Originally Posted by rlogan
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.
My design is heavily based on this:
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).