Wing Vs. Diffuser help needed

[bryancohnracing]

I am working with a driver that racers with NASA and he has the option of going with a wing or flat bottom/diffuser. The way the cars are classed he pays "points" for different modifications so while we could do both, doing so would cost too many points and bump him up to the next class.

More details:
2005 Mazdaspeed MX5 Turbo.
300 hp
2610 minimum weight (w/driver)
225/45/ZR15 DoT Hoosier R6 (or A6 for qualies)
Hardtop with lexan windows
Currently no aero, but a front undertray to the centerline of the front wheels will be added (and is free, no point mod).
An adjustable front splitter is being built as well.

So, the question is which is better? Flat Floor with small skirts and a diffuser or a rear wing?
Wing can be 8 inches above the roof line, so clean air is no problem. Drag is the main concern as the championships are at Miller in 2010 and straight line speed is key.
Diffuser has is low drag, but not adjustable and can't be changed.

The goal is to add downforce for high speed corners like Mid Ohio turn 1, or Miller, the end of back straight sweeper before Attitudes. These are examples of the kind of corners where we want to add grip.

Note: Tire size cannot be increased without drastic changes, i.e. fender flares, new wide wheels, etc. We want to stay with a narrow/skinny car.

Richard? Steve? Anyone?

Thanks for any and all help!

[glenn cooper]

that in order for a difuser to work properly it had to be somewhat close to the track surface - no problem on a formula car w 1" of ground clearance, a whole 'nother story w/ a Mi-Otter at what, 6 plus inches of ground clearance?

Seems the rear wing would be the way to go for this application.

PS - Not that it's the be-all/end-all, but what are the other guys in similar cars doing?

GC

[bryancohnracing]

The car has 5 inches of ride height and I figure I can get skirts/vanes/sides of the flat floor/diffuser around 3 inches off the ground and not damage them through normal use.

99% of the aero on NASA cars is done by the "it looks cool and others are/aren't doing it so we must have an advantage" method. I've yet to see anyone use a wicker for example. Most guys make large changes, i.e from say 1 degree to 3-4 degrees. Yeah, nothing like seeing a 250 hp M3 with 4 degrees of attack on the rear wing....

I'd guess that most cars have more wing than needed without enough knowledge to now how to use them, along with parts (wings, end plates, struts, etc) that are not efficient or even assembled correctly to be of much use. Its common to see a main cord that is say 6 long, exposed fasteners, minimal thickness, huge gap to the upper adjustable flap, etc etc etc.

At least I know enough to know what I don't know, and know what I do!

My thought is to hang a single element wing about roof height mounted by end plates with simple slots for adjustment and an assortment of wickers for fine tuning. (if we go the wing route)

For a diffuser I'd have to find something to adapt or go custom.

[RobLav]

I'd make a custom diffuser that is adjustable per the desired top speed / track conditions with an eye towards moving the center of pressure as far forward as I can.

The rear wing will add some downforce to the rear, which might result in the front not getting the same downforce as the rear, especially with a rear wing cantelevered behind the rear wheels. In this case, the car would tend to plow at higher speeds.

Make the rear of the diffuser in 2 or 3 sections to allow removal / changing the exit airflow under the car to either add downforce or reduce drag. The diffuser will need a good fence all around it on the sides as close to the ground as possible. It'd be nice to make the fence collapseable (sp?).

So I guess my idea differs from the Coopster.

[bryancohnracing]

One thing I have to keep in mind is that the owner/driver does not have the knowledge/skill to make big changes. We'll arrive at good all-round set up and run that till Nationals, where I can devote time to engineering for him. Not that he can't make changes, but if it gets too complicated we'll lose him for sure.

He has the $$, pretty decent driving skills and desire. What is missing is the mechanical skill/knowledge.
In other words, when I'm not there to twist the wrench (which is most of the time since my job as Race Director keeps me busy) he;ll be calling me asking how/what to do...

[Rick Kirchner]

There's a lot of good info on this topic over the years in Racecar Engineering but I can't point to a single article. As Rob stated, in many cases (like the COT) the car is dominated by front end downforce and the wing is used to achieve the desired balance.

A while back there was an article about the 350Z in a touring car series. Turns out it had handling problems because the rear overhang was short and the diffuser length was set by max rear overhang, so they could not build an effective diffuser.

Diffusers tend to be "free" downforce, in that you are not adding profile drag (although there is still induced drag). Adding the flat pan to make the diffuser work would do a lot for drag reduction anyway.

