Wind tunnel needed...

"The Stig's former college room mate" (das-stig) 07/21/2016 at 10:57 • Filed to: None

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perhaps there is an aerospace engineer Oppo out there, or maybe someone who’s just well versed with fluid dynamics.

I do 4 or 5 track days a year in my 2011 Dodge Charger. I am convinced that the factory rear spoiler either does nothing or is actually creating lift. I notice the vehicle get a little unsettled at high speeds (about 130 mph). And in 1 particular corner (NASCAR turn 1 at Pocono Raceway), if i push more than 95 through the turn, the stability control light flashes, like the back end wants to come out.

Up until now, this hasn’t been a huge problem, as it was only this one track. Pocono has the longest straight and fastest turn of any of the tracks i visit. and 130 was about my top speed on the straight. However, the horsepower bug has bitten and i recently installed a Procharger intercooled supercharger. The car is a LOT faster now, so aerodynamics are going to become a factor, even on shorter tracks.

I plan on adding a front splitter, but i’m not sure what to do about the rear. I’d like to find a way to test the airflow over the trunk lid. i found a company that makes a direct-fit duck tail style spoiler. since it is not on pedestals, it should at least not provide lift, and if it’s in the airstream, some downforce. But i don’t know if air coming over the roof will hit it. my other option would be a pedestal mounted racing spoiler (a la fast and furious). That style presents other issues, though. I don’t want to drill more holes, so i need one that either mounts to the edge of the trunk, or lines up with the existing holes. Mostly because i also drive this car on the street and i don’t want people thinking i riced out my charger, so i’d take the spoiler off for street driving. I don’t know how tall to have the pedestals to be sure it’s in the air stream. i would like to be able to adjust it properly, rather than just trial and error. i also don’t want to spend thousands of dollars on a carbon fiber racing wing, either.

TL;DR- Need advice on how to select a functional spoiler for my vehicle, that will actually create downforce at high speeds.

CalzoneGolem > The Stig's former college room mate 07/21/2016 at 11:06

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What if you took the spoiler off and then went to the track? That should show you if it currently is an issue.

RamblinRover Luxury-Yacht > The Stig's former college room mate 07/21/2016 at 11:07

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It’s possible that it’s not *there* for downforce - it may just be aiding the rear flow characteristics (lower drag overall) or doing a mixture of that and nothing. Alternately, it might give downforce only at speeds below a certain threshold, and once you hit a certain speed your trunk area is turning into turbulent nonsense. Who knows?

FWIW, the best actual results are probably with a big ol’ fat NASCAR lip like the allegedly-Charger NASCARs.

Nonster > The Stig's former college room mate 07/21/2016 at 11:16

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Most cars are going to produce lift at high speeds and its pretty safe to assume that a lot of the direct fit parts like that ducktail spoiler were only designed with style in mind.

I was about to start a long post explaining things but then I remembered this

http://oppositelock.kinja.com/wings-spoilers…

Aaron M - MasoFiST > The Stig's former college room mate 07/21/2016 at 11:16

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In terms of pedestal spoilers that are actually in the airflow of a basic three-box car, your WRX STI or Evo spoiler is a good starting point, for better or worse.

The ducktail is likely to be better than your stock spoiler aerodynamics-wise, but I can’t imagine it creating much in the way of downforce. Most flow separation is at the rear windscreen so improving aero off the trunk isn’t doing nearly as much as it would be if at the level of the top of your rear windshield.

But. Speaking of “top of your rear windshield”, have you considered vortex generators?

Less obtrusive than a massive pillar spoiler, purportedly able to reduce flow separation and drag. May also make your existing spoiler work, but I doubt the stock spoiler was designed for anything other than cosmetics so don’t quote me on that.

Also, in terms of obtrusive aero mods, pairing your front splitter with a rear diffuser is likely to be more effective at reducing lift than a big spoiler is, especially if you don’t want a giant pedestal spoiler.

yamahog > The Stig's former college room mate 07/21/2016 at 11:17

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Rule of thumb is balance. Most likely that factory rear wing is doing next to nothing, and perhaps my faith is too high in my former Chrysler colleagues but I don’t think they’d knowingly provide lift. You’d want an efficient wing as well as something on the front (such as a splitter) to prevent a pitching moment.

briannutter1 > The Stig's former college room mate 07/21/2016 at 11:42

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The roof canopy area creates a vacuum behind it. Airflow has an angle of divergence of between 7-14 degrees. Meaning if you were to run string at these angles from top and side view, they would project an outline of where a spoiler outline would need to be. You could build a plexi one based on scca a/b/c street and modified and CAM classes to get an idea. As they must be in a certain “box” size

When it’s assembled you could do string testing with a go pro to see how laminar the flow is and play with spoiler angle on hinges.

