brake ducting

Jason Mauldin · **Posted:** Sun Aug 31, 2003 10:28 am

On a FWD car with the calipers in front of the axles, where is the best place to route the air to?

Carl Fisher · **Posted:** Sun Aug 31, 2003 3:11 pm

This seems too obvious to be what you're looking for, but I'd have to say you should route the ducts to the calipers. They're what retain the heat, and that's where the fluid is...

Charlie Guthrie · **Joined:** Thu Aug 28, 2003 8:14 pm **Posts:** 832

Just to liven up the discussion, I'll disagree with Carl. If you have vented rotors, then a likely routing for cooling air would be into the center opening on the inside of the rotor. The caliper is "insulated" from the heat generated in the rotor by the brake pads, so most of the heat generated by braking is left in the rotors. As such, the rotor is the element operating at the highest temperature. If you study thermodynamics (I think I did that once a long time ago), you will note that heat transfer works better the greater the temperature difference between the cooling medium and the part being cooled. Therefore, using the same air flow rate, one directed at the caliper (which is cooler) and the other directed at the rotor (which is hotter) more heat will be dissipated by the rotor. So...for the same amount of air flow, you can get more cooling bang for the buck by increasing the air flow through the rotor.
Caveat, if you only cool one side of the rotor, that could lead to thermal stress. That's why I recommend ducting the air directly into the center of the rotor air inlet.
For some cars (BMW) there are kits available which duct air into the center of the hub.
(Actually the "technical" source for this was a course in Heat Transfer and not just Thermodynamics)
Charlie Guthrie
Geek in Residence

Rob Keehner · **Posted:** Fri Sep 05, 2003 3:57 pm

But you are absolutely wrong on this one. The rotor allready is ventilated in most cases. You know the section cutaway in between the rotor surfaces. The cool themselves quite quickly with air pumped by those vaines. The Caliper is what you need to protect from heat. The fluid is the problem with most cars. It's gets old loses it's boiling point boils when it gets hot. The Caliper is a big block of aluminum with poor airflow around it and retains most of the heat that goes into it. Let alone it is filled with the fluid that does all the work. Uncooled calipers can easily hit 500 or more degrees under hard long braking. This is bad considering most brake fluid boils before 500 degrees. If you still disagree then why does the Corvette C5-R cool the calipers only. Keep in mind they have carbon rotors that reach 1500 or more degrees in some cases.

Charlie Guthrie · **Joined:** Thu Aug 28, 2003 8:14 pm **Posts:** 832

You answered your own question regarding the Corvette brake rotors. The rotors can take lots of heat...PLUS, carbon does a very poor job of transfering heat. In this exceptional case, the cooling works better for the caliper. Also, the caliper for most of us is not aluminum, but is steel which makes for a poorer conductor and radiator of heat. I'll still put my money on cooling those iron rotors.
CG

Frank Catena · **Joined:** Wed Aug 27, 2003 1:08 pm **Posts:** 418

I hate to agree with Charlie, but ....

Fred Puhn, noted author of "How to Make Your Car Handle" (I think M. Whitney said: "Read this book, read it again, then again") has a book on braking systems too. He has a chapter on brake ducting and is a big proponent on ducting the rotor. Even recommends making sure that you duct a vented rotor with even airflow to both sides so uneven cooling does cause warping. Lots of pics, etc. I think the book is out of print, bought my copy at Amazon used.

Frank

Adam Ligon · **Posted:** Tue Sep 09, 2003 10:12 pm

Rob Keehner wrote:

But you are absolutely wrong on this one. The rotor allready is ventilated in most cases. You know the section cutaway in between the rotor surfaces. The cool themselves quite quickly with air pumped by those vaines. The Caliper is what you need to protect from heat. The fluid is the problem with most cars. It's gets old loses it's boiling point boils when it gets hot. The Caliper is a big block of aluminum with poor airflow around it and retains most of the heat that goes into it. Let alone it is filled with the fluid that does all the work. Uncooled calipers can easily hit 500 or more degrees under hard long braking. This is bad considering most brake fluid boils before 500 degrees. If you still disagree then why does the Corvette C5-R cool the calipers only. Keep in mind they have carbon rotors that reach 1500 or more degrees in some cases.

