RX7: RB Rotor Material
This reply was posted to RX7 forum on 12-16-2006 under this thread:
FD New Brake Options
We got into this RX7 brake project through Howard?s inspiration and enthusiasm which means a lot to a relatively unknown brake company like us.
I admire the respect that Howard received from you folks and from my experience in working with him only about two weeks, the diligence and eagerness he demonstrated and the efforts he puts into this project and shared with other RX7 enthusiasts what he found proven that he is well deserved.
I have just noticed user jimlab #32 asked some questions which I like to reply as follows:
BTW, most aftermarket 2-piece rotors use billet aluminum hats so this isn't exactly a new development...
RB rotor hats are made (machined) from "Forged" aluminum blank in stead of cutting from billet (aluminum bar) which is "extruded" and readily available from aluminum mfgrs such as Alcoa or Kaiser. We have to make various "blank" tooling for different sizes of hat. This is not a new development rather it's a process that we all know costs more to make, yet has the benefit of better stiffness and strength than those made from billets that offered by most competitions. RB commits to performance brakes. More and more enthusiasts start to realize that our extra efforts are worthwhile.
Which is? Most rotors are made from cast iron. How are these any different? Cryo treated? High nickel content? What?
RB rotors material are alloyed and heated from the base of SAE G3500 gray iron instead of G3000 which is the standard brake material for motor companies and aftermarket but unfortunately very few aftermarket brake meets this standard based on our past experience. Our rotors have higher carbon than regular gray iron for better heat handling but carbon will also weaken the strength of iron, so the alloys (Cu, Cr, Ni, Mo etc) may be added to make up the loss of strength resulting from the increase of carbon contents and enhance performance under extreme heat. But there are more important aspects than just the chemical compositions (% of elements) that predominately dictate the overall performance of a brake disc such as microstructure which deals with the formation (type and size) of graphite (carbon) and the matrix (distribution of pearlite or ferrite), that is beyond the scope of discussion here. Some reference link (scroll to Rotor Material)
At RacingBrake we believe in the basic material science, therefore we like to heat treat our iron because when you hit the brake, the temperature surged, so we developed a process to heat up the casting higher than the highest brake temperature a rotor can reach, then cool it down. The discs are cut from these ?treated? casting which might have some sort of deformation that is not noticeable with eyes. You may consider this is a process similar to ?pre-warp? the rotors then machine from those ?warped? castings. This explains to our customers why RB rotors will not warp and are more resistant to wear and heat checks. More on heat treatment but again it?s beyond the scope of our discussion.
The above made us to disbelieve on Cryo treatment which some companies are marketing to teat the rotors on opposite way ? cool it down. We don?t have enough experience to comment on Cryo treatment but we know from the experience we had with those companies supplying brakes to professional racing teams (NASCAR) they only use heat-treated rotors, not Cryo.
Link to Cryogenic treatment question asked by an EVO racer
More questions on rotor material
Originally Posted by whitey85mtu in RX7 under the same thread:
Due to the graphite formation in gray iron is in flake shape therefore the gray iron although is excellent in thermal conductivity and ?damping? capacity which is like absorbing the vibration (make vibration less noticeable by the driver) but is ?not? as good in strength and impact resistance comparing to ductile iron which the graphite is in spherical shape. Ductile Iron has better strength due to it?s ductility and is a more suitable material for structural (stationary) members such as calipers and caliper brackets where no heat cycles are encountered like brake discs.
So an ideal cast iron for a brake rotor is to have the good thermal conductivity like gray iron (flake graphite) but with good strength as ductile iron (spherical graphite). It?s called Compacted Graphite Iron (CGI). It?s a relative new casting process and still is not very popular yet due to more difficult to produce and cost more to make. However RB does offer CGI rotors in some selective club racing applications. (We believe we are the first brake company introduces CGI rotors in performance brake industry.)
Racingbrake CGI rotors helped Tom Long won his North American Title on his Pro Spec Miata race.
We prefer the fine pearlite matrix for better strength and hardness which make rotors more resistant to wear and heat handling.
Thank you for your questions.
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