Brake Disc Materials - EU Sources?

[markocosic]

Hi, another shopping/blagging question for y'all:


Brake discs/rotors - grey iron. Why? (as I understand it) Carbon (as graphite) is good at conducting heat. Pearlite, with big slabs (flakes/layers) of graphite going through it, conducts heat well, but is weakened/made brittle by these slabs. Ferrite (and mild, stainless etc) is strong/ductile but rubbish at conducting heat. Mild steel and stainless also aren't as dimensionally stable at high temperatures as cast iron, and have vastly inferior damping properties.

Make a non-pearlitic brake disc on, and when you hit the brakes the surface temperature of the disc will rise faster because it can't conduct heat internally fast enough. Once get the whole disc hot, and it can't conduct the heat out fast enough and so will stay hot forever. Make it out of mild/stainless and it'll probably go all wibbly on you too, as well as be more prone to squeal.


Now then - where can I source a suitable grey iron? I'm after two 220mm discs and one 205mm disc, all in 5mm thickness. A relatively small centre diameter means machining down existing discs isn't an option. SAE J431 G3000, a standard for automotive discs/drums, says peralitic structure, minimum tensile strength of 30,000 psi (206Mpa) and Brinell hardness 187-241. Something resembling this would be nice.

BS1452 Grade 220/250/260 are grades I've seen advertised for stockholders of 'flake' (pearlitic) cast grey iron bar - the number gives the yield strength. I don't think they'll be too amused if I ask them for a 5mm "slice" of iron though, and it'll involve a lot of surface machining before it's suitable for use as a brake disc. Ideally I'd like a 5mm plate(s) that can be snuk through the waterjet then pinned straight to the car.


Ideas? How have you sourced yours? (if, indeed, you use grey iron)


Cheers,

<edited to confirm details in question at time of writing>

[markocosic]

Hi, another shopping/blagging question for y'all:


Brake discs/rotors - grey iron. Why? (as I understand it) Carbon (as graphite) is good at conducting heat. Pearlite, with big slabs (flakes/layers) of graphite going through it, conducts heat well, but is weakened/made brittle by these slabs. Ferrite (and mild, stainless etc) is strong/ductile but rubbish at conducting heat. Mild steel and stainless also aren't as dimensionally stable at high temperatures as cast iron, and have vastly inferior damping properties.

Make a non-pearlitic brake disc on, and when you hit the brakes the surface temperature of the disc will rise faster because it can't conduct heat internally fast enough. Once get the whole disc hot, and it can't conduct the heat out fast enough and so will stay hot forever. Make it out of mild/stainless and it'll probably go all wibbly on you too, as well as be more prone to squeal.


Now then - where can I source a suitable grey iron? I'm after two 220mm discs and one 205mm disc, all in 5mm thickness. A relatively small centre diameter means machining down existing discs isn't an option. SAE J431 G3000, a standard for automotive discs/drums, says peralitic structure, minimum tensile strength of 30,000 psi (206Mpa) and Brinell hardness 187-241. Something resembling this would be nice.

BS1452 Grade 220/250/260 are grades I've seen advertised for stockholders of 'flake' (pearlitic) cast grey iron bar - the number gives the yield strength. I don't think they'll be too amused if I ask them for a 5mm "slice" of iron though, and it'll involve a lot of surface machining before it's suitable for use as a brake disc. Ideally I'd like a 5mm plate(s) that can be snuk through the waterjet then pinned straight to the car.


Ideas? How have you sourced yours? (if, indeed, you use grey iron)


Cheers,

&lt;edited to confirm details in question at time of writing&gt;

[Mike Cook]

Marko, we purchase pucks and machine them into brake rotors. As far as I know there is no real easy way unless you either a) buy a kart rotor (might work for your rear brakes - there small in diameter but are vented so are probably heavier) or b) find brake rotors off a motorcycle or ATV. If you have any specific questions about machining them, I may be able to help.

Mike

[Wesley]

As I understood it, gray iron was the one with graphite flakes in it - great for dampening vibrations due to internal friction, but lower on strength.

Nodular or ductile cast iron has graphite in little nodules/spheres that improve strength by limiting gap discontinuties in the material. Is this what you are talking about?

I'm trying to remember this from my last materials class, but I drew a brontosaurus (brontosaurite!) for the iron-iron carbide phase diagram...

I won't say what material we ended up using, but we just machined down our iron "pucks" to a close measurement and had them ground locally down to our desired thickness, finally machining our vents.

We really considered EDM machining, but the local guy couldn't do something our size. You might have something better nearby. We could've made all 4 rotors out of one piece doing it that way (our pucks were thick enough) but since we didn't have access, we had to waste 3/4 of our material due to the size at which they were delivered to us.

[markocosic]

Mike -

Kart rotors - tend to be too small in diameter and wide in width, but I'll have another hunt.

