Stainless rotors vs. ductile iron?

I see several manufacturers advertising aftermarket rotors. What's the difference between ductile iron vs stainless? Pros / cons?

Iron rusts like a mother#*cker. They work well for the track, but for the street use stainless.

ALANRIDER7 said:

Iron rusts like a mother#*cker. They work well for the track, but for the street use stainless.

Click to expand...

So you like stainless for the street? I'm curious about the friction co-effecient comparison. Without any data I would think that the stainless would have hardly any friction compared to the iron. I know the iron ones are heavier though. So you've got that going against you, suspension-wise. What is the composition of the stockers?

iron is for the track! they'll rust in an afternoon they are indeed heavier, but with so many racers running lighter aftermarket wheels the weight isn't a huge deal, plus the carriers are way light. some of the wheels on the honda FX bikes weigh less than the tire thats mounted on them

I know nothing about thew rotor question, BUT find out what type of stainless they use. I woeld imagine 304. See if they make them in 321 ss or 400 series it will hold up to the heat more often as plain 304 ss is comon and the other ss have a more compound of nickel/cromium/manganeese which brings the elevated temperature for where to a higher standard. THEY will not work as fast. Find some data on this subjest. Ask the manifactures about what test they have preformed. Or is it just some ones nice idea to make you pay more for the roters. Stainless is easyer to cut with a plasma or lased but takes more effort to machine.

ROBERT

Tech Talk

Sintered Pads on Iron Rotors

I'm glad to hear that the rotors will be OK, but why does the Ferodo web site recommend the Sintergrip pads to be only used with stainless steel rotors and not with iron? Just wondering, Jeff.
Joseph

It’s fairly simple really, we have to make recommendations that end up dealing with broad generalities, particularly when it is a safety issue. Historically, the majority of iron rotors manufactured over the past 20+ have been made from Gray iron, worse yet, individually molded Gray iron. This material has a relatively low threshold for thermal stress induced fracturing. Two main reasons, its molecular structure is of a coarse flake structure that is random in nature and as so often happens in the molding process, porosity is introduced during the cooling period creating even more random weak areas. The result is so often spider-web like cracks that can grow into catastrophic failure. As detailed below, ductile iron, particularly the high tensile strength variety from continuous cast bar (billet), exhibits neither of these failings.

DUCTILE VS. GRAY IRON SPECS:
The following specifications show our high grade Ductile iron from computer controlled continuous cast bar versus the best of Gray iron (also from bar). Be advised however, the actual strength values of individual mold castings are substantially lower by as much as 50%:

Tensile strength:
• Ductile Iron = 80,000 psi vs Gray iron at 40,000 psi
Fatigue strength:
• Ductile Iron = 40,000 psi vs Gray iron at 20,000 psi

Fatigue strength is primarily influenced by the graphite size and shape and will also be affected by the matrix structure. The continuous casting process and strict metallurgical controls of our Ductile iron results in a uniformly dense, fine-grained microstructure essentially free of porosity, sand and other inclusions, that can affect the endurance ratio severely lowering fatigue life. The BrakeTech Ductile iron specification from billet has optimal strength in tension, compression and fatigue versus traditional castings (including individual mold Ductile iron castings). Ductile irons having nodular graphite will have the highest endurance ratio, and gray irons with coarse flake graphite will have the lowest.

Sintered metal friction material by design has an extremely high metal content, and at the friction couple, operate at a thermal interface level as much as 150 degrees (F) higher than most organic pads. This further stresses the already relatively weak Gray iron rotor. As a result, and anticipating the litigious climate with which we live, must make pad recommendations that fall squarely on the side of prudence. As an example; a person buys a used bike with iron rotors of unknown origin (high degree of probability they're Gray iron as it is more common being much cheaper to produce), jurisprudence dictates we must strongly recommend a friction material least likely to cause problems. Hence the blanket recommendation against using sintered pads generically on iron rotors.

Hope this sheds some light.

Best regards,

--
Jeff Gehrs
BrakeTech USA
909-471-3476

More Info...

MY FRIEND TELLS ME CAST IRON BRAKE ROTORS WILL CRACK AND COME APART, IS THIS TRUE?
Traditional materials and manufacturing methods have kept the door open to problems associated with thermal stress fracturing or simply put, cracking. This phenomenon is primarily focused on the use of gray irons and particularly individual mold casting. That process creates difficult to control porosity problems during the casting procedure. Porosity weakens the structure and reliability suffers. We address this core issue in several areas by spec’ing high grade Ductile Iron, not Gray iron. Then using a computer controlled continuous cast process to produce high quality heat treated iron ingots (giant bars, often referred to as billet). These are then cut into plates with highly specialized saws and CNC machined. The rotors are double disk ground to a tight tolerance and finish to insure flatness and parallelism. This material and method eliminates porosity, creates a homogenous and consistent grain matrix structure that is both very stable and very strong.
See Tech Talk: Ductile vs. Gray Iron

WHY SHOULD I CONSIDER CAST IRON BRAKE ROTORS OVER STAINLESS STEEL?
Over the years, cast iron has proven itself to be an excellent rotor material providing superior "feel" and producing a higher coefficient of friction at the friction couple. If you're looking for genuinely improved performance in terms of both braking power and modulation, then this is for you. If rain and or high humidity is the rule and rust potential a concern, you may wish to consider the alternative in our new AXIS range which is now offered in premium 400 series Stainless Steel as well.

CAN I USE ANY BRAKE PAD MATERIAL ON CAST IRON?
Generally speaking, most organic (semi-metallic) brake pads are good for use on iron rotors. If in doubt, contact the manufacturer for details regarding specific use for this application. Ferodo Platinum, CP911, CP901, 4004F, 3485F, 3410F and I/D450 are all good for this. Although the various sintered metal pads will perform quite well, they generally operate at substantially higher interface temperatures (by as much as 150 degrees [F] at peak). The result is usually shortened life span of the rotor and as such, sintered metal pads are universally not recommended unless specifically denoted otherwise.

* Gray Iron rotors: Sintered metal pads ARE NOT RECOMMENDED for use under any conditions.
* Based on extensive dyno testing, results show it is acceptable to run Ferodo SinterGrip ST pads on BrakeTech Ductile Iron rotors (dual disc application only; Buell riders should use either Platinum or CP911 organic pads).

For interested in a more detailed explanation on this issue, click here for Tech Talk: Sintered vs. Organic

I'VE READ IN ADS THAT"LASER CUTTING" ROTORS IS BEST, IS THIS TRUE?
Laser cutting is simply a cheaper (read: less expensive) method of manufacturing in smaller volume. At the Ferodo R&D Facility, the same method is employed when doing prototype development of backplate shapes for testing. It's less costly to laser cut than to produce stamping dies for low volume production or testing.