Amateur Aerodynamics

Continuing our dissection of the untruths you'll encounter online , today we'll visit the idea that simple "rules of thumb" can be applied to any car for maximum drag reduction. "Rules of Thumb" are Not Reliable The claim: Modifying the aerodynamics of your car is as easy as following some simple rules for boat tail angles, diffuser angles, air dam height, and more. Lowest drag is guaranteed if you adhere to these rules. One particularly disingenuous feature of these "rules of thumb" is their common attribution to authoritative sources. Hucho did indeed include figures of various streamlined shapes in his book, but nowhere does he give a rule about maximum taper angle. The reality: Rules of thumb abound in online aerodynamics discussions and message boards. One well-known web forum has accepted, for example, that boat tail extensions should not deviate more than 22 degrees from horizontal and that air dams should not extend lower than the lowest co

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 squa