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Old 04-06-2022, 09:04 AM   #1 (permalink)
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Optimizing Aerodynamics of a Truck: Part 1

Read original here, and follow this truck's progress as I test more changes.

***

I’ve written before about my truck, a 1991 Toyota Hilux. This was my grandfather’s truck years ago; it went to my father and then to various of my brothers before I acquired it last year. It was in pretty rough shape, having sat at the back of my parents’ driveway for several years:



I had it hauled down to a mechanic to verify it would start and run and, several weeks later when I was able to fly out, I drove it back up to my parents’ house and set to work.



After a new timing chain, front axle, brakes, suspension dampers, front valance and rear bumper, and other odds and ends, I drove it over the mountains and back to Illinois.


Coeur d'Alene, ID, just down the road from where this truck was originally sold, in Kingston, ID.

Surprisingly, fuel economy wasn’t that bad! The truck averaged just under 25 mpg. But, I wondered, could I improve that? Is it possible to reduce the aerodynamic drag of a truck like this? Measurably reduce drag?

Making a Plan

Unlike my modifications of earlier cars, where I had followed conventional Internet wisdom and guessed at what to change, I wanted to test and measure the changes to this truck.

To do that, I decided to use throttle-stop testing to measure changes in drag directly. I wired in a throttle position sensor display and built a simple, adjustable stop that fits under the throttle pedal. This allowed me to hold the throttle opening at a constant angle, limiting engine power and giving the truck a top speed adjustable from ~90 kph to ~100 kph.

Next, I read several papers on CFD studies of generic pickup trucks, wind tunnel testing of a Ford Ranger, air dam and cooling drag optimization of the 2015 Ford F-150, and the development program of the 1988 Chevrolet C/K (which turned out to include a very interesting parameter study of a wind tunnel model). I made a list and sketched several devices I wanted to try: a deep front air dam, a tailgate spoiler, a cab roof spoiler, grill blocking, mirror removal, rear wheel air dams, and a tonneau cover.

Testing

First, I needed to verify that the throttle stop would give me good, repeatable results, allowing me to use this method to measure drag changes. I identified a few miles of straight, flat, low-traffic road (plentiful around here, fortunately) and went out several times, refining my technique each time until I was able to get good repeatability. Testing windows up against windows down, I found the following changes in speed at different throttle openings:



This suggests that lowering the windows on this truck increases drag by around 4-5%. I can’t be any more exact than that—but there’s no need! In real-world conditions, there’s too much natural variability to be as exact as a wind tunnel or CFD; what I’m after is seeing whether a change decreases or increases drag and approximately how big that decrease or increase is. An important benefit of testing in the real world is that these are the conditions the truck actually drives in: windy, on a real road, in a turbulent atmosphere, past trees and buildings and windbreaks and other cars.

Just as important, the consistency of the results showed that this method would indeed allow me to measure drag changes.

First Modifications

Since the windows up/down check was successful, I tested two simple changes to begin, in the same session: removing the external mirrors and blocking the air intakes at the front of the truck. I used the largest throttle opening to magnify the changes in drag and hopefully be able to measure them.


Mockups for testing don't have to be pretty! Make them quickly and cheaply; you don't want to spend a lot of time and effort on something that turns out not to work.

Results were as follows:



These are about what I anticipated; trucks are higher drag than cars, and on this truck—with its fairly small mirrors and grill opening—we should expect that changing these will have less effect than on a more streamlined car, where they will likely account for a higher percentage of the overall drag.

One interesting thing to note is the result of blocking the grill and removing the mirrors at the same time; there may be some interaction there, or it may be that the actual change of both parameters isn’t enough to show up in my measurements when added together. I expect subsequent tests (air dam, spoilers, etc.) to show greater change than these two, which means they should show up more clearly in results.

