| Iron Brake Rotors
The main reason that iron brake rotors are used instead of the OEM stainless steel is that for a given brake pad material they offer a higher coefficient of friction and consequently more braking power, better heat management, and better brake force modulation (feel). But they also have drawbacks. Iron, as a material, is more brittle (less ductile) than stainless steel, it develops a coat of surface rust quickly, and unless it has a high carbon content will often squeal during a slow stop. High carbon cast iron rotors are used on over a hundred different car and commercial vehicles today.
The intrinsic brittle nature of the material, weight and the product liability issue is the main reason that manufacturers don’t use them as original equipment today. Improvement to brakes have been mainly concentrated in other areas such as pad material, caliper stiffness and multiple pads and pistons.
The shift to stainless has allowed brake performance improvements mainly by allowing the use of sintered pad materials that are more abrasive and produce excessive heating in iron rotors. If the temperature rise is not uniform in different regions of the rotor, the differential thermal expansion can lead to warping and cracks that can eventually lead to disc failure.
In recent years, improved foundry and cryo-treating material processes have resulted in a more ductile formulation of iron giving it a more uniform and finer grain structure that is less prone to rusting, and cracking from rapid thermal cycling and carrier interface loads.
So, don't confuse cast iron rotors (Brembo) with ductile iron (Brake Tech) rotors.
With this in mind, if you have a set of cast iron rotors you should inspect them for cracks regularly, especially after a crash, paying special attention to the cross-drilled cooling holes. If you’'re unsure, have the rotors inspected using a fluorescent liquid penetrant process like Zyglo.
WARNING - Do not use sintered pads with iron brake rotors
There are two classes of brake pad compounds: sintered and organic. Sintered pad compounds were developed specifically for use with stainless steel rotors and should not be used with cast iron brake rotors. They produce excessive temperatures in cast iron rotors that can lead to cracking during severe braking conditions. They also produce premature cast iron rotor wear when used hard.
Some European manufacturers of cast iron rotors, like Brembo, after first trying to warn against the use of sintered pads, have simply stopped offering cast iron rotors for liability reasons. The brake pad manufacturers simply tell you don’t use them with iron rotors, but some people do anyway. If someone tells you that they’ve used them with no problems, tell them about Jimmy Adamo's death at Daytona after his cast iron rotors shattered. This tendency to shatter under severe race conditions is the main reason why a number of race organizations ban the use of non-ductile iron discs.
Using sintered pads on ductile iron disks is less problematic, but still risky unless specific testing has been done by the manufacturers. For example, according to Jeff Gehrs (who runs Brake Tech and designed their rotors,) the Brake Tech Axis rotors have been tested by Ferodo and found to be compatible with their sintered pads. The Axis rotors are cryo-treated during manufacturing to get good fracture-resistant material properties, but other manufacturer’s rotors may not be as suitable.
Some sintered pads (EBH HH, for example) are claimed to be designed for use with both stainless steel or ductile iron rotors. Be skeptical. Compared to organic pads, you will experience accelerated wear and elevated heat levels which means more thermal expansion-induced stresses in the disc. This means that using sintered pads for moderate-braking street conditions could turn problematic at a hard-braking track.
One more point about pad selection. By matching the brake pad to the brake disc, feel can be greatly improved. The pads used by racers can be broken into two catagories; Free and Not-Free.
The Not-Free pads are the ones that win races. Not-Free pads include HRC pads, Brembo sintered pads and Performance Friction Carbon Metallic pads. All of these pads offer improved modulation as well as ultimate stopping power.
[Edited on 5-26-2005 by Shazaam!]  |