Brake cooling

Dan:

I've picked up a set of wheel fans based on Corvair fans that will fit my MGB. Have not yet installed them, but they are on the shelf waiting. Barry Perkins also runs a set of wheel fans on his FP MGB.

Re: the ducting comment, I agree on the front brakes, but I have not yet figured out how to successfully get air ducted to the rears. I rented a FP Midget a few years ago and the car had Datsun defroster ducts mounted underneath the car to direct air to the rear rotors. Worked like a charm, and made a noticeable difference in reducing brake fade, until I had a minor "off" and stuffed them full of grass!

Rick
 
I agree with Kyle et al that brake fans are more or less the last thing I'd do to put some brakes under me and that heat paint or temp markers are key.

Here is the ordered priority list I use: Work on the brakes in this order and you will have a robust system that is not sensitive to small things that aren't perfect like a little water in the brake fluid. Also remember there are only 3 forms of heat transfer: conduction, convection, radiation....this list tries to maximize all 3.

1) Ducts- and I don't mean wimpy 2 or 2 1/2" ducts on front. Because I couldn't package a 4" duct on the front of my H car I had two 3" ducts on EACH front wheel....4 total front axle. Then one 3" on the rear but that was likely overkill. Get the air from a known high pressure zone on the car which means you might have to hook up a magnahelic pressure gauge to confirm this
1.1) Use them less.....you'll go faster and the brakes will be cooler ;^)
1.2) Mill a flat on the outboard half of the caliper casting big enough for a temp sticker in the 500F range.....you can view it through it wheel slot. If you ever get them that hot you have serious issues. Also use temp paint on the rotor OD edge
1.5) High BP brake fluid- but flush it out one or two days before the weekend. Don't fool yourself into thinking that because you use brarided teflon lines you won't get water in the highly hydroscopic DOT4 brake fluid. At Ford during warranty analysis studies I saw street cars with up to 10% water in the fluid!!
1.7) paint the caliper flat black for greater radiation heat transfer
2) Disc brake insulators- either super high density organic friction material counterbored into the the piston lip or another insulator
3) Stainless steel not carbon steel pistons- the coefficient of thermal conductivity for stainles is 1/3 of carbon steel so this REALLy helps
4) castle milled slots in the piston lip that contacts the pad- reduces thermal conduction path AND allows air to circulate inside the hollow piston
5) fancy thermal barriers sprayed inside the piston
6) Brake fans

Yes I did all of these on my H car since I wanted brakes that were nearly 100% reliable.....certainly overkill. I thought it would be embarassing for a brake engineer to run out of brakes!
 
Quick question for Jay...

How does the stainless compare to the stock pistons in my 06 Mustang calipers? They appear to be ceramic but tend to crumble after 10 races or so.
 
Consiering the stainless should last "forever" in theory, and if you don't have brake fluid temp issues now, I'd try to pick up a set of stainless pistons and see what they do. worst case is you have a set of backup pistons/calipers in the trailer, and swap them out between qual/race if the stainless ones cause you heat issues.
 
For the Prod classes , some rules allow slotting the piston to pad face to allow air to pass by, cooling the piston void a good bit.
FWIW if you are moving air more around the wheels, it will increase drag . There may be a drag decrease elsewhere by the increased flow of the air over the side of the car. My guess is that the fans will slow the car some.

I pull the pistons out every 6hrs or so and wipe them down clean. Rolling drag sux.
 
Dan, I have never made any but have entertained the thought more than a few times. I even bought some thin aluminum discs that I was going to make them out of earlier this year but never followed through. Maybe I will this next year. Noy sure how the aluminum would hold up in the long term, but we're not 'supposed' to be hitting things with our wheels anyway. :roll:
I know of a few other prod racers that use them to good effect, Chuck Mathis being one. He has some REALLY nice carbon fiber ones that he makes for his VWs. Find some pics of his car, both the Rabbit and his newer Scirroco and they are obvious on the front wheels.

I also ran across this photo sequence of some that were made for what looks to be a rally car, also Carbon Fiber. Not the best pics but the best I could find at the time.

8164512002_146727a158_k.jpg
 
The above Porsche pic looks almost identical to my sprite wheel fans except my vanes extended much further towards the wheel and their profile was quite complex to closely fit the 8 minilite cast wheel spokes and then curved and plunged into the area between the spokes. Like I said.....they were time consuming to make.

