Pickup air dam - too low? Jeep MJ

[Oceanic815]

Hello all,

I have the opportunity to pick up a fiberglass SCCA-style air dam reproduction from the original mold identical to the below Comanche race truck for my own Comanche, with the thought to improve aero.



Now as I understand it, there can be disadvantages to too large of air dam that extends below the underbody contours/parts. Would the SCCA one significantly hurt the aero/mileage vs. the modified S10 air dam on my rig (image below for comparison of the above, sorry for the shadows), and is this SCCA dam more for downforce than for streamlining?



Additional info: The above home-brew air dam's edges curve around to cover 2/3 of the tire and not completely cover the frontal area of the tire as the SCCA style does. Also, the S10 air dam is vertical; it looks curved back in the image.

Thoughts on this? I'm still new at this, eager to learn.

[aerohead]

Here's a link that ChazinMT shared,from a masters thesis on pickup aerodynamics.The author tested for affect of air dam height.http://csus-dspace.calstate.edu/bits...pdf?sequence=1

[kir_kenix]

Maybe it is just me, but the airdam you have right now "looks" more aero than the "race truck". I think a big part of the difference is because of ride height. Look at the "race truck," it looks lowered on much smaller tires than what you currently have. I would guess you would gain more MPG from trying to emulate the ride height than you would from the airdam.

[kir_kenix]

Also wanted to add...LOVE Comanches. I had one in highschool that the previous owner had put a mildly built 4.0 HO in with a manual, 2wd. It was a pretty darn good truck, but it was an electrical nightmare (OBD 1, Chrysler junk) and it was rusting apart.

I filled the box full of gravel one day, and the sides of the box swelled out and the rust broke free all over. That engine eventually found its way into a '93 2 door, where it served admirably as a fairly fuel efficient and capable fishing/boat hauling rig.

[Oceanic815]

The ride height I can change; it has a 1" puck on top of the coil spring up front and has 1" lift shackles out back. By removing the puck and reverting to the stock shackles I can get a little closer to the profile of the race truck. Since it's a 2wd it has lower springs than the 4wd models; however, several years ago the driver's side leaf gave up and the only replacement I could find was a set of 4wd leafs; the truck has a noticeable rake due to this.

I will see if I can get better pics of the homebrew air dam to post up.

[Thenorm]

looking at the race truck underbody, i would say that it is the right height. it is just low enough for the lowest hanging parts.
also it is a bit of a splitter design which moves the stagnation point downwards, further helping prevent air going underneath

[freebeard]

I'd follow aerohead's link, but they both look a little low to me. OTOH the bumper to fender flare transition is better, and you could just cut off the bottom 2 inches.

Have you thought about wheel spats and a half-tonneau?

[Oceanic815]

Originally Posted by freebeard

I'd follow aerohead's link, but they both look a little low to me. OTOH the bumper to fender flare transition is better, and you could just cut off the bottom 2 inches.

Have you thought about wheel spats and a half-tonneau?

Yes, I have considered a half-tonneau as well as building an aero topper for the bed. Wheel spats would take some doing as there is no good attachment points in front of the rear tires- it is only the bed wall without a flat surface on the bottom like under the cab.

Below is a better shot of the homebrew air dam. It will take some smoothing with some gorilla tape to get rid of the minute gaps.

Now that I am looking at it, I could extend it another 2" down for the lower control arms which are visible using some leftover lawn edging, and add more flaring for airflow at the tires withe remnants of the Rubbermaid trash can I used for the grill block strips.

[freebeard]

I've never built one of these, but I think it should work.



Fasten the conveyor belt material or whatever on two sides but not the third, and it should hold it's shape but deform on impact so it can fit closer to the tire and ground than otherwise.