Grey cast iron has better thermal properties -- conductivity and specific heat -- lower density, better castability, and better friction than martensitic stainless steel. Why are OEM parts made of martensitic stainless steel (AISI 4XXX series)? Are there any special required properties for a disc rotor that stainless steel has? Are drilled rotors feasible in stainless steel alone? I'm looking for ATV parts, so quick rusting is not a problem for me.

  • I"d like to assist but your use of acronyms is making it difficult to understand your question. Can you clarify please? What is SS? What is 4xxx? – DucatiKiller Jun 11 '15 at 17:27
  • @DucatiKiller : SS = Stainless Steel, The 4xxx is a reference to the grade of steel (e.g. AISI 4024, AISI 4016, etc) – Zaid Jun 11 '15 at 17:39
  • @Zaid I was actually trying to get the OP to make the question clean so it was universally understandable rather than just making corrections. Teach a man to fish and all. – DucatiKiller Jun 12 '15 at 4:12

I was looking at this from an automobile standpoint wondering What the heck is he talking about?? The vast majority of vehicle rotors and drums are made out of grey cast iron!, then went back and reaslized you are asking about motorcycles and ATVs. To that end:

Why are OEM parts made of martensitic stainless steel (AISI 4XXX series)?

The main reason for the use of SS over cast iron is that for the job, the SS can be made lighter, stronger, and thinner than cast iron can be. For a vehicle which weighs relatively little when compared to a car, the SS holds up great to the frictional/torsional forces created during the braking process. If you cast the grey cast iron in the same shape and size, it would not stand up to it. The main reason is, at that thickness, cast iron's brittleness would not stand up. The SS is a much better choice overall to do the job and stand up to the rigors.

Are there any special required properties for a disc rotor that stainless steel has?

If we were talking about cars instead of bikes, the cast iron wins, hands down. The main thing here would be the ability to absorb the heat from the friction. Since we are talking about bikes, the SS can be made as I described above, which you could not do in a comparable size with cast iron. Since overall vehicle weight is not as large of a factor, frictional heat is not as big of a problem for SS. It will maintain its true-ness without issue, whereas you'd see warping if you had a cast iron rotor of the same size in its place. You'd have to have a much larger cast iron rotor to compete in this arena.

Are drilled rotors feasible in stainless steel alone?

No, in fact you can find drilled rotors in just about any brake rotor material.

Carbon Fiber brake rotor on the new Chevrolet Camaro Z28:

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Cast iron brake rotor with drill holes:

enter image description here

NOTE: Obviously these are rotors for autos, but I am showing you that SS brakes are not the only ones which can be drilled.

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Grey cast iron possesses some traits which makes it less desirable than martensitic steel for brake disc applications:

  • it is more brittle, which means it is easier to crack
  • it has very low impact resistance, making it less durable under heavy braking
  • it has less hardness (400 Brinell vs 700 Brinell), so it wears more quickly
  • if left uncoated, it is more susceptible to oxidation, which means it is more likely to rust
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