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Steve Tarring · 01-27-2007, 07:16 PM

Warped rotor? You must be thinking of a condition know as "Brake Judder"

Judder is a phenomenon where the steering starts to shake when applying the brakes. Many think this shaking is due to the disc having been warped. This is a tale to in need of mythbusting...

Let say that if a rotor could warp just for argument. If a disc was un-true or had some sort of "run-out" the brake calibers are mounted on the suspension in such a way as to allow them to float and would simply follow the run-out of the disc under braking. No, classic brake judder is the cause of two types of problems associated with friction systems use on brake systems today. This condition is caused when the friction faces are no longer parallel to each other.

Judder is caused when adherent friction mechanism, used in street application, get to hot during braking. Abrasive friction is altogether different and is used in racing systems. However, the adherent system uses a thin layer of brake pad material transfered to the faces of the discs and sticks (adheres) to the friction faces. This layer of pad material, once evenly established on the rotor, is what actually rubs on the brake pad. The bonds that are broken, for the conversion of kinetic to thermal energy are formed instantaneously before being broken again. It is this brake pad-on-transferred brake pad material interaction at the molecular level that yields the conversion process. This is the mechanism used in a street applications of modern street vehicles today. But, this is were an unwary owner can get in trouble too.

Consider the following. You’re driving your vehicle a little too aggressively and the light changes on you. You start applying heavy pressure to stop, right? We’ve all been there before…

Now that the hard part is finished, what's do you think a unwary owners might do while waiting on the light?
1) Roll down the window;
2) Change the radio station;
3) Ease up on the brakes and let the vehicle move slowly so as to avoid the brakes pads from resting in one place on these "hotter the hell rotors".
If an owner fails to do the later he risks a phenomenal called "Pad imprinting" This is where most adherent friction or street brake problems start. What happens is the pad faces can bond to the friction faces of the rotors after this type of stop and when driven off, an uneven deposit of pad martial can results. What's worse under braking now when the pad(s) encounter this uneven build-up of pad martial, local temperatures can reach 1200F to 1300F degrees. If this continues, iron which contains inclusions of silicon-iron carbides and high percentages of carbon can develop deposits of "Cementite" (Fe3C) in the rotor's iron matrix.

Let’s use a metaphor here and say that the inclusion of silicon iron-carbides and the high content of carbon (30%-40%) in cast-iron is simply a recipe for the development of cementite. All we have to do for its development is locating a heat source and when working with brakes, this will not be a problem...

Cementite deposits are very hard and even a poorer heat conductor and if allowed to continue to develop the softer iron where cementite is not developing will begin to ware more causing a greater problem. It is at this stage where an owner will normally bring their vehicle to a shop or dealer and they will turn the discs. But, all this does is knock off the high points and after continued driving the disc will start to exhibit judder again sometimes as little at 7K miles depending on the owners driving habits.

So, what is the cause of judder with the above in mind? We need to revisit our school science classes. Remember when we learned fluids could not be compressed? Sure. This is a fundamental law in hydraulics and brakes systems used are based on hydraulics principles. When your foot depresses the pedal, it forced brake fluid thru to the brake calipers causing the pads to grab the discs. It is here where the judder is felt under foot on the brake pedal. Since fluids can not be compressed, and if the rotors are not parallel as they rotate into the caliper(s) this caused the pads to try and fight back the pressure against your foot on the brake pedal. Or, classic brake judder.

I'm not sure where you got the idea about slotted rotors but, I have been running them for years on my vehicles. The slots cause the vehicle to stop much better with good quality brake pads. However, the type I use are cyro-treated. After using them I noticed that the dealer never told me that the brakes discs needed replacement. From time to time they would only indicated that the pads needed replacing. I asked them to check the thickness of the disc to make user and they told me the the discs were fine. Now I thought this was very strange because I when thru the first set of brake discs at 15k miles or so, and there were close to 56k on the vehicle then. When I asked the service manager to double check the records he confirmed replacing brakes near 15k miles. When I asked how could he explain the milage on the rotors now any why they didn't need replacement he had no answer other they to say his service personal check the discs and they indicated they were OK.

Well, to make a long story short, I have been using these types of brakes ever since. I have sold three vehicles now using these brakes and never replaced a one. When the dealer informs me I'm nearing replacement of the stock rotors, I order a new set and I have the dealer installed them. Well, I have them on my Titan now... What a relief.

