Belly Pan Installation
by Victor Tikhonov ©2002
[This was performed on an electric car - low heat]
 I am saying "supposedly" reducing air drag because I did not make formal measurements with and without pan and covers, in non-windy weather both directions and record Wh consumed. I will probably never take the pan off just for doing these experiments - too much work and little practical benefit. To me it is intuitively obvious that the smoother underbody - the less C_d, and the fact that I can't attach a hard number to my thinking does not discourage me from doing it. Plastic is cheap (I use two 122x244 cm (4'x8') polyethylene sheets about 2 mm thick, similar material to what a gallon bottles for milk are made out of, just thicker) [sheetmetal for high heat areas] and easy to cut to required shape. Pick the material which is easy to bend and cut but not very flexible so it does not vibrate at high speeds.
All I needed is to cut out some material where suspension parts stick out and screw the sheets by the edges to the underbody with metal self threading screws and large fender washers. The rear sheet goes first and the front one - second so the pocket between them is facing rear of the car. Another side benefit - your under hood compartment is always clean and free of the water and road dirt spread around by spinning tires.
The process is somewhat labor intensive but straight forward. Preparation is a key - you'll need a scissors capable of cutting plastic, drill and sheet metal screws with large fender washers. Raising the car is almost mandatory. A grease pit comes very handy; realizing benefits of having it I decided to make it which by itself tuned into another big project...
So, first you make a templates for the pan shape. Take a large enough piece of clear wrapping plastic film and temporary stick it to underbody with a pieces of modeling clay. Ideally, it's 3 person's job, two stretch the film from the front and the back of the car, and the third one glues it to under body. When done, take a fat permanent marker and outline the cut outs for suspension members, driving shafts, anything which moves and would interfere with the pan if covered. Mark approximate places for screws bolting pan to the body. Don't forget to wrap the plastic to the wheel wells.
When that done, cut the real plastic based on the pattern you outlined leave the front part of the front piece and rear part of the rear piece uncut yet - you'll do it to fit after pan will be installed. Also don't drill any holes at this point yet - if you do, they will be in wrong place. In my case I had two pieces and since the front one will cover the the rear one overlapping about 10 cm, the rear piece gets installed first. Even if you find one solid piece of plastic I wouldn't recommend to make the pan as one piece - it is awkward to handle and unless the car is up side down, will be hard to line up and put in place.
The holes into the body for the screws should be drilled through the plastic as it pressed against the metal - this way both holes (in the pan and in the body) will match. Do one hole at the time because when a screw is tightened, it tends to pull and slightly shift whole piece and other holes will not match. Usually underbody is stamped not flat and have a profile designed for rigidity, so the bumps, pits and flat parts of the metal all arranged to maximize body strength. Try to pick sticking out areas for screws - this ensures that the plastic will stay flat and reduces risk of poking the carpet through from underneath.
Victor Tikhonov
www.metricmind.com
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