brake heat capacity modeling

[tonto]

is there a method to model the front brakes vs speed differential in braking zones?

my motivation is to figure out if i can get by with ducting my existing miata stock brake hardware as i increase power this winter. i am guessing there are two things i need to watch: fluid temps and rotor temps.

fluid i think i have some margin as i am running < 390* caliper temps w/o ducting.

the rotor temps i'm concerned about. it seems to me that for any given braking zone, if rotor temps go above a certain point (1400f?) you are done, you need to replace with a larger and/or more efficient rotor. is there any way of approximating this or are there too many variables?

the standby method is just to run them and see if i go through a set of rotors per day. if yes then buy bigger.

[Def]

Your rotor temp will always be LOWER than your pad temp. What rotor temps are you seeing now? My caliper temps are about 250-270 deg F up front, ~220-230 deg F rear. Rotors are about 320-330 front and rear. The peak temps are likely quite a bit higher up front , since they really cool down on track, and that's after a cool down lap.

As far as heat modelling, figure 70-75% braking done up front, perfect conversion of car's kinetic energy into heat energy. Take the weight of your rotor ring, or about 70-60% of your 1 pc rotor weight as absorbing the heat. Specific heat of cast iron vs. heat input gives you temperature rise. As far as end of session temperatures - no way you're going to model that, it's way too complex as a lot of it depends on flow through the wheels and rotors(excessively complex).

Best bet is just to give it a go and see what happens. That said, Miatas are all super slow, so you'll probably be ok. :D

[Matt93SE]

FWIW, Miatas run in GT Lite, GT3, and EP with 200+hp and stock calipers & rotors.

[Def]

Stock stuff works because they're all still slow. :D

[tonto]

u guys are mean. and correct lol i cant think of any cars on track with less hp.

ok lets me sees. figure 120 to 50 braking zone....



givens
120mph = 53.6448 m/s
50mph = 22.352 m/s
car weight 2286lb+190lb = 1123.0947 kg
1 rotor = 4650 g
c of cast iron = .46 (http://www.engineeringtoolbox.com/specific-heat-metals-d_152.html)

Ej = 1/2 m v^2

(1123*53.6^2)/2
1613167.04000000000000000000

(1123*22.3^2)/2
279228.33500000000000000000

delta E = 1613167-279228 = 1333939

figure delta degree K

(1333939/1000)/(.46*(4650/1000))
623.62739597942964001870

623 *K = 661* F

take 75% front bias, divide by 2 because 2 rotors
661*.75*.5 = 247.875 * delta F

250 degree rise? sounds fine. ill get some temp paint tho :D

(did i screw up any #'s?)

[Equinox]

Kind off off-topic but has anyone used this stuff?

http://tinyurl.com/3a8bh7y

Seems like a good price compared to the three seperate ones floating around.

[tonto]

^^ not offtopic at all. thats exactly the stuff i was thinking about getting (for rotors and hubs). was trying to talk some other local folks into chipping in cause thats like a lifetime supply right there...

[Def]

u guys are mean. and correct lol i cant think of any cars on track with less hp.

ok lets me sees. figure 120 to 50 braking zone....



givens
120mph = 53.6448 m/s
50mph = 22.352 m/s
car weight 2286lb+190lb = 1123.0947 kg
1 rotor = 4650 g
c of cast iron = .46 (http://www.engineeringtoolbox.com/specific-heat-metals-d_152.html)

Ej = 1/2 m v^2

(1123*53.6^2)/2
1613167.04000000000000000000

(1123*22.3^2)/2
279228.33500000000000000000

delta E = 1613167-279228 = 1333939

figure delta degree K

(1333939/1000)/(.46*(4650/1000))
623.62739597942964001870

623 *K = 661* F

take 75% front bias, divide by 2 because 2 rotors
661*.75*.5 = 247.875 * delta F

250 degree rise? sounds fine. ill get some temp paint tho :D

(did i screw up any #'s?)

No ideas on the numbers, my brain is fried from work. But keep in mind that temp increase is on top of your existing temperature, so it's constantly a game from shedding the heat before the next corner before spiking up again.

[tonto]

But keep in mind that temp increase is on top of your existing temperature, so it's constantly a game from shedding the heat before the next corner before spiking up again.

ya agreed. but to me, this is the "steady state" case. if i can get the cooling setup to reach this steady state, then i no longer have to worry about brakes of a certain "thermal capacity" which i see advertised.