Topic: Brake Ducting Q: Add Duct Fans or Not?

We have a heavy car at 3300 lb with driver and at our last race, one of our drivers boiled the fluid during a yellow. This is despite having 13" rotors and cobra calipers (biggest bolt on options for a 95 tbird). He had to pump twice to slow down. We measured a temp of 580F on the disc after he pitted.

FIX 1: upgrade the fluid from ATE Super Blue (dry temp 536F) to Motul RBF 600 (594F) right before the next race. This will clearly help.

FIX 2: I built some ducting that dumps air from a front bumper hole straight onto the rotor. I made this by fiberglassing in some ducts into two holes in the bumper cover and then welding two metal tabs onto some scrap 3" tubing. Ducting is RV sewer hose from Walmart. Interesting and satisfying project.

I'm now thinking that while fix #2 should lower disc temps at speed by forcing air onto the rotors at speed, it might not have the same effect under caution because of the reduced vehicle speed.

Q: Should I now consider adding two bilge pump fans (130CFM for ~$20 each) inline with the ducts or potentially as a separate duct?

PROS: would force more air during cautions onto the rotors... Arguably when we need it most.
CONS: fans themselves could/would be airflow obstructions when the car is moving at speed. See pic of how much of the inside of the tube is blocked by the fan.

Will the pros of active cooling outweigh the cons? This is what I'd like advice about.

I'm clearly over thinking this problem but this is the fun of Lemons for me. The cost and fab time don't bother me; I just want some advice if the added complication of the fan ducts (either inline with my existing new ducts or mounted on a second bracket aimed at the caliper) is worth it. A second bracket is added complication and imo one more thing that can fall into the track. If I go with inline mounting, I could probably secure the fans more securely on the inside of the fender.

Existing ducts and brackets I just made
http://www.gunn.com/images/race9_duct1.jpgWe have a heavy car at 3300 lb with driver and at our last race, one of our drivers boiled the fluid during a yellow. This is despite having 13" rotors and cobra calipers (biggest bolt on option for a third). He had to pump twice to slow down. We measured a temp of 580F on the disc after he pitted.

FIX 1: upgrade the fluid from ATE Super Blue (dry temp 536F) to Motul RBF 600 (594F) right before the next race. This will help.

FIX 2: I built some ducting that dumps air from a front bumper hole straight onto the rotor. I made this by fiberglassing in some ducts into two holes in the bumper cover and then welding two metal tabs onto some scrap 3" tubing. Ducting is RV sewer hose from Walmart. Interesting and satisfying project.

I'm now thinking that while fix #2 should lower disc temps at speed by forcing air onto the rotors at speed, it might not have the same effect under caution because of the reduced vehicle speed.

Q: should I consider adding two bilge pump fans (130CFM for ~$20 each) inline with the ducts or potentially as a separate duct?

PROS: would force more air during cautions onto the rotors... Arguably when we need it most.
CONS: fans themselves could/would be airflow obstructions when the car is moving at speed?

Will the pros of active cooling outweigh the cons? This is what I'd like advice about.

I'm clearly over thinking this problem but this is the fun of Lemons for me. The cost and fab time don't bother me; I just want some advice if the added complication of the fan ducts (either inline with my existing new ducts or mounted on a second bracket aimed at the caliper) is worth it. A second bracket is added complication and imo one more thing that can fall into the track. If I go with inline mounting, I could probably secure the fans more securely on the inside of the fender.

Existing ducts and bracketry I just made
http://www.gunn.com/images/race9_duct1.jpg
http://www.gunn.com/images/race9_duct2.jpg
http://www.gunn.com/images/race9_duct3.jpg
http://www.gunn.com/images/race9_duct4.jpg



Duct fans I'm considering
https://www.walmart.com/ip/Attwood-Turb … t/14150765
https://www.five-oceans.com/articulos/2835-M2/2835-M2-C3.jpg





Thanks in advance.
Regards,
-g

Myopic Motorsport's #888 Ceci n'est pas une Citron Thunderbird ("This is not a lemon" but a 1995 tbird w/ 93 V8 swap + shopping cart rear wing + engine mounted frito maker)
2017 Sears Pointless Organizer’s Choice
Frito Making Tbird from 2018 Sears Pointless Engine Heat BBQ - http://goo.gl/csaet4

Re: Brake Ducting Q: Add Duct Fans or Not?

I would go without the fans.  I think with the change in fluid and the ducts you should be good.  We have a lighter car (2700-2800 ish range) and we used the Motul 600 and did not have a problem, but there again, we only turned 65 laps.

Skip "Mongo" L.
Team DadBod

Re: Brake Ducting Q: Add Duct Fans or Not?

Yes, fans are an improvement. Look at the Detmar 4" fans instead of the Attwoods. Remember that they're more sensitive to restrictions on the intake than exhaust, so mount the fan ideally at the front duct. Second, your rotor duct needs to direct air into the center of the rotor, not the inside disc face. Can you move the duct inlet closer to the center of the car, say right about your radiator duct? You'll have a little higher pressure air there.

