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The Staff of Motorsport Marketing
The Staff of Motorsport Marketing Writer
6/22/17 11:20 a.m.

story by steve stafford

Steve Stafford is an aerodynamic engineer with Toyota Racing Development, where his current beat covers the NASCAR Nationwide Series. He has done aero work at nearly all levels of motorsport, from autocross and club racing to Indy and the Rolex 24 At Daytona.

On a fall day in 2001, amid the greater concerns of society, I …

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sesto elemento
sesto elemento SuperDork
6/22/17 11:52 a.m.

More of this please!

WAKman New Reader
6/22/17 8:18 p.m.

Great article! I especially liked the photo analyses of various cars.

David S. Wallens
David S. Wallens Editorial Director
6/22/17 8:31 p.m.

Glad to hear that you're enjoying these cool articles from past issues.

Fitzauto Dork
6/22/17 10:37 p.m.

Awesome article! Gonna go,mix up some test paint now.

David S. Wallens
David S. Wallens Editorial Director
6/26/17 8:34 p.m.
Fitzauto wrote: Awesome article! Gonna go,mix up some test paint now.

So, any data yet?

stafford1500 GRM+ Memberand HalfDork
6/27/17 1:49 p.m.

In reply to Fitzauto:

If you run into any issues with your chosen flow viz fluid let me know and I will try to help troubleshoot it.

DjGreggieP Reader
10/30/17 3:32 p.m.

I can foresee bookmarking this article at home so I can read it over many, many times so I can start playing around with aerodynamics in the spring AFTER I get in more seat time at the track so I have a good base line.

Thank you!!

stafford1500 GRM+ Memberand HalfDork
10/31/17 7:15 a.m.

In reply to DjGreggieP :

Feel free to post up any questions you may have.

Same goes for anyone else with aero questions.

10/31/17 9:29 a.m.

We had a couple mails a year or so ago then I had a heart attack that slowed down my project. I tried to PM you a couple times when this article came out to get your opinion on an underbody splitter/under tray/diffuser combination I've been working on. Unfortunately the PM system doesn't seem to work well for me and mail was returned. So I've been building a simpler version of the design for initial testing.

I've made quite a few aerodynamic modifications and most of my questions are related to what goes on under the car. What I'm doing is somewhat unconventional and more about me trying different things and learning.  No rules to stick to and I'm not trying to "win" anything.  If I had a race car running in a certain class I'd hire an aerodynamicist with the capability to run CFD programs etc.  rather than guessing and using some simple math.

Maybe I should start a different thread and show the modifications already done? Then continue as I finish.

Subjects I'm interested in are splitter, front diffuser, side skirts, controlling tire squirt, contoured trays, tunnels, and tripping the air under the car.

Everybody likes pics so here's what I'm working on now. Cardboard mock ups of diffuser are oversize so I can cut them to size when I put the car back on the ground for fitment before building the actual diffuser for testing. The area ratio at 3" ground clearance is 7.3 so I know they're too big to function properly as is and when I cut down to size I'll make sure they're under 5.








fearlesfil New Reader
10/31/17 2:01 p.m.

Great stuff, thanks!

stafford1500 GRM+ Memberand HalfDork
11/1/17 3:54 p.m.

In reply to NOT A TA :

I do recall the questions  few years back. The mock-up underbody looks great. Keep in mind you may want to roll the edges around the tire cutout a little bit to help evacuate the wheel tubs. And, don't forget to figure out where you are going to get some cooling air into and out of the underbody, so you don't cook the driveline and wiring.

I will try to PM you so you can ask your questions directly.

GTXVette Dork
11/2/17 6:07 a.m.

Un fortunately I missed the show,

But Baseball,  

 then when the Astro's Put up 5 I fell asleep (Happy for them)

My Brick will Likely exceed the Speed Limit Do to HP, but S M O O T H L Y would be Better     Sorry I missed It.

Daeldalus New Reader
11/2/17 6:38 a.m.

I was going to start a new thread but this seems like a better place to ask.


I heard last week that a properly designed rear spoiler (Not a wing) will both reduce drag and increase downforce. The increased downforce is easy to visualize but how it reduces drag isn't easy to see. The article I read claims that a good spoiler creates a pocket of stagnant air the effectively changes the shape of the car as far as airflow is concerned, as the flow will now follow a line from the top of the roof to the top of the spoiler and not dip down near the deck lid.

Is that actually what happens with a spoiler?

stafford1500 GRM+ Memberand HalfDork
11/2/17 8:17 a.m.

