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Old 05-27-2011, 02:18 PM
rdl rdl is offline
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Location: Ontario, Canada
Join Date: Jun 2010
Posts: 1,767
Mein Auto: 530i 2003
I believe that there is a definite difference between brake rotors. If the foundry does not have good technology and quality control, all sorts of inconsistent grain structure and inter-metallic compounds (hard spots) are probable. Besides, consider that many aftermarket brake manufacturers sell 2 or more grades of rotors with the price ratio approaching 2 to 1. While I doubt the cost ratio is that high, I'm pretty sure that there is a difference between the bottom & premium grades. Unless their product spec sheets and descriptions are pure fairy tales.

I think that many common brake vibration problems are a result of the irregularities throughout the rotor. Further, the temperatures created during braking can be high enough to cause changes in the grain microstructure and growth of inter-metallic compounds that will affect the surface condition of the rotor. Especially if the foundry practices are deficient. A lot of reading and a little personal experience leads me to believe that at the very least, many "warped" rotor are not in fact distorted, but rather have hard spots of intermetallics &/or resultant uneven brake pad deposits on the rotor friction surfaces due to the irregularities. Both will cause braking effect variation as the wheel rotates.

Have you ever had brake shudder/vibration without any pulsing in the pedal? Many of my brake problems over the years have been that way. Friction variation is the most likely explanation. Especially since I could make a good case that a pure warp, i.e. run-out, in a rotor should not pulse the pedal (particularly with single piston floating calipers), would create only insignificant braking variation and would cause only mild steering wheel shake. (Thicknes variation is another matter & would cause both pulsing pedal and brake vibration.)

A prior car (GM), bought brand new, went 100k miles on the original Delco pads & rotors without any vibration ever. No complaints there. I bought brand name premium grade pads and PG grade rotors. "PG" being premium grade, so I was told. After about 10k miles I had a vicious brake vibration, but no pedal pulsing. I had the rotors turned. OK for 10k miles, then vibration back. I checked into the rotor brand & grade. It turned out "PG" was professional grade. In other words, bottom line, enough to pass a safety check - just don't keep the car. A learning experience for me.
I checked run-out and thickness - all in spec. I then tried hand sanding the rotors with good abrasive paper to scuff up and remove any micro irregularities from the surfaces. Good for about 10k miles & then vibration returned. Eventually I put Delco pads & rotors on & problem gone for for another 100k until the car died.

For the extra hundred or so bucks on a brake job, I'm not going to take the chance on no name, unknown quality or low grade rotors. BMW, ATE, Zimmermann, etc. for me.

But I am willing to buy for as much off list price as I can find The adage that "you get what you pay for" needs very careful interpretation in real life.

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