Questions on the Aero article

I have 2 questions on the aero article in the new issue.

The gray sidebar says that " a wing can be used as an extension of the underbody tunnels." How exactly do you do that?

I was surprised to see that the front wing generates the least drag. The article mentioned a wing in the middle of the car. On a Locost that's a distinct possibility as the roll bar is in the cleanest air and is reasonably close to the middle. It seems that would generate the most balanced force as compared to a front or mid wing. Presuming that there wasn't a pressing need (oh, I made a funny) for downforce on either the front or the rear for balance, what would be the negatives from a mid wing?

I'm looking for answers from people who know, not more guesses to mess up the collective wisdom.

Now next month you need to follow up with an expansion of the smooth bottom data and diffusers. As I understand it the diffuser generates the greatest downforce at the point of highest speed of the air.

carguy123 wrote: ... The gray sidebar says that " a wing can be used as an extension of the underbody tunnels." How exactly do you do that?

The wing generates downforce by creating low pressure on its underside. If the wing is at the rear of the car, that additional low pressure below the wing pulls more air out the back of the tunnel/diffuser. It can indeed effectively extend the diffuser out past the rear of the car.

The Katz book, "Race Car Aerodynamics" has some good information on this.

So the wing and the diffuser work in tandem? Which needs to be the furtherest back? I'm guessing the wing if the diffuser helps the wing.

As far as flat underbody, tunnels and diffusers I know that at least one Ferrari model uses air from the front and somehow channels it to give downforce to the front wheels. I'd really like to see how much a flat underbody &/or diffusers can add to reduce drag &/or add downforce.

Not exactly related to your question, but for a better understanding of aero at autox speeds(vs. race track) look to R/C aircraft for some detailed studies.

carguy123 wrote: So the wing and the diffuser work in tandem? Which needs to be the furtherest back? I'm guessing the wing if the diffuser helps the wing.

Correct, the wing and diffuser when designed correctly will work together. As far as placement goes, its very dependent on the geometry of ....everything.

carguy 123,

The wing mounted at the back of the car (in order to extend the diffuser) requires that the wing be mounted reasonably close to the body (1" ish) and that the front edge of the wing only overlap the back of the body a little (also 1" ish). This turns the wing into a flap for the body of the car, which if fitted with a diffuser, can be considered a wing. Big end plates will also help this extension effect.

Front mounted wings generate the least amount of drag due to the fact that they are generally closer to the ground and can generate large levels of downforce in ground effect, which means they dont have to run at extreme angles (lower drag).

Mid mounted wings (and high mounted too) are very good for consistent distribution of downforce since they do not see ground effect and can be located to balance the load as needed. Check out some winged sprint cars to get an idea.

Diffusers: The lowest pressure does in fact occur at the highest speed location on an underbody. On the CART cars the highest speed occured about 12" behind the side pod leading edges, not at the start of the diffuser. The flow under the car wants to see smooth surfaces and gentle transistions. The diffuser can be too big. If the diffuser expands too quickly the air can seperate (stall) cause loss of downforce and increased drag. Just like the splitter choking and causing porpising, the diffuser entry can do the same thing if the car gets too low to allow enough flow to get to the diffuser.

Thanks, Steve Stafford (author of the article in question) p.s. the Katz book does a very good job of describing the ideas... nice guy too

Thanx Steve, that answered my questions and then generated a few more. I'll try to find Katz' book today.

I'm trying to incorporate some basic aero/downforce into the design of a street/HPDE/autocross Locosty type vehicle. It is homemade & won't really look like a Locost so maybe we ought to call it a GRMy vehicle. Woot! I just invented a new type of car! Partay! Partay!

I am at the design stage for the bottom of the vehicle and it's just as easy to design a good bottom as a poor one - well at least the physical construction part is just as easy. With a flat & level ride height I am able to design into it about a 2 degree rise along the full bottom of the car front to rear. The construction gives me a full, smooth underbody that also has side curtains or dividers whatever you want to call them to break the flow up into 4 different channels of air that will lead into the what is the real workhorse diffuser(s).

Gentle transitions I'm told include a diffuser angle of not more than 8 -12 degrees so that you don't get as much separation and drag. Is that factual information?

Let me say I realize that I can't/don't want to try to develop a full ground force dependent car for the street as there is just too much suspension movement and unequal roads to be able to utilize that fully and therefore could actually be dangerous due to varying levels of grip developed. Then there's also the fact that I don't know enough nor do I have the wind tunnel resources that would allow me to develop one either. BUT it seems a real waste not to try to incorporate some of what we know about the benefits of aero and the floor board seems to be the best place to work on flow & downforce.

