Iron Brake Rotors

[Shazaam]

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.

[amullo]

Most intresting.

I was advised by both EBC and ISR that that the EBC-HH pads where probably the best for ISR´s brakerotors.
http://www.isr-brakes.se/

//amullo

[Brakeman]

Great overview Shazaam.

Would like to take this opportunity to clarify a couple of points.

Traditional iron brake rotors such as widely employed in the automotive industry and to a much lesser extent, in the motorcycle aftermarket are Gray iron. That material is features a distinctive coarse flake molecular structure and typically suffers from issues of porosity that can lead to failures associated with thermal stress induced fracturing. To address these inherent material weaknesses, improve their failure resistance and increase thermal capacity, they are typically big and heavy. This whole reliability issue is made worse by the nearly universally employed individual mold casting technique...cheap and efficient to be sure, but also guarantee's even more problems associated with uncontrolled porosity.
The widely reported untimely demise of Jimmie Adamo is much as described, but that too was Gray iron and simply not up to the task.

The Ductile iron family is a different animal, developed in the mid 1940's specifically to deal with the inherent brittleness of Gray iron. Ductile iron features a unique spheroid - nodular molecular structure that introduced "ductility" into the matrix. This brought many benefits to the manufacturing industry with a much more reliable base material including substantially higher tensile strength, inherent resistance to thermal stress induced fracturing, improved thermal conductivity and more. Within that broad family of Nodular (ductile) iron, there are many variations, just like there are in the general steel family. Without getting too boring with unnecessary minutia, suffice to say the specific grade of iron used in the manufacture of BrakeTech AXIS/Iron rotors is an optimal alloy with a closely monitored and proprietary heat-treating process employed for both stress relieving and material hardness. We also utilize a specialized in-house Cryogenic treatment of LN in a computer control vapor system at -300F. Contrary to what is out in the ether sphere of the web, the cryo processing we employ converts austenitic carbon/graphite atoms to martensitic for much improved wear (abrasion) resistance. Our testing shows the cryo processing has little real-world affect on relieving residual material stresses. That aspect of manufacture is addressed in our proprietary and more traditional heat-treating.

Also, unlike the individually molded Gray iron mentioned at the outset, the BrakeTech ductile iron is manufactured from true billet, no individual molding here. We quite literally saw blanks from continuous cast bar containing virtually zero porosity. Every step of manufacturing is closely monitored to a remarkably high level and ISO 9001 standards (each finished iron blade is micro laser etched with all relevant batch and date info, look closely at the patented and trademarked Direct-Link "tooth" and you'll find that).

Quick note on perceived weight of iron versus stainless; contrary to popular belief, the misconception that iron is heavier than stainless steel is simply not true. The density of iron is slightly lower than that of common stainless, so for a given volume of material, iron will be somewhat lighter. Perception always plays a role; kinda like asking an unsuspecting friend, “which is heavier, a pound of feathers or a pound of lead?”

Lastly, the real reason the OE's don't use iron is primarily one of aesthetics; the potential of rusting rotors is very simply not palatable. A secondary reason is the greater expense of manufacturing from a proper grade of higher spec ductile iron, costs are always an issue. And lastly, advances in friction material technology for use on stainless steel have come a long way, their performance as well all know is nothing short of brilliant...it's just a bit better still on iron!

My apologies for the lengthy response...it's a convoluted subject so "sound bite" like replies doesn’t cut it.

Best wishes and safe riding to all.

Jeff Gehrs
BrakeTech USA