GusTA

Randy, please help me out with a technical issue....

QUICK BACKGROUND: Two of my colleagues own a BMW 745i and MB S430 - both cars are parked in the garage for the winter. [By the way, they disparagingly call the 2004 A8L 'bloody James Bond Car' - I think it is a great backhanded compliment]. I also admit to excessive gloating as I drive the A8L all over the place, regardless of weather or road conditions.

During one of our discussions [er, my gloating], I did mention that the 2004 A8L rotors are made of Aluminum, Silicon 7 and magnesium alloy, making it resistant to salt corrosion and rust - a first.

One of them indicated that good brake pad action requires some rust on the rotors. My personal experience is that the A8L brakes firmly, and accurately regardless of the road conditions.

The other said that an overpolished [no-rust] rotors will make the pads wear out faster.

Both of these statements do not make much sense to me, and I suspect they are sour grapes. I did agree to ask an expert

Randy, is there any truth in theses statements?

ryoung

Rust has no affect on brake performance one way or the other; it's just the natural result that you'd expect to see on a freshly polished cast iron (rotor) surface that's gotten wet. The rust is completely removed with the first moderate application of the brakes. Certainly the steel brakes on our Formula Continental are effective without rust.

High pad wear with highly polished rotors doesn't make any sense. In the case of drilled or slotted rotors, if you consider these highly unpolished, pad wear increases.

I didn't know that the rotor of the `04 A8L was an aluminum, Silicon 7, magnesium alloy. Do you have a link to this information?

The "common" rotor materials,

1. Cast iron - used on 99% of all street cars, excellent performance up to ~600 oC

2. Steel alloys - sometimes used on premium rotors for better high temperature stability.

3. Stainless steel - sometimes used on motorcycles ... to eliminate rust :-)

4. Aluminum - used on top fuel dragsters to minimize rotating weight. Note that the brakes are primarily used to hold the car on the start line. Parachutes provide most of the stopping power at the end of the run.

5. Carbon - used in F1 and other high end road racing where brake see extreme service and temperature. At low temps, carbon-carbon brakes have an extremely low coefficient of friction and very poor stopping power.

6. Titanium - used in Sprint cars to minimize weight. These rotors typically have a thin insulating ceramic coating to slow heat transfer into the rotor, keeping pad temps up in their desired operating range.

6. Ceramic composites - used on the Porsche GT2 to minimize rotating and unsprung weight.

In all cases, when new pads are bedded to rotors, a small amount of pad material transfers to, and forms a thin layer on, the rotor. As a result, braking forces are developed from friction between pad material and pad material. Page down to "The Nature of Braking Friction" in this <A HREF="http://www.stoptech.com/whitepapers/warped_rotors_myth.htm">Stoptech Article</a> for additional details.

GusTA

As usual, your answer was very informative.

The alloy composition of the 2004 A8L was obtained by my very helful Audi Brand Specialist from AoA Technical. I had asked him a direct technical question.

He also added that the ventilated brakes are unequal - 14.2" in front and 12.2" in the rear.

With this kind of innovation, you have to wonder why Audi does not tout its horn more often!