Hey GRM!
I'm putting together a DIY aero kit for my rwd DE vehicle. Currently the front consists of a 2" chassis mounted splitter, vented hood, side skirts and 64.5" chassis mounted GT wing (rear diffusor removed at this time, probably not very functional imo).
Next phase of the plan is to shape two 4'x8' sheets, having them meet in the middle of the vehicle at the trans tunnel and running the length of the vehicle from just behind the engine to the diff around the drive axles. There will be reinforcement material on the inside (for additional rigidity, etc) which I will also be using as part of the mounting structure for eight contact points on each panel.
How does this sound in theory? Should I leave a small gap between the two panels to allow some airflow for exhaust? Or would a NACA duct be appropriate to add here? I can upload my rough sketches on the design.
Thanks in advance!
Some dedicated flow for exhaust cooling will be needed. It does not need to be a NACA type duct, but some sort of inlet duct is a good idea. Roll the front edge upward at the back of the engine bay to smooth the flow to the floor (avoid sharp edges facing the incoming air).
The number of attachment points will depend of the material and thickness. 8 points per side is not really enough for large flat sheets, unless they are thick and/or stiff (your mounting structure may be enough additional stiffness). Lots of small fasteners is better than a few large ones to distribute the loads and provide redundancy/factor of safety. There is no need to leave a gap between the left/right panels. It may help with cooling/servicing/access.
Thanks for the input! Think I could get you to elaborate a little bit more on how to best approach placement of the inlet for the exhaust? I understand this can be somewhat akin to a moving target because all setups are different but if I can form a mental conception of an ideal setup, that will be very helpful to me. A pic here would speak a 1000 words imo.
For materials I plan to use somewhere between 4-6mm aluminum or 4-10mm carbon fiber (with some carbon furring / hat channels I found to use as additional support and mounting points). I'm personally leaning towards working with carbon because all other materials are 2x or 3x the weight by comparison and my car is rather under powered by today's standards so I can accept the related cost. In my brief research so far, I've come to understand that the general strength of the carbon will be greater with the materials bonded as opposed to riveted. I'm hoping to keep the entire flatbottom under 20 lbs total.
Rough measurements so far are around 36" x 84" and will have what I estimate between 30-34" hat bracket placed laterally at each front (3 mounting points across) with two 40" hat brackets (3 mounting points per side) running longitudinally parallel and finally two U-brackets at each rear corner. For the aluminum, I'd imagine spot welding thin bars in similar locations and using L or U-brackets. So that gives a new grand total of 11 mounting locations with the front bracket spaced approx 17.5" from center hat tunnel(s) (spaced 13.25 internally within the channel), followed by another 17.5"to the rear mounting brackets.
I also wanted to ask about how to approach the inclusion of "turning veins" and similarly to the exhaust, find a general approach to follow as a guideline. I also have splitter tunnels with fender deflectors, topside fender vents and removed/very modified liners so there's definitely a lot of dynamic stuff going on that I'd like to try and use to my advantage. Figure I'd just fully commit and throw in the kitchen sink at this point.
Just as kind of an example, I want to build a tapered and curved ground affect aluminum tray for my Volvo, I plan on using fabricated Z channels to attach the tray to the underside of the car's floor. I plan on making a small 2" wide template forming 2" wide Z channels by hand and attach a 2" wide strip of metal to it to get the taper and curve correct and then have a sheet metal shop bend up the full width Z channels out of galvanized steel which comes in various thicknesses, maybe out of 16-18 ga. For large voids, I will attach L brackets where needed for spacers to attach the Z channels to it and the floor. I will also use lots of screws/rivets. 