Bike rotors - started at Aprilla and spent an afternoon looking at everything sold here in any numbers over the last 25 years. The rears are where you want to look, but even then most discs have too large an internal diameter to machine what I'd like to out of them. (a disc that can be made to float around a hub flange designed for 100mm PCD wheels - without an intermediate carrier)

The one disc that might work is from a Piaggio scooter: either the 00-07 NRG, or the 98-03 Skipper. Listed as having OD 220mm, ID 58mm, with five 6mm mounting holes on an unknown PCD, but estimated as having 6mm between inner diameter and the edge of the mouting holes, so ~76mm PCD and they'd disappear if you machiend the disc to 82mm ID. Don't yet know the new disc thickness as of yet (minimum is listed as 3.5mm), nor the material, nor whether it's a floater or not. Doesn't /look/ promising though:

http://www.scootercrazy.com/images/nrg-power-new-model.jpg


Wesley -

Sounds like you remember the materials class to me! This site has some useful pictures:

http://www.thdick.co.uk/metallurgy.htm

Grey iron is the flake iron is the peralitic iron.

Ductile iron is the spheroidal graphite/spherical globule/sg iron and throws in some magnesium to change things from flake to sphere, so we can't switch between them by heat-treating alone.

Eventually turned up the folks across the road (but only via Google - typical!) and a wealth of info:
http://www.msm.cam.ac.uk/phase-trans/2005/pearlite.html
http://www.msm.cam.ac.uk/phase-trans/2001/adi/cast.iron.html
Check out the 2500MPa yield, 600-670 HV hardness and 30-40MPam^0.5 toughness 'hard bainite' if you get the chance - gorgeous stuff! http://fsae.com/groupee_common/emoticons/icon_redface.gif

Reasonably happy with the properties of the iron now, but it would appear that the only options for sourcing the material are still as disc diameter barstock or existing brake discs - bugger.


Other options - even if one steers clear of aluminium based materials for now, there are a few other iron-based options that are viable given how thin fsae discs are, the way they're usually supported better that automotive discs and with wear rates pretty irrelevant. Still need to some more thinking/reading/research to be sure of suitability though.


Note to self - don't assume that everything arrives off the shelf in the shape that you expected. Definitely don't assume that brake discs are a lump of any old iron... http://fsae.com/groupee_common/emoticons/icon_eek.gif http://fsae.com/groupee_common/emoticons/icon_smile.gif

[markocosic]

Re-visitng a typical formula student brake-system duty cycle again:

Formerly hosted here:
http://www.google.co.uk/url?sa=t&ct=res&cd=1&url=http%3...CchlwMTu6hc5EcfzAw3A (http://www.google.co.uk/url?sa=t&ct=res&cd=1&url=http%3A%2F%2Fwww.imeche.org.uk%2Fformulastuden t%2Fpdf%2FHonda%2520Data%2520For%2520Formula%2520S tudent.pdf&ei=iMwnRq3bMIvi0QSp6ZTHDQ&usg=AFrqEzd9iALWvrhVZWdARUyxn-cn2847_Q&sig2=BjCchlwMTu6hc5EcfzAw3A)

The IMechE site seems to be broken/undergoing upgrade, so I've now hosted it here:
http://www.cosic.org.uk/misc/Honda%20GPS%20Data%20For%20Formula%20Student.pdf

Converted (roughly) to Excel Format:
http://www.cosic.org.uk/misc/Honda%20Data.xls

and with some more info on acceptable temperature limits for modern brake materials (more later) I don't think the application is quite as demanding as initially designed for.


Kart discs are available with 100mm centrebores and in the correct form of grey iron/surface machined as appropriate up to 210mm in diameter. This is enough for the application in my view - though the line pressure do begin to get a little eyebrow-raising iat higher decelerations. (though the GPS data would tend to suggest cars barely break 1.3g) They're ~25 USD, which I'm inclined to call a no-brainer too! :-)


For what it's worth, when dropping by George Lister and Sons I found folks happily turning even very thin grey iron brake discs on a lathe, one side at a time and with no backing plates to prevent flex - the bloke doign it reckoned provided speeds were high and feeds/cut depths very small the deflection was a non-issue.

[markocosic]

Oh,

and if any established teams that have competed in a more recent (or non UK event) have this kind of forward velocity/average wheelspeed versus time data that they are willing to share, it'd be much appreciated.

[sbrenaman]

Where are people sourcing J431 G3000 grey iron?
I've been searching the interwebs and have come up with nothing.

[Adambomb]

I've found some Class 40 gray iron sheet at McMaster (40,000 psi strength), thinnest they have is 1/4 in. sheets, although that wouldn't be terribly much to machine down (seeing as how they should be finished anyway). Not super-cheap, but then again it is a sheet of cast iron that you can buy, and they ship as fast as is physically possible from the midwest.

Another note...looking at Wilwood's offerings, all of their solid discs (motorcycle, midget, or kart scale) all are steel. I did a bit of other research too, and honestly I'm not convinced cast iron is worth the trouble.

