The rotors I just had turned were the original ones on the car from
the factory and nothing was removed from the car in that area other
than normal wheel changes during that time.  And they were fine for
about 150k miles.  So something is happening other than a deficient
mounting of rotor on hub or a thickness buildup as the article
describes.  The ones currently on the car have been on about 40k and
symptom free for about 35k.  I wire brushed the hubs and used brake
cleaner when I installed them.  On these, I am having the symptom
where the braking effect varies with wheel rotation.  I can feel this
dramatically by holding steady pedal pressure as the car comes to a
stop.  I will swap these rotors and get them turned and see how they
look on the lathe too.  The auto tech instructor said he had
personally warped the rotors on his vehicle by driving into a deep
puddle with hot brakes.

Edward

At 03:22 PM 9/20/2013, Mike S wrote:
>On 9/20/2013 3:14 PM, Edward Maglott wrote:
>>This machine was turning both sides at the same time and it was easy to
>>stop it and see that the high areas on one side perfectly matched the low
>>areas on the other side.  I'll have to ponder this in my head for a while
>>to think of ways that the disk is not "warped" but has high and low areas
>>that correspond....
>
>If you stick a dime under the disk before bolting it on, the same
>thing happens. It's not warped, it only means that the plane of the
>disc isn't perfectly normal (perpendicular) to the axis of rotation.
>That could be either on the car, or on the lathe. A bit of rust is
>enough to do it, you're only talking mils. I always wire brush the
>hubs before installing new rotors (and inside the rotor hat if
>reinstalling old ones).
>
>When it's on the car, it can cause a bit of vibration, but it
>usually doesn't cause much pulsation (i.e. the brakes won't seem to
>grab harder at one point of their rotation) - the calipers simply
>move sideways to accomodate.