Rennteam 2.0 - EN - Forum - Re: What are 'glazed' discs? - Page1

Jul 12, 2007 11:08:29 AM

Rich C (UK): Veteran; Loc: Mortimer, UK; Posts: 1098, Gallery; Registered on: Jul 11, 2004

What are 'glazed' discs?

My car was in for some work recently - one of the items I raised was how I get a high pitch squeal - apparently from the brakes at low speeds after a spirited drive.

Dealer has told me the discs are glazing - and to rectify to do some vigorous stops from high speed (no problem

), but what does 'glazing' actually mean?

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Jul 12, 2007 2:04:16 PM

355Spider: Co-Pilot; Loc: Dallas, Texas , United States; Posts: 277, Gallery; Registered on: Jun 27, 2006; Reply to: Rich C (UK)

Re: What are 'glazed' discs?

I know that a glazed clutch disc is when the clutch is spinning really fast and you try and engage it. It slips and won't engage very well until it cools a bit. Probably a similar thing. My Z06 clutch glazes at the strip occaisionally.

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Jul 12, 2007 3:20:45 PM

Grant: Rennteam VIP; Loc: Denver, CO , United States; Posts: 12586, Gallery; Registered on: Aug 21, 2002; Reply to: Rich C (UK)

Re: What are 'glazed' discs?

Quote:
Rich C (UK) said:
My car was in for some work recently - one of the items I raised was how I get a high pitch squeal - apparently from the brakes at low speeds after a spirited drive.

Dealer has told me the discs are glazing - and to rectify to do some vigorous stops from high speed (no problem ), but what does 'glazing' actually mean?

I believe it refers to a depositing of brake pad material onto the rotors from high temperatures encountered with vigorous use (similar to clutch glazing as described above)...

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Jul 12, 2007 9:17:35 PM

911c2s: Newbie; Loc: MA, USA; Posts: 24, Gallery; Registered on: Jun 21, 2005; Reply to: Grant

Re: What are 'glazed' discs?

Quote:
Grant said:
Quote:
Rich C (UK) said:
My car was in for some work recently - one of the items I raised was how I get a high pitch squeal - apparently from the brakes at low speeds after a spirited drive.

Dealer has told me the discs are glazing - and to rectify to do some vigorous stops from high speed (no problem ), but what does 'glazing' actually mean?

I believe it refers to a depositing of brake pad material onto the rotors from high temperatures encountered with vigorous use (similar to clutch glazing as described above)...

There should always be a thin layer of brake pad material on the rotor. Proper brake bedding will achieve that with multiple high speed braking sessions followed by a cool down before you ever come to a complete stop. If the material is uneven, then you get the sensation of the often mis-diagnosed 'warped rotor'. Glazing occurs if the bed of brake pad material is stripped off by multiple stops in which the brake pad material has not heated up to a sufficient degree.

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Jul 24, 2007 8:22:14 PM

911digital: Veteran; Loc: Staffordshire , United Kingdom; Posts: 833, Gallery; Registered on: Dec 2, 2005; Reply to: Rich C (UK)

Re: What are 'glazed' discs?

Heres some info about the problem..

Pad fade occurs for several reasons. All friction material (brake pad stuff) has a coefficient of friction curve over temperature. Friction materials have an optimal working temperature where the coefficient of friction is the highest. Sometimes you can use the brakes so hard that you get the temperature over the point of maximum friction to where the coefficient of friction curve starts to decline.

The mechanics of this decline in the coefficient of friction are varied. At a certain temperature, certain elements of the pad can melt or smear causing a lubrication effect, this is the classic glazed pad. Usually the organic binder resin starts to go first, then even the metallic elements of the friction material can start to melt. At really high temperatures the friction material starts to vaporize and the pad can sort of hydroplane on a boundary layer of vaporized metal and friction material which acts like a lubricant. Pad fade is felt as a car that still has a decent, non mushy feeling brake pedal that won't stop even if you are pushing as hard as you can. Usually it builds somewhat slowly giving you time to compensate for it ,but some friction materials have a sudden drop off of friction when the heat is put on them resulting in sudden dangerous fade.

Green Fade
This is perhaps the most dangerous type of fade that has injured more race car drivers than any other type of brake incident.

