Why does this squeal happen anyway?
The brake rotor is the round metal object that the brake pads squeeze together like the white part of an Oreo cookie. The rotor is metal and has a smooth slick finish, and the brake pads are made of metal shavings and also have a smooth somewhat slick finish. The more metallic material found in the pad the greater the chance for noise, and vise-versa.

The other type of brake pad is called organic. There is no metallic material used in this kind of brake pad. Organic style brake pads can only be used on vehicles that are specifically designed to use them. Improper use of organic pads on a vehicle designed to use semi-metallic can severely reduce stopping ability. Organic pads are softer than semi metallic and usually do not have a squeal problem. Unfortunately due to shorter life expectancy, inability to stop larger vehicles, and the addition of substances like asbestos in their construction, they are not very widely used.

When are squeals signaling a problem?
Sometimes brake squeals are an indication that maintenance is required. Some common conditions that cause brake noise are:

  • Heat cracked or worn "un-true" rotors
  • Rough finish on resurfaced rotors
  • Loose fitting brake pads in the caliper
  • Lack of silicone compound on back of brake pad
  • Missing springs or anti-rattle clips that should be on the caliper or pad
  • Improper tightening sequence of lug nuts or caliper hardware

Most GM cars are equipped with a small thin piece of metal attached to the brake pad to act as a warning indicator when the pad material is getting low and the brake pads should be replaced. This inexpensive warning device can be deceiving though, because this warning noise is present when the brakes are not depressed. When the brakes are applied, the warning noise goes away because the indicator has now been forced against the brake rotor and is not able to vibrate which causes this whistling noise.

If you hear disc brake noises other than a squeal, it could mean your brake pads are worn out and need to be replaced. If your brake pedal feels different than normal or if you've noticed any change in the way your vehicle brakes (pulls to one side when braking or requires more pressure on the brake pedal), have the brake system inspected at once.

What should I do?
What can you do as a customer to reduce the chance of squealing brakes? First of all, noisy brakes should always be inspected to make sure there isn't a problem with the braking system. If the pads have worn down to the point where metal-to-metal contact is occurring, your vehicle may not be able to stop safely, and you may damage the brake rotors or drums to the point where they have to be replaced. Sometimes a few harder-than-normal stops can "de-glaze" the brake pads and help reduce the squealing noise for a while.
- Kris -

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Old 10-05-2008, 07:32 PM   #2
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Getting rid of an annoying brake squeal and other brake noise is like trying to cure a bad case of herpes. You can treat the symptoms and improve the patient's condition. But it is virtually impossible to eliminate the underlying cause. Fortunately, brake noise is not caused by a virus. It is caused by a combination of factors that sometimes add up to create noise.
Brake squeal is really a high frequency vibration. In disc brakes, it can be caused by vibrations between the pads and rotors, the pads and calipers, or the calipers and their mounts. In drum brakes, the vibrations can originate between the shoes and drums, or between the shoes and backing plates.
We can't say why brake noise is so annoying. It just is. To some people, it has the same effect as scraping your fingernails across a blackboard. They can't stand it, even if it is "normal" for many vehicles today. Most people want nice, quiet brakes that stop smoothly with a reassuring "shhhhh" sound. No scraping noises. No high pitched squeals that would make a dog howl. No concerned looks or stares from passengers, other motorists or nearby pedestrians who wonder if the vehicle is going to stop or not.
Brake noise can also be worrisome to many people because they fear something is wrong with their brakes and their vehicle might not be safe to drive. Noise may indicate trouble, but the only way to know for sure is to inspect the brakes. If you find nothing amiss (no worn linings, or loose, damaged or missing parts), then you can reassure your customer and recommend any or all of the following measures to deal with the noise problem:
  • Dampen the pads
  • Lubricate the calipers Click Here for info about Brake Lubricants
  • Replace the caliper hardware (slides, pins or bushings)
  • Replace the pads
  • Resurface the rotors
  • Clean the brakes (drums)
One way to quiet noisy pads is to make sure the pads fit tightly in the calipers. If the pads on a single piston caliper have mounting ears or tabs that need to be bent or hammered to hold the pad in position, make sure the pad can't be wiggled by hand. If the pads have clips, shims or antirattle springs, make sure the necessary hardware is in place and properly installed. If you see no such items when you inspect the brakes, do not assume that none are needed. The last guy who worked on the brakes may have left them off. It is always a good idea to look up an illustration or parts list for the brake system to make sure all the required parts are there.
If the pads are installed correctly but are still noisy, one of the least expensive and most effective ways to quiet them is to remove the pads and install insulator shims on the backs of the pads. The shims, which are usually self-adhesive, act like little seat cushions to dampen vibrations between the pad and caliper.
Another option is to apply a noise suppressing compound to the backs of the pads. Some compounds harden to a rubber-like consistency to cushion the pads. Another good choice is to apply a moly-based dry brake lubricant to the backs of the pads. This type of lubricant is long lasting and won't burn or wash off like brake grease can. If applying a lubricant to the backs of the pads, be careful not to get any on the front side of the pads or rotor!