I've got to find some CART car info to see exactly what it is you are referring to about the "leading edge of the side pod" because that's exactly the spot I've been scratching my head over. 2 of the 4 channels of air I've been able to develop are in what would be called a side pod on a real Locost and I've been trying to figure out what to do with that air to make it work for me. The rear tires get in the way of a smooth exit.

IIRC Ferrari used tunnels that generated downforce on the front as well as a diffuser to generate downforce at the rear. I don't remember where they exited the front air. I guess in theory they could have used a "pinch" in the air flow to generate the downforce and then dumped it into the flow towards the rear?

Great article Steve. I found the info. easy to understand and retain.

Awesome article, here's a suggestion for a follow up.

Start with an ST type vehicle and set a base autocross time, then start adding aero mods and look for a change times and or characteristics.
1. Front air damn
2. Rear spoiler or large lip
3. Bigassmotherberkeleying rear wing.
4. Splitter and canards to go with the air damn.
5. undertray.
6. etc

I realize an all in one day test is far from ideal, but it would be really interesting to see what real world results.

carguy123,

You are right about building a "good" underbody vs a "bad" one. Both are easy at the point you are at now. The 2degree main floor and an 8 to 12 degree diffuser are in the right direction. Keep in mind you can determine the main floor angle at speed (if you figure out the wheel travels), by diving the wheel base by 60" (that gives rougly 1degree per inch of travel difference front to rear). It sounds like you are going to use full length stakes(dividers). I would suggest using the ones most outboard first and do some flow viz to find out if you need to add any to the inside. You might even want to try tapered strakes from about mid car to the back (0 height to full height). The Side pods of open wheel cars are the bits either side of the driver that hold the radiators. The botom of these on CART and IRL cars is shaped to allow air to flow under the front and then "pinch" just behind the leading edge (a few inches to a foot). The air is then pulled back toward the center of the car as the diffuser trys to draw more air in. There are several 'modern' car that use diffusers at both ends. The air under a car generally starts to flow outward as it approaches the front wheels, then runs parallel to the sides before starting to pull back to center around the rear wheels. The direction is important, as I mentioned in the article, because the car is a 3D body and you have to try to manipulate the air to get your downforce out of it. As for wind tunnels, the interstates up near Texas Motor Speedway and Alliance Airport are pretty straight and flat!!!

Poopshovel, Thanks for the support and comments. There is a lot to say about the issue, but it was an article, not a book.

Adrian_Thompson, I have been watching for feedback on the article before I call David to see if there is another article out there... Some pretty simple test methods for big parts were discussed between myself and David.

Wonkothesane wrote: Steve - as an areo noob, I thought it was a great article. I'd love to see a continuance of it as Adrian said.. You've given us an overview of the "why" things work, now a bit more direction on some of the "how" (We found on this STi that changing the rear wing angle by 2* dropped our time by X.. Here's why it did that on THIS vehicle")

Agreed, some form of testing or documented data collection would rule!!! Splitter height, splitter length, wing height, length, position fwd or bkwd on the car....

You can also use vortex generators to prevent the air from stalling on a surface. Usefull and easy.

This is excellent infor for track/HPDE cars, but seems kind of pointless for autocross. Most cars can't go fast enough to be able to decern the gain/loss over the driver's lap time variations.

Poopshovel, Thanks for the support and comments. There is a lot to say about the issue, but it was an article, not a book.

Thanks for keeping that in mind while writing it. My brother has an aerospace degree. I did not inherit a substantial portion of that side of the brain.

stafford1500 wrote: carguy123, It sounds like you are going to use full length stakes(dividers). I would suggest using the ones most outboard first and do some flow viz to find out if you need to add any to the inside. You might even want to try tapered strakes from about mid car to the back (0 height to full height).

My stakes aren't add ons, but are actually structural components of the chassis. Instead of using 1' x 1' tubes as is common in a Locost my bottommost components are 1.5" X 3" (almost a ladder frame looking structure) so I have a 17" wide sidepod with a 3" deep area that allows me to taper the floor from front to rear. I do want to add a rubber skirt on the outside edge to help keep some of the turbulence out.

I loved your paint and oil method of checking flow. I'd done a bunch of work with yarn and you have to have a video going full time or be driving next to the car and have a good memory to be able to use the info.

stafford1500 wrote: The Side pods of open wheel cars are the bits either side of the driver that hold the radiators. The bottom of these on CART and IRL cars is shaped to allow air to flow under the front and then "pinch" just behind the leading edge (a few inches to a foot). The air is then pulled back toward the center of the car as the diffuser trys to draw more air in.