What do the rules state regarding the forward position of the wing? If it can be 8" above the roof, mount it as far forward as possible, like a sprint car wing!

[bryancohnracing]

Height rule is all there is. Wing can be placed as far back or far forward as desired. I've seen one other Miata built to this level (300 hp/2100 pounds) with a wing set back about 1.5 FEET from the rear bumper and at the full 8 inches above the roof. Serious ingenuity going on with that car....

The rules are really pretty open, in that everything you can think of is on the table, as long as your willing to take the points for the mod.

I like the opportunity the rule offer. Think Can Am for tin tops. For reference this will be a $100,000 Miata when done.......

[R. Pare]

Depending on what you have to swallow for "points" and not in any particular order of importance:

1 - Diffuser

2 - Skirts (flexible and easily replaceable so that you can get them closer to the ground)

3 - Full smooth bellypan

4 - Wheel well vents

5 - Gurneys at rear edge of roof as necessary for attached flow across the rear deck

6 - Engine compartment ventilation

7- Rear light valance ventilation

[JBlock]

Okay, I’ll take the bait.

Having been in racing for 40 years now I can tell you with a fair amount of certainty that aero is a lot like religion. You will evoke strong and heated exchanges over different beliefs. And if you truly believe it will work then you will go faster (special thanks to the folks at the Hawthorne Plant).

Now, let me put on my Engineer’s hat and give you the scoop. Step back and look at the big picture goals. If speed is key then drag is paramount. On the other hand you also want down force, which might be a problem because drag is a byproduct of down force. So, maybe the goals need evaluating.

What is the real problem you are trying to overcome? Do you have an actual understeer or oversteer situation. Adding a wing will no doubt increase the rear loading, but if you have an understeer condition you just made it worse.

If areo is such an itch you just have to scratch and you really want it to be an improvement then getting opinions and selecting one than meets these goals is likely worse than Powerball odds.

To do it correctly you need some basic aero mapping. Heck, you might be able to find the golden nugget with just the right rake and few little non noticeable mods. The point is that aero is highly interactive and you can make it worse by just adding something on faith.

If money is truly not an issue then I would suggest getting an Engineer (I can recommend a few) and really do it right. On the other hand if performance is the goal, then I would suggest getting someone really good with data acquisition to help and find bigger improvements in lap time, than aero bits blindly added to the car.

[Dick R.]

Do "you guys that actually understand this stuff and race" need to know anything about the types of cars which the Miata will be racing against to help determine priorities? How about being able to have a qualifying setup versus an ability to pass in the race with easy conversion between the two? Is he likely to be held up in corners in the likely traffic so he needs to 1)qualify high and 2) be able to pass back markers during the race?. What is the level of competition? Are off road excursions due to bumping, avoidance, etc. likely so "durability" during a race is important?

Dick

[Matt King]

Anyone willing spend $100K on a Miata ought to invest a few thousand in some wind tunnel testing and see what REALLY works!

[Rick Kirchner]

Do either of you have McBeath's or Katz's books on racecar aerodynamics? McBeath's in particular has a lot on touring car aero.

Mr Block's comments are spot on, but so are the Dicks' (sorry, being one I couldn't resist ).

The firewall presents a big flat wall to the air - you have to get it out somehow and most designs have it exit under the car - might be good, might be bad, but the wheel vents take it out the sides. If you have an air dam in the front the underhood air will naturally head for the low pressure you lovingly tried to create on the bottom unless you can entice it to go somewhere else. A friend of mine with a 911 saw some improvement with a row of holes across the bottom of the rear window like Richard suggests - air inside the car packs up back there like a big parachute. Another friend with a Z3 just left the gasket off the bottom of his hardtop - a "cheat" that was harder to notice than a bunch of holes.

And at the end of the day, any aero on a car where "rubbin is racin" is wholly acceptable might as well not bother if it gets bent up and the car is worse than without anything at all.

Of course you are hoping to find things that singly can be seen on the tach, given all the other vagaries of amateur drivers, track changes, tire wear, etc, but a lot of times aero is a bunch of little things adding up to something that an amateur can feel in total, but you're likely to need DAQ and a pro to see them individually.

[bryancohnracing]

Let's see, where to begin...

Miata's by nature have very little air in the cockpit. We run the vent windows and no outside mirrors. Holes in the rear window are allowed but cost points. Vortex Generators on the rear edge of the roof are part of the plan. Removing the seal is a damn good idea. There is a firewall closing off the fuel tank/cell from the drivers compartment (similar to what SCCA prod cars can do).

I had and have read Katz's book. Loaned it out about 2 years ago and it never found its way home.

