Brake Rotors

[Milky]

Has anyone had experience designing or using alternative materials for brake rotors?

I am doing some rotor design for the FSAE car at Auckland Uni and have come up against a distinct lack of local knowledge within race teams/car manufacturers, who generally say put the largest brake pad/rotor combination you can fit in, and do not care about weight. Having said that, the rules for a large proportion of the classes are quite strict in limiting the teams to a certain brake setup - Probably to make the technological gaps between cars smaller, and to get the focus onto the driver and race setup.

I am thinking of surfaced aluminium, Al-MMC (SiC/B4C), possibly fully ceramic - not cast iron or stainless. One of the big drawbacks with ally is the low thermal capacity, but I believe that with the weight of the car this year, and the type of speeds that it is generally doing, we should be able to make it work, given efficient ducting/airflow around the area. This is a second point where I am having some issues finding data - that of thermal capacity/conduction/radiation of heat generated in the system.

Any help that you guys/gals can provide would be helpful.

[erik]

http://www.fsae.co.nz/

You're only second year, right? And you're already designing rotors for a racecar?!

F***'n hell that's impressive.

[Motu]

Unless these cars are limited to 50kph you're going to need a shit load of cooling on ally rotors,it won't overcome it's own drag to run down hill.Think mean effective radius and multi piston multi calipers - like that means very big dia very narrow rotors ah lah Buel and say two small 3 piston calipers per wheel.

[HDTboy]

Won't aly wear a lot faster than steel?
You could use aluminium hats with iron rotors if you need to cut unsprung weight down

[Two Smoker]

If money is no object, go for the Carbon ceramic disks... otherwise it will have to be stainless or steel... Will debate this with you today when i come over with the RG...

[TonyB]

I thought the melting temp of aluminium was only about 800°C.

I looked at getting some laser cut once, the problem I ran into was finding out which grade of stainless to use. Buggered if I can remember which grade it was, but the problem with lil ol' NZ is while there's plenty of businesses that can cut it and grind it, there aren't many (or any) that stock the grade you need. From memory the cutting and grinding process stuffs the hardening, so it then needs to be anneald (sp...right word??) or hardened. Because bike disks are so thin it is difficult to do this without warping them.

[sAsLEX]

If money is no object, go for the Carbon ceramic disks... .

Question is where to obtain these from and at what cost??

I know there are issues with carbon brakes requiring a min temp to work but that shouldn't be too big a deal

[Ixion]

I thought the melting temp of aluminium was only about 800°C.

I looked at getting some laser cut once, the problem I ran into was finding out which grade of stainless to use. Buggered if I can remember which grade it was, but the problem with lil ol' NZ is while there's plenty of businesses that can cut it and grind it, there aren't many (or any) that stock the grade you need. From memory the cutting and grinding process stuffs the hardening, so it then needs to be anneald (sp...right word??) or hardened. Because bike disks are so thin it is difficult to do this without warping them.

660 degree C for the pure element. probably higher for commercial alloys and certainly fior duralium .

But a bigger problem would be that ali is hot short . So the hotter it gets the more "brittle crumbly" it gets. Got to be very careful welding it cos big castings can just collapse and break when they're hot. You'd need to be careful that the disk didn't have any risk of breaking up when it got hot with hard sustained braking.

[Kickaha]

I thought the melting temp of aluminium was only about 800°C.

I looked at getting some laser cut once, the problem I ran into was finding out which grade of stainless to use. Buggered if I can remember which grade it was, but the problem with lil ol' NZ is while there's plenty of businesses that can cut it and grind it, there aren't many (or any) that stock the grade you need. From memory the cutting and grinding process stuffs the hardening, so it then needs to be anneald (sp...right word??) or hardened. Because bike disks are so thin it is difficult to do this without warping them.

Good job no one told me this stuff when were doing ours. laser cut some out of mild steel on the 250 Superkart run a season and toss them out as they dish which was more to do with the caliper mounts not being square than the material,done the same on a mates KDX and they work fine didn't bother getting them surface ground either

Had a few stainless ones laser cut,didn't specify a material but they used some surgical grade,again we didn't bother getting them surface ground and they worked well enough for Diesel Pig to win the Pre82 Championship and the same will be going on my Pre82 RD400

I wouldn't have thought the cutting and grinding would soften stainless as I thought it work hardens? and we had no warping with the discs we did get ground they were between 4-5mm thick depending on what were using them for

[Ixion]

..
I wouldn't have thought the cutting and grinding would soften stainless as I thought it work hardens? and we had no warping with the discs we did get ground they were between 4-5mm thick depending on what were using them for

I think stainless work hardens. That might be the trouble, you don't want the disk to be TOO hard , or the pads will just "skid" over it. You need a bit of softness for the pads to "grip". But not too soft of course, cos then they'll wear out too fast. So work hardened stainless might indeed need to be annealed (softened) . But this is pure guess work I don't actually know.

Still reckon you can't beat cast iron for brakes.

[TwoSeven]

Ally pally is the show you want to go to - its where all the industry experts turn up and joe public can ask them questions.

In my case many years ago it was AP racing and it was pad compounds being the subject I wanted to know about.

Basically, you want to match the composite material in the pad, with the material in the disc. So if you have a steel disc, you need sintered steel pads. However, the basic material governs the friction level (and heat transfer).

