This covers how to make undertrays that work. It might not be the longest lasting (or the prettiest), but with a couple of hours and about $10 in materials (depending on scrounging ability) you can see if it's worth building properly.
Front undertrays are aerodynamic aids that serve to reduce drag and lift. The rear one does much the same. The theory is that the easier the underside of the car cuts through the air, the less positive pressure is present there, and less drag and lift is developed. That's a bit simplistic, but it will do.
The finished product will look like this... Yes the front one has a downward angle, but read step 1 to see why.
Step 1: Assess the Situation
Look under the front and rear of your car. I was confronted with this view. It's not the best aerodynamically, but that thing jutting down does actually have a purpose (first pic). In the second pic you will see that behind it is the oversized gap between the front of the engine and the radiator (that's what you get when you have a V6 where a straight 6 can fit), and in the third pic you can see the radiator and air-con condenser.
The plate currently does three things.
1. Provide a negative pressure behind it to extract air from the engine bay
2. Provide a slightly higher pressure in front of the radiator (to encourage airflow through it
3. Act as a stone guard for the cast alloy sump
I want to keep all 3 of those aspects while improving aero. Which means I have to run a sheet from the bottom rear of the front bumper to that plate. I also wanted to block air from going into the part in front of the wheels.
The rear section is just untidy. The other side is more fuel tank in place of the exhaust. I wanted to cover this, but unfortunately the exhaust meant that I could only block in the middle and right hand side of the car.
Step 2: Ingredients
Cable ties, $3 pack from auto parts store
"Sealing and joining" tape (non fabric-backed duc tape), $2 on special from the same place
Scrap sign material. Light, stiff, and free from my local signwriter. A 2400 x 1200 piece would have been $25 and been enough for the front and rear undertrays that I made that day.
Scissors (to cut tape)
Tin snips (to cut plastic)
Knife (to score plastic for folding)
Drill (slightly larger than cable ties, for drilling mounting holes)
All up cost should be around $30 if you bought everything new. I didn't, so it cost me $5.
Step 3: Making the Front Tray
To start with choose a piece of scrap plastic that was the same width as the gap between the rear of the front bumper. Trim the corners until it sits level with the plate, with the other (trimmed) end tucked underneath the bumper.Then drill holes in the plastic and in the plate to attach the sheet to the plate. Do the same with with the front edge of the sheet and the bumper. If you have places where there is a gap between the sheet and the bumper (where you pull down on the sheet to close it, not where the sheet is too short) put your attaching points there, as it makes it neater.
I didn't have a single piece that was big enough to do the whole front, so I needed to make separate side plates. Much like the main section. Cut to size, then trim around anything in the way. Drill holes then cable tie.
Tape up your gaps
Step 4: Making the Rear Tray
Again I didn't have a large enough piece to do the whole lot in one piece. So I had to persevere with what I had.
Cut it to the rough size that you want and hold it up to your car. Note the position of anything that will get in the way (such as the tow bar and its plug, the bumper clips, etc). Now cut these out, along with the rough shape of the rear of the bumper. Drill holes for the rear mounting points (on the lower edge of the bumper).
Now locate some mounting points for the front of your tray. You want them as close to the front of the tray as possible (so you have a smooth transition). I used my fuel tank mounting straps.
The right hand side was tricky for me, it required a compound curve, and the use of thinner (and slightly more flexible) material.
To make a compound curve in sheet plastic, mark one of the folds across the curve (i.e. the fold line).
Now mark closely spaced lines ALONG the second curve (i.e. the lines would go around the curve, not mark the edges) where you need it to be.
Cut along these lines, right to the end of the plastic.
Attach the flatter part of the plastic to the car, then fold the (very dangly and stringy) remainder to wherever it needs to be. Tape there, or otherwise attach it (I was lucky enough to be able to tuck it under one of the cable ties mounting the main part). There will be some that won't fit any more (such is the nature of compound curves), so cut them off and seal the gaps with tape.
Tape up your gaps.
Step 5: End Products
Stand back, admire, and think about what why the factory didn't do it in the first place.
Having driven the car with the new undertrays in place, I have found that it is ever-so-slightly quiter, indicating less turbulence. No fuel consumption testing has taken place, yet.
Why didn't they do it this way to start with?
For the front, everything that comes through that gap below my number plate is now trapped. This means rocks, dirt, bugs, birds and mitusubishi's will now have to be manually removed from my bumper. That would be the main reason this approach was not taken. And it would have cost them another $1.50 per car (over the 120,000 cars per year sold, that's a lot of money, and adds up even quicker when you save in other areas).
For the rear the money, and access to the fuel tank is the only real reason. Although the fuel tank never needs to be removed except in case of flooding or accidents, so it was probably just money.
Note that the rear undertray still needs some attention (the lip between the front edge of the tray and the fuel tank)
Next month (if my fuel economy testing provides a positive result): ABS Plastic versions of the same. These will look OEM, cost more, be harder to make, and last almost indefinately.