Question on using Racing Brake pads to cure rotor vibrations

Originally Posted by Test_Engineer

Rotors and pads are a whole bag of problems waiting to happen, let it be deformation OR pad deposits. Heat is usually a local elastic deformation(in extreme cases can become plastic deformation), while pad deposits I would guess have some local plastic deformation. I've never had the chance to mount a cross section of a pad deposit, so I can't say for sure, but you can measure thickness variation and runout do to heat damage.

I see. I had missed your ealier point that rotors CAN actually deform. How often does deformation occur vs just having pad deposits? Does deformation tend to cause more brake pedal vibrations than just uneven surface deposits?

Also, there's a myth (or maybe its reality) that BMW rotors "warp" more often than other cars. Can you think of any reason why this might be true (why they'd be more suseptible to deformation and/or pad deposits)? If it is true, my half-baked theory would be that it could be due to 2 things:
-use of vented rotors (which makes it possible for the surface to "collapse between the vanes" with extreme heat)
-Pad compound. I know OEM BMW brakes are more prone to dust than other manufacturers. Perhaps the pads are "softer" and more likely to form deposits?
The standard theory is that BMW rotors are thinner and thus more likely to warp. From what i've read here that theory likely doesn't make any sense.

Thanks for all your input! Its really interesting to hear what a real automotive engineer has to say about these things.

Originally Posted by robg

I see. I had missed your ealier point that rotors CAN actually deform. How often does deformation occur vs just having pad deposits? .

Usually Deformation happens more often on road cars, because they are not designed to handle extreme temps, while pad deposits are more common on race cars to do the extreme pressure and operating conditions.

Originally Posted by robg

Does deformation tend to cause more brake pedal vibrations than just uneven surface deposits?

YES, Deformation gives more pedeal vibration, because when the pad passes over the deformed area, it pushes the caliper piston in and out, cause the fluid to pulse in the brake lines. Pad deposites makes the car "shake" the same, but the pedal is usually not as bad.

Originally Posted by robg

Also, there's a myth (or maybe its reality) that BMW rotors "warp" more often than other cars. Can you think of any reason why this might be true (why they'd be more suseptible to deformation and/or pad deposits)? If it is true, my half-baked theory would be that it could be due to 2 things:
-use of vented rotors (which makes it possible for the surface to "collapse between the vanes" with extreme heat).

Vented rotors actually help this from happening. They keep the internal stresses much lower and allow for more better cooling so the rotor does not deform as much.

Originally Posted by robg

-Pad compound. I know OEM BMW brakes are more prone to dust than other manufacturers. Perhaps the pads are "softer" and more likely to form deposits?

Personally, I have never dealt with German brakes(in an engineering setting), so I don't know if the pad material is softer or harder. If I had to guess, I would say softer, but because they dust so bad, they may be more prone to material transfer.

Originally Posted by robg

The standard theory is that BMW rotors are thinner and thus more likely to warp. From what i've read here that theory likely doesn't make any sense.

I wouldn't say they are thinner. Go look at an American car, then look at your BMW.

Thanks Test Engineer. If you wouldn't mind-- I have another related question-- in my case I only get vibrations when braking at higher speeds (over 60). Why would this be (if its due to brake rotor "warping")? I would think that i should be able to feel vibrations at all speeds?

Originally Posted by robg

Thanks Test Engineer. If you wouldn't mind-- I have another related question-- in my case I only get vibrations when braking at higher speeds (over 60). Why would this be (if its due to brake rotor "warping")? I would think that i should be able to feel vibrations at all speeds?

There are 2 different types of brake judder. Hot judder and cold judder.
Cold judder is felt during all conditions and is plastic deformation of the rotor and or heavy deposits.
Hot judder is when the rotor is hot(braking from high speeds causes alot of heat) and is usually an elastic condition, where the rotor deforms with heat, but returns to relatively "normal" state when it is cool. Having said that it is an elastic condition, doesn't mean that it returns to new condition. Usually the cause is a previous plastic deformation from heat stress, so the rotor is no longer perfect anymore, and never will be. With the reheating of the rotor causing the rotor to expand in an abnormal fasion due to previous plastic deformation, it becaomes wavy and pulses the pad and piston....brake judder.
Cold judder is usually fixable by "turning" the rotors, while hot judder will never be the same again...they are junk.

Originally Posted by Test_Engineer

There are 2 different types of brake judder. Hot judder and cold judder.
Cold judder is felt during all conditions and is plastic deformation of the rotor and or heavy deposits.
Hot judder is when the rotor is hot(braking from high speeds causes alot of heat) and is usually an elastic condition, where the rotor deforms with heat, but returns to relatively "normal" state when it is cool. Having said that it is an elastic condition, doesn't mean that it returns to new condition. Usually the cause is a previous plastic deformation from heat stress, so the rotor is no longer perfect anymore, and never will be. With the reheating of the rotor causing the rotor to expand in an abnormal fasion due to previous plastic deformation, it becaomes wavy and pulses the pad and piston....brake judder.
Cold judder is usually fixable by "turning" the rotors, while hot judder will never be the same again...they are junk.