01-27-2007, 07:16 PM	#11 (permalink)
Steve Tarring Registered User iTrader: (0) Join Date: Jan 2007 Posts: 1 Thanks: 0 Thanked 0 Times in 0 Posts	Re: warped rotors Warped rotor? You must be thinking of a condition know as "Brake Judder" Judder is a phenomenon where the steering starts to shake when applying the brakes. Many think this shaking is due to the disc having been warped. This is a tale to in need of mythbusting... Let say that if a rotor could warp just for argument. If a disc was un-true or had some sort of "run-out" the brake calibers are mounted on the suspension in such a way as to allow them to float and would simply follow the run-out of the disc under braking. No, classic brake judder is the cause of two types of problems associated with friction systems use on brake systems today. This condition is caused when the friction faces are no longer parallel to each other. Judder is caused when adherent friction mechanism, used in street application, get to hot during braking. Abrasive friction is altogether different and is used in racing systems. However, the adherent system uses a thin layer of brake pad material transfered to the faces of the discs and sticks (adheres) to the friction faces. This layer of pad material, once evenly established on the rotor, is what actually rubs on the brake pad. The bonds that are broken, for the conversion of kinetic to thermal energy are formed instantaneously before being broken again. It is this brake pad-on-transferred brake pad material interaction at the molecular level that yields the conversion process. This is the mechanism used in a street applications of modern street vehicles today. But, this is were an unwary owner can get in trouble too. Consider the following. You’re driving your vehicle a little too aggressively and the light changes on you. You start applying heavy pressure to stop, right? We’ve all been there before… Now that the hard part is finished, what's do you think a unwary owners might do while waiting on the light? 1) Roll down the window; 2) Change the radio station; 3) Ease up on the brakes and let the vehicle move slowly so as to avoid the brakes pads from resting in one place on these "hotter the hell rotors". If an owner fails to do the later he risks a phenomenal called "Pad imprinting" This is where most adherent friction or street brake problems start. What happens is the pad faces can bond to the friction faces of the rotors after this type of stop and when driven off, an uneven deposit of pad martial can results. What's worse under braking now when the pad(s) encounter this uneven build-up of pad martial, local temperatures can reach 1200F to 1300F degrees. If this continues, iron which contains inclusions of silicon-iron carbides and high percentages of carbon can develop deposits of "Cementite" (Fe3C) in the rotor's iron matrix. Let’s use a metaphor here and say that the inclusion of silicon iron-carbides and the high content of carbon (30%-40%) in cast-iron is simply a recipe for the development of cementite. All we have to do for its development is locating a heat source and when working with brakes, this will not be a problem... Cementite deposits are very hard and even a poorer heat conductor and if allowed to continue to develop the softer iron where cementite is not developing will begin to ware more causing a greater problem. It is at this stage where an owner will normally bring their vehicle to a shop or dealer and they will turn the discs. But, all this does is knock off the high points and after continued driving the disc will start to exhibit judder again sometimes as little at 7K miles depending on the owners driving habits. So, what is the cause of judder with the above in mind? We need to revisit our school science classes. Remember when we learned fluids could not be compressed? Sure. This is a fundamental law in hydraulics and brakes systems used are based on hydraulics principles. When your foot depresses the pedal, it forced brake fluid thru to the brake calipers causing the pads to grab the discs. It is here where the judder is felt under foot on the brake pedal. Since fluids can not be compressed, and if the rotors are not parallel as they rotate into the caliper(s) this caused the pads to try and fight back the pressure against your foot on the brake pedal. Or, classic brake judder. I'm not sure where you got the idea about slotted rotors but, I have been running them for years on my vehicles. The slots cause the vehicle to stop much better with good quality brake pads. However, the type I use are cyro-treated. After using them I noticed that the dealer never told me that the brakes discs needed replacement. From time to time they would only indicated that the pads needed replacing. I asked them to check the thickness of the disc to make user and they told me the the discs were fine. Now I thought this was very strange because I when thru the first set of brake discs at 15k miles or so, and there were close to 56k on the vehicle then. When I asked the service manager to double check the records he confirmed replacing brakes near 15k miles. When I asked how could he explain the milage on the rotors now any why they didn't need replacement he had no answer other they to say his service personal check the discs and they indicated they were OK. Well, to make a long story short, I have been using these types of brakes ever since. I have sold three vehicles now using these brakes and never replaced a one. When the dealer informs me I'm nearing replacement of the stock rotors, I order a new set and I have the dealer installed them. Well, I have them on my Titan now... What a relief.