Actually you are absolutely wrong and Charlie, and others, are right. Some quick numbers for you best pure racing brake fluids go up to 600 degrees before boiling. Even seen rotors glow red from use? Even the faint red takes over 900 to be seen. Even seen them bright red? Over 1400 to get that bright.

If the calipers and the fluid were the limiting factor you can bet we would have seen manufactures make brake fluids with higher boiling pts. Take a look at where professional racers run the duct work, to the rotors. That's why vented discs are much less apt to fade than solid discs of the same size and with the same caliper acting on them.

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

I'm with Adam, Charlie, and Frank. I have never boiled fluid, but I have heat-soaked the hell out of some pads and rotors.

The best brake duct setups are the ones that create an almost-sealed chamber at the center of the back of the rotor by getting *really* close to the rotor surface. This creates pressure to force the air through the vanes. On several occasions I have almost broken down and bought these for the BMW:

More info:

http://www.bimmerworld.com/cooling_theory.htm

Mark Vitacco · **Posted:** Wed Sep 10, 2003 2:05 pm

From tests that I did with temp paint I have to agree with Charlie.
run ducts along the side of the LCA that is not blocked by
the caliper and direct the hose (as best you can) to the middle
of the rotor area but not directly on the cv joint.
Directing air to the outside of the caliper is not going to do much
if anything that I can measure because the heat path is from
the surface of the pads, to the backing plate, to the caliper pistons,
to the fluid. The outside of the caliper will sill be relatively cool while
the pistons temps is hundreds of degrees. The sorce of the heat
is the pads and the pads are cooled by the rotor surface. the better
you can control the temp of the rotors the better you can control
brake pad and fluid temps. If found that you need 3 inch ducts to
be effective or two 2.5's but anyting smaller is not effective.
With FWD it is slightly more difficlut because the CV joint and axle
is in the way but you shoud stillhave room to direct an air shroud
in there.

Frank Catena · **Joined:** Wed Aug 27, 2003 1:08 pm **Posts:** 418

You also want to make sure that your duct has good airflow. Sometimes its not intuitive where the high and low pressure areas are. Usually anything in the same plane as the radiator is a good place to start but Puhn is also big on actually measuring the airflow. Shrouds can also be used to enhance the flow.
Frank

Graham Jagger · **Joined:** Fri Aug 29, 2003 12:53 am **Posts:** 1718

Take at look at this article on brake cooling.

http://www.wilwood.org/ds254.pdf

Graham

Karl Shultz · **Posted:** Sat Oct 11, 2003 11:53 am

All this talk of pressure zones and $300 sealed brake ducting mechanisms and where to point the thing is great, but...Jason has a FWD car. So my suggestion is this.

Route them in such a way that they at least point somewhat close to the rotor (I too am in the "rotor" camp...good fluid doesn't boil). Your more important considerations are keeping the things out of the way. Brake ducting on a FWD car, from my experience with the GS-R, is a pain in the rear. You've got CV boots, calipers, tie rod ends, swaybars, shocks/springs, and lots of other stuff in your way. So get it the best you can, and then keep careful consideration of the following points...

1. Keep them away from the drivebelts and pulleys. Especially if you're using ducting with a metal spiral "core." Doesn't take long for a pulley to wear through the silicone, expose the metal, and start cutting things. Belts, your hands, CV boots, I've seen it all.

2. Keep them away from the tires. I know, "You never use full lock on a road course so who cares?" The reason you care is if the tire hits it, it moves the duct to a different location. Such as rubbing up against a CV boot or pulley. See item 1.

3. How do they work when the car is on the ground? A lot of configurations I've tried look fine when the car is up in the air. But enough stuff moves around when the suspension compresses that the configuration gets screwed up. Back to items 1 and 2.

4. Ground clearance. Once you've got them out of the way of everything, they may very well hang too low. The first time you go off, or even straddle a piece of curbing, you stand a good chance of ripping them off the car. Then anything can happen. Worst case, it wraps itself around a driveaxle, ruins the axle, and your weekend.

You may not be able to account for all of the above - I've yet to manage it on the driver side of the Integra.

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