Let's Talk Aero

[Grant 302]

Thanks @Grant 302 . I tried a session where I took the rear wing off. It balanced the car at speed again, but this time I was 3 seconds a lap slower at Buttonwillow. So, yes I will try to mix all those things together and see where it gets me. I think even some double digits increase in downforce/decrease in lift will fix it right up. Splitter drop, canards, wheel well splates, and some more venting should really do it.

Sounds like a good plan!

I also forgot to mention raising the rear and/or more rear spring. If the rear downforce is enough to change your trailing arm/LCA geometry toward more squat, it could also be contributing to your high speed push. One more thing to consider in addition to the front aero mods.

 

[modernbeat]

Regarding the radiator-to-hood ducting, I forgot that Vorshlag was able to accomplish this on their "Big Red" Mustang--you can probably pore through their blog and find the photos of how they laid everything out to clear stuff. It looked great but also very labor-intensive (read: expensive).

The good part about "labor intensive" is that it can often be done inexpensively by a DIY guy with a few tools and a little skill. Maybe not as fast or clean as we did it, but still achievable.

We actually did two different Mustangs. Our "Big Red" that was fully ducted on the front, and loosely ducted behind the radiator. It was a lot of work, but it really needed it for cooling efficiency and as part of the front downforce/drag-reduction package.

The other one was a silver Roush that was less of a track weapon, and more of a street car pushing it hard on track. We had to pay more attention to aesthetics, and had the extra heat exchangers to work around. There were no modifications in front of the radiator, and a loose duct behind it pushing air to the huge duct in a carbon hood.

The galleries have more detail than the build threads, though they don't have the commentary. I'll post links to the point in each gallery where we built the front ducting on each car.

The Silver Roush starts here, where we received the hood.
https://vorshlag.smugmug.com/Vorshlag-Test-Pilots/James-Meeker/i-BBT9ddZ
There is a small break, and then more photos showing us repositioning the underhood parts and building new tanks.
https://vorshlag.smugmug.com/Vorshlag-Test-Pilots/James-Meeker/i-8xVQF6Q

And here are some of the final photos.




Underhood showing the space we opened up at the front of the engine, to allow better airflow through the duct.


For the Red car, here is where we start moving things under the hood, and then build a new bumper beam to allow front duct space and to support the splitter. Then we build the front duct work and THEN we cut up the stock hood and build the super slide ducts in it.
https://vorshlag.smugmug.com/Projects/Vorshlag-2011-Mustang-GT-build/i-nKkkgQ2

Here's a good photo showing under the skin at a test day.


The brown thing in front of the radiator is a piece of Nomex honeycomb we used to protect the radiator during bug season. There were a couple events where we had to scrape off a couple inches of dragon flies off the radiator. We removed it when not needed.


A good shot showing how the tanks and intake were repositioned to make room for the hood ducts.


Neither of these projects were one-and-done. Both required some adjustments and changes to optimize them after the first iteration. Thankfully, small things, like different tanks that worked better, or fastener changes. And they allowed us to experiment and fine tune what we would do for future projects, like the V8 swapped BMW E46 or the '69 Camaro tube frame track weapon.

 

[blacksheep-1]

FWIW...A while back we had this car, It had always behaved and was never a problem child with regards to temps and pressures, well I could never get the rear pressures high enough when hot. So, because I do things the way I do, I kept inching up the rear pressures. Now normally, this car likes a 2 psi split front to rear when cold as well as hot, but as the rear pressures increased the car became more of a handful, especially when cold. The numbers would keep coming back all wrong.
We couldn't figure it out, so I just arbitrarily said screw it and put the cold pressures back where we used to set them, with the 2 psi split (plus whatever tuning adjustments and etc) The car still didn't handle.
Well upon further review, the wing had come off the car for transport and it was installed 3 degrees less than it should be. Once adjusted, all was right in the world.
So not only is this another reason to monitor your air pressures, but it also demonstrates how a simple aero mistake can really screw stuff up, this is why I recommend either the IMSA aero package or the PWC aero package in it's entirety, those cars have been in a wind tunnel, and are a known quantity. Now if you go some other route, that's fine, just be aware you could be opening a Pandora's box of issues that would take a lot of R&D to evaluate.
So, it pays to go down a road well travelled.