In reply to Daeldalus :

Generally, when you make the air do something it does not want to do, it creates drag. That would apply to stoppping it from spilling around the back of the car and swirling in the turbulent pocket behind the car.   

I can imagine a few situations where adding a spoiler may reduce drag, but they are generally due to badly designed rear bodywork to start with. If the rear spoiler manages to limit how big the wake behind the car is, then the overall drag will very likely be reduced. However the drag on the spoiler itself may overcome the reduction of the drag from the wake.  

Also, if the spoiler is reducing lift (at the rear in your example) and getting closer to zero lift/downforce then the effective drag could be reduced. This is because you have reduced how much lift the air is applying to the car and that should reduce the drag effect from the air working on the car.  

All aerodynamic drag is a function of how much the air gets pushed around by the body of the car and any stuff attached to it. Short stubby things hanging out in the breeze are very draggy, but may present the air better to things downstream.

Daeldalus New Reader
11/2/17 8:32 a.m.

In reply to stafford1500 :

the article actually did talk about the spoiler directing the air at less of a downward angle than having no spoiler in order to reduce lift. maybe that's what they were talking about. they also did say that this applied to a well designed spoiler, maybe that meant a spoiler designed to cancel out rear lift?

stafford1500 GRM+ Memberand HalfDork
11/2/17 9:00 a.m.

That sounds like a pretty good assumption. I have seen production cars in the tunnel that were modified to go from severe lift at the rear (like nearly the static rear weight, basically unloading the rear suspension) to downforce of roughly 50% of the static rear weight with simple underbody pans.

Daeldalus New Reader
11/2/17 9:44 a.m.

next topic I have been wondering over.

Roof spoilers.

The way I heard them explained is that they cause the air that would normally come straight off the roof line to follow the line of the rear glass more closely. which then makes the low pressure pocket of air behind the car smaller thus reducing overall drag on the car. BMW claims that the extra bump at the back of the roof of e30 m3's is for exactly that purpose.

I see that some STI's come with them factory.

It seems to me that they may more likely be for causing more air to go across the downforce designed rear wings on those cars.

to be clear I am not talking about those shelf-looking roof spoilers that some aftermarket companies make for cosmetic purposes.

stafford1500 GRM+ Memberand HalfDork
11/2/17 10:01 a.m.

I think you are referring to the small vertical fences on the rear of the roof. Those would be vortex generators. The idea behind these devices is to get some of the higher speed air away from the surface to swirl down next to the surface and re-energize it. That allows it to turn around curves and corners more easily.

With respect to rear wings, the roof vortex generators do help the air turn down the rear glass, which effectively changes the angle of attack of the rear wing (increases it) giving more downforce (as long as you are not near stall). However, vortex generators are notoriously draggy. They are short/stubby items AND they induce a lot of swirl which requires a lot of energy. By themselves vortex generators are bad, but as device to control the flow to other aero devices they can be good.

Daeldalus New Reader
11/2/17 10:43 a.m.

The STI spoiler I was talking about is this style. Not the one that looks like a diffuser.

Daeldalus New Reader
11/2/17 10:49 a.m.

This is the e30 m3 one

They claim that this spoiler is one of the key differences that lower the e30 drag coefficient from .38 to .33 for the m3

mazdeuce - Seth
mazdeuce - Seth MegaDork
11/2/17 10:52 a.m.

How do the big flat drag spoilers work in comparison to the upright ones we see on "normal" cars? Are they trying to accomplish different things? 

11/2/17 11:42 a.m.

Got your PM Steve, e-mail sent.

For those thinking about the cooling issues Steve mentioned, the sides between the tray and car body are open and the notched section in the middle of the rear diffuser will also allow air from between the tray and body out.

stafford1500 GRM+ Memberand HalfDork
11/2/17 1:17 p.m.

In reply to Daeldalus :

The STI one does just turn the air down the rear glass, but without adding lots of swirl like the VG's I mentioned. The e30 one looks like it generates a sharp change in shape, which could be helping to reduce the radius to the back glass and pull the flow down toward the decklid.

stafford1500 GRM+ Memberand HalfDork
11/2/17 1:19 p.m.

In reply to mazdeuce - Seth :

The flatter spoilers are effectively moving the wake down and back. The sharp trailing edge gives definition to where the wake begins and since the wake pressure is below that extended panel, the low pressure in the wake helps to pull the tail down. It's all about managing the pressure differences on a local level.

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