Optimizing Aerodynamics of a Truck: Part 3

Will this barn door reduce drag? Only one way to find out.

In Part 1 of this series, I measured drag reduction on my 1991 Toyota pickup with the grill blocked and mirrors removed. In Part 2, I did some qualitative flow testing, observing the behavior of wool tufts with a modified A-pillar and a bug deflector. Today in Part 3 we’ll go back to measuring drag reduction as we look at air dams.
How Do I Decide Which Test to Use?

Before we get into it, an anecdote. When I learned to drive, I remember my instructor taking us out onto a 4-lane highway to have us practice changing lanes. Look in the mirror, check that the next lane is clear, signal, and move over, she told us. When my turn came, I glanced at the mirror and began to move over. Driver’s ed cars had right-side brake pedals, and she stomped on it. There was a car in the other lane I was trying to move into. “See what you’re looking for,” she said.

When testing aerodynamic modifications, see what you’re looking for. That starts with identifying the type of test that will show what you’re looking for. When I first tested a spoiler strip on my Prius, I tuft tested the rear window—which didn’t tell me much, as the tufts didn’t change with the spoiler in its rearmost location (however, it told me a lot with the spoiler at the top of the rear window). So I went back and measured pressure changes on the window and roof, which gave me a clearer picture of what effect the spoiler had.

To test an air dam on the truck, I’m not very interested in seeing what tufts look like on the dam—it won’t tell me anything useful. Similarly, I could measure pressures on the dam but that won’t really tell me anything useful either; I already know that it will develop high pressures on its front. What I’m interested in is whether the air dam will reduce drag, and from reading various papers on the development of production trucks such as the 1988 Chevrolet C/K and 2015 Ford F-150, I know that this depends on complex interactions between the air dam itself, the shape of the front end and hood, the cooling air flow, and even the size and shape of the tailgate. A test that will directly quantify changes in drag will work best for determining whether building a permanent air dam is worthwhile, and just how big it should be if so. See what you’re looking for.

I mocked up a large dam out of an old refrigerator box. This dam extended 13 inches below the front valance, to around 6 inches from the ground. I wouldn’t make a permanent dam this low, even if this turned out to have the lowest drag, since this will impact the truck’s ability to go off road. Before taping it to the front, I marked off two intermediate heights, 9 and 4.5 inches below the valance. Over the course of the test, I trimmed the dam to see if there was a change in drag with different air dam heights.


Unfortunately, the dam mockup at its lowest height wasn’t usable on the road; it flexed so much that it pulled away from where I had taped it at the bumper and the whole thing folded in on itself. After adding some more tape and trimming to its intermediate height, however, it stayed attached and didn’t deform, so I was able to measure the drag change between a 9-inch dam, a 4.5-inch dam, and the stock front valance.

That turned out to be fine, since I had never intended on making a permanent dam as deep as my initial mockup. One of my objectives is to preserve the off-road ability of this truck since I use it for camping, and a 13-inch deep air dam would have compromised that. I was just curious if the large dam would reduce drag more or less than the intermediate heights.

What were the results?






91 kph

94 kph

95 kph

% change in drag




For a single modification, this reduction in drag is huge! I expected a fairly large change, but this surprised me. A 9-inch deep air dam will be permanently installed on the truck once I design and fabricate it.

One thing I’ll have to be careful of going forward is to compare percentage change in drag from any one modification with the original drag of the truck, since I’m implementing mods that work as I go along but want to get an idea how much total drag reduction I’ve accomplished to see if I can hit my goal of 20%. To do this, I’ll have to use math to manipulate my measured percentage change for any one modification.
Say I have a vehicle that has a drag coefficient of 0.50; I make a change and find that drag has been reduced 20%. In absolute terms, that represents a change in CD of 0.10 (we can assume for the purposes of this example that the reference area does not change).
Now, imagine I have that same car and that I’ve already reduced its drag 50%--that is, its CD is now 0.25. I make a change and measure a 20% reduction in drag. Does this same percentage change as before represent the same absolute change in drag? No. Now, that 20% change represents only 0.05 reduction in CD, half as much as before. The lower drag of the vehicle amplifies the percentage change of a smaller absolute change in drag.
To compare my measured percentage changes in drag with the original drag of the truck and keep track of approximately how much total drag reduction has resulted from the changes I’ve made, I will have to mathematically adjust my measurements. Fortunately, this is a straightforward process and not complex.
My first test, which found -4.0% change from the truck’s original configuration, is easy; I don’t need to apply any correction factor.
  100.0% (original drag)
–    4.0% (change from test)
    96.0% (percent of original drag the truck now has)
This second test, and all subsequent tests, will need to be corrected since my measured percentage change for any one of them is a percentage of the already reduced drag, not the original drag. This air dam, tested on the truck in its original configuration, may have reduced drag by 10% or 12% or 5% or 3%—who knows. Testing it with the grill block and mirror plates already installed accounts for any interactions with those changes, and with those installed the dam was worth 9.0%. But now I have to correct that mathematically to compare it to the original drag of the truck and try to keep track of my total drag reduction since that percentage change is not the same as the absolute change in drag.
   100.0% (original drag)
–     4.0% (change from grill/mirrors)
–     9.0% (change from air dam) of 96.0% (percent of original drag when truck was tested with air dam)
     87.4% (percent of original drag the truck now has)
It doesn’t make a huge difference here, but the total percentage reduction in drag is not
+  9.0%
It is actually
+  9.0%*0.960
Note that these are all approximate (even though I’m following the rules for significant figures). This is just so I can see if I’m generally on track to hit my drag reduction goal and not overestimating drag reduction by mistake (which will happen if I just add the percentages together with no correction and will be exacerbated as the truck’s drag gets lower). When I’m done, I’ll do a final test with all the mods on and off in the same session to verify the total change and get a more accurate number. In the meantime, I’ll start keeping a counter of estimated change at the bottom of these posts to keep track.

Goal: -20%
Total drag change counter: -12%

Update: I added a permanent air dam shortly after completing this test. I went ahead and ordered a 25-foot roll of HDPE plastic for this and future modifications. The dam is bolted to the front valance through rivnut inserts and braced where needed to the valance or underbody components.


  1. Some good thoughts there ,thankyou.

  2. Maybe a Gurney flap on the top of the tailgate next ?

  3. Back again, what thickness of HDPE did you use ? and height of the roll please .

    1. I used 1/8" HDPE, available online for not too much money; I bought a 50-foot roll of 22" wide plastic for around $50. You can even find it in different colors if you want something other than black.

  4. Nice writeup. Read all 3. Thanks for the inspiration!


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