Cheap DIY Brake Ducts

[adc]

Originally Posted by NikD

Great job. Looks very nice. I don't need any brake ducts but I'll join you for beer. If you need someone to measure the temps with you let me know.

Haha, that sounds like a plan!

[LiM3y]

Why did you choose not to hook up pipes to the back of the debris shield? This would be significantly more efficient, correct?

[BTW - it looks good and there are some nice touches

[adc]

Originally Posted by LiM3y

Why did you choose not to hook up pipes to the back of the debris shield? This would be significantly more efficient, correct?

[BTW - it looks good and there are some nice touches

Correct, it would be very effective. Matter of fact, I think they would be too effective for street cars - meaning in cold conditions they may prevent your pads from reaching optimum operating temps, plus may direct extra water to the rotor area.

I had other concerns as well - mainly having to do with routing of the duct and anchoring it.

No matter where it would exit, you'd have some 12" or so of it flapping about in a turbulent area, so you'd need to anchor it. I understand some people have anchored it with industrial zipties to the suspension arm, which I don't feel comfortable doing.

Another aspect is routing - it would be a bit more difficult to get it to route almost perpendicular to the rotor surface (you'd need that to have room for the duct when the wheel turns inward). Not impossible, but more difficult.

Then there's the backing plate - to anchor the duct to the OEM backing plate, you need to weld - which I'm not equipped to do. (Plus concerns like rust-proofing afterwards). I think if I'd have access to an OEM backing plate with integrated flange (like the ones in the GT4) I'd probably give it a go - but I think those cost 1200 Eur or some other stupidly large sum. Just not warranted for my use, I think.

So main concerns were: anchoring and bad weather (rain, winter) usage.


If anyone is considering going the full route to the backing plate and lives in the Northeast, I'd seriously consider some way to block the front flanges off in the winter.

[Bubbles]

Any updates?

[adc]

Originally Posted by Bubbles

Any updates?

Not yet, time is on short supply... I'm planning on changing to race pads this weekend to clean up my rotors (detected some slight vibration on hard braking) and I'll be driving on those for a couple of days. After that I'll immediately bed in the OEM pads and repeat the testing...

[Richbot]

I wonder how this effects the aero balance? Ducts are great as long as they don't increase lift. There may be a reason the 335 has ducting and the M3 doesn't...probably wouldn't matter at most tracks though, even if it did increase lift.

One of the reasons a ducted backing plate can be so effective is that it employs the pumping action of the vanes in the rotor to both move more air and fling it outward through the spokes of the wheel, which helps get rid of some of the high pressure air you're introducing to the wheelwell. In this case you're getting the fire hose effect, which has been SOP on most BMW's for a long time (simple hole in the splash guard with a duct leading to the lower edge of the bumper). Has to be better than nothing for brake cooling anyway.

[adc]

Originally Posted by Richbot

I wonder how this effects the aero balance? Ducts are great as long as they don't increase lift. There may be a reason the 335 has ducting and the M3 doesn't...

Probably wouldn't matter at most tracks though, even if it did increase lift.

I've thought about that - however the lower vertical front dam in the M3 (the vertical "face" of the furry black undertray) doesn't seem to perform any valuable aero service - the GTS covers it with a nice front lip - and so does the GT4 if I remember.

And at least the GT4 seems to have some ducts in the front lip, right around where that black front dam would sit. If you look at it, there really aren't many places where BMW can place brake duct intakes at the front of the car.

The M3's front ducts (in the bumper fascia) are already used - one for an oil cooler of some sort, the other for the air intake. To me it just seems that BMW designed a pretty face and could have easily included some ducting, but at the last moment decided that most owners wouldn't know what to do with them (remember, the E46 M3 US-spec came with blocked brake ducts). Or perhaps they were weary of people scraping the front lip. After all, in Europe there are the GT4 for serious racing and the GTS for the serious "gentleman weekend racer" - both which have different front aero (with ducts).


But the more important thing to me is that my ducts only take about 20% of the front airdam vertical surface. If that means loosing 10 lbs of downforce, I have to say I haven't felt it - it feels just as before at any speed.

[adc]

Originally Posted by Richbot

One of the reasons a ducted backing plate can be so effective is that it employs the pumping action of the vanes in the rotor to both move more air and fling it outward through the spokes of the wheel, which helps get rid of some of the high pressure air you're introducing to the wheelwell.

Agreed - but as I've mentioned elsewhere, I am not sure this is a good idea on a street car. You're also bringing more water & other crap to your rotors, and most likely cooling the brakes too much for winter driving.

In this case you're getting the fire hose effect, which has been SOP on most BMW's for a long time (simple hole in the splash guard with a duct leading to the lower edge of the bumper). Has to be better than nothing for brake cooling anyway.

Yep - my E30 had the firehose, so did the E36, E46 and I believe the 335i.

