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Old 08-30-2011, 10:25 PM
Evlengr's Avatar
Evlengr Evlengr is offline
Ukemi - that's how I roll
Location: MD
Join Date: Feb 2007
Posts: 4,252
Mein Auto: 2013 Audi S4 2010 MT MCS
Originally Posted by Supercourse View Post
I'll play Devil's advocate here .......

Rotors do build up a microscopic layer of pad material, because of the nature of disc brakes and the heat involved.

They are both sacrificial components, but not independent of each other.

This is why bedding in of new pads can be important - to get that initial transference as evenly distributed as possible.

And it's why using old pads with new rotors is not normally advised, because the old pads are not likely to be worn perfectly evenly.

Using new pads with old rotors is more acceptable, but also not optimal, regardless of min. rotor thickness considerations.

New pads and new rotors provide the best starting point for establishing long-term braking performance.

So, after a few months with new pads on old rotors, could the rotor thickness really have increased?

Nah, not really - but other that inaccuracies in measuring, they could be measuring now at a spot that has a high point of pad material transfer (as already mentioned by madurodave).

So that could account for the 0.01 mm. but the point is that there is still a way to go until min. thickness is reached.

More than measuring inaccuracy I would say, so in hindsight it might have been best to wait until closer to the 8K miles remaining under no-charge scheduled maintenance.

If an independent measurement finds min. spec. reached, would this really lead to the dealer readily coming up with new rotors?

Or more time-consuming back and forth discussion?

Scuff up the rotors with emery cloth for a fresh start, do the belated bedding in process, and try the dealer again in a few months?
I agree on all accounts. I wouldn't even bring up the point if they weren't so dogmatic about their approach. Kind of live by the sword, die by the sword. If you are going to belabor the point over a 100'th of a mm, then you better be darn sure you are correct.

If a private ASE cert mechanic comes back and say hey man these are 22.xx, then yes I will make them measure them right in front of me as braking is not something you argue over. I'd rather crack the engine than find out my brakes won't work the way I expect them to.

If he says eh 23.xx then I will wait a bit and come back in Feb.

So far BMW has lied to me more than told the truth so I think everything they do is subject to scrutiny. Chances are I will borrow a micrometer from work and check myself as well.
2013 S4 Loaded (and NO RFT's) Awesome is an understatement
2008 MT X3 RIP you were a faithful sidekick and will be missed.
2007 AT X3 RIPOS
2005 GC
2000 Jeep Cherokee
1997 Twin Turbo RX-7
1984 GTI Wolfsburg Edition Neuspeed and more

Last edited by Evlengr; 08-30-2011 at 10:27 PM.
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