Survey about purchasing a Carbon Fiber Discs

[Chang]

Hi guys..

I contacted carbon composite company (http://www.dacc21.com)
They can make at least 30 discs. So I hope how many teams want to buy carbon discs and pads.
Our team used it front 2 discs and rear 1 disc.
It was useful for us in terms of braking performance.

I wait your reply.

Ikwhang chang
2006 Yeungnam University

[raska]

At what cost?

[Jersey Tom]

My philosophy in engineering is to use the cheapest, most available, easiest design alternative that meets all functional requirements. In this case for us, its machined or water jetted HRS rotors and regular pads. So I can't imagine we'd be usin any.

Curious to see how much they cost out to..

[Ehsan]

I might be wrong, but don't carbon fiber discs need to be raised to a high temperature before they are effective? Would like to see how anyone would be able to do that on a cold rainy day while doing the brake test.

And I agree with Jersey Tom. Cheap, correctly sized discs are the way to go. I designed some discs we are making ourselves this year and it saved us 10 times compared to the price of the ones we purchased last year. plus, if you are aggressive with the sizing, they will be lightweight even when made of steel.

[Chang]

Hi

Thank you for your reply.
I know your anxiety.
First of all, the price of disc will be $200~300. and then the weight of disc is just under 300 gram.

About peroformance of disc, when we designed carbon disc, we changed its density. It means that temperature of performance is lower than F1 and LeMans. The temperature is just 300 degree celcius.

2006 YU FSAE

[Jersey Tom]

300C.. 570F..

Judging from the surface oxidation on our steel discs I'd say we get up there, barely (some purples and a little blue throughout the disc). But it would take a while to get up there.

Whats the difference in grip, hot and cold? Brake test, is gonna be on mighty cold brakes.

Only time you'd get your brakes good and hot I'd think would be the endurance race.

[Homemade WRX]

looking at the use of the car and the temps the rotors get to, I don't see us having much use for carbon...but less inertia is nicer http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Matt Gignac]

I seem to remember from chemistry class that the heat capacity for carbon varies very much on temperature, it is ridiculously low at room temp (like .17) and at around 600deg C it's something like .5.

Steel varies too, but less so, it's something like .5 at room temp, and maybe .6 at operating temps.

So let's assume that you can make a carbon disc that weighs half as much as a steel one, and you have the heat capacities from above. Let's say that all the kinetic energy of the car (1/2*mv^2) goes into heat in the brakes (m*c*delta(T)).

For cold carbon brakes, m is about half that of steel and c is about a third. So we'd expect six times the temperature difference initially for carbon discs.

Now I don't know the kind of temperatures that carbon discs like, and most probably friction limitations with your discs will not allow you to brake as hard initially, but maybe the fact that it will increase in temp faster will kind of cancel out the higher operating temp.

And I'm pretty sure carbon also has a lower thermal conductivity, so you'd keep more heat in the discs longer.

Matt Gignac
McGill Racing Team

[Wright D]

The disks seem to be only half of the story in braking.

I have several additional questions to think about when switching to a Carbon Carbon braking system.

Where would you find calipers that can handle the elivated temperatures, or would you have to use insulators?

What fluid would you use in the calipers?

What temperature would the calipers see?

Where can you find carbon carbon pads?

How much do these pads cost, and how fast would a team use them up?

How sensitive are the disks to chemicals, and oils? If they get contaminated can they simply be cleaned like steel rotors?

And; aren't there already small clutch disks, the carbon carbon type, being used by some FSAE teams for brake rotors?

[JPS]

You say they were useful to your team, do you have figures as to how much? Was it more in endurance or in stopping distance?

It seems like the company could provide pads, they are picutred on the site. Life doesnt seem to be a problem, discs are rated for 300000km and pads for 60000km

Can someone elaborate on the use of carbon clutch plates? is this successful?

[Jersey Tom]

With regard to specific heat capacities..

Carbon, and Silicon Carbide, have a specific heat capacity of yea .17 at room temperature..in English units. Steel's heat capacity is also about .5 at room temperature.. in METRIC units.

As it turns out.. heat capacity of steel is ~.48 J/gC, while SiC and C are ~.7 J/gC.

A total BS approximation, which I'm going to use anyway, is that for the most part these specific heats are constant since we wont see ridiculous temperatures. As it is, I eyeball a deltaT of about 450F / 250C for our front, steel rotors.

So, goin with half the weight, and 1.5 times thes heat capacity, you'll see what, deltaT of 600F / 500C.

Puts your discs/pads at 675F instead of 525F, peak temperature.

[Matt Gignac]

Oops, stupid units.

Guess it makes sense though, thinking about it a bit more now.

Matt Gignac
McGill Racing Team

[Jersey Tom]

Knew I should have checked this as well before I went to bed..

Also worthy of note is that steel is closer to 2.6 times heavier than SiC.

deltaT then becomes 775F / 430C, putting rotors at 850F. That's considerable. Hell you've still got the same amount of energy to dissipate either way, but getting that damn hot you'd think there would be much more heat transfer through the pads to the caliper and the rest of your wheel assembly.

[RacingManiac]

I remember reading an article on upright design in Racercar Engineering that talked about how running carbon disc will start getting into thermal material limit for Aluminum and Magnesium based design and thus those material can't be use in such a car....

[KU_Racing]

my stance on carbon brakes:
-cool, but unecessary for formula SAE
-expensive (our stainless rotors are $65 + 2hrs machine time)
-hard to find pads
-dont work well in the wet
-weight savings is negated by the use of larger rotors

On one more note, last year we had a bolt come loose on day 1, which through a long chain of events caused us to shatter a brake rotor. We drove to our shop and cut another one, got the car back together in less than 6 hours- enough time to go to sleep and still compete day 2. If our shop had not been so close, it is very likely that we would have been able to purchase a spare from another team, since many teams use the exact same rotor size and hat design. ON the other hand, if we had used carbon rotors, we basically would have been screwed. Granted, you can purchase spares, but they are fragile (cant break a stainless rotor by dropping it on the floor!!!) and the price doesnt fit into our budget.

[Chris Davin]

Originally posted by Jersey Tom:
With regard to specific heat capacities..

Carbon, and Silicon Carbide, have a specific heat capacity of yea .17 at room temperature..in English units. Steel's heat capacity is also about .5 at room temperature.. in METRIC units.

As it turns out.. heat capacity of steel is ~.48 J/gC, while SiC and C are ~.7 J/gC.

A total BS approximation, which I'm going to use anyway, is that for the most part these specific heats are constant since we wont see ridiculous temperatures. As it is, I eyeball a deltaT of about 450F / 250C for our front, steel rotors.

So, goin with half the weight, and 1.5 times thes heat capacity, you'll see what, deltaT of 600F / 500C.

Puts your discs/pads at 675F instead of 525F, peak temperature.

For a given average rate of heat input into the rotors (can be approximated to be independent of the brakes system), the equilibrium rotor temperature will be a function of your rotor surface area and your convection coefficient alone, not your heat capacity. Under transient conditions (ie. in the middle of a braking zone), heat capacity has an effect, but on a typical FSAE track I think you'll find the temperature rise in any particular braking zone is small.