belly pan thrash 2008 - Fuel Economy, Hypermiling, EcoModding News and Forum

lovemysan · 05-03-2008, 11:23 AM

It was time to renew my inspection so I decided I put in fresh fluids, turn the front rotors and fix some nagging issues with the pan. I had one major advantage this time. I found a drive on lift to use for free.

I removed the side panels over a year ago and had yet to reinstall them. The issues I was having were as follows. I needed a radiator duct , 50% of the air through the grillblock was spilling under the radiator. No jack ports. NOISY, floppy aluminum down the center and unnecessary vent holes. BP frame striking roll front roll bar when jacked up. Duck taped extra vent in front. Wheel spoilers crumpled, and falling off. I added the typical unibody jack points plus one in the back under the center control arm mount. The radiator duck was cake to fab. I bought 3/16" polyboard for the main side panels and had some 28g steel to run down the center under the exhaust. Its much quieter now. The polyboard was 24.99 from oshleins and works great! I like it better than coroplast.

The pics were taken with a D rebel XTI. I borrowed this camera and obviously it was set to expose shots differently than my a410 in automatic mode.

As you can see from this picture the exit section of the pan is too severe an angle. I believe this why I don't see stronger results. I intend to fab a mild diffuser and a spoiler also. Along with tuff and coastdown testing eventually. As also believe that I need fences running inside the wheels to keep air from spilling out.

tjts1 · 05-03-2008, 12:43 PM

I don't think there is anything wrong with the ramp angle at the back. Its still much less severe than the angle of your back window and its helping leave a smaller hole in the air behind the car. Thats one of the best belly pan I've ever seen. Nice work.

aerohead · 05-03-2008, 02:09 PM

From the work on bellypans published in Hucho's text,2-1/2 degrees is reported to be the ideal angle for the upswept diffuser.If you'll measure the "run" of your proposed diffuser,from origin to termination,and multiply that by the sin of 2-1/2 degrees,it will spit out the proper "rise" for the panel.For,say,a 36-inch long diffuser,the vertical component would be just a little over an inch and a half ( for a meter,it would be about 38-mm ).
Or you can draw it on the floor of the shop with a protractor and measure it directly.

lovemysan · 05-03-2008, 06:23 PM

Currently its 6" of rise over 36" run. Not optimal. The rear control arms are the reason for the extreme angle. I really need rework the back of the pan. I also need to get hucho's book.

tasdrouille · 05-03-2008, 06:42 PM

Phil, I would be inclined to think that you are half right on this one as the optimal diffuser's angle is dependant on the diffuser's lenght. Hucho's book showed 2 diffusers of different lenghts. For the longer one the optimal angle was around 2.5 degrees, while for the shorter one it was closer to 3.5 degrees.

Fabrio posted an image from Hucho's work on this here

MetroMPG · 05-04-2008, 12:13 AM

Thanks for posting the pics. I have to agree with tjts1: one of the cleanest installations I've seen.

stevey_frac · 06-24-2009, 07:02 PM

I'd like a bit more discussion on the diffuser angle. I kind of think that a diffuser angle of 2.5 degrees is unpractical. to extend all the way back. It would scrape everytime i left my driveway. Is it an all or nothing kind of function? If i were to use say, 5 degrees, would I end up with no aerogain at all?

winkosmosis · 06-24-2009, 07:45 PM

15 degrees is supposed to be the upper limit for a rear window right, so why wouldn't that same angle apply to a diffuser?

NeilBlanchard · 06-24-2009, 09:47 PM

Hi,

I'll take a stab at the reason for the difference in the angle: on the top, there is more "available" air to flow in and fill the lower pressure zone. Underneath the ground itself cuts off the largest source of replacement air, so it can only flow in from the sides to fill the lower pressure zone.

lovemysan · 06-25-2009, 04:05 PM

The main problem with my belly pan is not the rear exit angle. But the lack of front airdam. The problem is too much air is taken under the car and as it works its way to the rear it piles up, till it touches the ground and then tumbles. This problem is exaggerated when the car is loaded because the ride height in the rear can be reduced a further 3". I've found that a belly pan requires more design than I originally anticipated. I know what I need to change but lack real motivation now.