Why is it that most of the brake rotors I see that advertise the type of steel they use are stainless steel?
Stainless steel has the worst thermal conductivity of almost any metal except titanium alloy.
Aluminum is very good for thermal conductivity, but too soft to last, which is why shimano's sandwiched design makes sense.
But, why not just use regular old carbon steel, which has 3 times the thermal conductivity of SS at a lower cost?
Is it because most bikes sit around in bad weather, and would rust too much between uses?
Since I keep my bike inside, and use it daily, are there some decent light weight carbon steel rotors available? It seems the higher thermal conductivity would allow using less material(more holes, or smaller diameter) without worrying about overheating.
The brake track itself is kept clean & rust free by the pads, the rest of the rotor, well, that's a bit of a problem. In a car or other motor vehicle it's not a big deal since the disc itself is over an inch thick, it's going to take quite a few years for the rotor to rust to the point where its strength is compromised. On a bike where the rotor is only 2mm thick at most, it doesn't take much rust before the rotor gets weakened to the point where it's unsafe.
Manufacturers know that many owners are going to leave wet mud splattered on the rotors, why build from a material that's guaranteed to result in lots of consumer complaints and returns?
How many people dry their bikes off with a cloth or hair drier? Water left on a surface for even half a day as it dries off can result in rust (check out what happens to a chain that is stored indoors but isn't lubed well).
If you really want carbon steel rotors, take a set of generic rotors to a good metal store and have them cut something out of a sheet.
Rust can really destroy it but if you take good care of your bike, you don't really need to use stainless steel or worry about rust.
I'm a mountain biker, not an OCD mechanic. Unlike frames, gears, chains, and various nuts & bolts, I can't oil or rust proof a brake rotor in any way. Unless I dry the rotor immediately after every ride and store the bike in a nice dry place it's going to rust, and I don't have that kind of time & energy. Well, I do, but it would piss me off to the point where I give up mountain biking or go back to V-brakes.
Last edited by aerius; 02-24-2013 at 07:25 AM.
Reason: typos
I'll put other peoples words into different words, and hope it also makes sense to the OP.
Because the material choice was made based on a marketing decision, and not an engineering decision.
Because stainless works just fine, and doesnt rust. The decision was definitely made from an engineering perspective, carbon steel would be a terrible choice.
A carbon steel rotor would rust. I dont know about you, but I dont want to be wire brushing my rotors after every ride to keep rust off. Regular carbon steel rotors will rust from moisture in the air. The rusting is removing material from an already very thin rotor. You'd be replacing rotors often, and we'd see people breaking rotors off the hub frequently.
Because stainless works just fine, and doesnt rust. The decision was definitely made from an engineering perspective, carbon steel would be a terrible choice.
A carbon steel rotor would rust. I dont know about you, but I dont want to be wire brushing my rotors after every ride to keep rust off. Regular carbon steel rotors will rust from moisture in the air. The rusting is removing material from an already very thin rotor. You'd be replacing rotors often, and we'd see people breaking rotors off the hub frequently.
While it is true that stainless works just fine ... Without modeling one up and performing thermal emmissivity vs thickness , the choice to go with stainless is a factor of marketability (keep it pretty), and not one of performance.
Gray cast iron that is precision flat ground has dominated the disc brake industry for years, on every vehicle except a bicycle.
Second place goes to ductile Iron ... Blame the cosmetic conscious, and weight conscious for the decision ... Also those who don't like shattered rotors (gray cast), after a crash
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.
Corrosion is a non issue, it's all marketing... you don't need an engineer to figure out that iron can be galvanized or coated with other stuff such as Titanium Nitride.
Safety takes strong priority over pure performance. Rust is real. Its also quite easy to break a 2mm thick piece of ductile iron.
Cars and motorcycles dont use 2mm thick rotors, its not the same conversation with them.
We have lots of super high performance near-disposable bike parts, but brake components shouldnt be one of them.
Safety to a consumer acceptable degree, but more so to a manufacturer lawyer acceptable degree ... If the average consumer didn't have a seemingly unlimited supply of disposable income, the near-disposable bike parts would not exist.
Corrosion is a non issue, it's all marketing... you don't need an engineer to figure out that iron can be galvanized or coated with other stuff such as Titanium Nitride.
LOL... How far into the metal do you think a coating will go? Nevermind that some of these coatings will act as lubricants in certain instances.
I'm pretty surprised that nobody has flamed the OP or any of their others here, Stainless Steel serves the purpose very well. Its low maintenance, it looks nice and its relatively cheap, to mfg.