Green fade is a type of fade that manifests itself on brand new brake pads. Brake pads are usually made of different types of heat resistant materials bound together with a phenolic resin binder. These are thermosetting plastic resins with a high heat resistance. On a new brake pad, these resins will out-gas or cure when used hard on their first few heat cycles. The new pad can hydroplane on this layer of excreted gas. Green fade is dangerous because many people assume that new brakes are perfect and can be used hard right off the bat. Green fade typically will occur much earlier than normal fade so it can catch a driver that is used to a certain car's characteristics unaware. Typically the onset of green fade is rather sudden, further increasing the danger factor. I was a victim of green fade once. The crew forgot to tell me that new brake pads were installed on the car and when I went out on the track, I was flying down the escape road at about the third corner! Some teams have a new pads warning sign that they place on the steering wheel to inform the drive to be careful on his first few laps.

Green fade can occur if you change the pads and drive on the street for a few hundred or even thousand miles, never braking hard, then suddenly start using the brakes hard. I think that this is the fade that many list member complain about on their own cars.

Green fade can be prevented by bedding the pads. This is a simple procedure to boil off the resins and break in the pads under controlled conditions

Just some info which i found on the net about glazing.

Normaly glazing can be reduced or even irradicated by using better quality pads and drilled and grooved discs..

Or use the porsche ceramic discs, and pads, mike W8MM said that in the CGT after repeated hard braking the brakes felt as sharp as they are normaly..

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Jul 25, 2007 2:39:02 PM

Walter: Pilot; Loc:; Posts: 453, Gallery; Registered on: Jan 20, 2006; Reply to: 911digital

Re: What are 'glazed' discs?

Quote:
911digital said:
Heres some info about the problem..

Pad fade occurs for several reasons. All friction material (brake pad stuff) has a coefficient of friction curve over temperature. Friction materials have an optimal working temperature where the coefficient of friction is the highest. Sometimes you can use the brakes so hard that you get the temperature over the point of maximum friction to where the coefficient of friction curve starts to decline.

The mechanics of this decline in the coefficient of friction are varied. At a certain temperature, certain elements of the pad can melt or smear causing a lubrication effect, this is the classic glazed pad. Usually the organic binder resin starts to go first, then even the metallic elements of the friction material can start to melt. At really high temperatures the friction material starts to vaporize and the pad can sort of hydroplane on a boundary layer of vaporized metal and friction material which acts like a lubricant. Pad fade is felt as a car that still has a decent, non mushy feeling brake pedal that won't stop even if you are pushing as hard as you can. Usually it builds somewhat slowly giving you time to compensate for it ,but some friction materials have a sudden drop off of friction when the heat is put on them resulting in sudden dangerous fade.

Green Fade
This is perhaps the most dangerous type of fade that has injured more race car drivers than any other type of brake incident.

Green fade is a type of fade that manifests itself on brand new brake pads. Brake pads are usually made of different types of heat resistant materials bound together with a phenolic resin binder. These are thermosetting plastic resins with a high heat resistance. On a new brake pad, these resins will out-gas or cure when used hard on their first few heat cycles. The new pad can hydroplane on this layer of excreted gas. Green fade is dangerous because many people assume that new brakes are perfect and can be used hard right off the bat. Green fade typically will occur much earlier than normal fade so it can catch a driver that is used to a certain car's characteristics unaware. Typically the onset of green fade is rather sudden, further increasing the danger factor. I was a victim of green fade once. The crew forgot to tell me that new brake pads were installed on the car and when I went out on the track, I was flying down the escape road at about the third corner! Some teams have a new pads warning sign that they place on the steering wheel to inform the drive to be careful on his first few laps.

Green fade can occur if you change the pads and drive on the street for a few hundred or even thousand miles, never braking hard, then suddenly start using the brakes hard. I think that this is the fade that many list member complain about on their own cars.

Green fade can be prevented by bedding the pads. This is a simple procedure to boil off the resins and break in the pads under controlled conditions

Just some info which i found on the net about glazing.

Normaly glazing can be reduced or even irradicated by using better quality pads and drilled and grooved discs..

Or use the porsche ceramic discs, and pads, mike W8MM said that in the CGT after repeated hard braking the brakes felt as sharp as they are normaly..

wow thanks a lot! Very informative!

But what does 'bedding the pads' mean?

edit: Does the brake pedal feel very mushy when green fade occurs?

