Are you ready to have your mind blown off? Your warped brake rotors don’t actually warp. There is a long, involved science story explaining this ahead, so be ready. We’re going to get technical.
While heat transfer is indeed an issue that causes brake judder from rotor runout, it is not because the rotors themselves become “warped”, a common diagnosis in your average auto shop.
Essentially, brakes work something like this:
Because there are two types of friction happening between your tires and the road, there are two types of friction happening between your brake pads and your rotors. Friction is the mechanism that converts dynamic energy into heat. Let’s start from there.
Your tires create friction with the road in two ways: when the elastic rubber tire compound grips irregularities on the road surface, and when molecular adhesion occurs between the rubber and the road, where rubber is transferred to the road surface. Because of the way your tires react to the road and vice versa, your brake pads will interact with both tires and rotors two different ways, and create two different types of friction. Discerning what type of driving you do, and therefore which type of friction you’ll be experiencing more of is crucial when choosing the right type of brake pads for your vehicle. To put it simply, are you going to be hauling a** everywhere, or driving within the confines of the law (conservative)?
The two types of friction brake pads can cause are abrasive friction, and adherent friction.
Abrasive friction involves the breaking of the crystalline bonds of both the pad material, and the cast iron of the disc. The breaking of these bonds generates heat, ergo friction. With this type of friction, the bonds between the disc material and the pad material are permanently broken, and the harder (disc) wears the softer (pad) away. When the pads reach their effective temperature limit, they start to wear (transfer material from pad to disc) quickly, and randomly spread. This causes runout, roughness, and judder as the brake pads wear.
Adherent friction happens when the pad material diffuses across the pad and the disc, forming a super-thin but uniform layer of pad material on the surface of the disc. With both pad and disc being comprised mostly of the same material, the bonds will break and reform continually. While this still creates runout, it is much more uniform, and even across the disc than abrasive friction.
While it is impossible to have a purely abrasive or adherent brake pad, most material formulas found for everyday driving is abrasive enough to keep the disc surface even and smooth, but still uniform enough to keep them from vibrating the second you put them on your car. Choosing which type of brake pad will minimize rotor runout best will depend on where your brake pads optimize at which temperatures. After-market bargain-type organic or semi-metallic brake pads tend to be more abrasive than adherent, causing them to be severely limited by temperature (the faster you drive, the harder you brake, the hotter they get). Luckily today, most higher-end, OEM, or metallic carbon racing pads intentionally utilize one type of friction versus the other, and are able to be stable through much higher temperatures.
Sadly, there is no such thing as a brake pad that performs optimally in both street and racing capacities, and if you use one but do the other, they will wear unevenly and cause rotor damage. That doesn’t mean by any means that if you tend to be on the Fast and Furious side you should get racing pads just in case. These pads are designed to maximize at much higher temperatures than regular driving brakes, thus simply won’t work in lower temperatures. What we recommend for such drivers is high-end ceramic pads, you’ll just go through them more frequently than a normal everyday driver. And even if you can’t feel a vibration, have your rotors resurfaced every time you replace your pads. If you get to the point where you can feel them, than they need to be replaced.
Regardless of what kind of pad and rotor you use, they’re not going to perform if they’re not properly-perfectly- installed. If they’re not, right out of the box they will be producing uneven deposits, which get hotter than the metal beneath it. Every time one of these deposits comes into contact with the pad (thousands of times per minute as the wheel turns), the overall temperature increases. As this happens, an extremely hard iron carbide called cementite begins to form out of the cast iron disc, which will increase brake roughness and uneven wear, tearing through pads and rotors even quicker.
So if the guy doing your brakes tells you that you’ve got warped brake rotors on your hands, stop him right there, explain all of that science stuff we just went through to him, and then take your car to someone else. When choosing the proper pads for your vehicle, choose high-quality aftermarket or OEM pads, have your rotors resurfaced every time you replace pads, and don’t buy racing pads for your daily driver, even if the driving that you do daily is super fast. Stay safe, Road Warrior!