Thanks. very interesting. So, it would seem that I have "hot judder". What still puzzles me is this: If i brake at speeds over 60, it happens. But if I slow down, and brake again within a few seconds there's no judder. I'd thikn that since the brakes are still hot (thus causing the deformation), that I should still get the judder at the lower speed? Or are the brakes cooling fast enough in that few seconds that they return to their "normal" shape?

BTW, I never thought brakes could be such a complicated and interesting topic.

Originally Posted by robg

Thanks. very interesting. So, it would seem that I have "hot judder". What still puzzles me is this: If i brake at speeds over 60, it happens. But if I slow down, and brake again within a few seconds there's no judder. I'd thikn that since the brakes are still hot (thus causing the deformation), that I should still get the judder at the lower speed? Or are the brakes cooling fast enough in that few seconds that they return to their "normal" shape?

BTW, I never thought brakes could be such a complicated and interesting topic.

What you might be feeling is the brake juder being amplified by matching the natural frequency of some other system in the car. Lets say the natural frequency of the engine mounts is about 20Hz, and if the pulse created by the judder near 60 mph is 20Hz, a small vibration can excite the 20Hz engine mount and shake the engine like a SOB. This could also be the natural frequency of the steering gear mounting....many things. This condition is same as when glass breaks during a very high frequency noise occurs(opera signer breaking a wine glass), it is because it matched the natural frequency and all hell breaks loose.

Originally Posted by Test_Engineer

What you might be feeling is the brake juder being amplified by matching the natural frequency of some other system in the car. Lets say the natural frequency of the engine mounts is about 20Hz, and if the pulse created by the judder near 60 mph is 20Hz, a small vibration can excite the 20Hz engine mount and shake the engine like a SOB. This could also be the natural frequency of the steering gear mounting....many things. This condition is same as when glass breaks during a very high frequency noise occurs(opera signer breaking a wine glass), it is because it matched the natural frequency and all hell breaks loose.

Interesting. Make sense. Although-- if I brake and take my hands off the steering wheel (briefly), i still feel a judder through the brake pedal; so it would seem that the vibratino is mainly coming from the rotors, right?

BTW, I completely understand that I almost certainly need new brakes. Out of pure curiousity, I'm going to try "re-bedding" them before I replace them just to see what happens) , I just find it interesting to really understand exactly how and why these things happen.

Originally Posted by robg

Interesting. Make sense. Although-- if I brake and take my hands off the steering wheel (briefly), i still feel a judder through the brake pedal; so it would seem that the vibratino is mainly coming from the rotors, right?

That's right!

Originally Posted by robg

I just find it interesting to really understand exactly how and why these things happen.

This is the exact reason why I chose to be an engineer! It is a very rewarding field of work, and you get to drive cool cars all the time.

Just to throw in what I've picked up over the years from the guys at StopTech: You may have to re-bed in pads on occasion. I've had to this with my BBK (Pagid Orange pads).

As for the so called 'warpage' of rotors, I had this too, with both my stock rotors and pads and the StopTeck BBK but I no longer call it warped, I call it pad deposits. They usually went away after X amount of miles because of two factors.

1. - I changed my braking behavior in order to NOT create hotspots/deposits on the rotor's surface.
2. - The hotspots that were there eventually wore away from step #1.

In all honesty, what creates all these braking issues is the driver. Change your braking habits/methods and you'll see that you won't have these issues coming up.

Originally Posted by SergioK

Just to throw in what I've picked up over the years from the guys at StopTech: You may have to re-bed in pads on occasion. I've had to this with my BBK (Pagid Orange pads).

As for the so called 'warpage' of rotors, I had this too, with both my stock rotors and pads and the StopTeck BBK but I no longer call it warped, I call it pad deposits. They usually went away after X amount of miles because of two factors.

1. - I changed my braking behavior in order to NOT create hotspots/deposits on the rotor's surface.
2. - The hotspots that were there eventually wore away from step #1.

In all honesty, what creates all these braking issues is the driver. Change your braking habits/methods and you'll see that you won't have these issues coming up.

Yes, this whole thread is really about the "myth of warped rotors". But, one thing I've learned is that its not just pad deposits that can cause vibrations-- the rotor surface can become wavy due to heat defromation.

Regardless, I do agree w/ you that its mostly driver issue. I probably have some bad braking habits that brought this problem on. WHen you say that you changed your braking habits/methods what specifically do you do differently now? I'm assuming that has something to do w/ never "riding the brakes", and applying them fir,my and quickly when coming to a stop?

The "heat defromation" is typically caused by a hotspot. Once an uneven transfer of material takes place, you get these.

How to avoid them: You can use your brakes at their fullest if and only if you can properly cool them afterwards. During remotely spirited driving, I never hold my foot on the brake pedal if the car is stopped. When coming to a stop (light or sign) I always taper off the pedal pressure. During brake application, do not 'slam on the brakes'. This intial pressure will cause an instantaneous increase in pad temp and uneven pad deposit on the rotor's surface. This is especially true if the rotors have not gone through the bed-in process correctly. Application of the brake pedal must be gradual. The same holds true for release.