Fabrication

Since these first tests showed that blocking the cooling air opening and removing the mirrors would in fact reduce drag, I went ahead and made the changes permanent. I had to make a blanking plate to cover the mounting holes for the stock mirror, so first I traced the outline onto a piece of scrap paper:


Yes, those are the installation instructions for the driving lights. Remember what I said about being cheap?

Then I cut that out and traced it onto thick plastic sheet, trimming that carefully with a pair of shears:


If you're careful, shears can be a very versatile and delicate tool. Cheap plastic can be procured by purchasing storage bins and cutting them up.

And finally, I caulked it into place:


That looks "almost professional," as we used to say in one organ shop where I worked.

For the grill block, I reinstalled the sheet aluminum pieces I had fabricated over the winter (the 22R-E is a cool-running engine, and I wanted to speed warmup times and keep as much heat in the engine as possible; now I know there's an aerodynamic benefit as well):



The grill isn’t blocked completely, but the lower grill has just two small openings and only the center section is open on the upper grill. I’m happy with the small drag reduction this combination of mirror removal and grill blocking gives; the bigger gains will come in the future with air dams and spoilers.

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Old 04-07-2022, 01:53 PM   #2 (permalink)
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Quote:
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Have you never considered blocking the grill from the outside instead of the inside? Wouldn't it lead to a greater aerodynamic benefit?
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Old 04-07-2022, 02:25 PM   #3 (permalink)
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The air is technically stagnant at that location.
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Old 04-12-2022, 03:01 PM   #4 (permalink)
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Quote:
Originally Posted by cRiPpLe_rOoStEr View Post
Have you never considered blocking the grill from the outside instead of the inside? Wouldn't it lead to a greater aerodynamic benefit?
We don't know without testing, and unfortunately the difference between the two will likely be much smaller than what I can measure on the road.

I did, however, redo the grill blocks--but not because of that; purely for aesthetics. I used the same heavy plastic I made the mirror blanking plates out of and was able to cover part of the center section of the upper grill in addition to what I had before:



Quote:
Originally Posted by freebeard View Post
The air is technically stagnant at that location.
We can't know where the stagnation point is without measuring, or how large an area is stagnant. I don't even know where it is/how big it is yet, since I haven't tufted the front of the truck or measured pressures. Plus, if this truck behaves like the 1988 Chevrolet C/K and 2015 Ford F-150 (two of the papers I mentioned above), the stagnation point will change with other modifications such as an air dam (I'll be testing various air dam configurations in the coming weeks).
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Old 04-12-2022, 04:00 PM   #5 (permalink)
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I suspect it's shaped like a bubble. Even the air that hit exact center has to go somewhere. Sideways at first and then around a corner.

The chamfered front end on the Cybertuck is instructive.
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Old 04-13-2022, 08:00 PM   #6 (permalink)
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I did, however, redo the grill blocks--but not because of that; purely for aesthetics.
The previous grille block wasn't bad-looking either. Actually I'd bet more folks would prefer its more concealed look.
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Old 04-14-2022, 03:27 AM   #7 (permalink)
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Hello Vman455,
Did you consider trying to block just the above bumper hole. Then the below bumper hole?
It might give you an idea of the stagnation area?
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Old 04-14-2022, 03:38 AM   #8 (permalink)
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I am surprised that just taking off the mirrors gave a two percent indicated gain.
Did you try with the mirrors folded?
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Old 04-14-2022, 08:45 AM   #9 (permalink)
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Quote:
Originally Posted by Whitworth View Post
Hello Vman455,
Did you consider trying to block just the above bumper hole. Then the below bumper hole?
It might give you an idea of the stagnation area?
Hmm...I'm not sure what you mean by "bumper hole"?

Quote:
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I am surprised that just taking off the mirrors gave a two percent indicated gain.
Did you try with the mirrors folded?
I didn't bother testing drag with mirrors folded after tufting the window and seeing that the flow didn't really change with the mirrors folded. (It cleaned up significantly with the mirrors removed). Anything smaller than 2% isn't really measurable with the throttle-stop technique.

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