Common Misconceptions in Aerodynamics: Part 6

Last time, I wrote about the myth of waxing your car for lower drag. Next, we'll look at a misconception that's a little more nuanced.

The Front of the Car is Just as Important as the Back for Reducing Drag
 
The claim: The front of the car doesn't really matter for low drag. As long as there is attached flow over the front, modifying the back of the car is the only way to reduce its drag.

Spoiler alert: "Hucho's book" says nothing of the sort.

The reality: The half-truth that “the back of the car is aerodynamically more important than the front” is repeated over and over online. Simple statements like this are more often wrong than right, and you can probably guess why this one is more nuanced in reality than many claim.
 
Think back to Part 3 of this series, on the simplicity of aerodynamics. Aerodynamic forces are generated by the action of the fluid moving over the entire surface of a car. If we divide that surface into a bunch of tiny, imaginary squares and measure the pressure on each square, we can multiply that pressure by the tiny square’s area and get a force vector—ie an imaginary arrow with a magnitude (its “strength”) and direction (pressure always acts perpendicularly, or “normal,” to a surface at every point on that surface exposed to the fluid). Map the entire car this way and it will have a bunch of arrows sticking directly out of it everywhere on its surface, the strength of each arrow/force proportional to its length. Summing all these arrows together by simple addition in each of the three dimensions gives one resultant force, but that resultant is purely imaginary; it is only a convenient way to think about the force that simplifies measurement and calculation.
 
The important concept here is that the action of the aerodynamic pressure everywhere on the car’s surface produces the force acting on it. Clearly, what happens at any one point on the body is as important as what happens at another point in terms of generating this force. In other words, changing the airflow such that one of these force vectors sticking out the front of the car is increased by, say, 10% in x (bold type indicates a vector, in this case the vector that aligns with the car’s longitudinal axis) has the same result as if another vector on the back is reduced by the same amount in the same direction: both actions will have the same effect on the total drag, the resistance to motion in x. One isn’t “more important” than the other; the result is the same.
 
Now, this isn’t to say that more of the drag, to pick an arbitrary axis, isn’t generated at the rear of the car; because those surfaces point backward, and because aerodynamic pressures at the top and back of a car are usually lower than atmospheric i.e. negative, they can contribute more to drag than the pressures/forces at the front of the car. But online commenters think this means that only the rear is important when it comes to reducing drag, and that the front doesn’t matter much if at all.
 
This is simply not true. R.H. Barnard points out in Road Vehicle Aerodynamic Design, “Because of the high flow speed and consequent low pressure, [the front surface] can even provide a small negative contribution [to pressure drag]. This fact appears contrary to common sense because it means that the front of the vehicle and the windscreen are actually pulled forward, and not ‘blown’ backwards as would be expected; however, experimental measurements show this to be true.” After reading this for the first time, I decided to investigate his claim by measuring centerline pressures on a 2013 Toyota Prius. It turns out that, aside from a small area at the base of the windshield, there is negative pressure from the front emblem all the way up to the roof—negative pressure which, because of the direction those surfaces face, contributes thrust and reduces drag! Compared to a classical shape (flat hood, upright windshield), a continuously curved front like this is important in reducing drag, and probably the reason more and more cars tend to have nicely curved front ends and windshields that avoid abrupt changes in orientation.


Yet online commenters don’t seem to understand this. I posted the results of my pressure measurements on a well-known web forum and was immediately met with skepticism, argument, and outright disbelief. One person accused me of misunderstanding pressures and aerodynamic forces in general (incidentally, this person is a vehement “template” promoter). I believe this is the natural result of the community never bothering to test things for themselves, and their lack of familiarity with any current resources or textbooks on car aerodynamics. In effect, they shout into a self-supporting echo chamber. Don’t be like them. Read, verify, design, and test (seriously--don't believe me! Go measure pressures on your own car).
 
What may seem true at first blush is the idea that it is easier to modify a car for lower drag by focusing attention at the back. But even this is not necessarily so. Various home modifiers have shown it is perfectly possible to add things like turning vanes and air curtain ducts, reshaped noses and/or hoods, different windshields, air dams, and undertrays to the front of their cars—and some have been tested and shown to be effective in reducing drag and/or lift (for example, I reduced drag on my pickup by more than 10% modifying only the front). This idea that you can or should only focus on the back is, unfortunately, an erroneous one.

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