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Old 11-07-2011, 05:33 PM   #21
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One thing to add, if you add rake into the mix, the flat bottom of the car can add a bit of downforce...but this would be negligible in anything but a prototype (true flat bottom) car.
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Old 11-07-2011, 05:40 PM   #22
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If you want reduced drag, then don't run a splitter. Well, I guess I should say, don't run a splitter that protrudes past the lip/bumper.

The height of the splitter will affect downforce far more than drag.

I hate making these generalizatiosn, but...air passing under the car does not add drag (other than the fact that the bottom of the car is not completely smooth)

By adding the splitter you divert more air over the top of the car which increases the speed/pressure differential from air going under vs over the car. This creates the downforce...but adds drag.

If you simply smooth out the bottom of the car...you will reduce drag, but actually decrease downforce(add lift). Great for a straight line/salt flats car, not so much for track car.

There are a couple of interestng books you should read if you are really interested. One in particular talks about the primitive nascar/oval track bodies and how the front downforce is generated in a very unorthodox way. The air coming uder the car is so turbulent (because they have no splitter) that it creates a low pressure area resulting in downforce.

http://www.amazon.com/Competition-Ca.../dp/1844252302
My design is heavily based on this:

http://www.nptec.com/imgsti/aerobytes.pdf

V.1: I feel a difference when there is an airdam only (like TR lip only) with nothing but the OEM radiator panels/shrouds.

V.2: With the splitter to smooth the airflow underneath right below the air dam, not protruding past the bumper, lift was decreased even more. More stability.

V.3: Essentially I added another airdam in front of the splitter, so the "splitter" became more of an undertray. This is the last picture I posted.

I believe Cl/Cd can be improved and from my rudimentary findings with my design, it is exactly what happened. For example, if I remove the front-most airdam, lift is probably decreased more-so than with the airdam. With these variations, smoothing the front til the subframe was the most beneficial in front stability, a splitter as a staganation point reduced lift the most (but made the rear the lightest), and the result was that the rear end was much lighter.

I have a basic diffuser beginning at the rear of the eccentric bolts to the bumper at 9.82 degrees. At 6.52 degrees, I found heavy turbulence at speeds 50mph and above. 9.82 was more ideal and the car is very balanced 90-100mph. I haven't hit Buttonwillow yet with this stuff but am anxious to.

The more I read on the topic, the more I realize application-specific testing is the only way to progress further, so I was hoping someone else had done Miata-specific testing.

EDIT: I do have about 11/16" rake at the pinch welds. I'm not too far into flat paneling the bottom of the car, mostly due to cost (my rear diffuser was basically $12-20 in parts lol), the next step is using aluminum underneath the front and rear subframes and then the rest later on.... but I am trying to do more testing with my stuff as is before proceeding further (for example the diffuser is still a prototype that I threw together one Sunday evening).
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Old 11-07-2011, 06:11 PM   #23
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The more I read on the topic, the more I realize application-specific testing is the only way to progress further, so I was hoping someone else had done Miata-specific testing.
Bingo! All of my experience thus far was on my Cobra, I had planned to just go spec miata after spending so much time with "development" on the Cobra, but the dark side is luring me back in. I'm already looking at aero for the turbo car.
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Old 11-08-2011, 05:27 AM   #24
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Bingo! All of my experience thus far was on my Cobra, I had planned to just go spec miata after spending so much time with "development" on the Cobra, but the dark side is luring me back in. I'm already looking at aero for the turbo car.
I see. I've yet to see any Miata-specific testing unfortunately. I think doing aero for a turbo car is a lot easier because the drag consequences are not as severe. I'm not sure where you're located at, but I plan on testing my max speed around the roval with and without my aero. On all season tires to reach mechanical grip limitations sooner than later. Coast down tests seem to have too many variables for me to consider trying.
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Old 11-08-2011, 02:04 PM   #25
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hm arjay already chimed in with what i was going to say lol
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