4 (edited by Fishah 2018-05-14 12:44 PM)

Re: Brake Ducting Q: Add Duct Fans or Not?

I was always under the impression that cooling ducts should be aimed at the calipers rather than the rotors. They house the fluid that is going to boil... That's where we aim ours but I'm no engineer.

I think fans are unnecessary and will be a restriction at speed. We run ATE in our 3100lb 928, with no fans in our ducts, and we've never even had a hint of fade. In fact, I'm not even sure we're getting them up to operating temp...

And we just raced at 100 degrees at Willow Springs, with repeatedly braking from 120+ MPH.

Granted we use very expensive pads, OEM Porsche 911 rotors, and have four-piston Brembo calipers, all of which manage their temp very well and provide far more braking than our tires can handle, so they don't get pushed overly hard.

#928 Porsche 928 - West German Pushrodders

5 (edited by ross2004 2018-05-15 07:59 AM)

Re: Brake Ducting Q: Add Duct Fans or Not?

Admin edit. No linking to other race series forums please.

Ok...... google search "the other race series" thread discussing brake ducts & fans.

Re: Brake Ducting Q: Add Duct Fans or Not?

Thanks for the differing opinions. The reason I aimed my ducts at the rotors (vs the caliper) was largely dictated by packaging and the mount points I had on my spindle. These holes were originally used for rivets to hold the brake shield to the spindle (which I sold off years ago) and were basically the only place to mount the duct. FWIW, maybe half of the airflow coming out of the 3" duct should dump into the center of the rotor but because of the spindle design (and where the ABS sensor is mounted) I can't really dump more and still maintain range of motion. If I used the caliper bracket as a mount point to dump air on to the calipers, anything I fab will likely hit the front strut that sits behind the spindle assembly when you turn the wheel too much.

As far as mounting the intake more closer to the center of the car, this would be problematic because I a pretty big radiator & oil cooler in the way.
To move the brake duct intakes up higher I suppose I could cut a hole in the actual front bumper (what sits behind the cover) but honestly, I think having a complete front bumper will be more useful to us than having slightly higher pressure air going onto our rotors.

We use decent brake pads (Raybestos ST43) so I suspect with the new fluid our system will be good enough. If not, thanks for the suggestions and I'll look at mounting the Detmar's as an "upgrade"

-g

Myopic Motorsport's #888 Ceci n'est pas une Citron Thunderbird ("This is not a lemon" but a 1995 tbird w/ 93 V8 swap + shopping cart rear wing + engine mounted frito maker)
2017 Sears Pointless Organizer’s Choice
Frito Making Tbird from 2018 Sears Pointless Engine Heat BBQ - http://goo.gl/csaet4

Re: Brake Ducting Q: Add Duct Fans or Not?

Rotor temp doesn't necessarily equal caliper temp or fluid temp.

A lot of race pads are happy at 600+°f. I wouldn't consider that too hot for the rotor or pads but definitely too hot for fluid. Don't forget there are seals in the pistons that need to deal with those high temps for 8+ hours if you're doing well.

Are the calipers painted?

You could add stainless or titanium shims between pads and pistons to help reduce heat transfer.

Are you sure the brakes aren't dragging slightly when up to temp? Can check sliders, sticky pistons, collapsing brake hose that holds pressure like a check valve, a master cylinder adjusted a bit too tight..

-Killer B's (as in rally) '84 4000Q 4.2V8. Audis never win?

Re: Brake Ducting Q: Add Duct Fans or Not?

We use just ram air ducts and RBF 600 in our 88 T-bird with 11” sn-95 mustang brakes and havent had issues.

The problem you’re havving is hot soaking, not the slow times under yellow. Those just help the heat hang around. With ducts, even going 30 mph os going to help a lot where you have them installed in the fog light holes like we do.

Fans would only be for hard fast runs followed by coming to a complete stop. For vented rotors, throwing air at the inner hub is ideal since that’s where the rotor pulls it’s air from then slings it out the edge. So you cool bothe the thermal mass of the rotor, and move air over the caliper along the edge of the rotor.

Mistake By The Lake Racing (MBTL)
88 Thunderbird "THUNDERBIRDS ARE GO!"
A&D: 2014 Sebrings at Sebring (NSF), 2014 NJMP2 Jurassic Park (SpeedyCop), 2012 Summit Point J30 (PiNuts)
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Re: Brake Ducting Q: Add Duct Fans or Not?

Someone, I think one of the 3PM TR7/RX7 guys, told me that the pros claim brake ducts don't really work until you're close to triple digit speeds, so fans are a good plan in a series where you're not usually going that fast.

How true is that? I'm not sure since I've never seen the source for the claim. But I'd believe it to some degree.


As far as where to blow air, I've always seen it ducted to the center of the rotor, even on professional race cars. Sure the fluid lives in the caliper, but it's far easier to cool the rotor. And heat travels from hot to cold. So if you can get the rotor cooler than the pads, the heat generated from braking will try to heat the rotor more than the pad. obviously it's not a perfect all or nothing, but it does help.

The center venting on rotors draws air from the center and exits it through the outer edge as it spins. Get the extra cool air to the center and you'll help that process.

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