Brake ducts too big??

[1BADBET]

Is there any down side to running "too much" brake duct or is there such thing? I have some ideas for brake ducting but I'm not sure if it's going to be too much air or if there are any possible negative effects.

[Ron Sutton]

Is there any down side to running "too much" brake duct or is there such thing? I have some ideas for brake ducting but I'm not sure if it's going to be too much air or if there are any possible negative effects.


Hi Jordan,

I rec'd your email, but figured I'd just answer the Q on here.

The only draw back, and it can be significant depending on how much airflow we're talking about ... is increased drag and/or lift from putting excessive volumes of air into the rear end area under the car. Otherwise, no.

Frankly, we always duct cool air to the rear brakes, but nothing compared to the volume to the front brakes. How much air the rear brakes need is somewhat dependent on how much the rotors weigh. (Rotor mass is a heat sink) In Road racing we still duct air to the rear brakes, but mainly to keep the pads working in a consistent heat window. The rear brakes can handle much higher temps than we run them at. The front brakes are where we need to focus maximum cooling efforts.

If we run 14# rotors front & rear ... the rear rotors will be about 1/2 the temperature of the front rotors ... if they had even cooling. (Hard to achieve) But IF we had even cooling ... and the front rotors showed to be 700° ... the rear rotors would be "around" 350° +/- depending on the brake bias you arrive at. Said another way ... about 2/3 of the total brake heat is in the front & 1/3 of the total brake heat would be in the rear (67/33) ... plus or minus depending on the brake bias you arrive at ... again, if the front & rear brakes had even cooling.

But it's not typical, nor practical, to have even brake cooling front & rear.

If you ran dual 3" ducts to the front brakes from ducts in the bumper or grill ... in clean force fed air ... and a single 3" duct to the rear brakes pulling attached airflow off the side of the body ... the front brake to rear brake heat difference would shrink. Instead of a 67/33 heat ratio F/R ... it could be 65/35 to 60/40 F/R.

What throws a big change in this line of thinking ... is when we run lighter rotors in the rear. I have a client's car where we run ...
* 13.06" x 1.25" Rotors weighing 12.3# in the front ... with 1 duct of cooling
* 12.19" x .81" Rotors weighing 9.6# in the rear ... (narrow rotor has smaller holes pulling less air through the rotor) ... and no ducted cooling
... and the rotor temps are typically 350° front & 300° rear ... which works out to 54% front & 46% rear ... but all the brakes run cool enough to avoid any fade.

**The reason to run a smaller, lighter, rotor in the rear this is to reduce rotation weight on the rear axle. We always want to have plenty braking & run rotors large enough to manage the heat well. But any more rotor size or weight is a negative, as it adds to the rotating weight. This client didn't care what the brakes looked like behind the wheels. He just wanted optimum performance.

So ... back at the ranch ... I would not go hog wild on ducting a high volume of air to the rear brakes ... because:
A. They don't need as much
B. The additional airflow can create underbody drag & lift

Make sense?

[1BADBET]

My thought was to have ducts in the package tray of the car to feed the rear brakes (maybe the front also) this way I can vent the air from inside the cockpit with out modifying the back window and possibly disrupting air over the rear of the car and hurting down force. The thought was to vent it out the side to cool the brakes and to try not to disrupt air flowing under or over the car. Could this be a viable solution or could I change where the package tray ducts exit, if so where would be best?

[Ron Sutton]

My thought was to have ducts in the package tray of the car to feed the rear brakes (maybe the front also) this way I can vent the air from inside the cockpit with out modifying the back window and possibly disrupting air over the rear of the car and hurting down force. The thought was to vent it out the side to cool the brakes and to try not to disrupt air flowing under or over the car. Could this be a viable solution or could I change where the package tray ducts exit, if so where would be best?

For road courses ... are you going to run the car with or without side windows?

[1BADBET]

I thought about running polycarbonate Windows but I want to keep it more real deal street car so I'll keep all factory glass and probably have the Windows up on a road course. I was thinking of making a polycarbonate rear side window that I could roll the window down set the poly glass in its place and roll the window back up to hold it in. (Hope that makes sense) the poly window would have 2 naca ducts one to feed cool air to my helmet the other would have an extension hose and feed the package tray duct.

[NOT A TA]

Hi Jordan,
So ... back at the ranch ... I would go hog wild on ducting a high volume of air to the rear brakes ... because:
A. They need as much
B. The additional airflow can create underbody drag & lift

Make sense?



Not to me. Shouldn't this read

So ... back at the ranch ... I would NOT go hog wild on ducting a high volume of air to the rear brakes ... because:
A. They DON"T need as much
B. The additional airflow can create underbody drag & lift

[Ron Sutton]

Not to me. Shouldn't this read

So ... back at the ranch ... I would NOT go hog wild on ducting a high volume of air to the rear brakes ... because:
A. They DON"T need as much
B. The additional airflow can create underbody drag & lift

Haha. Yes. I'll correct it.

[Ron Sutton]

I thought about running polycarbonate Windows but I want to keep it more real deal street car so I'll keep all factory glass and probably have the Windows up on a road course. I was thinking of making a polycarbonate rear side window that I could roll the window down set the poly glass in its place and roll the window back up to hold it in. (Hope that makes sense) the poly window would have 2 naca ducts one to feed cool air to my helmet the other would have an extension hose and feed the package tray duct.

I was asking if you're running side windows on road courses, because:
A. If you don't run side windows, there is significant airflow into the cockpit that creates pressure ... and ducting from the panel between the cockpit & trunk is an effective way to duct cool air to brakes & rear end coolers. (Hard to do with a rear seat.)

B. If you run windows ... there is no significant airflow or pressure to do that ... but the car's aerodynamic drag is much less. :)
So then I prefer ducts from the belly pan or side of the body.

C. For autocross, it just doesn't matter, because the brakes don't get hot enough.

[1BADBET]

If windows up is the best way I'll just do that and make the extra rear side glass with vents to cool the driver. It's a factory non ac car but maybe I'll splurge for an under dash unit at some point. So cal really doesn't get that hot lol