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Jim,
Sorry for the lingo. I go through a few of the terms and define them.
Crr = Coefficient of rolling resistance - Works just like a Cd for mechanical drag (tires, gears, bearings, etc.) except that the drag here is only proportional to velocity NOT velocity squared.
rho = density of air (~1.2 or 1.3 kg/m^3)
Quadratic Equation = Equation of the form: Y = a*X^2 + b*X + c
Acutally if you can just get me a set of speed versus time data for your car as it slows down in neutral, I can do a quick excel and send it to you so can see how I would do it.
I can tell you have been reading up on aerodynamics. I guess I'll elaborate on what you mentioned.
The back of the body is key to reducing drag. If any signifigant separation of the airflow from the body occurs the drag is going to go up as a result. From Jim's CFD work you can see very signifigant separation of the flow behind the top on the S2000. (I'm acutally suprized it's that bad with the top up.) The front is acutally important, as it can help with one big thing, keeping the boundary layer laminar (non-turbulent) for as long as possible. A laminar boundary layer has much less drag than a turbulent one.
As to why we don't have full bellypans on our road cars, that's an easy one. Three big reasons:
1) Maintence. If covers were in place, things like fluid changes would be much more difficult and much more expensive. (Price out an oil change for a Mercedes 300 SL Gullwing sometime. Last time I heard a number it was $500 in labor alone to remove the belly pan and put it back.)
2) Heat. The air flow over the exaust pipes (esp. the catalytic converter), oil pan and other hot parts help keep them cool. Also the bottom is a very convenient place to direct the hot air that comes out of the radiator/condenser assembly.
3) Not much gain. Most car designers know they need to watch their Cd. If they don't their engineers remind them of it. They try to keep the bottom of the car relatively smooth as is. (S2000's already have a partial pan at the front, and the floorboards work that way too.) If they keep all the 'bumps' inside the boundary layer the drag penalty is isn't too bad.
My thoughts on wheel fairings. In my experience, cover as much of the wheel as possible as tightly as possible, without risking either ground or tire contact when the suspension travels. The fairing should also keep air from entering from the inside of the wheel (suspension side) as well. The spokes of wheels tend to ask a pump impellers if not shielded. One downside to this is cooling the brakes is a much more complex proposition. However even a simple fairing will get most of the benefits of a more complex one.
No offense taken, no worries. I'm going on your points one by one below.
-First and foremost is the underbody. A flat underbody would help a lot. So would lowering the car.
* Agree on flat underbody, disagree on lowering the car. We're at 4.5 inches now. If we any lower the boundary layer of the car will be interacting with the ground and big aerodyanamic penalty will result. (Goro Tamai's book (The Leading Edge) suggests that for a flat bottom car the minimum ground clearance for no interaction at highway speeds is about 6 inches.) Lowering the car should help with downforce though!
-Skinny wheels a tires.
*Agree, good way to cut stagnation and mass
-Taping gaps
*Agree, especially near the front of the car. I don't think it is as important after the boundary layer goes turbulent, say three feet from the front.
-A smoother front end with smaller cooling orifices (and no useless ones)
*Spot on, we don't need as big of a radiatior inlet at 150 as we do at 15.
-A rear underbody diffuser would help at very high speeds (and looks sweet at any speed)
*Agree that it could look sweet, and help with downforce at speed, but disagree on a Cd reduction. (Downforce = drag)
-A mooncraft-style hardtop.
* Absolutely, but we would have to watch for lift effects on the attached flow. (This is why Crossfires, Audi TTs and 911's need spoilers at high speeds!)
-A subtle rear lip spoiler could reduce drag slightly.
* Not sure on this one, the theory is sound (clean, no eddy separation off the trunk), but I'm not sure how well it could be implemented in the low pressure area above the trunk.
-Smaller mirrors
* It would depend heavily on the shape. The interactions between the main body and the mirrors is hard to predict... Hucho spends an entire chapter trying this one and still ends up with 'Try it in the tunnel and see what you get'.
I'm not sure what you mean by "tire data". The tires are high speed specials, rated to 400mph, i.e., only about 1mm to 2mm of rubber, no tread, and inflated rock hard to 70 to 90 psi so there is as little distortion as possible, since distortion (we're running 4000rpm at the tire) can pull the bead away from the rim.
For the record, my tires are 24.5 inches in diameter.
And for time over distance, the best I can provide at the moment is 225mph at the beginning of the mile and 198mph at the end. "Average" speed 211, for a time of 17 seconds for the mile.
As for belly pans whys and why nots, I have a full belly pan on my 355 Ferrari with small "tunnels" at the back. Now it's rear engined and side mounted radiators, so that helps, but I believe the V12 front engined Maranello is fully panned also. And you have to do something with 500hp worth of heat plus the cats. It's really no big deal to take the pans off for maintenance, and I believe with only minimal engineering you could make regular maintenance simple.
As to cooling, two reverse NACA ducts dumping the heat out into the turbulence behind the front tires might work.
Also as to driveline efficiency, I can only say that in these forums the most RWHP I have seen from a stock S2000 is about 215. Now I did add an Electromotive TECIII to run all the injectors, ignition, etc., so I believe we picked up some ponies there. But I mostly think it's from the extra efficient Chevy 7.5" rear end. It's got a very high pinion shaft placement to minimize drag for better mileage. That and a 2.28 rear end ratio (and 80 psi tires) might give us quite a leg up on a stock S2000 4.11 rear end.
Phew!
I'm tired.
Hopes this helps you help me, and as always, thanks!
holy crap, you guys are smart.
if you need finance help im your man... kinda.. not really...
im a finance major, but i still got another year and a half left.
And Eric thanks for your efforts. And yeah the underside of a 355 is roughly the same as the Enzo with rear diffusers that produce 267lbs of downforce at 170mph (or something like that). The suction produced is because the underside is similar to the rear section of an inverted wing and is running in ground effect.
And the Cd of my race car is mostly just for the fun of knowing. I'm going to start on a new one (any year now) that will have even a smaller frontal area (which should be lower wetted surface simply because less sheet metal for the body), but the fact remains that a long wheelbase with the Center of Gravity forward of the Center of Pressure is still the most reliable way to live through a high speed run.
Now I have had a bunch of discussions about whether to build a scraper with 3/4" ground clearance (see photo below of a one liter car that goes 321mph; using a turbo on fuel, but still), or one up in the air with more or less a boat shaped bottom with the lowest point about 3/4" from the ground so that the boundary layer buildup spills to the sides before generating any substantial drag. It's a bit harder to produce wheel fairings, and a little less stable, being higher, but overall more reliable aerodynamically, I feel. I mean what if a scraper gets the nose airborne a couple of inches at speed? Instant back flip, etc.
Anyway, I don't want to wear out my welcome, but your replys are always well thought out.
And don't fret about the 355 it was only high 5 figures. But until you've spent some time driving one you won't realize just how inexpensive that is. You just have to believe it when I say there is NOTHING like driving a Ferrari. Come to San Diego and I let you experience (drive) it. You'll never be satisfied with anything else.
