Disc brake rotor materials and shape?

ejls2

Hi all,

If anyone has any info or experience of this I'd be most grateful.

The current rotors on my disc brakes are steel. They're absolutely fine for what I do but they are quite poorly machined and rust tends to build up on the edges where the machining marks are unless I clean them immediately after every ride.

I've seen some people using alu rotors for weight saving and I've seen a couple of titanium rotors for sale. I've also heard bad things about alu rotors warping under heavy braking etc. Does anyone know if this problem affects titanium rotors too? I have a 180/160 Hope mini/mono mini setup and ride xc/marathon. My race weight is under 75kg.

My other query is about the shape of the edge of the rotors. I've seen a couple of nasty accidents: spinning rotor + body parts = deep gashes. I thought the chances of this would be partially reduced if rotors were rounded at the edges.

I can see two problems with this: difficulty with machining and minor decrease in surface area. Has anyone got any thoughts on this? I might try it with one of my rotors and see how it works.

Cheers,

Ed

nicklouse

Most rotors are actually stainless steel. I can only think of a few really cheap discs that are not stainless.

Alloy is poor to use.
Ti needs special pads as does carbon rotors.

Are you sure it is rust and not just drying mud?

re the shape i would be more concerned with your chain ring.

I have not found much difference in performance between round and wavy other than the wavy may clear mud better of the pads.

ejls2

My rear rotor is stainless but my front rotor definitely isn't. I got the brake 2nd hand and I don't think the rotor can be an original.

Cheers for the info on the other materials - carbon rotors!!! Crikey!

I've never found that chain rings keep spinning after a crash so the damage they do is mainly limited to impaling (still bloody nasty) whereas discs can cut very deeply. I don't exactly think it's a big issue but if there aren't any side effects it might be worth trying!

Thanks,

Ed

Matteeboy

Nick - stainless steel is an alloy!

Car brake disks are made of steel on all but the most racey, then they are carbon ceramic composites.

While I reckon rotors will, start using these more, they have poor impact resistance and are prone to cracking. Only really good for sponsored riders.

Titanium has very similar properties to steel but doesn't rust - however steel is more resistant to wear.

Proper stainless steel should do the job fine - maybe just upgrade to higher quality ones (spheroidal carbon steel would work a treat but is very expensive to use - no one does them yet as far as I know).

Alunium has much less heat resistance (and melts at a much lower temp), and it prone to stress cracking and fatigue - not good for braking surfaces!

Agree that they could do with being less sharp on the edges though.

nicklouse

Matteeboy
Err yes so are most metals. (and yes i know what you are on about).

and what is Alunium ? Aluminum! and the reason for the sharpness? the cutting process and the price you lot are willing to pay. you want a better finnish then expect to pay loads more.

ejls2

Thanks all. I'll replace the front rotor with a genuine hope stainless steel one and I'll have a look at smoothing out the edges. At least that'll be easier with stainless steel than with titanium!

Cheers,

Ed

Matteeboy

nicklouse wrote:

Matteeboy
Err yes so are most metals. (and yes i know what you are on about).

and what is Alunium ? Aluminum! and the reason for the sharpness? the cutting process and the price you lot are willing to pay. you want a better finnish then expect to pay loads more.

Spelling error of course... :oops:

Materials Engineering degree, Swansea 1997, forgotten most of it...

dave_hill

Matteeboy wrote:

Car brake disks are made of steel on all but the most racey, then they are carbon ceramic composites.

Not true - most (note - MOST!) car and motorbike discs are cast iron (apart from the carbon-ceramic ones of course).

ejls2

Really? Why is that?

Matteeboy

dave_hill wrote:

Matteeboy wrote:

Car brake disks are made of steel on all but the most racey, then they are carbon ceramic composites.

Not true - most (note - MOST!) car and motorbike discs are cast iron (apart from the carbon-ceramic ones of course).

You are completely right and I'm kicking myself for being such a muppet!
We did so much stuff on iron, steel and other metals that I sometimes trip myself up!

Oh well - cast iron is very close to steel. Very wear resistant and durable.

Lotus tried MMCs for a little while on the Elise disks but had too many problems.

dave_hill

ejls2 wrote:

Really? Why is that?

- High co-efficient of friction
- High thermal stability (doesn't warp as easily when heated)
- Reasonably durable
- Cheaper to manufacture than steel

Cast iron isn't an alloy BTW - it's iron as nature intended it.

