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1) Interesting that it seems to have reduced aerodynamic lift. (right?)

2) The car is lowered, right? (so likely not an issue at stock height)
1) yes - at the current diffuser angle, I expect near maximum downforce due to the diffuser. I think an important note here is the bottom of the car is flat with my current suspension setup, so I'm getting no downforce due to body tilt. One of my aerodynamic sources emphasized that the diffuser's most important job is to "drive the underbody airflow." The 13 degreee angle is too steep and isn't the best angle for dowforce or efficiency.

As a side note, by blocking the cooling flow under the car and through the wheelwells, I might be getting some front downforce due to the hood vent vs underbody flow. This is covered at the racelouvers website.

2) yes, the car is lowered and running performance wheels/tires.

I plan to adjust the fore/aft tilt to about 0.3 degrees by lowing the front 0.25" and raising the back 0.125". I might see a 10% increase in downforce ( see post #16)
I also plan to reduce the diffuser angle to improve the airflow. If the angle's too steep, the air will separate in the diffuser, effectively forcing a smaller angle. (see post #16)
 

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I'd like to provide a bit more details to answer ETS's question number 2 ( the care is lowered, right? )

This might help provide some context to compare it to others who have lowered their cars and seen no change, or a lot of change in drag.

I've removed the OE airdam, and replaced it with small deflectors which help channel the air under the car in front of the front tires. I plan to do this for the back as well.

As I've said previously, the battery hangs down lower toward the back of the car; you can't just measure the rocker panels to establish the "bottom."

So, what I did is make some adjustable spacers that could be used to measure the space under the car; one for the front, and one for the back. I then got a piece of straight aluminum angle, and fit it between the spacers and the car's bottom. I placed the front spacer approximately 4" in front of the front axle - this corresponds to the chassis stiffening bar. The rear spacer was placed about 19" in front of the rear axle, which corresponds to the rearmost battery support beam or that rib I mentioned previously. These two points are the inflection points that match the front and rear low points of the car. The distance between these points is 73".

The space between the ground and the front inflection point is 3.71" The space between the ground and the rear inflection point is 3.725" This shows a slight forward tilt, but for all practical purposes, we could call it 0 degrees.

Another reference is the space between the top of the tires ( I'm using 205/50 ZR15 tires ) and the top of the fenderwells. In the front this is 2", in the back that space is 2 7/8". So, if you looked at the tire/fender space or the rocker panel/ground angle, the car has a pretty pronounced forward tilt. However, the actual bottom is pretty flat.

So, if i want to increase the forward tilt, I could do what I posted in the previous post. This would result in an a tilt of 0.294 degrees.
 

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Thanks, but what I meant was that others who might want to streamline their chassis wouldn't scrape any more than stock, at the original ride height, right?
From that standpoint, yes the stock ride height would be no problem.
I was careful to add the minimum thickness; in this case, the ABS was 0.125 thick" + the button head bolt's head height of 0.125"
There are no brackets that add to the overall thickness.
I suppose there would be ways to reduce the thickness further, using aluminum panels with carefully placed stiffening ribs and countersunk fasteners. Done properly, it would reduce the aerodynamic drag of all the bolt heads and brazier head rivets I used. See post #17 about this; it was a big enough issue for the early aircraft builders to study...

The scraping issue I had with the diffuser was due to the sides being too low; this is a totally adjustable parameter - not limited by fasteners, material thickness, etc. You can see what I'm talking about in post #31 - the sides of diffuser near the jackstands are still too wide (before I trimmed them. This sides are important to limit the amount of air pulled in sideways to the diffuser. If I add vanes in the middle to direct the air, I will have to be careful to provide more clearance than provided for the diffuser sides. Other things, including the bulges in the diffuser for the battery pack ribs are no problem.

Even in a lowered car, the scraping issue is not much different than it would be without the bottom panels. For example, I said that the front tire-fender clearance in the front was 2". The ground clearance in the front is 3.71 inches. If it bottomed out the suspension in the front so that the fenders rubbed the tires, there would still be 1.71 inches of clearance. This is why I thought it would be reasonable to lower the front 0.25" more. However, one does need to be careful about speed bumps and steep driveways.

Similarly, the back tire-fender clearance is 2.875" and the ground clearance is 3.725". If this were to bottom out, I would have 0.85" of clearance. In this case the clearance is a bit tight, and in order to get the correct body angle, it might be best to raise the rear about 0.125". The benefits: more downforce and a bit more clearance; depending if I adjust the bottom out bumper as well and the spring mount.
 
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