The Stig's former college room mate > CalzoneGolem 07/21/2016 at 12:00

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I’ve considered pulling it off at the next event to see what changes it makes.

CalzoneGolem > The Stig's former college room mate 07/21/2016 at 12:04

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Probably your easiest option.

yamahog > Aaron M - MasoFiST 07/21/2016 at 12:23

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Vortex generators and canards on street cars make me dry heave.

Rock Bottom > The Stig's former college room mate 07/21/2016 at 12:43

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I’m an automotive aerodynamicist, so I may be able to help.

Adding lift (or downforce) always adds drag, but don’t worry about that. You have excess power, use it. Most high performance cars (like Ferrari, McLarren, Corvette Z06) have Cd well over .40 and often closer to .45 because stability costs drag and they have the power to live with that. Note that the manufacturer claimed Cd and independently tested Cd are almost never the same...

Roof VGs seem to be a gimmick. I’ve never seen them do anything.

I’m not sure what your stock spoiler looks like, but you may be able to shim it up at the rear to increase it’s angle of attack.

Buy another factory spoiler (to drill holes in) and make a few different trailing edge Gurney Flaps of varying heights. Think of starting with a 1 inch height increase in 1 or 2 inch increments out to maybe 6 inches and see what that does. It won’t be pretty, but I guarantee it will work (see the pic at the bottom of this comment)

You may be able to work with the underside of the car. Often, if the underbody is well shielded everywhere aft of the front axle line, the rear lift decreases.

If Mopar offers a high-performance spoiler, it probably reduces lift. Most of the OEMs will at least try to spend a little engineering time on their hi-po parts. They won’t be as well optimized as the regular production stuff, but it’s better than nothing!

This would probably be a really cool university undergrad project! Find a university near you that offers aerospace or aeronautical engineering and contact a professor there with your problem. A little time designing a small-scale wind tunnel experiment or a CFD project might be right up their alley.

If you have the money, contact a wind tunnel like Aerodyne in North Carolina, or ARC in Indianapolis. They both do customer tests and probably have a lot smarter things to say about all this than I do!

Note: If you add a splitter without changing the rear, you may make this thing scary due to the upset lift balance. You definitely want to be careful until you reduce the rear lift.

Aaron M - MasoFiST > yamahog 07/21/2016 at 13:03

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Nowhere in the requirements did I see “pretty”.

gogmorgo - rowing gears in a Grand Cherokee > The Stig's former college room mate 07/21/2016 at 13:10

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Spoilers don’t create downforce. They’re simply to clean up the rear air and reduce drag, although on street cars they’re mostly aesthetic, and sometimes even increase drag.

If this is the one you’re talking about, it doesn’t look like it does much of anything.

If you can modify it to get a better angle of attack, that might help a bit, but you’d be better off getting something else. Really, you want a wing, not a spoiler, but anything that kicks the air up will be better than nothing.

Disclaimer: I’m definitely not an expert.

yamahog > Aaron M - MasoFiST 07/21/2016 at 13:13

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What exactly makes you think it’s “pretty” I’m concerned with?

Aaron M - MasoFiST > yamahog 07/21/2016 at 13:16

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Nowhere in either of my engineering degree studies did I get to the point where bad aerodynamics made me want to vomit.

yamahog > Aaron M - MasoFiST 07/21/2016 at 13:22

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It wasn’t my aero degree that did it, it was overexposure to asshats in flatbrims slapping faux carbon fiber ones on any old fartcanned POS.

Aaron M - MasoFiST > yamahog 07/21/2016 at 13:23

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I still say vortex generators is the lesser of two evils when the other one is a three-foot tall pillar spoiler.