All steels are alloys of iron.

Most aluminium and titanium in common use is an alloy of some sort.

Metals are alloyed with others to change their physical properties - resitance to corrosion, ease of machining, hardness, ductility, malleability, torsional and shear strength, and so on and so forth.

Each alloy generally has positives and negatives - for example if you increase a metal's hardness, it becomes more brittle.

ride_whenever

cast iron is an alloy, because it has shed loads of carbon in it.

omegas

Cast iron contains 2—4% carbon, 1—6% silicon, and small amounts of manganese

Oh what would we do without Wikipedia ………………

Dave Hill is as good as correct though as cast iron is as raw as you get iron as a usable material .

ride_whenever

But you can use the 99.999% iron that you get from sigma, just not for anything practical, only scientific research

Also doesn't steel have fewer impurities?

ejls2

Cheers! I figured that cost would factor into it somewhere!

Matteeboy

Iron ore is chucked in a blast furnace and melted at very high temps and a number of things are extracted including cast iron.

Cast iron is very runny when it is molten and doesn't shrink much when it solidifies. It is therefore ideal for making castings - hence its name. However, it is very impure, containing about 4% of carbon. This carbon makes it very hard, but also very brittle. If you hit it hard, it tends to shatter rather than bend or dent.

Most of the molten iron from a blast furnace is used to make one of a number of types of steel. There isn't just one substance called steel - they are a family of alloys of iron with carbon or various metals.

Mmmkay?

Been to the Port Talbot steelworks before - when they "tip" the 300 tonnes of molten metal out of the "cauldron" it's like an earthquake has hit the place - add that to massive flames and molten "lava" and it feels a bit like a taste of hell!

ride_whenever

but do not all the steel alloys have less carbon in them than cast? Presumably if they had roughly the same they'd be brittle too?

dav1

carbon will strengthen iron up to a point where the amount of carbon disrupts the nicely orderd packing of the iron atoms.

The carbon atoms are small enough to get in the gaps between the atoms to prevent them slipping so easily.

Think of having an orderd layer of large balls then stacking another layer on top. If you then place smaller balls in the gaps between them they will help to lock the layers in place.

If you had too many small balls you would fill up the gaps with some over which have to go somewhere. These will push the layers of iron atoms out of that orderd layer which is a weaker conformation leading to a brittle metal.

when iron is made as much carbon is removed as possible to give a pure metal. This metal is then used or made into an alloy by mixing with exactly the right amount of other substances.

Matteeboy

You need to know your way around a Eutectic diagram to truly understand iron and steel.

Trust me, it's complicated!

ride_whenever

well i do, so could you explain...

Matteeboy

Have you got three years, a large beer budget and a desire to live in a scabby flat?

There are so many variables that affect the properties of iron and steel from numerous heat treatments to almost infinite alloying possibilities.

To try and explain it here would:

a) kill my brain.
b) take me about a month to write.
c) kill my brain.

ride_whenever

Okay, i was really bored last night.

And yes to the first three, i'm hoping to start a D.phil next year!

Matteeboy

Ride - where you planning to study?
I did MatEng at Swansea.

My sister is back at Uni to a BEd (she's 23 now) - quite tough in many ways having had a "proper" job for a while!

ride_whenever

I'm currently finishing my undergrad at oxford and hoping to stay on.

Matteeboy

Brookes or Uni?

We've just done the website content for Brasenose College
Was one HELL of a job! Huge huge website :shock:

ride_whenever

Uni, the college websites are pretty big, the chemistry site had 14million distinct users last year :shock:

dav1

did some physical chemistry in my first year at uni that went into steel a little.

was hoping you would go into it as well.

unfortunatly i dont have the beer gut (rode it off )

Matteeboy

Dav - I honestly think it's too big a subject to go into properly.

So I'm going to cheat and put up a Wikipedia link!
Had a good look at it and it's pretty comprehensive:

http://en.wikipedia.org/wiki/Steel

ride_whenever

Don't remember anything about steel, might hit up the library tonight.

Or i might get drunk and pass out, who knows...

batch78

My vote would be beer and pass out, your all boring me and being geeky nerds now!!

Blundell

Ah cementite, what a fine name for a phase! Back in the geology days we used to joke that one of our lecturers had the world's reserves of cementite down his kecks; as he never changed them and they were an offensive pale grey colour!

Ah the days!