The car has a slight loose condition from mid corner to exit in high speed corners that so far has not been able to be dialed out with shocks/bars. The car has Afco's double's, being revalved to Jesse Prather's specs. (he and John Buck, Chief Engineer at Afco designed them).

Current competition consists of:C4-C5-C6 Corvette's, 997 CS, M3's, etc. It all depends on the mods one does to their car, as cars are base classed as stock and then mods add points to find the final class. Its a bit of a knife to a gun fight, but at 300hp/2610pounds is competitive as it sits. We have some points to use hence the reason to go to aero as there are no suspension or tuning mod points left on the table.

Thanks for the info and ideas. All good stuff and though provoking.

[Larry]

It's about balance! No sense putting a big, honkin' rear wing on that thing, plasterning the rear tires to the pavement if the front tires can't match them! What does the car do now? Fix whatever gross, evil characteristics it has, and then work on fine-tuning.

...or does the guy just want to have a sexy-looking, hot Miata?

Larry Oliver

[LAJ]

Rear wing !!
A diffuser will do NOTHING ,I repeat NOTHING on a production car !! maybe smooth out some air flow but not enough that you could even measure it. Lots of sports racers have to do a LOT of work to get air to the top back of the car to help the diffuser work SOME JUST LOOK AT ALMS cars!! That should give you a clue!!
A diffuser has only a small effect even on a sports 2000 and they are as close to the ground as a formula car.
the reason is a diffuser needs air going over the top of it (more critical than the bottom)to help create air pressure difference . I find it very funny that a lot of formula cars bend there exhaust up and away from the top of the diffuser instead of wrapping it and laying(small megaphone) it right at the top of the end of the diffuser.This will make the diffuse work a lot better!!! Jag did this on there LeMans car and it worked great!!

go with the rear wing!!
Lee

[George Main]

I am working with a driver that racers with NASA and he has the option of going with a wing or flat bottom/diffuser. The way the cars are classed he pays "points" for different modifications so while we could do both, doing so would cost too many points and bump him up to the next class.

More details:
2005 Mazdaspeed MX5 Turbo.
300 hp
2610 minimum weight (w/driver)
225/45/ZR15 DoT Hoosier R6 (or A6 for qualies)
Hardtop with lexan windows
Currently no aero, but a front undertray to the centerline of the front wheels will be added (and is free, no point mod).
An adjustable front splitter is being built as well.

So, the question is which is better? Flat Floor with small skirts and a diffuser or a rear wing?
Wing can be 8 inches above the roof line, so clean air is no problem. Drag is the main concern as the championships are at Miller in 2010 and straight line speed is key.
Diffuser has is low drag, but not adjustable and can't be changed.

The goal is to add downforce for high speed corners like Mid Ohio turn 1, or Miller, the end of back straight sweeper before Attitudes. These are examples of the kind of corners where we want to add grip.

Note: Tire size cannot be increased without drastic changes, i.e. fender flares, new wide wheels, etc. We want to stay with a narrow/skinny car.

Richard? Steve? Anyone?

Thanks for any and all help!

The biggest problem in a sedan is Lift under the car. Keeping air out is of the highest importance. In an "Ideal" situation"
, an air dam that reaches the ground is best,

next skirts that also reach the ground.
Next a full flat bottom with a diffuser, as close to the ground as possible (at certain height from the ground it will do nothing no matter how it's designed)

Next would be to prevent the wheels from pumping air into the engine compartment and the diff area. Louvres on the hood above the front wheels extended into part of the engine area to relieve the high pressures getting into these compartments.

Next Rear wing...


Just adding a "well designed" rear wing without doing any one of the above will open a new cans of worms for your car, though will warrant the highest number of downforce at the wing provided it is as high as possible from the trunk lid (preferable above the roof line).
The wing will cause downforce that will pitch the car rearward at speed, allowing more air to get under the car.. leading to huge amounts of understeer.
So if you put a wing on it, you will need to add "blockage" to the front (full air dam, better with a splitter on it of 4-5" out the front.), preventing more air from getting under the car and creating more lift.

Just remember that of all the devices you will add, only one will add downforce, the wing. The rest will be anti-lift (air dams, spoilers, diffusers) or air flowing smoothing devices (flat bottom), helping prevent additional high pressure areas from forming. The added devices will allow a wing to increase it's downforce by reducing the lift.

The simple answer is downforce, and the wing. if your going to select only one, adding the rest will greatly increase the wings performance through their anti lift properties or reduce the amount of lift you have now.