Custom car aero

[theunchosen]

I've been sketching how I want to layup the carbon fiber for my Atom wannabe, but I'm not sure whats the best for the rear.

Its going to be extremely light, no extra frills, chassis, half canopy, engine bay, suspension, drivetrain, manual steering and probably manual brakes with massive rotors. In that sense I can't sacrifice any of the front end wedge shape and maintain the neccessary downforce for. . .aggressive cornering.

Obviously the best aero is a full boat tail, but I'd be sacrificing lift and it would require more CF(I would have to use FG for something with that much surface area thats not structural) than I want to pay for. So my question is how badly would 12.5 square feet of rear flat surface detract from FE and top end speed?

The exhaust, engine bay venting and pre-cooler will all be dumping heat into this flow to lower the pressure difference slightly. The oil coolers for the transmission and engine will be dumping heat into the underbelly air flow near the tail and the radiators will be dumping heat into the sides of the CRX-like tail. Those factors will not influence its Cd because thats tested sitting still in a wind tunnel but realistically with such a small vehicle with mid-rear its going to be dropping a substantial amount of heat into that flow.

I'm open to any suggestions but I do have some inclinations. If those get in the way I apologize ^_^.

[theunchosen]

sorry forgot to drop in the sketchup I've been playing with. ..

Obviously I don't want the sharp edges but I wait till I am finished playing with the model to round those out otherwise fab is sometimes annoying.

I'm not going to be shipping it off from CAD to have it fabbed so its a guideline for how I want to make the molds.

Its color coded for different CF layers. Obviously you can only layup so much at one time and I can't do shapes that are extremely complicated with a 2 hour layup, so I broke it up into layers. Blue, red, yellow are CF(Chassis, suspension mounts/lower mounts, engine upper mounts) Green and Teal are likely to be FG to save $. The boat tail components(unpainted) will be FG and I don't know if they are visible in the pics but there are two side-mounted radiators, bottom mounted oil coolers for trans and engine and a precooler mounted on the rear deck at the top all of which are also unpainted.

[aerohead]

What is the frontal area of the car? At what point,as a percentage of the overall length of the car does the frontal area occur when measured from the nose? Or does the body continuously "wedge" to an ever-increasing cross-sectional area to a maximum of 12.5 Ft-Sq at the back?

[theunchosen]

The frontal area almost all occurs before 3/4 of the length of the vehicle, from there it wedges back down on the top and up on the bottom. The almost part is for the radiators that protrude slightly along the rear after the frontal area.

Frontal area is 14.5 sq ft.

It wedges up until 6 feet from the nose and then it starts boat-tailing. Max chassis height difference is 3' which makes a very tight fit for the engine and trans. chassis width is 5 feet. It boat tails at a somewhat steep angle on top and a shallow one along the underbelly for 1.5 feet after the apex of cross sectional area.

Total length of 7.5 feet as it sits now, 5 feet wide and just 3 feet tall. Obviously I haven't integrated the wheels and suspension yet so it will be a bit taller than 3 feet ^_^.