 

[modernbeat]

I'm a lot less enthusiastic about using the PWC or IMSA packages. They have all sorts of restrictions that don't apply to most of us. And their packages will work IF your suspension and tire are the same AND if you have the setup sheet they used. Otherwise, you will be doing all the R&D anyway to tune the setup.

FWIW, there are decent aero testing procedures in Neal Roberts book "Think Fast". Stuff than can be done by a couple guys and simple tools.

 

[LERCH]

@modernbeat is the hood vent location on the silver Roush better than say the tiger racing hood?

Is the scoop on the back of the radiator there just to direct the air upward to keep it from getting down under the car?

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[modernbeat]

Lerch, correct. We were trying to keep air out from under the car, and also trying to direct it to the duct in the hood. We ran it without the rear scoop for a while and found that the scoop helped with the cooling, not just the aero.

 

[AJ Hartman]

Woo hoo! An aero thread! You guys are basically well on track vs. some other aero threads I've seen in the past on other forums. Aero is probably one of the trickier things about race cars since its very difficult to accurately measure, coupled with all the wind tunnel and CFD can basically go out the window when you are near another car, have a head wind or cross wind, the car pitches or rolls, whatever. It will get you pretty damn close tho. I've always liked hood vents rather then louvers on a hood. Only reason is that louvers on a hood are open to whatever is below it and water could get on some sensative bits. A hood vent is closed on top and only open in the front, so water intrusion is probably a little less of an issue. You can also go to "outie" louvers vs. "innies" if you want more flow. The theory there is the same as adding a wicker, or gurney, in front of a vent and will force more air out rather then relying on the pressure differential. Then there are canards, dive planes, splitter tunnels, larger splitters, lower splitters, splitter end plates, splitter strakes, radiator ducting, tire reliefs...

I just got back from nationals as got a chance to do runs with and without aero, relatively speaking. I pulled my canards off and put the wing at a degree or two and basically what I expected to happen, happened. I was quicker in the straights by a few MPH, but overall lap times increased. This made the car a little more "raceable" as I could hang with other cars in the straights but was slower overall. The next morning I piled all the aero back on, cranked the wing back up, and immediately went 1.3 seconds quicker then I did all weekend. Granted it was the next morning so weather was a tad cooler, but otherwise no changes so there are a few variables aside from the extra Df. I may do a blog post or YouTube video going over the data of the full aero vs. less aero runs. Sebring is one of those tracks where there are only one or 2 turns aero will help (aside from just about every brake zone). Otherwise its kinda a hard brake zone and then a drag race to the next turn.

Heres some links to blog posts I made regarding some aero topics.

Monthly book review:
http://www.ajhartmanracing.com/blog/april-book-review-competition-car-aerodynamics/

Post about canards. At the bottom is a link to a Kyle Engineers video that is also super informative.
http://www.ajhartmanracing.com/blog/front-bumper-canards-and-dive-planes-explained/

Here I use AiM data to confirm the benefit of my splitter tunnels. I may look into making carbon splitter tunnel inserts. This way you can make a plywood splitter on the cheap, then just cut out a slot and install our tunnels. Kinda a happy medium of a cheap splitter with good performance vs. a complete carbon splitter.
http://www.ajhartmanracing.com/blog/utilizing-gps-data-to-confirm-aerodynamic-gains/

Heres my splitter setup now. I always have a spare.
IMG_1095 by AJ Hartman, on Flickr
IMG_1067 by AJ Hartman, on Flickr

Our Innie vs. Outie louvers
IMG_1105 by AJ Hartman, on Flickr
IMG_1104 by AJ Hartman, on Flickr

This picture is a little older but here is our hood vent setup.
IMG_3057 by AJ Hartman, on Flickr

 

[blacksheep-1]

AJ Hartman, I thought that was an interesting comment concerning removing aero, we did this at Daytona with some surprising results, especially in the bus stop...very entertaining..lol.
and Modernbeat, what class are you racing?

 

[ChrisM]

Aero seems like one of those things that you would start tweaking after you've figured out 90% of what your suspension is doing. The benefits of an optimized aero setup are very obvious, but I see a lot of people who start adding wings and splitters and all sorts of other stuff before they've even understood how their suspension system works. How can you really begin to optimize your aero if you're not even sure if it's the aero or the suspension that's the problem? Of course, they work together as well, but understanding each system individually probably helps to create a whole car that is greater than the sum of its parts.

Edit: out of my element here because I'm not smart on most of this stuff. Just an observation I've had in my limited experience. I love hearing from the experienced people here, it's a great learning opportunity.

