Optimizing Aerodynamics of a Truck: Part 2

 
In Part 1 of this series, I showed how to use throttle-stop testing to a) get reliable measurements of changes in drag, and b) how to use this measurement technique to test some simple modifications—blocking the cooling air intake at the front of the truck and removing the side mirrors. Together, those amounted to a 4% reduction in drag, so after the test was completed I made permanent versions of both aerodynamic modifications:


Next up, we’ll look at one outlandish modification and one typical change that a lot of pickup truck owners make.
 
Modifying A-Pillar Flow
 
When I go to auto shows, I take lots of pictures of details on production cars. One of those fascinating details is the prevalence of what I call “fenced” A-pillars; that is, the windshield is slightly inset below the A-pillar and a sharp edge on the pillar stands proud of the glass:

Here's an example, on the 2019 Hyundai Elantra.

My truck is an older design and doesn’t have this edge. Instead, the windshield blends smoothly into a rounded A-pillar. What happens if we change that to a more modern design?

There’s only one way to find out: test. No amount of guessing will determine whether adding a permanent fenced A-pillar—which would entail a large investment of time, design work, fabrication, and money for materials—will be worth it. Could it reduce separation at the A-pillar? Minimize flow to the side over the windshield? Or would it make the flow worse, exacerbating the A-pillar vortex or increasing turbulence further back on the window? How on earth would I know just by guessing at something so complex? So to start, I spent five minutes mocking up a fence out of scrap cardboard that I could tape to the pillar:



Look at that, three-dimensionally shaped and everything. Not bad for literally 5 minutes of my time.

Then I taped tufts to the window and windshield and headed out to my usual tuft-testing location, a 4-mile long, square route on the University of Illinois’ South Campus.
 
Here’s the flow over the side window without and then with the fenced A-pillar:


Top: no fence. Bottom: fence in place.

And here’s the windshield, again without and then with the fence:


Top: no fence. Bottom: fenced.

No change! And if you think you saw an improvement with the fence: the first set of images are actually reversed, with the fenced flow above and no fence below. Here they are again, in the proper order:


Top: no fence. Bottom: fence. (For real this time).

But here’s the takeaway: testing will save you time, effort, and money. Don’t guess; something you thought would work, like fencing the A-pillar to improve flow over the side window and/or windshield, might not. And if it turns out not to work, as it did here, you will only be out a little of your time and the cost of some leftover cardboard (this was actually the same piece of cardboard I used to test grill blocking). As importantly, you won’t have to BS to curious individuals about how you “think” a modification is reducing drag or changing airflow when you actually have no idea!
 
Bug Shields
 
Bug shields—small plastic pieces that advertisers claim send bugs over your truck or SUV instead of splattering on the windshield—used to be more popular. A quick Google search reveals that you can still buy them for most current models and nearly all older trucks, but they just don’t seem to be as common anymore. It used to be that bug shields were flat plates:

(Image source: f150forum.com)

But now they are always molded plastic that conforms to the shape of the hood and fenders:

(Image source: anchortruck.com)

So, what do these do? Is there any truth to sellers’ claims? Again, the only way to find out is to test; an ounce of testing is worth a pound of theorizing.
 
Using a slightly larger scrap of cardboard—left over from an IKEA futon box—I cut out a quick approximation of a bug shield and taped it to the front of the truck:



Then I taped tufts to the hood and drove it. Here’s the flow over the hood and windshield without the shield:



And here it is with:



Holy crap—something that I expected would have a minor effect on the flow over the hood and none on the windshield turns out to be disrupting flow over both surfaces, almost in their entirety! So yes—the claims that bug shields can send insects over your truck instead of onto the windshield may, in fact, have some truth to them. They may also be trashing the flow over your truck’s front surfaces, so if you’re thinking of fitting one, test it. My quick mockup is exaggerated in its height, so it may be that a commercially-produced shield is better than this cardboard example—but may also be worse at actually deflecting bugs.

There’s only one way to find out.

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