The plastic pistons are called phenolic pistons, quite common on many street cars. While excellent heat barriers they tend to crumble with high temps and be unsuitable for racing applications due to the problems cited. It would scare me too much to use them in a racer until I knew the temps were low enough. They also tend to leak/nick more and can suffer from more brake pad drag....long story here

My brake fan circular plates were made from aluminum and they held up fine........until I rubbed them up against another car. Then adios.
 
Great suggestions Jay, thanks!

One question for you please -

How would you relate rotor temp as measured by paint dots on (non-vented) rotor OD to the temp cited by pad manufacturers. IE if a given pad is said to be good for 1000F, would that mean a 1000F reading on the disc OD, or are they talking pad face temp and thus another thing altogether?

btw, I was at Link Test Labs in Dearborn two weeks ago and saw the big million buck brake dynos - did you use to work with those things? (No, I'm not considering one for my garage!)

Regards,

Al Seim
HP VW Scirocco
 
Al,

Yes I did work with big expensive brake dynos, most of them manufactured by Link.

Actually a small hole is drilled in the center of the brake pad FM and k type (I think) thermocouple is pressed in the hole and backing plate maybe .060" below the friction material rubbing surface. The wire comes out in the hollow part of the caliper piston and exits out a small slot in the contact face of the piston. That is the temp brake manufacturers are concerned about.

Here's why SS is such a good material beyond the obvious corrosion and mechanical strength properties. Thermal conductivity in watts/(M hr degF). Lower is better ie a better thermal "resistor" impeding conductive heat transfer:

Gold 310 oops it melts
Aluminum 205....amazing how many aftermarket calipers have aluminum pistons!!!! and it sucks!
Nickel 91 where on earth do you get a chunk big enough to to machine a piston?
Iron 80
Magnesium 156
steel, carbon 43
stainless steel 17
Titanium 22 $$$$ but the stuff looks way cool
Firebrick 1.4 oops it shatters at the worst possible time
plywood 0.13 oops it catches on fire
nylon 6-6 0.25 oops it melts and then catches on fire
porcelain 1.5 oops it cracks but luckily no fire ;^)

I think you get the idea here.
 
Guys go to about 1/2 way down this page

http://www.corvairforum.com/forum/viewtopic.php?f=13&t=4225&view=unread

and look at the 65-69 magnesium fan version on the right. These things weigh nothing, have a nice flat center mounting surface cast in, can be turned down to just about any size on a lathe, can be bought 4 for $80 all day long, and have been used off and on as wheel fans on everything from club to TransAm to CanAm cars off and on for last 40 years.

Remarkedly similar to what Jay is building in his posted pics.
 
I considered pizza pans, then looked around and saw that just about everyone wanted more for a flat (essentially) piece of aluminum than I thought they were worth. Which is when I found aluminum pan separators, obviously what restaurants use in the kitchens to separate food pans (DUH) :wink: . I found several restaurant supply stores online that sell them for $3 to $5 each.
I bought some of them in 14" diameter, which should be perfect for 13" wheels. They're made from 18 gauge (~0.040" thick) aluminum.
 
Jay Lutz said:
4) castle milled slots in the piston lip that contacts the pad- reduces thermal conduction path AND allows air to circulate inside the hollow piston
5) fancy thermal barriers sprayed inside the piston
Click to expand...

Good stuff, a couple questions.

Re castle milled slots. Just to be clear . . . make the piston "skirt" look like a castle nut, correct?

Re fancy thermal barriers. Any specific recommendations?

Thank you,
L
 
Larry F. asked:

"Re fancy thermal barriers. Any specific recommendations?"

Years (decades?) ago I used a 2" fibrous asbestos tape material between the metallic thermal barrier and the pad. Remember it would char in a circular ring where the puck contacted the metallic barrier.

Asbestos NLA, but high temp, ceramic fiber material is. McMaster-Carr #87575K83. .032" thick, -300-2300 deg., 0.38 Btu/hr. x in/sq. ft., 12 lbs/cu. ft. $24 for 3" x 50 ft roll.

Have seen it used as welding mask effectively. Very similar to old asbestos material.

RJS
 
If the pan is under the rim size, you could attatch the thing to the outer edge with small machine screws into tapped holes.

Chris, could you feel any output air?

I use a whizz wheel to slot my pistons. 2 to 4 pretty good size slots.
 
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