Rotors do not warp - that's a myth that even many of the most experienced mechanics continue to push since they really do not understand what is taking place. Ever wonder why turning "warped rotors" never really helps and the issue comes back again and again? It's because on a molecular level the brake rotors have changed due to improper use. Let me back up a few steps and begin by talking about how to bed brakes in.
When new brake pads and rotors are initially used they are not familiar with one another - there's an exchange that must take place prior to them becoming most effective. Again, this occurs on a molecular level and largely involves depositing a layer of the brake pad onto the surface of the rotor. This may sound strange but bear with me. In order for the braking system to work properly a uniform layer of the brake pad must be bonded to the rotor. Each brake pad manufacturer normally is able to provide a guide on how to bed (introduce) their brake pads. This normally involves repeated stops in quick succession from 60-5 MPH. Depending on the pad it may take five to ten stops. Now this is the most critical part...
You can not come to a full stop during the bedding nor can you stop after bedding the pads until the rotors cool down. This is either sink or swim folks - you either have a happy relationship between the new pads and rotors or if you have stopped you have began to create inclusions...that's fancy word for problems in the cast iron brake disc.
Remember above I stated that this new molecular friendship must be uniform? If that pad material is transferred properly at elevated temperatures and the system is permitted to cool at a uniform rate then all is well. If you come to a complete stop while bedding the pads or before the rotors cool down guess what happens? The majority of the rotor is now exposed to ambient air while a certain section remains insulated under the brake pads. This hot spot leaves us with a non-uniform deposit. Now we are beginning to head in the wrong direction. At this point you may begin to feel the slightest of shudder or pulsation with your brakes simply because there is a non-uniform deposition of brake material on the surface of the rotor. Some argue that rotors can be saved at this point but I have never seen this being successfully accomplished.
Now that the rotor has a non-uniform deposit of brake material the braking system begins to operate differently. As the pad reaches this area slightly more friction and heat is created and this problem begins to compound itself. Very quickly the problem goes beyond a non-uniform deposition of brake pad material and the cast iron rotor itself begins to change in that area to a cementite phase. This phase is a description of the microstructure of the gray cast iron rotor. I am working from memory but it is suffice to say that the change in phase in this one area is now really taking us into what can be described as a downward spiral. This new microstructure in one area is hard with a high level of friction so guess what happens when brake pads attempt to do their work? That area becomes hotter and the cementite phase grows and begins to penetrate deeper into the rotor. And this is why turning rotors is only a temporary solution - the microstructure of the rotor itself had changed in one area.
Of course, when we purchase a new vehicle we drive it very carefully and are light on acceleration, turning, braking, etc. This is good but it never really fully introduces the brake pad and rotor to each other in a proper way through bedding. But after some time, when you are working your truck like a mule, dragging heavy loads up and down hills, maybe bringing the brakes up to elevated temperatures, etc. And stopping while waiting for a light shortly after really heating the brakes up...the seeds are planted and the downward spiral begins.
We could go one into much greater detail but I would encourage each of you to read up on brakes and learn more. The truth is out there although you must wade through much unintentional misinformation to find it. Me, I raced for years and my life depended on brakes so I am quick to find manufacturers information regarding the bedding of their brake pads, or I have done it so much I know when I smell the outgassing and begin to feel the brakes lose effectiveness that just another cycle or two then I can drive without stopping and permit everything to cool down.
And just remember, never come to a complete stop with very hot rotors. If you promote uniform deposition of brake pad material and gradual cooling of your gray cast iron rotors your brake system will love you.