Even Porsche has a firehose for their street cars, but some models (the S versions?) have an aero blade stuck to the front lower control arm, which directs some of that lower airflow towards the back of the brakes. Sort of like the windshield wiper arm aero blade...

I wonder if cooling would be further improved if I removed the backing plate altogether.

[bfetch]

I've been curious whether there's enough airflow below the undertray to support using a NACA duct instead of the front-on porthole everyone's attempted.

NACA operates on the principle of creating a low pressure area at the opening to cause the flow to be "sucked" into the output side. However, if there's not enough flow to be had on the horizontal surface of that undertray then you won't get hardly any air through whatever hose you have at the end of the duct.

Any engineering types reading this thread?

Otherwise your mod strikes me as a cleaner, better executed version of what I'm looking to do to the wife's car.



Thanks for the write-up and pics. I'm curious of your thoughts post-HPDE.

-Brandon

[gr8000]

wow.... nice and clean job!

[JAJ]

Originally Posted by bfetch

I've been curious whether there's enough airflow below the undertray to support using a NACA duct instead of the front-on porthole everyone's attempted.

...

Any engineering types reading this thread?

Otherwise your mod strikes me as a cleaner, better executed version of what I'm looking to do to the wife's car.

-Brandon

I'm interested as well. THERE ARE people on this forum that have extensive backgrounds in fluid dynamics, and it would be interesting to hear from them.

Until we do, we can chat, though.

I stuck a fan in front of my car and looked at how the air flowed around the front end. The design of the fascia and the step under the bumper pushes the air around the outside of the fender past the front wheel. The airflow is sideways, not up or down. The step extends the channeling process down as close to the ground as possible, presumably to reduce drag and improve economy.

If you take a careful look at the bottom of the car below the radiator from underneath, you can see that the under-bumper covering actually forms an airfoil shape that will create a low-pressure area under the front of the engine. This accomplishes two things - first it pulls the front of the car down, countering lift from the air going over the top of the hood. Second, the low pressure zone sucks air out of the exit vent from the engine oil cooler.

There's a third feature under the front of the car that seems to be really important too. ALL of the factory modifications (except the GT2) leave it untouched, and even the Schnitzer and the Brabham BT92 don't mess with it. I'm talking about the little round cutout in the under-bumper fascia right next to where the GT4 and adc brake duct openings go. It must be a critical part of front-end lift management because everyone that has access to a wind tunnel leaves it alone.

So, back to the topic of brake ducts. Everything I've read or figured out about the front of the M3 suggests that any air that gets ducted into the wheel wells won't have a material effect on lift. To the extent that the air doesn't go around the outside of the car, it WILL increase drag.

The NACA duct is a good idea, but the only place to put a NACA duct is in the middle of the low pressure zone under the bumper. Air might just flow the wrong way through a duct in that location. I've studied this layout quite carefully because I have a couple of conveniently sized molded NACA ducts off another car sitting in my garage, but when I looked at how I might use them, I couldn't.

Lastly, a clarification: somewhere in this thread, there's a comment about the GTS and GT4 having brake ducts. I believe that while the GTS has a splitter (same as the ZCP competition package?), it doesn't have brake ducts. On the other hand, the GT4 has brake ducts but no splitter. Could anyone who's actually seen a GTS confirm one way or the other?

[Richbot]

Removing the backing plate altogether could cause some issues with the rubber parts at the upright, like ball joint boots. I'd be sure to wrap them in some gold tape or other heat reflective material if I were to take the backing plates off.

NACA ducts probably wouldn't work under there, it's a turbulent flow region even with all the nice air control pieces on the front of the M3. IIRC NACA ducts are only really effective when they're used to induce the boundary layer in a laminar flow region to "break" allowing faster moving air from beyond the boundary layer into the ductwork, which is the reason for the curvature of the "reverse scoop" on the NACA duct. So in other words, in turbulent regions like the underside of a car, a NACA duct isn't much better than a hole in the bottom of the bodywork and might even be worse.

[adc]

Originally Posted by JAJ

I stuck a fan in front of my car and looked at how the air flowed around the front end. The design of the fascia and the step under the bumper pushes the air around the outside of the fender past the front wheel. The airflow is sideways, not up or down. The step extends the channeling process down as close to the ground as possible, presumably to reduce drag and improve economy.

That's very interesting, I've been wondering what the intent was...

If you take a careful look at the bottom of the car below the radiator from underneath, you can see that the under-bumper covering actually forms an airfoil shape that will create a low-pressure area under the front of the engine. This accomplishes two things - first it pulls the front of the car down, countering lift from the air going over the top of the hood. Second, the low pressure zone sucks air out of the exit vent from the engine oil cooler.