The same approach can be used on the calipers. Cleaning and lubricating the caliper mounts can also help dampen vibrations to quiet the brakes. Vibrations here can be caused by worn or loose mounts or mounting hardware.
If the calipers are badly rusted or worn, they may have to be replaced. But in most instances, you can probably clean them up, lubricate the mounting points and return them to service. You may have to replace the caliper slides, pins, clips and/or bushings, though, if there is too much play or looseness between the caliper and knuckle. Be sure to use a high temperature brake grease so the grease stays where it belongs.

Some pads are noisy than others. Semimetallic pads are typically the ones that cause the most noise problems because they are harder than nonasbestos organic (NAO) pads. Their high metallic content often makes them squeal when metal rubs against metal.
The amount and type of fillers and binders in a friction material can make a big difference in the amount of noise a given set of pads or shoes produce. Ingredients such as graphite, carbon and "rubber modifieds" may be added to reduce noise. Brass is another ingredient that helps dampen noise (it also has a cleaning effect on drums and rotors). Many of the newest pads designed for quiet operation use a nonmetallic "ceramic-enhanced" formula to eliminate noise.
The design of the pads themselves can also affect the amount of noise produced. A chamfered leading edge on the pads eliminates the sharp edge so the pads will slide across the rotor without grabbing and vibrating. Slotting the pads changes the frequency at which the pads oscillate so noise can be tuned out of the brake system.
Some pads also have a special coating that transfers to the rotor surface when the pads are first used. The coating material leaves a film on the rotor that reduces noise, vibration and also rotor wear. The transfer film also makes the pads less sensitive to variations in the surface finish on the rotors.
If the original pads are too noisy and can't be quieted by insulator shims, noise compound or grease, therefore, replacing them may be the only way to get rid of the noise.

Some brands and grades of aftermarket brake linings are quieter than others. So you may have to experiment with several different ones to find the quietest set for a given application. But whatever you do, do not substitute asbestos or nonasbestos organic (NAO) pads for semimetallic pads unless the friction material supplier says it is okay to do so. Asbestos and NAO do tend to be quieter than semimetallics but can't withstand the heat that semimetallics can. Consequently, if you swap asbestos or NAO for semimetallics in a front-wheel drive car or minivan where the brakes run hot, it can reduce the life of the linings significantly (up to half or more!) and increase the risk of overheating and brake fade.
After you have installed the new pads, it is a good idea to break them in (unless the pads are the "fully cured" type that do not require an initial break-in period). Not breaking in a new set of pads increases your risk of pad glazing and brake noise. A driver can glaze and ruin a new set of pads if he fails to go easy on the brakes for the first 200 miles. If he overheats the brakes by mashing down on the brake pedal at every stop light, he can cook the resin in the pads before it can cure and glaze the pads.
Pads that require an initial break-in can usually be seated by making 20 to 30 easy stops from about 30 mph with at least 30 seconds between brake applications so the brake pads have enough time to cool.
Whether a new set of brake pads need to be broken-in or not, you should still test drive the vehicle to make sure the brakes are operating properly and that your efforts to eliminate the noise problem have been successful.