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Jul 25, 2007 10:27:02 PM

911digital: Veteran; Loc: Staffordshire , United Kingdom; Posts: 833, Gallery; Registered on: Dec 2, 2005; Reply to: Walter

Re: What are 'glazed' discs?

To bed in the pads some suggest you take it easy for the first 200 miles to allow the pads to ''bed'' in so that the surface of the pad is worn in gradual.

You may have found that when new pads are fitted they dont work very good well this is just that, you have to wear the pads in.

Some also suggest that you do a couple of heavy stops or to press the brake pedal hard a few times for 1 second pulses, this is intended to remove the crap layer and help to bed the pads in.

Also try not to keep the brake padal pressed when slowing down, but rather press the brake pedal using on and off intervals this helps to prevent brake fade and glazing.

Another tip is after hard braking if you come to a complete stop dont keep the pedal pressed, reason is the discs and pads get extremely hot and by doing this you sandwiching your pads and discs together and this creates hotspots in the areas of the pads and disc which helps to warp the disc and cook the pad material, so if you ever come to a stop after hard braking ie like pulling into the pits on a track then use the handbrake.

I'm not sure if the pedal feels mushy on green fade probably not the pedal may feel ok but no matter how hard you press the vehicle may not stop although with newer brake assist hydraulics this would help in this situation.

A mushy brake pedal is normaly a sign of either a master cylinder problem or air in the brake lines or you may have a rubber pipe swelling, although i've developed a spongy and excessive pedal travel when i have replaced the pads on my vw, and it's took a fair bit of driving to return the padel feel to normal? which is strange although it may have something to do with pushing the pistons back in and the brake fluid gets pushed back through the lines and back in to the master cylinder and res.

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Jul 26, 2007 2:47:14 PM

Walter: Pilot; Loc:; Posts: 453, Gallery; Registered on: Jan 20, 2006; Reply to: 911digital

Re: What are 'glazed' discs?

Quote:
911digital said:
To bed in the pads some suggest you take it easy for the first 200 miles to allow the pads to ''bed'' in so that the surface of the pad is worn in gradual.

You may have found that when new pads are fitted they dont work very good well this is just that, you have to wear the pads in.

Some also suggest that you do a couple of heavy stops or to press the brake pedal hard a few times for 1 second pulses, this is intended to remove the crap layer and help to bed the pads in.

Also try not to keep the brake padal pressed when slowing down, but rather press the brake pedal using on and off intervals this helps to prevent brake fade and glazing.

Another tip is after hard braking if you come to a complete stop dont keep the pedal pressed, reason is the discs and pads get extremely hot and by doing this you sandwiching your pads and discs together and this creates hotspots in the areas of the pads and disc which helps to warp the disc and cook the pad material, so if you ever come to a stop after hard braking ie like pulling into the pits on a track then use the handbrake.

I'm not sure if the pedal feels mushy on green fade probably not the pedal may feel ok but no matter how hard you press the vehicle may not stop although with newer brake assist hydraulics this would help in this situation.

A mushy brake pedal is normaly a sign of either a master cylinder problem or air in the brake lines or you may have a rubber pipe swelling, although i've developed a spongy and excessive pedal travel when i have replaced the pads on my vw, and it's took a fair bit of driving to return the padel feel to normal? which is strange although it may have something to do with pushing the pistons back in and the brake fluid gets pushed back through the lines and back in to the master cylinder and res.

Thanks again!

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Jul 26, 2007 3:21:28 PM

95jersey: Senior; Loc: US; Posts: 578, Gallery; Registered on: Feb 20, 2006; Reply to: Walter

Re: What are 'glazed' discs?

Most pad manufacturer's bed in process are very similiar. Get them HOT FAST, then let them cool for 24 hours before driving the car again. You simply want to transfer pad material to the rotor. This is what really stops your car. Try to stay with a particular brand.

Take the car our and tape any ducts. Stop with medium pressure from 60-20, progressively providing harder pressure. Do this about 4-5 times, then drive without applying your brakes for a couple miles to allow to cool, then get into them HARD. Big stops from 80-10, never stopping completely. Do this about 4-5 times, then you should smell the pad burning. When you get this smell, your job is done, don't touch the brakes bring the car home and let sit overnight. You will improve the life of the pad, it's stopping power, and the likelyhood of glazed or warped rotors as it is often referred.

Warped rotors are nothing more than pad matieral applied unevenly to the rotor.

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