Mayayana wrote, on Tue, 04 Nov 2014 18:22:11 -0500:
This question is *always* the next question when the conversation comes to the question of what people 'think' is warp.
First, before I answer that question, do realize that there are many (i.e., scores) of reasons for pedal and steering wheel vibration upon braking, which range from myriad suspension components to unsprung components such as worn bearings and bent components, etc.
However, if we *limit* our discussion to *brake rotor* related issues, then the number of things that can cause pedal pulsation is reduced.
The main three components that are disc-only related are: a. DTV b. BTV c. runout
Let's forget runout, for our purposes, because that's a mechanical problem, e.g., someone mentioned lug nuts disproportionately tightened. Runout can also be caused by rust developing under the rotor between the hub and the rotor. Let's ignore runout, which is generally easy to measure with a dial gauge attached to some sturdy anchor point.
DTV and BTV are harder to measure, simply because they are caused by really small variations on the rotor.
We're talking in the range of ten thousandths of an inch for pad deposits to build up such that they make a "hill" or a "slick spot", where the hill is the DTV and the slick spot is the BTV (both of which cause pulsations).
How is this "hill" or "slick spot" most commonly caused?
Well, think about what happens when you cruise down an exit ramp, at a starting speed of 80 mph and you stop at the light at the bottom and hold your foot on the brake pedal for 30 seconds or more.
What can you get? A pad imprint is what you may get.
What happens if you do this over and over again? More pad imprints.
Sometimes they form hills (I'm not really sure, chemically, why, but they do). Sometimes they form slick spots.
The hills cause DTV and the slicks spots cause BTV. So, what do you do to prevent that?
Two basic things:
Mentally change your braking habits:
- Coast to a stop, and roll if you can, and lift off the pedal if you can, and,
- Rebed your brakes, in effect, constantly.
In suggestion #1 above, the common argument is you don't have the room, but, I have been doing it for years, and you *often* don't even have to stop, because a light is finite, so, you can often roll up to the light until it's green, and, even so, you at least are allowing the rotors to cool if you have to stop, and even then, if you left a few feet, you can roll a bit more also. It's a mental decision to change your braking habits.
In suggestion #2 above, if you already *know* what rebedding is, you can do mini rebedding, simply by braking hard, once a month or so, on that very same stop, and, in effect, redistributing the bedding layer on the rotors. This is more subjective, and, here in California, I do it on the five-mile long hills of the mountains, as it's difficult to do on a flat straightaway, but, that's the second mental challenge. Rebed your brakes all the time.
That's really it.
Of course, take none of this from me. Take all of this from the literature. From people who should know.