Air dam thoughts?

[ExDelayed]

This is on my old Dakota. Her belly is quite messy, and with the layout of the drivetrain, both axles, t-case, exhaust and differing cross member heights, a belly pan would be somewhat difficult to pull off. I still may consider trying to fill the void on the opposite side of the truck, equal to the fuel tank, but that's not what this thread is about.

I've been driving it for most of the 16000 miles I have put on it, with a partially damaged front air dam. The previous owner must have tried to drive over something, idk. Needless to say, it was very filthy, from an aerodynamic standpoint, with exposed suspension, open frame rails, etc. I finally got that replaced, and am now toying with the idea of an air dam.

With the number of parts that hang below the bumper, the air dam would be quite tall (~6-7") to make it hang equally to the lowest points of the truck (differentials, front cross member). With this being my work truck, and occasionally needing to go offroad, I don't want to lose too much of my approach angle. Optimal placement of the dam would be equal to the factory one, but what if I place it further backward? Flat across from tire to tire would probably be bad since the air wouldn't naturally have a curve to follow. What if I place it between the factory dam and flat against the tires? I have included a link to an Imgur gallery with my thoughts, it's at the bottom of this post.

I already have material for the dam that will handle the resistance and the mounting hardware. I would need to find something the bridge the gap between the factory dam and the new one, to keep the deflected air from getting back into everything I am attempting to cover.

Would I be hindering my MPG, since any air going under the factory dam would now be dumped directly next to/in front of my tires, instead of having a chance to go over the pickup?

Click for pics!

[Grinder74]

I would think you're going to trap a lot of air under the front. If you have to set it that far back I would make it one big radius, so some of the air bleeds off around the tires. I think the tire tread air dam would be great for a truck like yours, would take a beating.

[ExDelayed]

The trapped air is what I'm concerned about. I'll end up pushing the air, creating an invisible bow wave, instead of it being compressed under that angled cross member and heading under the truck.

The truck itself has a fairly flat nose, though I have about a foot of space to play with. I could build some tire spats on the factory air dam, or possibly heat it and force it outwards, allowing the air dam to be a little wider. That would also help the new air dam from dumping all of its air on the center of the tire.

[California98Civic]

Have you studied aerohead's pickup?

Regardless, though you increase drag area with a big airdam like this, I still think it is likely a win... air to the side is still better than under. Why not build and test a prototype with cardboard? You could do some A-B-A testing coasting down hill from 0 mph or along a flat 50 mph-0 mph. In the first case measure speed differences. In the latter case measure time and distance rolled to a stop in neutral.

Also, you could add a tonneau cover to the bed--even make one fairly cheap. If you have great skills there are dudes here who have built inverted boat tail shapes onto their beds behind the cab.

[ExDelayed]

I've read a lot of his posts. Actually had the chance to inspect Gumby79's boat tail in person as well. A boat tail is eventually on the list, but is slightly impractical at the moment with how much stuff this bed hauls around. I do smile a bit when there is plywood in the back, giving me somewhat of a tonneau.

With how irregular my normal trips are, I'm half thinking of just giving it a go. I've got plenty of supplies, and they were all free, it's just time.

[freebeard]

What I would do is a half-tonneau, their almost as efficient as a full aerocap. Except for having all the weight behind the rear axle if you use it for lockable storage it would be advantageous. The airflow is 6-8 inches above the tailgate typically. You could knock the corners off or round it.

In the front the stagnation point rules. That's the point where the air has to decide which way to go. Adding frontal area that far back will have little effect. It needs to be furthest forward. Which leads to problems with the approach angle.

Consider an air dam that only covers 90% of what hangs down. That should reclaim an inch or two. Then leave it long in front of the wheels, with a notch in the center for clearance on unpaved roads.

[ExDelayed]

Being that I had all the parts just sitting in the shop, I built it just to see. It ended up sitting about 5" behind the leading edge of the factory air dam, and making a fairly decent curve to directly in front of the tires. I eyeballed it, and ended up making it hang a bit too low, but it handled the freeway just fine today, and only appears to have touched down a few times. I'll run it through the week, and trim it down this weekend. I should have its first tank of fuel through it too, so I'll be able to see if there is any improvement.

Thanks for all your insight, everyone.

[ExDelayed]

I've only ran two tanks through it since I installed it, and it seems to be working. I eyeballed the height when I built it, and made it slightly too tall. It sits about 6" behind the leading edge of the factory air dam, and has a curve that has it meet the corners of the stock piece. I have not installed the filler panels between the factory part and mine yet, since I wanted to see if it needed changes.

I think I will nudge it forward slightly and trim it down a bit, but I'm fairly happy with the results.

The first tank with it installed got 18.5 MPG, and was mostly freeway. The second was 16.5, but was almost all city, with me hauling around some bricks and a small box trailer full of tools.

I'll post pics later, I'm on mobile right now, and they are on the laptop. Thanks for your advice everyone.