Once you start mixing compounds you get two different problems that can either work in your favour or against depending on the set up. The first problem is heat transfer from the pad. You want the heat to travel to the disc, not the other way round, you do need heat in the pad for it to work, but excess heat causes expansion of the caliper and boiling of the liquid (which causes fade).

The next problem you have is hardness of the compound - its kind of like tires - soft heats up quickly and works in a low heat range (good for riding in the wet) and hard heats up slowly and works with high temp ranges (not quite true but close enough). You cant use carbon on the road because it just wont get enough heat to work - so you'll have no brakes. Even on race bikes on cold days they often mix a carbon and iron disc to make a balance.

Next the surface area of the pad is important because thats what gives you the braking ability. Long and thin is the most ideal because it requires a narrow rotor which can then be lighter, however, the trade off is that it might not deal so well with crud and glazing. Wider shorter pads require a wider rotor which means more rotating mass which is not good (one of the main factors where it differs from cars).

Rotors also need to be designed to clean the pads - that is, they have waves or grooves or holes cut in them that are designed to shave the surface of the pad to remove any glaze or built up crud. Hence the hardness of the pad must be less than or equal to the disc or the pad will wear the disc down (very expensive). Its the pad thats designed to wear and thats what pad compound is all about.

Disc expansion is another important criteria - hence mounting the rotor on a floating or semi-floating mount. Last thing you want is the disc expanding with nowhere to go as it will warm the disc which prevents the pad from sitting on the surface properly.

The leading edge of the pad seems to have some importance as well as the shape of the grooves in the pad. I suspect that the leading edge of the pad has to push crud and water out of the way or it wont create an ideal surface. I know in the wet you have to drag the brake a bit (to keep heat) as well as pump it slightly to remove crud and water. I've never investigated exactly how it works.

The last thing you need to look at is airflow - both for cooling and for removal of dust (ask anyone with a cibby 250 about rear brake squeal caused by crudded up rear disc).

[Milky]

You're only second year, right? And you're already designing rotors for a racecar?!

Aye.. second year, but this is a big learning curve for me, and there are a heap of other talented people around to help out here and there.

Question is where to obtain these from and at what cost??

I know there are issues with carbon brakes requiring a min temp to work but that shouldn't be too big a deal

The car is likely to be running around 120-140ks, and braking is not likely to consist of repeated high speed stops - more a quick squeeze here and there.Temperature can be controlled by ducting up to a point, and if we can avoid this, all the better for aero.
The money issue is important, and in keeping with this we are looking at manufacturing our own if possible.

Re the wear rate - MMC rotors wear slightly faster at higher temperatures, but below 200 they are very similar to conventional cast iron.

With the enquiries that I made into MMC, it seems hardly anyone uses it here in NZ... The properties of pure ally aren't suitable for disks for obvious reasons. Inclusion of additional ceramic particles (13% to 30%) in the metal increases the temperature at which the material begins to lose it's steady friction coefficient from roughly 400 to 500C. There are some issues with finding the correct pad compound to match the rotors, but I understand that the MMC rotors were used in racing overseas and on the Elise S1 so surely there will be some available.

Re the work hardening of stainless - yes it does, and yes when you cut it it will lose some of that hardness. If you weld it and get it above the eutectic? eutectoid? temperature - I can't remember which off the top of my head - the base structure changes, and you get the Nickel/Chromium migrating towards grain boundaries in the metal - gives a brittle lattice through a softer iron grain structure and also leads to differences in chemical reactions on the surface of the metal, leading to loss of rust resistance.

And finally:
Yes pad material is just as important as disk material, as is matching the two to get the right transfer layer properties on the disc. we have done some tricky shit in the caliper design, and I dont think fluid boil will be an issue at all.
The grooves or holes in the disc or pad have different properties depending on the orientation - I can't remember exactly what goes with what, but they reduce squeal, clean the other surface, remove gases from the organic material in the pads and so on.
I was thinking of full floating rotors, not sure if they will be run on the rim or the hub yet though
Airflow - see up there somewhere ^_^

[Motu]

Slots and holes in rotors have a similar function to fluffy dice - lots and lots of theories on what they do - but in the real world they have been proved to have no effect at all....but they sure look cool eh? And you can't buy high performance rotors without them so that just goes to prove they work! Do a double blind and come back with facts and figures....

[F5 Dave]

Well one theory with wavy discs is that the leading edge provides most of the cooling of a disc so increasing its area helps it’s cooling. Drill holes & slots may do this a little too.

An extra argument for 6 pot callipers & longer thin pad areas to augment 2-7s points is that the thinner the pad the less the difference in radius from the top to the bottom of the pad swept area, hence speed difference & heat difference (top of the pad has more leverage from bottom so will heat the disc differently, which may encourage ‘coning’ or warping of the disc.

[pete376403]

Kawasaki used plasma sprayed aluminium disks on their KR750 (two stroke triple) racers years ago (ie late 70's -early 80's) when it was felt that iron or stainless disks were too heavy. Not sure about the coating unless it was similar to the electrofusion coating they put on aluminium cylinders.
Or maybe beryllium disks as used on aircraft (eg C5a Galaxy) and the space shuttle.
"Other beryllium alloys are used in the windshield, brake disks and other structural components of the space shuttle."

http://education.jlab.org/itselemental/ele004.html