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A Practical Guide to Aerodynamic Modification

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Updated August 15, 2023 Tuft testing shows the streamlines on a car as the yarn aligns itself with airflow while you drive. Gas prices have recently reached their highest level in nearly a decade. You may find yourself looking at your car, wondering if it’s possible to use less fuel on your long commute and keep some money in your pocket. You may have heard of people who modify their cars to get better fuel economy. You might have even seen cars like the Aerocivic, a weird-looking contraption that was reported on in mainstream media articles during the gas price spike of 2008-09. Would doing something like that work on your car? Can you modify the aerodynamics of your car at home? The good news is, you can! The better news is, you don’t have to (and shouldn’t) make your car look like the Aerocivic. Air drag has an influence on the fuel economy of cars, and that influence is greater the faster you typically drive. You can also do a lot more with airflow than just reduce drag. Many peo
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Aerodynamic Modification Doesn't Have to be Expensive or Complicated

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Reading through a Youtube post recently on the perceived roadblocks to aerodynamic modification, I was disheartened to see many commenters espouse the idea that effective mods require huge expenditures of money and time, lots of trial and error, or won’t be effective at (legal) road speeds and therefore aren’t worth doing. These attitudes couldn’t be more wrong: aerodynamic modification of road cars can be done very simply and cheaply, and the results can be dramatic.   Myth: To effectively modify your car’s aerodynamics requires a powerful computer and CFD or thousands of dollars’ worth of wind tunnel time.   Reality: CFD and wind tunnels are the two test environments most amateurs are somewhat familiar with and put a lot of faith in. However, there are huge problems with both of these, as I’ve intimated before in several posts .   CFD, or computational fluid dynamics, uses mathematics to model airflows. This can be a very useful tool in the initial development phase of a new ca
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Optimizing a Tail for Low Drag: Part 4

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Curvature Now that I know from my first round of testing which panel orientations have attached flow, how the pressure behaves with changes in angle, and a rough prediction of pressure drag reductions from all that information, I’ll move on to a larger test buck that will start to approximate a full tail. This buck is almost as long as my maximum length requirement and, rather than a flat panel like my first board approximations , has some curvature in it. Specifically, the extension here bends from an angle of about 20° from horizontal at its front to 23° at its trailing edge, in between the shallowest and middle angles I tuft- and pressure-tested: “Conventional wisdom” says to bend the tail in a convex curve like this for lowest drag. But is that “wisdom” correct? Only testing will answer that. I’ll test this buck in this configuration as well as with an added spoiler that, when affixed to the tail, is horizontal: More fun with the miter saw. All in, I’m still sitting at $0 inve
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Optimizing a Tail for Low Drag: Part 3

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Initial Testing To start the process of designing a drag-reducing tail, I threw together some boards that would give me some adjustability of their angle relative to the rearmost surfaces of my car. Then I headed out on the road for initial testing. Before you start testing, it’s important to recognize potential shortcomings and what your tests can actually show you (as well as what they can’t ). In this case, separate boards at the top, bottom, and side of my car do not replicate a complete, solid tail; rather, I’m using this test to get an idea of what might be appropriate dimensions and taper angles to start my investigations of a full tail. I’ll use these data to try and predict the drag changes from the various angles and then use those as a jumping off point to design the real tail rather than just guess at a shape.   You will see people misunderstand this all the time online. Commonly, someone will make a change to their car and then use one tank’s measured gas mileage as “pr
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