Brake myths?

[verse]

http://www.stoptech.com/technical-support/technical-white-papers/-warped-brake-disc-and-other-myths

 

This just sounds like complete bs. I've machined thousands of rotors and physically seen large amounts of runout. How do they try to explain that?

[Mensan]

I've machined thousands of rotors and physically seen large amounts of runout. How do they try to explain that?

 

In fact every case of "warped brake disc" that I have investigated, whether on a racing car or a street car, has turned out to be friction pad material transferred unevenly to the surface of the disc. This uneven deposition results in thickness variation (TV) or run-out due to hot spotting that occurred at elevated temperatures.

 

Not saying he's right, I'm just pointing out that they specifically answer the question you asked.

[greg1647545532]

I have a personal theory. I used to work at a dealership and one of my tasks was helping sell people on brake jobs. I was told then, and I told lots of people, that rotors warped form panic stops or abuse when the brakes were overheated. And I believed that.

 

But then in my own cars I would sometimes get a pulsating pedal when I knew I hadn't done any panic stops or abuse. And the only car that I never got any kind of brake shudder was my track car, where I regularly did threshold braking from north of 100mph, which should cook the shit out of the rotors. If that didn't warp them, then the overheating thing has got to be bogus.

 

So my theory is that shitty pads can leave pad material behind (this can happen with shitty race pads too, like the link shows). Once a small amount of material has been deposited on a rotor, it starts to act like a washboard dirt road. The pad comes up to the lump of residue, gets launched off the rotor like a little ramp, them smacks back down. Where it smacks down it removes a little bit of extra rotor material, and where it flies off the rotor it removes a little bit less. That little variation then forms a feedback loop, creating more little hills and dips in the rotor surface, just like one little dip in a dirt road can eventually create a mile long washboard due to the feedback loop in the springs of the cars driving over it.

 

Those variations are the runout you're measuring, and that's why the rotor needs to be turned. But my theory hypothesizes that the inner plane of the rotor is still true; that is to say, the rotor hasn't warped like you're thinking, like a soggy paper plate or something; instead, the surface has just worn unevenly.

 

So when the guy says,

 

"In fact every case of "warped brake disc" that I have investigated, whether on a racing car or a street car, has turned out to be friction pad material transferred unevenly to the surface of the disc. This uneven deposition results in thickness variation (TV) or run-out due to hot spotting that occurred at elevated temperatures."

 

I agree 100%.

[Mr. Jones]

Bear in mind, those high spots of deposit, if not addressed early enough, will glow overtime and become so hard, no amount of high friction compound will level it back out, short of a true turning.

 

On my cars that have more serious pads, I will occasionally get some judder and a few abs inducing applications later, it's as smooth as butter.

[Draco-REX]

One problem is the infamous "Warped Rotor" is as in-grained into the psyche of Automotive Repair as "Fan Death" is in Korean minds. You can't explain to someone that the rotors aren't defective, that they just need to rebed their brakes. So I just nod and give them another pair and hope they don't come back.

[Berto]

When I interned at Honda I worked with brake test engineers. Material deposits as mentioned were one of the culprits but I remember the major culprit they discussed was Weathering/rust.

 

You notice when you go outside after rain and your car has been sitting (even just a few hours), depending on the rain there is already rust on your rotors, but if you were to take your caliper off there would be no rust under it because it was not exposed to water under the pad.

 

When you drive off you obviously burn off the rust but where the caliper/pad was there is a minimal high spot. After hundreds of this cycle depending on the random nature of where your rotor/caliper line up when you stop you will develop one (or many) high spots which result in the shaking feeling everyone calls 'warped rotor'.

 

I tend to believe this. I like one of the other posters knew for a fact, on my daily driver I had not done any panic stops yet my rotors were warped as crap. The car sat outisde all the time.

 

I purchased my civic new, and it has always been garage kept. I have 125k on it with never a hint of brake shake. I really do believe that for most folks this rust build up/removal is the biggest culprit of brake shake. Additionally, I don't know if the metal composition between cheap rotors, OEM, and racing have any property to reduce likeliness to rust, but that might explain why the cheapo advance auto ones 'warp' faster.

 

good topic.