My old truck pushes about 38.5 square feet of air going down the strip. In the past with a low power 440 powering it areodynamics really wasn't to much of a concern. It lost a few mph and a tenth or two in the 1/4. Now that I've got a little more steam under the hood I've run into some concerns. Tonite I took it out to get some WOT recordings with the LM-1 meter. I ran it up to 7000rpm in high gear, (4.56 gear, 31" tall tires, Torqueflite, 10" converter). It really gets there quick I might add. Anyway I noticed the front end start coming up on the truck and it swapped lanes on me. Not something I care to do every day by the way.
I read a while ago about Dick Boynton and his Max-Wedge D-100, he said he cut some slots in the pickup bed to release air from under the truck. Now this was almost 50 years ago and i'm wondering if it had any merit. I'd like to find a way of keeping the front end planted without lowing the truck. Any suggestions or ideas? Dave
A front air dam will stop a lot air from going under the truck, and almost certainly improve high speed stability, but it will also slow you down. Another, and probably better way to do this might be to lower the front of the truck as much as you possibly can, and leave the back sitting up high. A combination of lowered front springs, air dam, and smaller front wheels might work.
Another problem could be the low pressure area right behind the cab. If you can fit a cover right across the top of the tray, and completely close in that area, it might help too. A lot neater and less destructive than cutting away huge holes in floor of the tray.
Basically you want to reduce the high pressures under the vehicle, and minimize the low pressures behind the cab in the rear tray region.
I agree with warp speed. lower the truck, a front spoiler that dishes air off to the side, and a top for the bed. Maybe even a rear spoiler to help close the air coming around. I always count the space under the front bumper as part of the frontal area. If you get the right blending off the front spoiler in the front wheel area, you can suck the front down if springs allow and it will do wonders for your cooling system. The basics is make low pressure in engine bay an under truck, you get a natural high pressure for the rear part of hood to roof line, then you want to blend them together as best you can in the rear.
Something I found interesting is that the previous Nascar cars was getting around 480 lbs of down force on the front tires, mostly from blending the air at the front half of the wheel wells.
One thing to remember is where ever you make high or low pressure it exerts that force equally in all directions. Even against on coming air flow. This is how boundary layer air is formed and how air planes fly.
You better do something-like it or not-or pretty soon you may not have that truck-in 1 piece.
Don't forget about the wind, with high speeds and a gust of wind blowing the right way, the truck could lift a bit, come down on a angle and start rolling--not a pretty sight.
Air dams typically reduce drag, because the airflow under the vehicle is so turbulent. As mentioned, unless the ground clearance is up around a foot or so, the effective frontal area goes right down to ground level anyway, so the dam does not increase it.
A forward-extending horizontal lip ('splitter') up to 4" or 5" long (if it's not too out of character for the 'look' of your truck) adds further downforce, since the high pressure built up ahead of the pretty much vertical nose acts, as noted, in all directions, including down on this surface. (brace it well, we're talking hundreds of pounds at 150 MPH)
Caution: The above will work so well that if cornering too fast at high speed, the back of the truck will break away first (oversteer) (not a Good Thing!) unless a big spoiler (also as mentioned) is added at the rear.
One way to reduce lift is to carefully seal the grill area to the rad with filler plates, so that no air can bypass the rad, then seal off any redundant air entries and reduce grill area as much as possible while still maintaining adequate cooling. Excess underhood air builds pressure which causes huge lift on the hood area.
Testing has shown that a bed cover over the rear half only provides the greatest drag reduction. (don't know about lift)
In addition to the useful suggestions already offered, I would consider the side profile of the truck. When viewed from the side the truck profile is an airfoil.
For a pickup, the center of lift is located well forward which means that at higher airspeeds, lift will be experienced at the front. Air dam type spoilers (splitters) are very useful to try to keep the front down. But more important would be any method that would move the center of lift rearward.
