So, if I am turning my rotors and on the first cut, parts of the rotor are being cut and parts of the rotor (as in whole sections) are being completely skipped over, this is not a warped rotor?
My Q45: 154 mph, heavy car, stock brakes, had "Warped Rotors" on several occasions (also boiled the brake fluid numerous times). Each time I cut the rotors (until they got too thin, then new rotor time) whole sections were skipped over on the first cut.
The reason I ask is the article from stop tech you linked says that the author has never seen warped rotors, it's always a misdiagnosis!
Not arguing, just want to learn.
I totally understand why seasoned mechanics believe in warped rotors. Everything they see and do reinforces their misconception. For example, if they turn the rotors, the judder goes away. If they replace the rotors, the judder goes away. In both cases, if you believe the root cause to be warped rotors, the "fix" you just did will cure the problem. The catch is, the "fix" also cures pad deposition!
As for placing a rotor into a lathe and watching the blade cut into one section and not the other, that can also be explained. If you are making an aircraft quality part on a lathe, you don't use a cone shaped mounting fixture. Instead, you have a mounting surface that is completely perpendicular to the cut and you measure it with a dial indicator prior to mounting anything on it. If it's not within specification, you cut the mounting surface with your lathe. (Have you ever seen a Pep Boys employee measure and adjust a brake lathe?) And you use a 4-jaw chuck with a flat shoulder that holds the piece perfectly flat. If the piece has a section that is not going to be cut (analogous to the rotor's hat portion), you use a dial indicator to make sure the piece is being clamped perfectly flat. Only then do you begin to make the cut.
An automotive brake lathe is a terribly inaccurate device that, when used without meticulous setup technique, will usually induce several thousandths of an inch of runout into your rotor. This is runout that wasn't there to begin with. That perfectly flat rotor will appear warped because it isn't laying flat against the mounting fixture.
Back to diagnosis of warped rotors. Often, a mechanic will call and tell me he's just taken measurements of a rotor on the car and insists there is at least 8 or 9 thousandths of an inch of runout. So the diagnosis is warped rotors. I ask how the runout measurement was taken. Were the wheels simply removed and the dial indicator placed on the face of the rotors? If the only thing holding the rotor to the hub is a rotor retaining screw (or the metal clips on the 300C studs), you will get a false runout reading. You need to use washers and wheel nuts (or wheel bolts) torqued to the same setting (it doesn't have to be all the way to 100 lb-ft, you can torque them to 40 lb-ft as long as they are all the same) and only then will the dial indicator give you an accurate measure of rotor runout. If there is real runout on a newly installed rotors, I usually find a chunk of rust or debris (e.g., a metal shaving) between the hub and the rotor hat. Properly clean the hub mounting surface and reinstall the rotor and the runout goes away.
The problem is, the mechanic is expecting
the rotors to be warped as an explanation for the customer's symptoms. So when he sees excessive runout during an on-car measurement, he doesn't question his measurment technique. And when he slaps the rotor onto his lathe and sees the blade only cut one section of the rotor on the 1st pass, he doesn't question his set-up. When you know the true cause of most judder problems to be pad deposition, you go back and question everything and that's when things start to make sense. For example, you can set up a fixture and take careful thickness measurements at about 20 evenly spaced locations around the rotor (same distance from hub) and you will find regions of thicker transfer layer in excess of four ten-thousandths of an inch.
If the problem was really warped rotors, then it wouldn't be cured by any of the following techniques:
1. Re-bed the brakes.
2. Install Hawk Blue race pads and drive around normally for 30 minutes of stop-and-go traffic.
3. Remove all pad transfer with a Flex-hone tool.
Once I've cured a "warped rotor" problem with brief use of an abrasive track pad, my customer becomes a true believer.