Xyl0c41n3 > Aaron M - MasoFiST 07/21/2016 at 13:26

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Dude, you do realize that Yamahog is an engineer, too, right? Might not be the best idea to dick wave your engineering degree when she's got the same credentials (and probably better ones) to back up what she says, too.

yamahog > Aaron M - MasoFiST 07/21/2016 at 13:31

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It’s possible to get a functional, removable, track wing. That and a decent splitter would be the best route for his purposes IMO.

yamahog > Rock Bottom 07/21/2016 at 13:34

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LD underbodies are decently covered. Chrysler is a big fan of trying to make up FE with low drag. CD on the 200 was 0.28 IIRC.

Aaron M - MasoFiST > Xyl0c41n3 07/21/2016 at 13:35

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Dude, calm down. Physical features of a car inducing vomiting have little to do with education...it was a joke.

Aaron M - MasoFiST > yamahog 07/21/2016 at 13:36

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Probably right. There’d probably be plugs for the holes as well. I’d assume the vast majority of wings would be designed to use the existing mounting holes for the spoiler, but then again I have no idea how extensive the aero aftermarket is for a Dodge Charger.

Rock Bottom > yamahog 07/21/2016 at 13:43

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Yeah, I’ve seen as much as a 20 count hit to rear lift when pulling mid and aft shields from some cars. Same shields are probably worth about half that on drag. Most of us pay attention to the shield effects on drag but we miss the lift benefits!

The Stig's former college room mate > Aaron M - MasoFiST 07/21/2016 at 14:24

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it’s basically non existent. there are (not surprisingly) very few of us that like to take our big American muscle cars and make them go around corners. Most LX/LC (that’s charger/challenger for those that don’t speak mopar) performance parts are for drag racing. So anything in the aero department is going to be custom.

I like the idea of a removable wing. Anybody ever see one where the width between the pedestals is adjustable? i’d rather not drill more holes in the trunk, but i have no problem modifying the wing to fit the existing holes.

Aaron M - MasoFiST > The Stig's former college room mate 07/21/2016 at 14:28

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I’d go and measure the distance between your spoiler pillars to get an idea then start looking online. It’s *possible* that there may be a universal fit product out there, but it’ll take digging.

But yeah, removable wing will serve your needs better than a slightly less obtrusive permanent aero piece.

Sweet Trav > The Stig's former college room mate 07/21/2016 at 15:17

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Pocono is a banked tri-oval. Is this happening on a banked section of track?

The Stig's former college room mate > Sweet Trav 07/21/2016 at 15:38

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this is the configuration we typically run at Pocono. We use NASCAR turns 1 and 2 (tunnel turn). Yes it happens on NASCAR 1. I feel the car should be much more planted in that corner with the amount of banking it has. I feel like it should be able to take it at 100+ but the car feels like it wants to get loose over 95. i’ve never actually had the tail step out, it just feels like i’m on the edge of grip.

Levitas > The Stig's former college room mate 07/21/2016 at 15:44

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Another aero engineer here (with the typical masters >_>) with some experience in automotive and high speed low drag streamlined motorcycles.

What Rock Bottom said is very good, so I’ll add/reiterate some thoughts:

Most likely the looseness is in face the tail lifting at high speed. This can be countered by adjusting suspension/tires, but since aero forces are the square of velocity, any changes in suspension/tires are quickly overpowered by aero again.

Again, the stock spoiler is most likely useless. The easiest way to find out is to go slow and ramp up at another track day. If FCA did engineering work on it, it does most likely just reduce drag.

Typically, adding a spoiler ala NASCAR does little to change lift/downforce, it usually helps form a bubble behind the rear window and pull flow down behind the rear of the vehicle to reduce drag. Very similar to pickup trucks and the bubble that forms in the bed.

Another thought that just popped into my head: what is the weight balance of the Charger? Too much weight rearward can cause the vehicle to be unstable as well and bring the rear end around.

If you’re looking for eliminating lift and increasing downforce, front lip and a real diffuser is your best way to go; instead of depending on a small area (like an STi wing) the whole vehicle becomes a wing.

If you’re near Indiana, Purdue has a great aero professor that I worked with on a land speed motorcycle; he’d love to talk aero on cars and can also make a semester long class project as Rock Bottom suggested.

