Optimizing Aerodynamics of a Truck: Part 5

After my last round of testing, I made a permanent version of the air dam extensions I tested:

This brings my truck’s aerodynamic drag down to around 84-85% of what it was before I started this project:
 

Modification	Percent Change
mirrors removed, grill blocked	-4.0%
9” air dam	-8.6%
air dam extensions	-3.8%
Total	-16.4%

 
I didn’t expect to get that much with these changes, especially because I haven’t even tested anything on or around the bed, rear undercarriage, tailgate, rear wheel housings, or rear bumper.
 
Tailgate
 
Many websites claim that lowering a truck’s tailgate increases its drag. This was even explored on an episode of the popular television show Mythbusters; the show’s hosts put a model truck in a water tunnel to illustrate the recirculation that happens in the bed (and ran some fuel economy tests with the tailgate down, removed, and replaced with mesh). You can see this recirculation if you tape tufts to your truck’s bed floor, like I did a while back:

The tufts generally point forward—showing that flow along the bed floor is moving forward. And, although it seems counterintuitive, this happens with the tailgate lowered as well. Notice the tufts on the gate itself in the images above; they still point sideways/forward when the gate is down, not out the back.
 
What does this mean for drag, though? Well, that isn’t clear from the tufts. I’ll use a throttle-stop test to see if there’s a change in drag associated with lowering the tailgate on my truck. I expect there will be; one study on production trucks found as much as a 5% increase from putting the tailgate down, on a second-generation Ford Ranger.
 
Spoiler
 
Less clear, though, is what effect adding a spoiler to the tailgate will have on drag. Twenty years ago, truck tailgates didn’t have any sort of elongation on their top edge. Starting with truck redesigns after the turn of the millennium, flat spoilers began to be molded into the sheet metal of the tailgate with a plastic cap added to the top of it. On the current generation of trucks, these tailgate spoilers are generally a few inches long, rounded (longer in the center than at the edges), and may have an additional short, vertical flap:

2022 Chevrolet Silverado. The Silverado has a short flap in the center of the tailgate.

2022 Ford Maverick. The Maverick's flap extends almost the full width of the tailgate.

2022 Toyota Tacoma. The Tacoma has no flap...

...while the 2022 Tundra has a flap similar to the Silverado's.

I suspect that any change in drag on my truck won’t be measurable if I mock up a spoiler similar in size to these because they’re so small. Since I’m not constrained by any total length requirements (believe it or not, this truck is a few inches shorter than my Prius. Remember when compact trucks were actually compact?), I decided to use a 10” long plywood board for testing. I used some scrap 2”x4” to cut supports to hold the spoiler at four progressively steeper angles:

I’ll test at 0°, 15°, 30°, and 45°. If you’re investigating an easily-changed parameter such as this, test it in a range of positions; this will be more informative than picking a single one since it will show how an aerodynamic characteristic (in this case, drag) changes with the parameter (spoiler angle)—real-world differential calculus!
 
The board will be screwed to each support and everything taped down with Gorilla tape to hold it in place for testing.

What will these do? I suspect that the steep spoiler:

…will increase drag. The lower angles and flat spoiler? No idea; hopefully they'll reduce drag. Let’s find out.
 
Results

Configuration	Speed	% Change	% Change (corrected)
standard	93 kph	0.0%	0.0%
tailgate down	91 kph	+4.3%	+3.6%
0° spoiler	91 kph	+4.3%	+3.6%
15° spoiler	91 kph	+4.3%	+3.6%
30° spoiler	89 kph	+8.4%	+7.0%
45° spoiler	88 kph	+10.5%	+8.8%

 
Analysis

Turns out, I’ve saved myself a lot of time designing and building a permanent spoiler right now—because all of these increased drag! Even the flat spoiler, which surprises me. Given the prevalence of flat tailgate spoilers on production trucks, I thought this might work to decrease the drag of my truck. What gives?
 
Well, perhaps the optimum drag-reducing spoiler on a truck with an open bed is shorter than what I tested. Perhaps the dimensions and shape of the bed, bed sides, and side rails affect spoiler performance. Perhaps the shape of the roof and cab height matter. Perhaps the shape of the tailgate itself and rear bumper affect things. Perhaps the underside of the truck and the bumper height from the ground influence the spoiler. Who knows? Any one or a combination of these things (and more) might affect what a spoiler of this size did on my truck, and they are all parameters I can investigate through further testing.

Since I tested at a range of angles, I have a pretty good idea now what a spoiler this size does as its angle from horizontal increases. Plotting drag change versus spoiler angle makes this clearer:

The shallower angles don't really change in how much they increase drag over no spoiler; as the angle steepens, the change in drag per change in angle increases and then starts to taper off as it approaches 45°. In mathematical terms:

This is all "differential calculus" is, fundamentally: relating rates of change.

Any time you have an opportunity to discover relationships like this in your testing, use it! If I find in future tests that, say, the steep spoiler reduces lift and I want to build a permanent version, I will already know its effect on drag. All that knowledge cost me was a little time.

Previous: Optimizing Aerodynamics of a Truck: Part 4