I can confirm this, it's an extractor vent. The pressure differential is most likely also the reason for the presence of the bowden cables, to maintain the shape of the undertray at high speed.

Originally Posted by Richbot

NACA ducts probably wouldn't work under there, it's a turbulent flow region even with all the nice air control pieces on the front of the M3. IIRC NACA ducts are only really effective when they're used to induce the boundary layer in a laminar flow region to "break" allowing faster moving air from beyond the boundary layer into the ductwork, which is the reason for the curvature of the "reverse scoop" on the NACA duct. So in other words, in turbulent regions like the underside of a car, a NACA duct isn't much better than a hole in the bottom of the bodywork and might even be worse.

Not only that - but the side (end) pieces of the undertray are too short for a NACA duct (even if the airflow were laminar). And the center piece has no room to spare, it sits too close to the radiators.

Originally Posted by bfetch

Thanks for the write-up and pics. I'm curious of your thoughts post-HPDE.

-Brandon

HPDE coming up at the beginning of October - I'll post a report.

[Radiation Joe]

Ahhh. Some engineering speak. That was refreshing.

[AP Racing - Chris_B]

Originally Posted by Richbot

NACA ducts probably wouldn't work under there, it's a turbulent flow region even with all the nice air control pieces on the front of the M3. IIRC NACA ducts are only really effective when they're used to induce the boundary layer in a laminar flow region to "break" allowing faster moving air from beyond the boundary layer into the ductwork, which is the reason for the curvature of the "reverse scoop" on the NACA duct. So in other words, in turbulent regions like the underside of a car, a NACA duct isn't much better than a hole in the bottom of the bodywork and might even be worse.

+1 on the NACA duct analysis. They like laminar flow and are sensitive to flow direction. Paint drops are a fun way to watch which way air is flowing across surfaces, although that method does not help with the off-body flow field. The nice thing about using drops is that you can check flow with the car moving. Monitoring flow while the car is standing still does not tell the whole story, which is why it is important to develop vehicle aerodynamics in a rolling road wind tunnel.

Chris

[Richbot]

Originally Posted by Radiation Joe

Ahhh. Some engineering speak. That was refreshing.

Oops, almost forgot. Don't want to get banned from M3forum:

Needs 20's, tints and a drop.

[Jeff_DML]

Originally Posted by adc

HPDE coming up at the beginning of October - I'll post a report.

looks good, where is the report?

[adc]

Originally Posted by Jeff_DML

looks good, where is the report?

Ouch, tough crowd...

Did a track event with Trackdaze on Summit Point Shenandoah and the excellent news is that I pretty much forgot about the brakes. What this means is that for the first time with the E90 I had to concentrate on all the other aspects of driving and the brakes were there for me, lap after lap, no matter how fast I went.

I still cannot compare perfectly with Summit Main, where the speed at the end of the main straight can reach 150-155mph (and I've seen 145-150) and there's hard braking for Turn 1. In contrast, the highest speed I saw at Shenandoah was 125mph on the back straight, and if I went that fast I had to manage my tires and was on the absolute limit of creating an M3-shaped hole in the surroundings.

Still, Shenandoah has probably more braking areas than Main due to the extra 5-6 turns. All in all it might be similar, not sure.

Bottom line, brakes were a lot more confidence-inspiring than ever before. Pedal feel remained excellent throughout, there was no grumbling or other initial signs of deposits and pad wear was pretty even.

I tried jumping out of the car to measure rotor temps but they were outside the range of my IR thermometer (meaning over 950 F). I'll purchase a better thermometer this season and do a few laps with one of the ducts blocked off, to see if there is any difference. So, more info to follow...

[ersin]

Sounds promising. As per one of the previous posts, if you need someone to help you measure temps then hit me up. I'd love to see this up close anyway. You can PM me.


Cheers.

[adc]

Originally Posted by ersin

Sounds promising. As per one of the previous posts, if you need someone to help you measure temps then hit me up. I'd love to see this up close anyway. You can PM me.

Will do, ersin - thanks!

One of the things missing from making this more aesthetically pleasing is some sort of semi-flexible plastic flange, to install at the exit of the duct in the fender liner. I've searched last year for an animal like this and couldn't find it, I'll search some more this year.

[008]

Originally Posted by Richbot

Oops, almost forgot. Don't want to get banned from M3forum:

Needs 20's, tints and a drop.

On the dust shield issue, I removed front and back on my E46 M3 and replaced the front with ducted backing plates and I never had a heat issue of the suspension in 3 years and extensive tracking. I've already removed the plates on the E92 and ran hose along the suspension arms fastened with zip ties peeking out at the bottom to catch airflow. They terminate as close to the center of the rotor as I could get, pushing out through the internal vents. I'll remove the rears when I do a pad swap before the track.

[alefcole]

Looking forward to hear your report OP with a better thermometer and one side closed, one side open.