That's a very good question. In answering it, many car dealerships make up all sorts of bull about "softer" rotors which are specially designed to wear at the same rate as the brake pads in order to provide better performance. In fact, almost all rotors (excluding exotics such as Porsche ceramic or Ferrari composite) are virtually identical in terms of metallurgy.Northern Rider said:
So why does a Mercedes E320 rotor last almost 50 percent longer than a BMW 530i rotor?
The answer is partly to do with the physical dimensions of the rotors. Mercedes allows 2.4mm of total rotor wear before they are supposed to be discarded. BMW allows 1.6mm of rotor wear. It comes down to design philosophy. BMW makes sportier cars and they want lighter rotors that have a larger air gap in between the friction plates in order to pump more cooling air. So a 30mm thick BMW rotor has a bigger air gap, but thinner friction surfaces. A 30mm thick Mercedes rotor has more "meat" on it, but the air gap is smaller, restricting convective cooling through the narrower passageways. And the Mercedes rotor is heavier, impacting the handling.
Pad selection is also important when determining rotor life. Axxis (REPCO) Metal Master pads are middle of the road performers in most categories, yet they last an incredibly long time. So they might outlast your rotors, depending on the application. Axxis Ultimate pads are more aggressive on the rotors, but they also wear faster. So it's a crap shoot.
To add even more complexity to the mix, the rotor wear is also a function of the factory balance of the brakes and the configuration of the car. A Porsche 993 has much more contribution by the rear brakes than your typical front-engined American muscle car. My Impala SS used to go through front brakes very quickly and the rear brakes would have lasted forever. That was the result of laziness on the part of Chevrolet by using the same proportioning valve on the 4-wheel disc Impala as on the front disc, rear drum Caprice. On a Hunter brake force measuring rig, the Impala had something like a 97:3 front/rear brake force ratio when the brakes were used moderately. Modifying the proportioning valve to remove the drum brake delay (on a car that had no drum brakes!) fixed the problem and gave us something like 75:25.
So the bottom line is that it depends on the physical dimensions of your fleet's rotors, the pads being used, and the type of car. How much allowable rotor wear before they must be discarded? What type of pads are you using? What kind of cars? And are your mechanics bedding in the pads properly before handing the keys over to the motor pool? Bedding brakes has an impact on rotor life.
Bedding technique: http://www.zeckhausen.com/bedding_in_brakes.htm
Bedding theory: http://www.stoptech.com/whitepapers/bedintheory.htm
As for your questions about the 300C and the StopTech rotor longevity - the 300C front rotors are 28mm thick. The minimum thickness stamped on the edge is 26.5mm. So that's even less allowable wear than BMW. Not knowing anything about the pad compound that comes with it, I'm not sure my guess is worth much, but here goes: I'm thinking that one set of pads won't finish off the rotors. But there won't be enough meat left on them to make it worth leaving the rotors in place. So you'll probably end up swapping rotors and pads at the same time. We'll know soon if my prediction is any good!
The StopTech rotors are 32mm thick and the discard thickness is 30mm. In my experience with customers running Axxis Ultimate pads (the standard pad that comes with the StopTech BBKs) they can expect to go through almost three sets of pads for each set of rotors. That's assuming no track events or driving schools. And it also depends on the type of car and the age of the driver. (I know, I know! Too many qualifiers.)
