Are my Rotors Warped?
When you begin to feel pedal pulsation during braking, the common thought is that you have a warped brake rotor. However this is rarely the case unless you have an unusually low quality brake rotor on the car, or the product was severely overheated due to a seized caliper or some other mechanical fault.
And yes, we know that your uncle ‘who knows cars’ said it was a warped rotor, and everyone on your favorite web forum said so too. But unfortunately, they are most likely confused about the root cause.
Generally speaking, with today’s engineering, technology and material quality controls, you’d have to go out of your way to create a brake rotor that would warp during typical use. The vane structure would have to be wrong, the materials would have to be problematic, and there would have to be some other significant contributors to allow a rotor distort and become warped severely enough to feel it in the pedal.
So now that we’ve eliminated a warped rotor as the cause of your pedal pulsation, what else could it be?
More than likely, you are experiencing either a runout problem, or a pad transfer issue.
The first question to ask is, how many miles were on your brakes when you noticed the pedal pulsation? If they were recently installed, with under 5000 miles, chances are good you have a pad transfer problem. So we’ll start with that.
When you first install new pads and rotors, there is usually a specific process that is recommended to follow to properly mate the pads to the rotors. This procedure is often referred to as the bed-in process. During the bed-in the pads and the rotors are carefully brought up to operating temperature to allow excess oils and resins from the rotors and pads to be burned off safely. Additionally, during bed-in, the pads apply a fine film of pad material to the rotor surface which is what the pads actually come in contact with when braking. This very thin coating of pad material is critical to optimum braking friction, and can really wreak havoc when it doesn't transfer properly.
If the pad transfer layer is uneven or has hot spots in it, the pad can skip or slide over certain areas of the rotor surface, which can show up in the form of pedal pulsation. The ‘skip-bite-skip-bite’ as you brake will feel like a surging in the pedal
and is often described as a warped rotor.
The good news is that this is typically an easy fix. Often times this can be solved with some mild sandpaper and some elbow grease. Use a fresh piece of 240 grit sandpaper and sand down any thick spots, or just even-out the entire friction surface. Then go ahead and re-do the bed-in procedure specified by your pad manufacturer.
This commonly solves the issue. But, in the off chance it doesn't, you can also have your rotors turned at the machine shop to reveal a completely new friction surface where you can start over again as if the rotors were new.
If the brakes are older, and have more miles on them, your problem is more likely Disc Thickness Variation or DTV.
DTV is when the rotor itself has become unevenly worn. This can be either due to hot spots in the rotor where the iron itself has become hardened and is more difficult to wear down, or from the rotor having excessive lateral runout, which is where the rotor wasn't properly installed on the hub for one reason or another, and the rotor has rubbed on the high spots more often, effectively wearing down the high spots more than the low ones leading to a rotor with different thicknesses. When the pedal is pressed, the wavy rotor fights against the caliper causing pulsing to be transmitted back to the pedal, again feeling like what people describe as a warped rotor.
DTV really only has two possible solutions, first is simply to replace the rotors and make sure they are installed with no lateral runout. Or, you can have the rotors turned at the machine shop, assuming there is enough material remaining to do so. All Sparta rotors have their minimum operational thickness etched into their outer edge, so you can have a reference for how far rotors can be machined to resolve issues like these before they are too thin to be safely used.
A pulsing brake pedal is a fairly common automotive issue, and there are quite a few different things that can contribute to this type of a problem. But bear in mind that in the majority of cases, the issue comes from something other than a warped rotor, as those have a tendency to be pretty rare these days thanks to the generally higher quality of automotive products these days.