[HoggyN]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Another note...looking at Wilwood's offerings, all of their solid discs (motorcycle, midget, or kart scale) all are steel. I did a bit of other research too, and honestly I'm not convinced cast iron is worth the trouble. </div></BLOCKQUOTE>

Neither am I. We have used laser cut steel gauge plate for the last few years with absolutely no problems at all.

[Stocky Fast 1]

A good material, not sure if you can get it in the UK is G2 Durabar(Gray Cast Iron), Google it and you may find a distributer in the UK.

[Richard Pare]

I'd advise that you stick with Ductile iron for your disks. In comparison to Grey CI, they are much less prone to heat-stress cracking because of the spherical graphite. Unfortunately, the heat transfer rate is a bit less, but for the most part, since you are not heat soaking the disks to their max all the time, you won't really notice the difference.

The most common alloy used (in Ductile) is 85-55-06 (also commonly caller Ductile 80).

Steel is used on many karts and motorcycles only because the manufacturer doesn't want the liability of rotors breaking. Stainless gets used on motorcycles only because riders really hate the sight of rusty rotors after a rain. Both are crappy rotor materials from the performance standpoint.

If you can find a source, Compacted Graphite Iron ( a modified form of ductile that has a worm-like graphite structure) is the best CI rotor material available, and seems to be more in use commonly on your side of the pond.

If push comes to shove, I can supply disks of D80 sliced from 10.5" bar stock - I usually have them in stock.

[Adambomb]

Looking at Matweb, here's what I came up with (granted Matweb isn't gospel...but is the best "internet bench racing" reference I know of):

Ductile Iron (80-55-06):
Brinnell hardness: 187-269
Yield strength: &gt;= 55 ksi
Elongation at break: 6%
Specific heat: 0.506 J/g-C
Thermal conductivity: 13-32 W/m-K
Max service temp: 649 C

1018 steel, 16-22 mm round:
Brinnell hardness: 143
Yield strength: 60 ksi
Elongation at break: 18%
Specific heat: 0.486 J/g-C
Thermal conductivity: 51.9 W/m-K

Did a little high-temp strength studying too, looks like up to 500 C there's not much effect on yield strength of low-carbon steel (same for gray iron). Performance-wise, DI looks to have slightly higher specific heat, although much lower thermal conductivity. Strength gives a slight advantage to steel (although if you use Grade 50 steel like we do, in this case iron is slightly better). Also, Ashby's materials charts actually show low-carbon steel to have a slightly higher coefficient of friction (for steel on steel or iron on iron...haven't found anything on steel vs. brake pad or iron vs. brake pad, outside of generic brake pad specs). DI has significantly higher hardness, which I'm assuming would mean longer life.

So basically, the only real differences worth worrying about that I can see (from what I've looked at) is that ductile iron is much harder, and steel has much higher thermal conductivity and ductility. Aside from that, low-carbon steel is available for next to nothing from almost everywhere, and anyone with a laser cutter will gladly carve them into whatever shape you want.

So.....what am I missing? Why should I run ductile iron?

[Richard Pare]

The Cf of steel against pad material is about 1/2 that compared to CI. Since the idea behind brakes is to be able to slow the car as rapidly as possible with the least amount of pedal effort - along with the fact that most brake pads are formulated for use with CI and not steel - what other reason would you have for going to an inferior performing material other than possibly cost?

CI (of various types) is the most common rotor material for street cars, trucks, trains, and race cars, up to and including (but only on rare occasions nowadays) F1.

PS - Standard laser cutting of rotor material is not a good idea unless you plan to cut or grind off the layer of martensite that is left over on the surface. SOME of the latest high-wattage pulsed lasers CAN vaporize the material fast enough to not leave that brittle layer behind, but good luck finding a company that has one. Waterjetting is a better process for this purpose.

[Adambomb]

Good to hear on the Cf, I have heard rumors of CI being higher but haven't found any hard data on it. Haven't had a chance to do testing yet either, what with not having random bits of CI laying around. Also with the laser cutting...feel dumb now for not having thought of that; didn't give it much consideration in the past since low-carbon steel isn't hardenable. Water jetting sounds good. Thanks!

[Vlado]

for my project car, which is a bigger version of a FSAE car, I bought 4 brembo solid discs from alfasud.... as a "blank" to be machined..

http://bremboaftermarket.com/I...?ModelIDMaster=11733 (http://bremboaftermarket.com/It/Car_Disc_Catalogue/Catalogue_Detail.aspx?ModelIDMaster=11733)

I had them CNC machined into floating disc of a slightly smaller diameter.. I'll have some pics of that when I get them back from the machine shop..

project log:
http://www.locostusa.com/forum...=7683&p=85389#p85389 (http://www.locostusa.com/forums/viewtopic.php?f=10&t=7683&p=85389#p85389)

[Chapo]

Here is an idea from Claude...

Go to the wreckers, find some cars that use carbon clutches, but the clutch plates and machine them down to the right size, hey presto carbon discs.

Might not be as simple as that but could be something worth looking at.

We just use a low carbon steel that is given the correct profile through wire cutting and NC milling, and then is ground to the final thickness. Never had a problem with them and we run quite small rotors.