In addition to replacing the brake pads, it may be necessary to resurface the rotors to cure a noise problem. To brake quietly, the rotors have to be in good condition, relatively smooth and flat. So if the rotors are rough, glazed or have not been finished properly, they'll have to be resurfaced.
How smooth do the rotors have to be? OEM requirements vary, but generally speaking a surface finish that ranges anywhere from 15 to 80 microinches should be acceptable, though GM recommends a surface finish of 60 microinches or less. A smooth finish will reduce the risk of brake noise and brake squeal.
Unfortunately, there is no easy way to measure surface finish short of buying an electronic "surface profilometer," a device which drags a calibrated stylus across the surface to measure roughness. Profilometers are expensive and nobody except brake part and equipment suppliers can afford to buy them. So the next best way to check your work is with a "surface comparator gauge." This type of gauge, which is available from machine shop suppliers as well as some gasket manufacturers, generally costs less than $50 and has sample patches on a metal plate that you feel or scratch with your fingernail to compare finishes.
Another way to check the surface finish on a rotor is the ball point pen test. Write you name on the rotor. If the ink leaves a continuous line, the surface is smooth enough. But if the ink line is broken up into dots, the surface is too rough (or coated with grease!).
How the brake rotors are resurfaced does not matter as long as they end up with a high quality smooth finish. Bench lathes and on-car lathes are both capable of high quality finishes when used properly. But both require sharp tooling and the right feed rate and spindle speed or drive speed to produce a good finish that resists brake noise. For best results, many experts recommend using round lathe bits. These will produce a smoother finish (up to twice as smooth as a new rotor!).
If the crossfeed rate is too high, the lathe bit can groove the rotor like a phonograph record. There will be too much space between the peaks and valleys on the surface of the rotor making it unacceptably rough and noisy. What you want are narrow peaks and valleys. This may require you to slow down the crossfeed rate or adjust the spindle or drive speed (if possible).
Specific operating recommendations will vary with the type of equipment you are using, but if you are turning rotors on a lathe with a fixed spindle speed (100 rpm) and a fixed crossfeed rate of .003 in. per revolution, you should get a suitable finish. On lathes with adjustable spindle speed and crossfeed, a speed of 100 rpm with a crossfeed of .002 to .008 in. should give satisfactory results. A silencer band or vibration dampening attachment should be used while turning the rotor to eliminate tool chatter.
After the rotors have been turned, you can sand them with #150 grit sandpaper to smooth the surface finish. Press two sanding blocks against both sides of the rotor while it is turning slowly on a bench lathe for about 60 seconds. Or, you can do the same thing with a "Flex-Hone" tool for rotors made by Brush Research Manufacturing. The flexible beaded abrasive on the Flex-Hone tool works better than an abrasive pad in a drill. The key point here is to achieve an EVEN surface finish with no high spots or low spots on the rotor.
A final step that is often overlooked but is just as important as any of the others just mentioned is to clean the rotors after they have been turned. Use warm soapy water and a stiff brush. Aerosol brake cleaner is NOT as effective and won't remove metallic debris that can become embedded in the new pads you have just installed. To check cleanliness, wipe a clean white raga across the surface of the rotor. If you see any gray streaks on the rag, the rotors are NOT clean.
To reduce the risk of brake noise during pad break-in, there are aerosol "brake silencing treatments" that can be applied to rotors to help suppress noise. These are spray-on coatings that are applied to the rotors after they have been resurfaced. Some contain microfine aluminum particles, graphite and moly that fill in the valleys on the surface of the rotors and act as a temporary lubricant to help the new pads burnish in more gradually. This not only reduces brake noise but also helps the pads develop a better cure which actually increases the coefficient of friction slightly, according to one supplier of this type of product.

One of the leading causes of brake squeal in drum brakes is brake dust inside the drum. Removing the drum and cleaning the brakes, therefore, may be necessary to eliminate this kind of noise. Use an aerosol brake cleaner or brake washer to clean the brakes. Never, ever blow out the drums with compressed air because doing so blows zillions of microscopic fibers into the air, which you certainly want to avoid if the vehicle has asbestos linings.
Another cause of brake noise in drums is poor contact between the shoes and drum. Heel and toe contact between the shoe and drum is often the culprit, and the cure is to either replace the shoes with new ones or to resurface the drum slightly to increase its inside diameter. New shoes are ground with a slight eccentric to compensate for drum wear. This moves the point of contact away from the ends of the shoes toward the middle. In the old says, mechanics used to arc shoes to match their shape to the drum. But with the concerns about asbestos, shoe grinding is pretty much a thing of the past (though it might make a comeback if and when asbestos is totally out of the picture).
Other causes of drum noise can include weak or loose hardware (replace it), and vibrations between the shoes and the raised pads on the backing plates (apply brake grease).
That about sums up the techniques for eliminating brake noise and brake squeal. The war on noise is an ongoing battle, but one that can be won.
- Kris -

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Old 12-20-2008, 03:56 AM   #3
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Thanks! Good info
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Old 04-01-2009, 05:49 PM   #4
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Originally Posted by Eazy-E46 View Post
Getting rid of an annoying brake squeal and other brake noise is like trying to cure a bad case of herpes. You can treat the symptoms and improve the patient's condition. But it is virtually impossible to eliminate the underlying cause.

good info though. i had aftermarket pads replaced recently and the squealing is driving me mad. I'm just going to have to live with it i guess.

i know it's pretty uncool, but it could be worse.... i may not be drivin an e46!
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Old 04-01-2009, 06:00 PM   #5
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good stuff, but it was constantly repeating the same stuff.

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