If I get really bored next week, maybe I’ll trace a charger similar to yours into a CFD package and see what affects removing the wing does.

yamahog > The Stig's former college room mate 07/21/2016 at 16:26

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I just checked for my own curiosity and yea, not too much LA/LD stuff. One of my coworkers recently had to order a cheap ebay carbon fiber wing - it was in fact real carbon fiber, sturdy and functional. Looking at the 4th image on this listing:

http://www.ebay.com/itm/56-Inch-Un…

it seems like they give you a nice angle of attack adjustment, but I can’t tell if the actual mounting point is adjustable. Some CF suppliers mold the threads right in, I’m sure there’s a way to homebrew a solution.

ETA - or find a wing you really like, keep it on all the time, and start covering the back window in track stickers ;)

The Stig's former college room mate > Rock Bottom 07/21/2016 at 16:35

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I’m pretty sure the factory wing is supposed to emulate the “Go wing” from the late 60's and early 70's. there are many retro design cues on the car. The Hellcat has a very low spoiler, mounted pretty much flush to the trunk lid. That car has a 202mph top speed, so i’m sure aero was considered by the engineers. it is not a direct replacement for mine, but can be made to work. After reading people’s suggestions in this thread, and knowing how the car feels at high speed, i would offer and educated guess that the stock wing is neither producing lift or drag, but disrupting airflow off the back of the roof and creating a large amount of turbulence behind the car. In theory, simply removing the wing should reduce turbulence. but again, since i’ve bumped the horsepower up significantly, i’m also looking for more grip.

Vortex generators look too fast & furious to me. And any extreme aero i add to the car needs to be removable. As in the case of a tall wing. i don’t want to give the impression that i’m some poser that bought a wing off ebay to look cool. mostly because there’s a guy here with a honda civic that did exactly that and i like to make fun of him.

A gurney flap might be a way to go. that wouldn’t be hard to test and tune at a track day.

no wind testing facilities in my area. maybe this could be done in scale? i could probably fab something up with a model, some plexiglass and a leaf blower. hell, if mythbusters can make their own wind tunnel, so can i.

Sweet Trav > The Stig's former college room mate 07/21/2016 at 16:51

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Are you sure that the stability control light isn’t coming on due to the fact that on a banked corner/straight the abs wheel sensors are showing vastly different rotational speeds?

The Stig's former college room mate > yamahog 07/21/2016 at 16:55

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i already have a dozen track stickers on the back window!

The Stig's former college room mate > yamahog 07/21/2016 at 16:57

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Something like that wing could be easily modified to work.

The Stig's former college room mate > Sweet Trav 07/21/2016 at 17:12

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the difference in wheel speed should be negligible. even with the system turned completely off, it feels like it wants to get loose.

yamahog > The Stig's former college room mate 07/21/2016 at 17:25

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see! already legit!

Sweet Trav > The Stig's former college room mate 07/21/2016 at 17:55

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I was under the impression that only the SRT models received full TCC + Stability off.

Have you experienced this phenomenon on non banked tracks? My Monte Carlo gets light in the front at about 120. Rear always feels planted.

The Stig's former college room mate > Sweet Trav 07/22/2016 at 07:55

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even on the srt models you can’t completely turn off the esp/tcc. it’s mostly off, but still on. however, if you pull the 5amp mini fuse to the ebcm, it’s 100% off. i wired in a switch to that fuse that allows me to turn it on and off at will. does ridiculous burn outs with it off.

There aren’t any other tracks i visit with any turns that fast, so it’s hard to say. i do notice it a little on the tunnel turn at pocono, which is much flatter, but also much shorter, so i’m in and out of the turn quicker.

Rock Bottom > The Stig's former college room mate 07/22/2016 at 09:34

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You could certainly do some A to B testing at scale, but in order to get anything meaningful you’ll want to use an actual wind tunnel. I used to work at NASA Ames, where the Myth Busters hacks guys used to test and they’re generally using well calibrated government equipment! Most universities with aero programs have small scale tunnels with well understood aerodynamic performance, too.

I think your best course here is to spend a track day experimenting with Gurney Flaps. They’re cheap, effective, and easy to tune. You might even find that you want different flaps for different tracks. Just make sure you re-test any time you make changes to the front of the car, vehicle attitude, or ride height. It’s easy to change the lift balance and make a car go from happy to scary!