 

[302BOB]

ChrisM hit the nail on the head when it comes to Aero. A lot of folks add wings & splitters to their cars and think they're good to go. But the truth is it's much more complex than that. I speak from observation & not experience so take this with a grain of salt. I believe true Aero comes from all the dynamics working on the car when you're driving it to the max.Air flo, suspension, driver ability and other variables all come into play. Why is it some NASCAR drivers can "see" the air around their cars on super speedways and others don't have a clue?

 

[Grant 302]

out of my element here because I'm not smart on most of this stuff.

The truth is, nobody is 'very smart' on this stuff. None of it is truly intuitive. Any time I've explained how a splitter works, the direction(s) of airflow over it, and that it does not literally split the air...it is most often followed by some amount of blank stare or disbelief. We can't see the air and people have trouble understanding what they don't see.

This is why so much of aerodynamic knowledge comes from empirical data from race teams. You know something is actually working when you go faster and/or win. It's also why it takes hefty computing power to even model the theoretical effects on a complete vehicle. Even then, finding 'optimal' settings and adjustment needs to be validated from testing on track.

For example, I'd challenge anyone to explain all the functions of the canards on the Mustang GT4 *without* CFD analysis. It's hard to understand what we can't immediately visualize.

 

[ArizonaBOSS]

The truth is, nobody is 'very smart' on this stuff. None of it is truly intuitive. Any time I've explained how a splitter works, the direction(s) of airflow over it, and that it does not literally split the air...it is most often followed by some amount of blank stare or disbelief. We can't see the air and people have trouble understanding what they don't see.

This is why so much of aerodynamic knowledge comes from empirical data from race teams. You know something is actually working when you go faster and/or win. It's also why it takes hefty computing power to even model the theoretical effects on a complete vehicle. Even then, finding 'optimal' settings and adjustment needs to be validated from testing on track.

For example, I'd challenge anyone to explain all the functions of the canards on the Mustang GT4 *without* CFD analysis. It's hard to understand what we can't immediately visualize.

Like I said earlier (or in another thread)--air is a fluid. If you explain it to people that we are, in our own way, "underwater", it may start to make more sense.

 

[Grant 302]

It does help visualize streamlines and flow patterns, but compressible/incompressible fluids make it a bit more difficult to compare. A long enough splitter might actually split the flow in a water tunnel or have a significantly lower stagnation point than in a wind tunnel.

 

[Fabman]

One elementary way help understand airflow is to go to a creek and watch the water flow around rocks and debris. Pour some food coloring in different spots and watch what happens. Pour different colors in different spots and watch how they merge. Sure there are some variables with speed/density etc. but as a general rule, the flow patterns will be helpful to understand basic principles.

 

[Grant 302]

That's great, if you're car is shaped like a rock! Maybe a Ford Taurus?

Maybe I should throw some of my 1:18 and 1:24 Mustang models in there.

 

[Fabman]

That's great, if you're car is shaped like a rock! Maybe a Ford Taurus?

Maybe I should throw some of my 1:18 and 1:24 Mustang models in there.

Like I said, an elementary understanding. That method was very helpful to me when I was designing racing mufflers back in the 80’s....but that’s another story for another time.


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[Big Black]

Thank you AJ Hartman for the links, and +1 Chris M, I think you're right. Great topic!

Now if I may, I'd like to ask for an opinion about a wicker I mocked up before I go ahead and cut and bend the aluminum (.040"). I have a carbon fiber KR hood pictured. I'm sure it helps with cooling, and I suspect it adds a bit of down force. I'm wondering how much of an effect, and what kind of effect it will have if I do what's pictured below:

 

[Grant 302]

If you tape that up and put some yarn/string/tufts in and in front of the vent and do a before/after test drive, you should be able to see the difference like posted earlier:

I'd also add some before or on the wickerbill to see how it changes the flow coming off of its trailing edge.

My rough estimate is that the change would net less than 3lb. increase in downforce (each) compared to the vent alone. @100 mph. assuming vent area of ~40 sq in.

 

[302BOB]

I'm not sure if I'm right here, but the "path of least resistance" is the key to free all things to flow their best! It's up to us to figure out what that is............I know, If it was that easy.................

 

[Big Black]

I'll do a test with some ribbon and evaluate -- a teeny bit of downforce with a little more air "pulled" through the vent surely can't hurt, I suppose.