Whomever is heating their brakes beyond the limits of the rotor material, the fluid or the friction material is breaking new ground in cycling...
any number of coatings would prevent rust in all areas except the frictionsurface, but would wear away pretty quickly there, unless fairly thick and hard. I aagree that stainless seems to work ok for most uses, but I would think downhill racing would have fading performance with stainless steel, and rapid pad wear, since the pads are mostly cooled by the rotor. I am sure that normal wear from use will affect the structural integrity of the rotor long before rust can if used even once in a while. I think that as thin as bicycle rotors are, ductile iron would work, but still have some issues of breaking from impacts. I was suggesting regular high carbon cold rolled steel, which doesn't have as good of thermal properties as grey, or ductile Iron, but much better than stainless, while being stronger than the Iron rotors. I do see how it would be unappealing to see an orangish patina on the friction surface when you pick up your bike that's been sitting out in the rain, but I don't see any practical reason to use stainless. In fact a light scale of rust between uses would probably help keep things roughed up a little, preventing that aweful howling noise when it's humid.
You're trying to solve a problem which doesn't exist for the vast majority of mountain bikers. The percentage of riders who have issues with brake fade is somewhere in the low single digits, and for those folks we have vented rotors from Hope and Ice Tech rotors from Shimano. Everyone else gets along just fine with stainless steel rotors (aside from the Avid turkey gobble) which are low to zero maintenance and provide all the performance they need.
You're trying to solve a problem which doesn't exist for the vast majority of mountain bikers. The percentage of riders who have issues with brake fade is somewhere in the low single digits, and for those folks we have vented rotors from Hope and Ice Tech rotors from Shimano. Everyone else gets along just fine with stainless steel rotors (aside from the Avid turkey gobble) which are low to zero maintenance and provide all the performance they need.
Naa,
I got the question, and it seems valid.
Originally Posted by autodoctor911
But, why not just use regular old carbon steel, which has 3 times the thermal conductivity of SS at a lower cost?
Is it because most bikes sit around in bad weather, and would rust too much between uses?
Why Stainless when a non-stainless would work just as well, or better ... And the answer really is,
Cosmetic appeal ... Most people like shiny, and hate rust.
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Hey !
Cool $10,000 bike, but the brakes are all rusty ... You should upgrade
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Why Stainless when a non-stainless would work just as well, or better ... And the answer really is,
Cosmetic appeal ... Most people like shiny, and hate rust.
As a former bike mechanic, I know how well the the average person takes care of his/her bike. Give'em a non-stainless rotor and I guarantee that rust-through failures will become a common occurrence.
As a former bike mechanic, I know how well the the average person takes care of his/her bike. Give'em a non-stainless rotor and I guarantee that rust-through failures will become a common occurrence.
I'm not prejudiced against stupid people, but take away the labels and let the solution take care of the problem
Corrosion is a non issue, it's all marketing... you don't need an engineer to figure out that iron can be galvanized or coated with other stuff such as Titanium Nitride.
You don't have to be an engineer .... Too figure out that this is a pretty stupid* comment. Coating? A brake rotor disc? C'mon... Think about that for a moment. Its difficult to hold close tolerances when you hot dip galvanize steel or iron, and electroplated galvanize is expensive and leaves a very thin coat. Both methods are a few mils thick and would wear off rapidly.
* Note- Stupid, but only in my opinion, but l do have a degree in metallurgy and materials engineering. CRSS is a fantastic material for a mountain bike brake disc rotor. It's the material I would select if I was spec'ing a rotor. (Titanium Nitride? Hmmmmmm wonder what that would cost?). Stop reading this thread and go do something productive. Like riding. Just saying.
Last edited by Scott In MD; 02-24-2013 at 04:55 PM.
I know about the kettle sicc rotors, and I will be getting some of those, which are far superior to any metal rotor. I was just posing the question because I noticed during my research of the thermal properties of various materials that stainless steel seemed to be a poor choice. Iron or high carbon steel would give better friction and thermal properties than stainless steel, and reduce the weight by about 10%, but the SICC rotors will give even better friction and reduce weight by 50%. Of course, they will cost twice as much, but the reduced wear will make up for that somewhat.
corrosion, in the normal sense will also not be an issue with the SICC rotors, although they will degrade with time, and sunlight.
I also learned recently that most stainless steel brake rotors are made of high carbon stainless of the 420-430 grade, which is much better at thermal conductivity and friction than the shinier 3xx series stainless steels. Thus the desparity in performance compared to iron is not as much as I had thought, and may be comparable to regular cold rolled high carbon steel.
Last edited by autodoctor911; 02-25-2013 at 04:08 AM.
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Originally Posted by Max24