Originally Posted by SergioK

During remotely spirited driving, I never hold my foot on the brake pedal if the car is stopped.

so what do you do? parking brake?

Sunny So Cal is relatively flat but on the occasion that the car rolls a bit, I'll use the hand brake but only so much as to stop the car from rolling.

Originally Posted by SergioK

Sunny So Cal is relatively flat but on the occasion that the car rolls a bit, I'll use the hand brake but only so much as to stop the car from rolling.

aren't you worried that you are not lighting up your brake lights though? i mean what if someone hits you from behind and then says it was cuz your brake lights weren't working...especially at night. and if in the case you did get rear ended at a stop your car would just be pushed forward, possibly damaging your car even more....

forgive my ignorance...but what you are advocating doesn't sound quite so safe...just to prevent pad transfer.

No, I don't worry about it. Not all cars have automatic transmissions. In fact, most vehicles in the world have manual transmissions. Thus, not every driver behind the wheel must have the security blanket of standing on the brake pedal at an intersection. This is uniquely 'north american' and too bad if my brake lights aren't lit. If someone can't see my stopped car and hits me from the rear, that person should immediately have their driver's license revoked! It doesn't take much to see a stopped car at an intersection... err.. maybe half a brain and one working eyeball???

Originally Posted by SergioK

No, I don't worry about it. Not all cars have automatic transmissions. In fact, most vehicles in the world have manual transmissions. Thus, not every driver behind the wheel must have the security blanket of standing on the brake pedal at an intersection. This is uniquely 'north american' and too bad if my brake lights aren't lit. If someone can't see my stopped car and hits me from the rear, that person should immediately have their driver's license revoked! It doesn't take much to see a stopped car at an intersection... err.. maybe half a brain and one working eyeball???

Yeah, but if someone hits from the rear, its not going to pleasant for you. Even if they have their driver's license revoked, you still have a damaged car and probably some painful injuries.

I never noticed whether drivers hold the brake pedal at lights or intersections in Europe. I agree, it shouldn't be hard to notice that a car is stopped-- but drivers in this country are generally not paying too much attention, and are likely driving large vehices that will inflict serious damage.

Idiot drivers are everywhere and you are kidding yourself if you think two or even three brake lights are really going to matter if someone doesn't pay attention and notice a stopped car in front of them.

Originally Posted by SergioK

Idiot drivers are everywhere and you are kidding yourself if you think two or even three brake lights are really going to matter if someone doesn't pay attention and notice a stopped car in front of them.

probably true.

Originally Posted by robg

My steering and brake pedal vibrate slightly when I apply the brakes over 60 mph. I'm assuming that this is likely due to uneven pad deposits (I've read enough from reputable sources to believe that there's really no such thing as "rotor warping"). To solve this, I could either buy new rotors and pads, or I could take the rotors to a shop and have them "turned" (not a good idea from what I hear)

I've also read that racing brake pads are much more abrasive than regular street pads. So, I'm wondering if a cheap and easy way to solve this issue might be to buy some racing pads for my front brakes, put them on, and apply the brakes at speed enough to "grind" down the existing deposits (but not enough to heat up the racing pads and allow them to form their own deposits).

Do you think this would work?

Also check your tires condition, and balance of the front wheel.

I had similar problem with my previous car. First, I thought it is brake pad, I replaced it, the problem seemed to go aways, but come back after a month. Then I thought it was the roter, I replaced it, problem went away for a month and came back. Then I replace my front tires. Problem went away and hasn't come back for 2 years.

eel

Originally Posted by robg

probably true.

It's even worse when I ride my motorcycle!

http://www.bimmerfest.com/forums/showthread.php?t=86401 (not me)

I was poking around StopTech's site today and came across this:
http://www.stoptech.com/whitepapers/warpaway.htm

Originally Posted by doeboy

I was poking around StopTech's site today and came across this:
http://www.stoptech.com/whitepapers/warpaway.htm

interesting article....but from a cost perspective why wouldn't you just pop the rotors off and get them turned at a local "butcher" shop....i mean just about anybody can turn a rotor for cheap...but thanx for posting the link, interesting to see what stoptech had to say about this.

Originally Posted by doeboy

I was poking around StopTech's site today and came across this:
http://www.stoptech.com/whitepapers/warpaway.htm

INteresting. I guess my theory wasn't totally crazy.

Originally Posted by Ace

interesting article....but from a cost perspective why wouldn't you just pop the rotors off and get them turned at a local "butcher" shop....i mean just about anybody can turn a rotor for cheap...but thanx for posting the link, interesting to see what stoptech had to say about this.

No way I'd send my StopTech rotors to get turned; each one costs $225! And, with only 2mm of wear possible, I'd be crazy to throw away 1/2mm (about $100 easy) on 'turning rotors'. This is why I've learnd to elminate the causes of pad deposits instead of trying to find a remedy to it.