So the side pods you were referring to are more towards the rear tires? (I still haven't had a moment to google CART cars, but that will change once I get thru moving my mother in law) My "sidepods" stretch from behind the front tire to the rear tire. They would only be considered sidepods to someone building a Locost and they only look like a sidepod from underneath. My construction is basically a Locost type frame with an extra compartment on both sides to make up the extra width. I'm glad to hear about that "pinch" because that's the only solution I'd come up with and I wasn't sure it would work.

As far as the dividers of the inner 2 tunnels, it's a removable aero shaped cover that covers the oil pan and drive train back to the rear diff. I don't believe the turbulence introduced by those elements would be an option. The shape of the cover is a variable.

stafford1500 wrote: As for wind tunnels, the interstates up near Texas Motor Speedway and Alliance Airport are pretty straight and flat!!!

So are you from around here? If so you also know those wind tunnels are manned by the police dept of Northlake. I've got better wind tunnels on the west side of Fort Worth that don't have those pesky patrol cars.

Glad you guys liked it. We have known Steve Stafford since he went to school in Daytona, was part of our local sports car club and drove a rather rough Triumph Spitfire.

I like the follow up article idea on some real world testing and have asked David and Per to make that happen.

Steve, are you up for some real world testing. We could use our MINI or Tommy's BMW. He would probably dig that.

Liked the article as well, very informative.

On the subject of real world testing: it would be nice to know how to figure downforce without load cells etc. IIRC the olde skool way was to attach a rod to the suspension with markings, run it through a hole in the body or against some other fixed reference point on the chassis, then note the distance it stuck through the hole with various mods. Like, at 50 MPH if the rod stuck about 1/4" further out with Wing B as compared to Wing A then that told you Wing B had more downforce (at least at that particular speed).

A lot can be done with the air inlet at the front. I just read where Chevrolet on the Cruze will have cutoff plates to limit flow above something like 37mph.

kb58 wrote: This is excellent infor for track/HPDE cars, but seems kind of pointless for autocross. Most cars can't go fast enough to be able to decern the gain/loss over the driver's lap time variations.

The argument I've heard for aero on an autox car is that it is a way to dynamically change the handling over the range of speed. What you can do is set the car up to be looser than you normally would, and then add an effective rear wing. This makes the car faster through tight stuff, since loose = fast, but then adds some stability once the wing stats contributing, so that it is more stable in high speed offsets and slaloms.

Whether that pans out, I have no idea, but it's the argument I've heard that makes sense.

kb58 wrote: This is excellent infor for track/HPDE cars, but seems kind of pointless for autocross. Most cars can't go fast enough to be able to decern the gain/loss over the driver's lap time variations.

So in the unlimited class, or whatever they call it, those huge wings are just for grins?

It has been a few days since I have had time to respond...

But a few notes to all the questions: Autocross speeds are enough to generate aero forces, but the forces are low, that is why the A-mod type cars run such big wings (more force thru greater wing size)

Downforce can be determined without load cells (reasonably well) as described in the article, by determining the change in shock/damper travel. It needs to be tested in a controlled evinornment however, not on track in race conditions.

Component testing will generate specific data, but it will only apply to the car (or type of car) being tested. In addition, each componet added to a car affects all of the others, so the combination has to be kept in mind too.

Vortex generators do help keep flow attached, but can be draggy, depending on thier location. Don't use them where the air is reasonably attached, just before the speration occurs.

Tim, thanks for the invite. looking forward to it...

Thanks all for the input and questions

so what do you think of these: www.airtabs.com

?

As an aside to the more serious aero stuff being discussed, I'd love to see some real-world evaluation of some of the low-buck stuff that's been tried by the ecomodder crowd. There has to be some cross-over to the more high performance-oriented people here.

There's a good thread going on over on Miata.net about autocross areo. Some one has done some simple modeling based on a 2D side view of a Miata with links to the results on Youtube. I don't normaly cross link threads between different forums, but this one is really interesting.

http://forum.miata.net/vb/showthread.php?p=4455135#post4455135

I was really really happy to see that article. One of my pet peeves is when people freak out over aero mods on an AutoX car.

Between "autoX is too slow for areo mods to work" and "OMG now I am going to have to put my car in a wind tunnel to be competitive" there is a lot of misinformation and unnecessary freak outs, out there.

I really liked that some simple means of testing were articulated.