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Originally Posted by diss7
1: Quite wildly different information...
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Based on my understanding of aero, there are two main goals.
1) You want fast, laminar/smooth air flow under the car. Fast flow --> lower pressure --> downforce (as it acts on the lower surface of the car). There will be flow separation and turbulence, whenever you have to accommodate a wheel, cooling ducts, or the ground. Your goal is to keep the bulk air flow as laminar as possible. Sir Mix-a-lot would have my head for this, but... "A flat bottom is ideal."
2) Reconnecting the air paths (upper and lower) should be as gradual as possible, and it should minimize the "dead" volume of air behind the license plate. The Kammback/teardrop shape is ideal, but impractical. If you want downforce from a spoiler, you can exaggerate the diffuser angle to take advantage of the low pressure zone that creates.
The optimal angle is usually one that expands the cross-section as rapidly as possible without causing flow separation, turbulence, or instability. Vague, I know. Numbers like 6-10 degrees are often thrown around, but a gentle curve will usually outperform a piece of flat plate.
Quote:
Originally Posted by diss7
2: Exhaust flow.
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Adding air below the car will only disturb the flow and increase the pressure (minimally). Both are bad things. That's why designs like the LFA, Lotus, etc look like this:
Quote:
Originally Posted by diss7
3: Leading to the diffuser. I understand that the smoother this path, the better.
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Yup!
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Originally Posted by diss7
3a: Is it then beneficial to undo the factory engine bay "exhaust" path (where hot air escapes) put new non vented panels in, and instead redirect air upwards and out a vented bonnet.
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Interesting idea! Heat management and pipe routing would be a challenge. It might be reasonable if you have a turbocharger and turn up from there. There's no room for exhaust... but you said it's a racecar. The NHRA had alot of people doing this, and there's been consideration for banning it, since fluid spray and soot can get be rather tough to see through.
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Originally Posted by diss7
3b: Again related to question 3. A front air dam is going to reduce underbody air flow, good or bad thing for the diffuser / overall ground effect?
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Air dams are generally a good thing, as long as you're not scraping the ground... and don't care about fuel economy. Frontal cross-section is a major player in drag force (not coefficient).
Quote:
Originally Posted by diss7
4: Seeing that it appears universal that the lower the overall car is, the more compounded these effects are, why then are more people/racers not using a 15" wheel and a 580 slick?
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Smaller wheels/tires will lower the car and improve final drive ratio. Lighter wheels/tires will reduce rotating mass and unsprung weight. If you can get the braking force/cooling and optimal tire width in a 15" package, by all means, go for it. You may sacrifice sidewall stiffness, but the upside is definitely there.
Quote:
Originally Posted by diss7
5: If now had car very low, low airdam, diffuser, the car will then try to seek to suck in air from the sides?
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Yes, but not that much. In the CFD that Hancha Group has done, there didn't seem to be much effect... but similar to air dams, the lower you can go with sides skirts, the better you'll isolate the two air paths.